A High Pressure Time Projection Chamber with Optical Readout
Alexander Deisting, Abigail Victoria Waldron, Edward Atkin, Gary Barker, Anastasia Basharina-Freshville, Christopher Betancourt, Steven Boyd, Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi, Adriana Dias, Patrick Dunne, Jennifer Haigh, Philip Hamacher-Baumann, Sebastian Jones, Asher Kaboth, Alexander Korzenev, William Ma, Philippe Mermod, Maria Mironova, Jocelyn Monroe, Ryan Nichol, Toby Nonnenmacher, Jaroslaw Nowak, William Parker, Harrison Ritchie-Yates, Stefan Roth, Ruben Saakyan, Nicola Serra, Yuri Shitov, Jochen Steinmann, Adam Tarrant, Melissa Uchida, Sammy Valder, Mark Ward, Morgan Wascko
AArticle
A High Pressure Time Projection Chamber with OpticalReadout
A. Deisting * , A. V. Waldron * , E. Atkin , G. J. Barker , A. Basharina-Freshville ,C. Betancourt , S. B. Boyd , D. Brailsford , Z. Chen-Wishart , L. Cremonesi , A. Dias ,P. Dunne , J. Haigh , P. Hamacher-Baumann , S.B. Jones , A. Kaboth , A. Korzenev ,W. Ma , P. Mermod †, M. Mironova , J. Monroe , R. Nichol , T.S. Nonnenmacher , J. Nowak , W. Parker , H. Ritchie-Yates , S. Roth , R. Saakyan , N. Serra , Y. Shitov , J. Steinmann ,A. Tarrant , M. A. Uchida S. Valder , M. Ward , M. O. Wascko Cavendish Laboratory, Cambridge CB3 0HE, UK; [email protected] DPNC Université de Genève, 1205 Genf, Switzerland; [email protected] (A.K.); [email protected](P.M.); The Blackett Laboratory, Imperial College London, London SW7 2BW, UK; [email protected] (E.A.);[email protected] (P.D.); [email protected] (M.M.); [email protected] (Y.S.);[email protected] (M.O.W.) JINR, 141980 Dubna, Russia Lancaster University, Bailrigg, Lancaster LA1 4YW, UK; [email protected] (D.B.);[email protected] (J.N.) Department of Physics, Oxford University, Oxford OX1 3PU, UK Queen’s University, Kingston, ON K7L 3N6, Canada; [email protected] Royal Holloway, University of London, Egham Hill, Egham, TW20 0EX, UK;[email protected] (Z.C.-W.); [email protected] (A.D.);[email protected] (A.K.); [email protected] (J.M.); [email protected] (W.P.);[email protected] (H.R.-Y.); [email protected] (A.T.) III. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany;[email protected] (P.H.-B.); [email protected] (W.M.);[email protected] (S.R.); [email protected] (J.S.) University College London, Gower St, Kings Cross, London WC1E 6BT, UK;[email protected] (A.B.-F.); [email protected] (L.C.); [email protected] (R.N.);[email protected] (R.S.) University of Warwick, Coventry CV4 7AL, UK; [email protected] (G.J.B.); [email protected](S.B.B.); [email protected] (J.H.); [email protected] (S.V.) Physik-Institut, Universität Züriche, Rämistrasse 71, 8006 Zürich, Switzerland; [email protected](C.B.); [email protected] (N.S.) * Correspondence: [email protected] (A.D), [email protected] (A.V.W.)† Passed away on 20 August 2020Version 15th February 2021 submitted to Instruments
Keywords:
Time Projection Chamber, Optical Readout, Neutrino Detector Development, Hybrid Chargeand Optical Readout, Gaseous DetectorsMeasurements of proton-nucleus scattering and high resolution neutrino-nucleus interaction imagingare key to reduce neutrino oscillation systematic uncertainties in future experiments. A High PressureTime Projection Chamber (HPTPC) prototype has been constructed and operated at Royal HollowayUniversity of London and CERN as a first step in the development of a HPTPC capable of performing
Submitted to
Instruments a r X i v : . [ phy s i c s . i n s - d e t ] F e b ersion 15th February 2021 submitted to Instruments these measurements as part of a future long-baseline neutrino oscillation experiment such as the DeepUnderground Neutrino Experiment. In this paper we describe the design and operation of the prototypeHPTPC with an Argon based gas mixture. We report on the successful hybrid charge and optical readout,using four CCD cameras, of signals from
Am sources.
1. Introduction
High Pressure Time Projection Chambers (HPTPCs) are an area of growing international interest.The Deep Underground Neutrino Experiment (DUNE) envisions the use of an HPTPC as part of its neardetector and European groups have held a series of workshops on HPTPC development over the last fiveyears. Given the recent indication of non-zero CP violation in the Tokai to Kamioka (T2K) experiment’sdata [1], it is timely to quantify the potential impact of HPTPC neutrino ( ν ) detector technology onmitigation of the dominant neutrino-interaction cross-section uncertainties for the future long-baselineneutrino oscillation programme.Final State Interactions (FSIs) of nucleons produced in neutrino interactions are among the leadingsources of systematic uncertainties in neutrino oscillation experiments. Gas TPCs are ideal for preciselycharacterizing FSI effects because of their high track reconstruction efficiency, low momentum thresholdand 4 π angular coverage of final state particles, which are all key to distinguishing between interactionmodels. For example, the proton multiplicity and momentum distributions for neutrino charged currentinteractions on argon calculated by the neutrino interaction Monte Carlo generators NEUT [2] andGENIE [3] are highly discrepant in the fraction of events with few ejected protons, and at low protonmomentum, below 250 MeV/c [4]. This is below the proton detection threshold in water Cherenkovdetectors (1100 MeV/c) and below that of liquid argon TPCs, around 400 MeV/c [5]. A gas-filled HPTPChowever has a low enough momentum threshold to resolve FSI model discrepancies, and therefore anHPTPC has unique capability to address the driving systematic uncertainty in neutrino cross-sections.This paper describes the design, commissioning and calibration of a prototype HPTPC detector. Section 1.1describes the prototype detector and readout design, Sec. 2 motivates the choice of gas target, Sec. 3describes the high pressure vessel and the gas system, Sec. 4 details the TPC hardware including itshigh voltage supply and data acquisition system. In Sec. 5 and Sec. 6 the analysis of camera images andcharge signal waveforms, respectively, are explained and results of the commissioning measurements arepresented. Section 7 contains a combined analysis of the optical and charge readout signals. The need for lower momentum measurements [4] motivates the choice of a gas-filled detector for thetask of measuring neutrino-nucleus scattering. Another key consideration for the detector is that it hassufficient target mass to achieve a low statistical error on measured final state kinematic distributions. Thisrequirement drives the choice of a high pressure gas as it has higher density and therefore higher mass.The momentum threshold goal for our HPTPC prototype is designed to probe the discrepantlow-momentum region of parameter space [4]. The threshold goal for a well-reconstructed proton in argonat 5 barA (10 barA) is ∼
50 MeV/c (about 70 MeV/c). This drives choices in the readout design, such thatat ∼
50 MeV/c a proton track is sampled by ∼
10 measurements in the readout plane. We also aim to coverthe momentum range above 320 MeV/c (50 MeV kinetic energy) where no measurements currently exist[4]. The track length of a 50 MeV/c proton in a 5 barA argon target is ∼
10 mm. To achieve 10 samplesalong such a track, a readout plane with a granularity of order 1 mm is needed. Conventional segmentedpad planes of current experiments (e.g. T2K) have a pad size of order 1 cm at a cost of about 8 EUR per ersion 15th February 2021 submitted to Instruments channel. Given that an area of 20 m is realistic for the readout plane of a future HPTPC near detector at along-baseline neutrino oscillation experiment, a solution with a lower cost per channel is attractive.The transverse diffusion in pure Ar at 5 barA is too large to allow for drift lengths of several 10 cmwhilst permitting 1 mm track sampling. When adding a quencher such as CO to the argon gas, thediffusion is reduced, allowing for 1 mm track sampling and a 50 cm drift length (Sec. 2). A relatively new development in TPC readout technology that offers a low cost per channel is opticalreadout. TPCs have been in use since the late 1970s, typically with direct readout of the drifted charge.CCD optical readout of time projection chambers was first demonstrated in ∼ segmentation over a readout plane with an area larger than 1 m [11]. For a recentreview, we refer the reader to [12].An optical TPC is instrumented with a cathode and (several) anode electrodes which define its signalcollection and amplification regions. Ionisation electrons from charged particles propagating throughthe TPC move in the drift field to the amplification region where avalanche charge multiplication andscintillation photon production occurs (Fig. 1a). A schematic of how an optical TPC operates is shownin Fig. 1. The anodes may also be equipped with charge readout to provide high resolution tracking inthe drift direction, as in [11]. CCD or CMOS cameras view the amplification plane through lenses fromoutside of the pressure vessel containing the TPC and target gas, collecting the scintillation light andsubsequently providing tracking information in the amplification plane. The design considerations foroptical TPCs are described in detail in [12].In an optical TPC, the track reconstruction resolution in the amplification plane depends on the optical platescale. This scale is determined by the requirement that the object be in focus, which sets a minimum objectdistance given an image distance and focal length of the lens, and on the optical system demagnification,which is the ratio of the object to image distances. Typical demagnification values are 5-10. The area of theamplification region imaged by each CCD pixel (a ‘vixel’), determines the smallest unit of track segmentmeasurement possible with a given optical system and detector geometry. We define a vixel to be a boxwith an area of A vixel for the sides parallel to the readout plane and a height corresponding to the lengthan electron drifts during one CCD exposure time.The minimum sensible vixel size is determined by the transverse diffusion of the ionization electrons froma particle track in the TPC, as they drift to the amplification region. The track reconstruction resolution inthe drift direction is determined by the number of samples along the track, which depends on the tracklength, drift velocity, and readout rate.The momentum threshold for track reconstruction depends on the minimum deposited energy at which acluster of vixels can be identified as a track. This depends primarily on the signal-to-noise ( S : N ) ratioper vixel. In general, vixels with S : N > N S = (cid:20) ε particle W × G × ( γ / e − ) (cid:21) × (cid:20) T anode × T cathode × T window × T lens (cid:21) × (cid:20) × f × ( + m d ) (cid:21) × QE ∗ , (1) ersion 15th February 2021 submitted to Instruments where the first term in brackets is the number of photons produced in the amplification region, whichdepends on the ionization energy deposited per vixel by a particle with energy ε particle , the energy toliberate one electron-ion pair in the gas W , the gas amplification factor (gain) G , and the scintillationphoton-to-electron ratio ( γ / e − ) of the gas. The second term in brackets is the total photon transmission ofthe system, which depends on the transmittance of the lens ( T lens ), the pressure vessel window ( T window ),and the cathode ( T cathode ) and anode meshes ( T anode ) through which the CCD views the amplificationregion, averaged over the scintillation emission spectrum. The third term in brackets is the geometricacceptance of the optical system, which depends on the lens aperture to focal length ratio ( f stop ) and thedemagnification ( m d ). The last term QE ∗ is the CCD quantum efficiency averaged over the scintillationemission spectrum. Other elements which enhance ( e.g. reflections) or reduce the signal are not taken intoaccount.The noise per vixel depends on the quadrature sum of the shot noise which is (cid:113) N signal , the read noise N read , and the dark rate of the camera times the exposure time ( N pixels · R ( T ) · t exposure ): N N = (cid:113) N signal + N + N pixels · R ( T ) · t exposure . (2)In the dark noise term, N pixels is the number of CCD pixels grouped into a readout bin, t exposure is theexposure time of a pixel, and R ( T ) is the dark rate which is a function of temperature T . Here, a readoutbin is a group of camera pixels which is grouped together and read out as one. Typically a cooled CCD cansuppress the dark current to < (cid:113) N pixels × A vixel can be thought of as an effective pad size of the readout, where A vixel is the vixel area imaged by one CCDpixel.In the prototype detector described here, the vixel size is ∼ ×
236 µm in the readout plane, and thereadout binning operated was 4 × N pixels = 16) and 8 × N pixels = 64), producing an effective padsize after readout binning of ∼ and ∼ respectively. In this way, a 10 mm long track,corresponding to a 50 MeV/c proton, is sampled at 5-10 points, as the vixel area in the readout plane is asquare. The area A vixel is calculated by dividing the area imaged by one camera ( ∼ ×
71 cm ) by thecamera’s pixel layout of 3056 × and accounting for the readout binning. The height of a vixelcorresponds to the full drift length, since we operated the cameras with an exposure times of 0.5 s to 1 s. The prototype HPTPC detector described here is housed in a stainless steel (type 304L) vessel (Sec. 3)of volume 1472 L which is rated to 5 barG. We use barA to denote absolute pressure and barG for pressuremeasurements relative to ambient pressure. The vessel features feed-throughs for high voltage andinstrumentation, optical windows and camera mounting hardware. The detector layout is sketched inFig. 1. The vessel’s internal rail system supports a TPC, which has 44.7 cm drift length and 111 cm diameter(Sec. 4). The TPC drift region is enclosed by the cathode mesh at negative voltage and the first anode mesh.Two more anode meshes at increasing positive voltage follow in order to amplify primary ionisations.The working principle of the detector is illustrated in Fig. 1. A particle entering the drift volume ( e.g. a neutrino) scatters at a time t on an atom or molecule, thereby ejecting protons from the struck nucleus.These final state particles ionise gas atoms and molecules along their path (indicated schematically withan arrow in Fig. 1a). The resulting primary ionisation electrons drift in the electric field E towards theanode meshes ( t in the figure) and are eventually amplified in the high electric field close to the meshes’wires and between the meshes ( t in the figure). In the avalanche, electrons and photons are producedand the latter can then be recorded by the cameras, which provide an image of the interaction (Fig. 1b) ersion 15th February 2021 submitted to Instruments (a) (b)Figure 1.
Cross-sectional view of the HPTPC through the (a) plane parallel to the drift field E and the (b)plane perpendicular to E . A particle (dotted line) scatters on an atom or molecule in the gas at the time t ,ejects a charged particle form the nucleus which in turn ionises gas atoms along its trajectory (arrow, Fig.(a)). These ionisation electrons are moved by E towards the anode meshes ( t and t ) and are eventuallyamplified. Photons produced during the amplification are then imaged by cameras and provide the 2Dprojection of the interaction (Fig. (b)). with the locations as well as the intensity, where the latter is proportional to the energy deposited in thedrift volume. Reading out the charge signals induced by the avalanches at the anode meshes providesadditional time information, i.e the extent in time of the particle’s track in the TPC, which is proportionalto the track length projected onto the drift direction.The optical readout system for the HPTPC prototype described here uses 4 CCD cameras, which aremounted onto the high pressure vessel and image the amplification stage from the cathode side, throughthe windows of the pressure vessel, as well as through the cathode and anode meshes. Each camera viewsone quadrant of the amplification region, through lenses focussed on the amplification plane (Sec. 4.6).The HPTPC’s charge readout system reads the charge induced on the whole (un-segmented) planeof each of the three anodes. The signals are decoupled, amplified and shaped by commercial front endelectronics, and subsequently digitized synchronously in time with the CCD data acquisition.Throughout the paper we use a Cartesian coordinate system in which all electrodes are x / y planesat a constant z and where the z axis is parallel to the electric field direction. The origin is located in thecentre of the anode 1 mesh and z increases towards the cathode. In the x / y planes we occasionally usepolar coordinates where r points from the centre to the edge of the TPC.
2. Gas Requirements
The typical wavelength sensitivity range of CCD cameras is 350 nm to 850 nm (Sec. 4.6), and thereforethe gas is required to have a high photon (or electro-luminescence) yield in this wavelength range. Anoble gas is the obvious choice for the dominant part of the gas mixture, since it lacks the rotational andvibrational degrees of freedom which absorb photons.Gaseous argon has been shown to emit not only light in the Vacuum Ultra Violet (VUV), but also in the nearinfra-red (NIR) wavelengths [14]. Scintillation light measurements at pressures higher than 1 barA showthat the NIR light yield normalised to the number of amplification electrons decreases with increasingpressure [15]. This can however be compensated by a larger gain of the amplification stage. In [15] theauthors show that additions of CF serve as wavelength shifter to the visible (VIS) and NIR: In Ar gas witha small (5 %) admixture of CF , the scintillation photon yield in optical wavelengths is 0.1-0.3 per avalanche ersion 15th February 2021 submitted to Instruments − E/P (V cm − Torr − )10 t r a n s v e r s e d i ff u s i o n ( µ m / √ c m ) Ar , 5 bar Ar , 3 bar Ar , 1.0% CO , 5 bar Ar , 1.0% CO , 4 bar Ar , 1.0% CO , 3 bar Figure 2.
Transverse diffusion for pure argon and different Ar/CO mixtures simulated using M AGBOLTZ [18]. electron, and is a weak function of the reduced electric field. Neon, on the other hand, emits as well in NIRregion [16]. Admixtures of nitrogen have been shown to result in a higher intensity electro-luminescencein the VIS, as compared to the NIR neon electro-luminescence. A Ne/N mixture is therefore also a goodcandidate for a TPC with optical readout.We chose argon as the principle component of our gas mixture because an Ar based mixture is foreseenfor the HPTPC of DUNE’s near detector. Demonstrating the technological readiness of an HPTPC withthis gas mixture makes a strong case for using this technology as part of a near detector in a long-baselineneutrino oscillation experiment with far detectors with identical targets. Argon has already been proven toemit light at high pressure in the wavelength range our cameras are sensitive to [17]. Furthermore argon isconsiderably cheaper than neon gas.Operating a TPC with pure argon comes with the disadvantage that stable operation is notoriouslydifficult at high gains, and that the transverse diffusion is high. For a drift field of ∼
200 V cm − the thetransverse diffusion in pure Ar at 5 barA ( E / P ∼ √ cm [18] as can beseen in Fig. 2. An optical readout with cameras provides an effective segmentation of the readout planeinto segments of less than a 1 mm , as discussed in Sec. 1.1. The diffusion in pure argon for drift lengths ofseveral 10s of cm is too large to exploit the advantages of a fine segmentation. Adding a quencher reducesthe diffusion and enables higher gains under more stable operating conditions. For example, in Ar/CO (99/1) the diffusion is reduced by an order of magnitude as compared to pure argon (Fig. 2). This allowsdrift lengths of up to 50 cm whilst retaining the requirement that the transverse diffusion not exceed twicethe readout segment length of 1 mm.The typical quencher for an Ar mixture is carbon-dioxide, however CO has been shown to lower the lightyield [17]. N on the other hand is not a good quencher in Ar, but provides small reduction in the lightyield [17]. In this paper we experimented with pure Ar and different Ar/CO , Ar/N and Ar/CO /N mixtures to establish which gas provides the largest light gain in the NIR (Sec. 5.4). Other gases andadmixtures are also interesting to study, however, these studies are not part of the measurements for thispaper. ersion 15th February 2021 submitted to Instruments
Figure 3.
CAD model of the pressure vessel: The view on the door side (left), on the right left side – asseen from the door – (middle) and on the side opposite to the door (right).
3. High pressure vessel design
The pressure vessel design is shown in Fig. 3. The vessel is 304L stainless steel, with an inner (outer)diameter of 140 cm (142 cm). The total length of the vessel, including the domed ends, is 138.6 cm; thelength of the domed sections is 32.5 cm each, leaving a length of 73.6 cm in the cylindrical straight sectionwhich hosts the TPC. The weight of the vessel (empty) is 2370 kg.One of the domed ends of the cylinder is fully detachable to gain access to the vessel’s interior, e.g for the TPC installation. The detachable door is connected to the body of the vessel via a large DN 1500flange. The door and the body of the vessel are mounted to separate steel frames with wheels, both withadjustable feet for elevation adjustment. A double O-ring seal of viton and a rectangular silicone layerbetween the door and body flange sides provide gas tightness. The door and body flanges are clampedtogether with 8 hydraulic pistons and 8 screw-able clamps, with a force up to 50 N m. The helium leaktightness specification is 2.5 × − mbar L s − .The vessel flanging is indicated in Fig. 3. The door is equipped with five DN200 and four KF40 flanges(Fig. 3, left), while the body features one DN200 and four KF40 flanges on the side opposite of the door(Fig. 3, right), four KF25 flanges and one KF40 flange on the left side of the body (Fig. 3, middle), andfour KF40 flanges on the right side. The KF25 and KF40 flanges are used for High Voltage (HV), gas andvacuum system feed-throughs. The 5 DN200 flanges on the door are each equipped with a custom opticalwindow flange and camera mount incorporating a 60 mm thick quartz optical window. The body flangeshost two independent pressure relief systems. The first is a 5 barG burst disk backed by a 5 barG pressurerelief valve. The second, on an independent body flange, is a 6 barG burst disk.The interior of the vessel houses three steel rails along the walls, separated at approximately 120 ◦ , toallow mounting of equipment inside the chamber. The interior surfaces are shot blasted.The vessel is rated to a 6 barA. To verify this after construction the vessel was filled with water andsubjected to the test pressure of 7.2 barG for 10 minutes. No evidence of leaks or material deformationwas observed. The hydrostatic pressure was subsequently decreased to the working pressure of 5 barabsolute pressure and maintained for 90 minutes to verify the tightness of the pressurized vessel. Afterthe test the vessel was emptied and dried with nitrogen gas. All optical windows were installed duringthis test, validating the design of the custom DN200 optical flanges. Given the 1472 L volume, thevessel is a category IV pressure vessel. This hydrostatic test was used to follow conformity assessment ersion 15th February 2021 submitted to Instruments
HPTPC
Air compressorBurst discPneumatic valvePressure regulatorPressure gauge c o m p r e ss e d a i r T P C g a s v a c uu m li n e vacuum pump6 bar5 bar safety valve Pirani1.7 bar g a s A g a s B g a s C g a s D A-105 bar on bottles(to exhaust)
Figure 4.
Diagram of the gas fill and evacuation system for the HPTPC vessel. procedure MAT-17-CE-G-CRTO02/17 to obtain the declaration of conformity with pressure vessel directive97/23/CE.
The gas and evacuation system for the HPTPC prototype detector described here is shown in Fig. 4.The gas filling strategy for the HPTPC foresees to evacuate (and purge) the vessel prior to the targetgas fill. An Agilent Triscroll 800 dry vacuum pump is used to pump down the vessel to a pressure ofapproximately 1 × − barA before gas operations. The same pump is used to evacuate the fill line fromthe gas system to the vessel to reduce contamination, during the filling procedure or in case a gas fill istopped up to a higher pressure. The system enables mixing of gases from four different inputs, using 8Aura gas pressure regulators with manometers and threaded connections (4 in the primary 200 −
10 barstage and 4 in the secondary 10 − × − barA to just below atmospheric pressure. Since the Pirani gauge is not suitable for over-pressureit is protected by an electronically controlled valve when the pressure exceeds 0.8 barA. The slow controlsystem logs the gas pressure from the two gauges as well as the ambient laboratory temperature, measuredwith sensor SynAccess TS-0300, for later use in the analysis. ersion 15th February 2021 submitted to Instruments (a) (b)Figure 5. (a) The field cage before insertion into the pressure vessel and (b) after insertion. The latter pictureis photographed through the high-transparency cathode towards the amplification region and shows thefull TPC.
4. Time Projection Chamber
The principal components of the time projection chamber are the field cage and the electrodes thatdefine the drift and amplification regions. Figure 5 shows the field cage ring structure, and amplificationregion before the assembly is inserted into the pressure vessel (left), and in-situ – including the cathode –before the pressure vessel is closed (right).
The field cage (Fig. 5a) is constructed of 12 copper rings with an inner diameter of 111 cm, and lengthof 1.0 cm in z and 0.6 cm in r . The distance between two neighbouring rings is 2.5 cm. Each ring is suppliedwith HV via the cathode in series with 3 M Ω resistors held in place with compression fittings betweensubsequent rings. The last ring on the field cage facing the amplification region is connected to ground viaa resistor who’s value is chosen depending on the spacing between the final ring and the amplificationregion to maintain field uniformity. The total length of the field cage is 42.4 cm, resulting in a 44.7 cm driftdistance between the cathode and the amplification region.The field cage assembly is supported from the three internal rails on the pressure vessel by machinedDelrin parts. One set of these supports houses the resistor chain. The size of the support between thevessel rails and the field cage is adjustable. The cathode electrode is a 25 lpi (lines per inch) steel mesh made from 27 µm diameter wires. Dueto its low wire density the mesh has a calculated transparency of ∼
97 %, which allows for cameraimaging of the amplification region through the cathode mesh (Fig. 5b). A 122 cm ×
122 cm square of thismesh, was stretched to a tension of 6.4 N cm − on a Grünig G-STRETCH 210 mesh stretching machine.After stretching, the mesh is epoxied using DP460 epoxy to a circular stainless steel ring, with 118 cmouter diameter, 112 cm inner diameter, and 0.3 cm thickness. The tension measurement employs a TerenInstruments HT-6510N tension meter; measurements are made at 9 points on the mesh. The average ersion 15th February 2021 submitted to Instruments
10 of 42 tension reported here is the average of measurements at these 9 locations, after the stretching and relaxationprocedure is completed. The standard deviation of repeated tension measurements across the 9 spatiallocations is measured to be 0.4 − − [19].The cathode assembly is supported on the three internal rails of the vessel with machined Delrinpieces. The spacing of the cathode to the first field cage ring is constrained by the Delrin supports of thecathode and the mating support parts of the closest field cage ring. The amplification stage is constructed from three electrodes (anodes), separated by two resistivespacers. The resistive spacers are 121 cm outer diameter, 112 cm inner diameter rings, with 24 0.1 cm widebeams crossing them, laser cut from polyester shim stock. The spacer beams are visible as the vertical linesin the anode plane in Fig. 5b.The amplification region flatness is constrained by its support frame, which consists of two ring-shapedframes made of Nylon bolted together, which sandwich the anodes and resistive separators. The framedimensions are 118 cm outer diameter, 112 cm inner diameter, with thickness 1.6 cm. The two rings, andeach amplification mesh and resistive spacer ring have 88 aligning drilled holes. A stack is formed withone support frame on the bottom, followed by alternating the three amplification meshes with the tworesistive spacers and finished with the second support frame. Nylon bolts are passed through the 88 drilledholes in the stack. Finally, on the side facing away from the camera readout, a hexagonal aluminiumstiffener is bolted to the framed assembly, attached to the nylon bolts at 16 of the 88 drilled holes points.The three anodes are constructed from steel meshes with 121 cm diameter. Anodes 1 and 2 are made from100 lpi meshes with a wire diameter of 27 µm. The optical transparency of the anode 1 and 2 mesh is 89 %.The third anode is made from a 250 lpi mesh with 40 µm diameter. We chose the meshes with the smallerwire diameters for the two meshes closest to the field cage in order to achieve the highest gas amplificationin the first stages, and minimize the loss of light because of imaging the amplification region through thecathode, anode 1 and anode 2 meshes. The meshes are epoxied to stainless steel rings (outer diameter118 cm, inner-diameter 112 cm, thickness 0.1 cm) after stretching the meshes as described for the cathode.The procedure for stretching the anode meshes takes approximately a week of successive stretching andrelaxation of the mesh. Following this procedure, the average tension force on the anode 1 and 2 meshes is16.8 N cm − . The measured tension is uniform over the plane of the anode mesh to better than 5 %. Theaverage tension force on the anode 3 mesh is 38 N cm − . After stretching, the meshes are epoxied to thestainless steel support ring in the same way as described for the cathode [19]. The goal for the distancebetween the anode 1 and anode 2 (anode 2 and anode 3) meshes is 0.5 mm (1 mm). A measurement of thecapacitance of the amplification region is described in Sec. 6.3.1. The capacitance measurement implies thedistances achieved were approximately 1 mm (2 mm) spacing. This is likely due to the epoxy and spacerthickness tolerances as well as flatness variation.Like the cathode, the amplification region assembly is supported on the three internal rails on the pressurevessel using machined Delrin parts. These supports constrain the amplification region distance to theclosest field cage ring. The anode meshes are provided with positive high voltage (HV) by either a CAEN NDT1470 or aCAEN N1470 multi-channel Power Supply (PS), which is controlled through a serial link over USB. Thecathode power supply is a Spellman SL 30 PS with a maximal output voltage of 30 kV. The resulting limiton the electric field in the field cage is over 600 V cm − . The cathode PS voltage is controlled by varying ananalogue input from 0 to 10 V, which results in an output voltage from the PS of 0 V up to its maximum ersion 15th February 2021 submitted to Instruments
11 of 42 voltage. This analogue signal is generated by the slow control system using a LabJack U3-HV USB DataAcQuisition (DAQ) device which is connected to the Spellman PS control input.The various meshes are connected to the power supplies through the following chain: Inside thepressure vessel all meshes are connected to Kapton coated copper wires which in turn are connected tothe HV feed-throughs that pass through the pressure vessel wall. To provide extra insulation, these wireshave ceramic beads threaded along their entire length, and the resulting assembly is also surroundedby a fibreglass sheath. In the case of the anodes the HV feed-throughs are rated to 10 kV; in the case ofthe cathode the feed-through is rated to 20 kV. Outside the pressure vessel, each anode’s feed-throughconnects via coaxial cables to a custom ’bias box’. These bias boxes decouple the charge signals from theconstant current HV as shown in Fig. 6. Therefore each bias box connects to the respective PS and eachbox has a signal output which is fed into the TPCs charge readout system. Signals are routed from thesesignal outputs through a preamplifier, described in Sec. 4.5. The RC constant of the R in resistor and therespective mesh capacitance of ∼ The pre-amplifiers used for the detector’s charge readout are charge-sensitive CREMAT CR-113 (orCR-112) hosted in CR-150-R5 evaluation boards. The specified gains of the pre-amplifiers are 1.3 mV pC − (or 13 mV pC − respectively). A measurement of the agreement of our preamplifiers with this value canbe found in Sec. 6.3.1. The output signals from the preamplifiers are digitised by a CAEN N6730 8-channeldigitizer, with 2 V dynamic range and 500 MHz sampling frequency. The optical readout system uses four FLI Proline PL09000 CCDs, each of which contains afront-illuminated Kodak KAF-09000 chip with 3056 × ×
12 µm . The chip has a Quantum Efficiency (QE) in the range of 50 −
70 % for photons with awavelength between 475 nm and 750 nm. In the wavelength range from 350 nm to 925 nm the QE is alwayslarger than than 20 %. These wavelength ranges cover the full VIS part of a spectrum and extend towardsthe NIR and UV, which makes the TPC sensitive in the regions of the spectrum discussed in Sec. 2.Each camera’s field of view is centred on a quadrant of the amplification plane. Each camera iscoupled to a Nikon f/1.2 50 mm focal length lens with a 54.8 ◦ angle of view. The cameras are mounted tooptical flanges as shown in Fig. 7. Quartz windows of 6 cm thickness are used to allow for the desiredoverpressure in the vessel. The transmission of the optical flanges is measured to be 97 + − % for red light.The camera lenses have a transmission of 70 % (90 %) at 420 nm (750 nm) wavelengths, where the lenstransmission includes also photons lost between the 7 elements of this compound lens.Adding up the TPC drift distance, the non-active area between the cathode and the vessel door as wellas the path through the camera assembly, the total object distance is approximately 102 cm which is largerthan the minimum focal distance of our camera lenses. At this distance the system images a 71 ×
71 cmfield of view with a vixel size of ∼
40 µm, when no extra readout binning is applied. Considering the full ersion 15th February 2021 submitted to
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12 of 42 V anode1 V anode2 V anode3 S anode3 S anode2 S anode1 B anode3 B anode2 B anode1 V anode i R bias R in C filter C dec S anode i B anode i Figure 6.
Schematic of the circuit to bring high voltage ( V anode i , i ∈
1, 2, 3) to the anode meshes andto decouple the signal from the high voltage lines. The signals are decoupled in bias boxes via a 10 nFdecoupling capacitor ( C dec ) and are then fed to the signal line ( S anode i ). These bias boxes B anode i featurealso a protection and filtering circuit consisting of a bias resistor ( R bias =
200 M Ω ), filter capacitor ( C filter =
10 nF), and input resistor at the detector input ( R in =10 M Ω ). ersion 15th February 2021 submitted to Instruments
13 of 42 (a) (b)Figure 7. (a) Drawing of the optical flange with the camera mount. The thick quartz is necessary to ensurethat the assembly can withstand the pressure difference between the vessel pressure and ambient pressure.(b) A photograph of the assembly with the camera removed. optical path including quartz window and lens, we estimate a geometric acceptance of the optical system –the third term in brackets in Equation (1) – of approximately 1.1 × − . Achieving a high enough gain inthe amplification region to produce enough photons for signals to be detected above the noise, given thisacceptance, is key.To achieve optimal noise performance the CCDs are cooled to − ◦ C to − ◦ C. The cameras areequipped with an internal thermoelectric cooler which can cool the CCD to approximately 50 ◦ C below theambient camera temperature. This is supplemented by a water cooling system attached to each camera toreduce its ambient temperature by 15 ◦ C. At − ◦ C operating temperature, the read noise per pixel is inthe range of 9.6 to 11.3 e − , and the dark rate is 0.006 − − /pixel/s (the range of variation is acrossthe four cameras).The CCDs digitize the number of electrons collected in each pixel in each exposure. For scale, thetypical conversion gains of the cameras are 1.52 − − /ADU. To mitigate the dominant effect ofreadout noise, pixels are grouped prior to digitization. This grouping reduces the noise per pixel in thegroup by approximately 1/ (cid:113) N pixels given the relative scales of readout noise and dark current rate in a1 s exposure. Typically we use 8 × N pixels =
64) as this gives an acceptable balance betweenreadout noise and readout pixel size, with the effective vixel width (pixel width in the amplification plane)being approximately 2 mm.
The slow control software sets and monitors the detector voltages, gas pressure and ambienttemperature. The software has a web based user interface, and uses java and C++ software to interfacewith an SQL database. The database contains the values of the monitored variables as well as the desiredset points for these variables. The detector control code reads the set points from this database andcommunicates with the high voltage power supplies to set the required voltage and read out the measuredvoltage and current into the database. Control of the gas system is also achieved through the same webinterface, which is able to launch code communicating with the valve control hardware to perform filling,venting and evacuation automatically. ersion 15th February 2021 submitted to
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The DAQ system triggers and acquires data from the charge and optical readout hardware. DAQcommands are sent from the same web interface used for slow control to a DAQ PC which communicateswith the cameras and the CAEN N6730 digitiser used for the charge signals to initiate each run. A runconsists of a user-specified number of camera exposures (data frames), which are acquired simultaneouslyfrom the four cameras as well as the charge waveforms digitized during the exposure time. Additionally, atthe start of the run a specified number of frames are acquired while the camera shutters are closed. The useof these frames is to subtract off the baseline behaviour of each pixel in the CCD chip when it is not exposedto light (Sec. 5.2). After these empty frames the data frames are taken with the camera shutter open. Thedetector can run in two triggering modes. In the first mode the data frames are taken immediately aftereach other, separated only by the CCD readout time. In the second mode the data frames are taken basedon an external trigger signal. During the CCD exposure time the charge waveform digitiser (see Sec. 4.5)is triggered by signals larger or smaller than a user-configurable threshold amount above the baselineon each channel, and then records waveforms of typical duration 100 µs around each trigger, including aconfigurable period of time before the trigger event. The digitiser can also be triggered externally. In bothtriggering modes any trigger causes all eight channels of the digitiser to be read out simultaneously. TheDAQ system stores the configuration of all of the parameters described in this subsection for each run.
5. Optical Readout Analysis and Performance
In this section we report on the results of data taking with
Am sources mounted in the pressurevessel. The HPTPC was also tested using a beam at CERN, the analysis of which is ongoing and notpresented here. After explaining the CCD calibration (Sec. 5.1 and Sec. 5.2) we show a first scan of variousgas mixtures (Sec. 5.3) to establish the most promising mixture for a more comprehensive light gainmeasurement. This in-depth measurement with a single mixture and its analysis is then shown in Sec. 5.4.Am-241 predominantly emits either a 5.486 MeV (84.8 %) or 5.443 MeV (13.1 %) alpha particle ( α ) anddifferent energy gamma-rays ( γ ), where the most probable ones have an energy of 59.54 keV or 26.34 keV[20]. Furthermore there is a substantial amount of x-ray radiation in the range from 10 keV to ∼
20 keV.The α particles pass through a foil before they enter the gas volume, therefore their energy is reduced byabout 860 keV to ∼ α particles are stopped inside the gas volume and deposit theirfull remaining energy. The γ -rays have high enough energy to escape the active TPC volume. Accordingto a HEED [22] and G
ARFIELD ++ [23] simulation which takes the HPTPC’s geometry into account only1.2 % of all γ -rays interact in the counting gas. The lower energy x-rays are more likely to interact – whenintegration over all x-ray energies we find that 58 % are absorbed in the active gas volume. Their overallcontribution is still not large, since the ratio of the x-ray count over γ -ray count is about 12 %. The emissiondistribution of the Am in the forward hemisphere is roughly isotropic for the different kinds of radiation.Furthermore there is a contribution from cosmic rays.Figure 8 shows the result of a
HEED and G
ARFIELD ++ simulation of the expected energy deposits bythese different sources of radiation, which does not take any trigger effects, electronic noise, gas gain or anamplifier response into account. For the simulation we assume a quadrant of the HPTPC’s volume with asource location similar to the location in the experiment. The normalisation of the three different kinds ofradiation in Fig. 8 is given by the result of the simulation: For 1000 Bq of
Am decays, all 1000 α particlesinteract in the active volume every second as do 70 x-rays and γ -rays. Note that the x-rays and γ -rayscontribute only at the low energy end of the spectrum. The contribution from the cosmic rays per secondis scaled up by a factor of 60 to make the shape of the cosmic ray spectrum better visible. The most distinct ersion 15th February 2021 submitted to Instruments
15 of 42 energy deposit (10 electrons) c o un t s ( /27311 e l ec t r o n s )
70 x-rays s − cosmic s − × α s s − Figure 8.
Simulated energy deposits of
Am decay radiation and cosmic muons inside a gas volumefilled with Ar/CO (90/10). Energy deposits are measured in the number of liberated electrons duringthe energy deposit. This is the result of a HEED [22] and G
ARFIELD ++ [23] study taking into account theapproximate layout of the HPTPC and the information in [20,21]. feature of the spectrum is the α -peak from the Am decay at ∼ × electrons. a For a gas pressure of1 atm these ionisations are created along a 5 cm to 10 cm long trajectory, yielding a high ionisation densityalong the track. For larger pressures, the track length de- and ionisation density increases. When amplified,this high ionisation density will result in many photons produced in a small area. An α particles’ energydeposit in the detector is thus more easy to image with cameras than less ionising forms of radiation.Furthermore, a gain measurement is possible since the total energy deposited in the detector is known.In the amplitude spectrum of the charge readout ( cf . Sec. 6.3.2), we expect to see something qualitativelysimilar to the spectrum in Fig. 8. However, the simulation does not take into account the energy resolutionof the amplification plane, hence the actual measured quantity – amplitudes or light intensity – will exhibita spread larger than what is shown in the plot. Furthermore electronic noise is not included, which is asubstantial contribution at threshold.Two different configurations were used in our measurements: one using five Am sources and one usinga single
Am source. In the single source configuration the source is either visible in the overlap region ofthe top two cameras or the bottom two cameras. In the five source configuration the sources are arrangedin a cross configuration and are distributed such that there are always two sources in the overlap region oftwo cameras and that the central source can be seen by all cameras. Occasional sparks can be used to mapthese positions in the recorded frames (Fig. 9a).
A major source of noise comes from sparking in the chamber. These sparks cause a large fraction ofpixels in an image to become significantly brighter, an example is shown in Fig. 9a. The camera pixelsmeasure charge in analogue-to-digital units , which is represented by the acronym ADU. Images withsparking are rejected form the analysis as follows. First, events where one of the CCD images has an RMSabove ∼
100 ADU to 300 ADU are rejected. Of the remaining images, those with events where one or more a It turned out to be not feasible to simulate stopping of α particles in HEED . Therefore we ultimately simulated 11.8 MeV α particles, evaluated their most probable energy loss and scaled this energy loss to 4.56 MeV.ersion 15th February 2021 submitted to Instruments
16 of 42 (a) (b)Figure 9.
Both images show the readout plane of the HPTPC, so the vertical (horizontal) image axispoints along the y ( x ) direction. The colour information encodes the light intensity in arbitrary units. (a)Simultaneously recorded frames during a spark event. The locations of the Am sources (marked bycircles) inside the TPC are visible during the spark event as well as the field cage rings and the anodesupport, cf.
Fig. 5b. (b) Light yield from the calibration sources for 200 s exposure time in pure Argon at3 bar absolute pressure. The intensity of the image in the top left frame differs from the other three frames,because the corresponding camera has a different conversion gain. of the CCD images have 100.000 pixels above 100 000 ADU are also removed from the analysis. The exactthresholds depend on the actual CCD camera and detector settings, e.g. the RMS thresholds vary from133 ADU to 300 ADU between the four cameras. The exact values for each threshold have been identifiedby comparing the properties of spark images selected by eye to images without sparks.
The CCD camera calibration removes variations in pixel gain, transient phenomena, and timedependent noise sources. The first step of the CCD calibration is the the subtraction of bias frames ,which deals with persistent features and noise sources, and accounts for variations in pixel gain. At thestart of each run we take between 5 and 10 bias frames with the shutter closed. These are averaged andthen subtracted from all exposure frames with shutter open in the same run, where a typical run consists outof 20 to 100 images (per camera) with an exposure time of 2 s per frame.Another source, this one transient, is hot pixels , created e.g. by cosmic muons passing through the camerachip and saturating pixels. These hot pixels are usually confined to individual frames but can remainsaturated over several exposure lengths. They are corrected for in the bias frames before their subtractionfrom the exposure frames. Each pixel value of each bias frame is compared to the values of the same pixelin the other bias frames, and if its ADU reading has changed by more than five standard deviations of itsmean ADU value, the value of the pixel is set to that of the previous bias frame.The next step of the CCD calibration is the temperature dependent image mean correction. This correctsfor the change in the CCD pixel charge-digitizing electronics’ gain which is temperature dependent. Thetemperature of the CCDs is seen to increase with the number of events taken in a run. This results ina natural upwards drift in the pixels’ intensities with time which contributes to the noise. This effect iscorrected for by calculating each CCD frame’s average pixel value and then subtracting that value fromevery pixel within that frame. This process is applied to every frame in all runs. ersion 15th February 2021 submitted to
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17 of 42
Pixel Intensity (ADU) N u m be r o f P i x e l s (a) - - - - - Pixel Intensity (ADU) N u m be r o f P i x e l s (b)Figure 10. Analogue-to-Digital Unit (ADU) distribution of all pixels of an exposure frame before (a) andafter (b) bias subtraction.
The impact of these calibration steps is to reduce the pixel variance. The distribution of pixel values beforeand after bias subtraction is shown in Fig. 10.5.2.1. Calibration without closed-shutter bias framesThe measurements in this paper are grouped into two data taking periods: One, where we exploredifferent gas mixtures to find the most promising gas for an in depth measurement campaign (Sec. 5.3),and the second period where only the gas identified in the first period is studied (Sec. 5.4). During thebeginning of the second period it was discovered that camera 2 (which was set up to take the light gaindata) had a stuck open shutter. Due to time constraints we continued with data taking despite this andhave adjusted our calibration accordingly as detailed in this section.To address mechanical shutter failure, a procedure was developed to acquire bias frames for calibrationwith the shutter open. To avoid stray light from the sources or sparks, 1000 2 s shutter-open frames wereacquired daily with the TPC voltages switched off. The anode meshes need to be slowly brought up tothe desired voltages in order to reduce the probability of sparking and the subsequent need to reduce thevoltages for some time. Reaching the target voltages in a gas mixture with low or no quencher contentcan thus take on the order of hours, when starting from zero. For this reason we decided to take theseshutter-open bias frames not before every run. These frames are then used to produce a single, low noise super bias frame to be subtracted from each event taken that same day.
Super bias frame creation
The method used to create each super bias is to first remove any anomalous pixels by the methoddescribed in Sec. 5.2. Next, a 1D distribution for each pixel in the super bias is created and filled with the N bias = σ pixel ) of that distribution is calculated and any ADU value above 3 σ pixel of the mean are removed. AGaussian is fitted to the remaining 1D distribution of each pixel. The centre of the Gaussian gives theADU value of that pixel in the super bias. As mentioned in Sec. 5.2, bias subtraction using a bias frame, ersion 15th February 2021 submitted to Instruments
18 of 42 (a) (b)Figure 11.
Example of the average of 100 bias subtracted events with event and bias frame taken daysapart (a) before row correction (demonstrating row CCD artefacts) (b) after row correction (demonstrationcorrection of row CCD artefacts). The colour in both plots encodes the ADU value at the position of a pixel,while the horizontal and vertical axis shows the y and x coordinate, respectively. taken close in time to the event frame can help to reduce temperature (and therefore time) dependentnoise. Due to the significant time difference between bias and event frames additional corrections oftemperature/time dependent effects need to be implemented before the super bias frame can be used as abias frame for exposure frames.Figure 11a shows an example of row pedestal artefacts. The scale of the effect has been artificiallyincreased for demonstration purposes by using exposure and bias frames taken days apart. This effectoccurs when the pedestal of each pixel within a row changes by some amount between taking the biasand exposure frame. These effects are not observed when the bias frames are recorded directly beforethe exposure frames as part of the same run, since the pedestal value shift occurs only between runs. Inorder to use the super bias frames we apply a row correction to every row in the super bias subtractedimage. For this correction the average ADU value of a row is calculated whilst omitting any anomalouspixels or any pixels located within the region of interested for the analysis, i.e. the source locations. Thisaverage is then subtracted from each pixel in that row. Figure 11a shows the same image as Fig. 11b afterrow correction has been applied.Applying row correction to a super bias subtracted image also corrects for any time dependent drift ofpixel intensities ( e.g. because of temperature). Figure 12 show an example of the pedestal drift in 100subsequent exposures (events) for 150 different runs before and after row correction. The bi-modal natureof Fig. 12a is likely due to temperature differences on different days. It should be noted that the rowcorrection can fail for regions on the CCD where a differential pedestal drift is present. The lower leftcorner of the CCDs experiences such a non differential shift and care has been taken to ensure that thesource positions do not overlap with affected rows.Overall, super bias frame subtraction significantly reduces the pixel intensity variance in an event which isnormally introduced by the classical bias frame subtraction, because the mean pixel value error in a super ersion 15th February 2021 submitted to Instruments
19 of 42 (a) (b)Figure 12.
Mean ADU value of exposure frames versus event number for 150 runs (of 100 events, i.e. frames) taken over a number of days (a) before row correction (demonstrating pixel pedestal drift) (b) afterrow correction. The latter demonstrates the correction of the pedestal drift by the row correction procedure. bias frame is reduced by 1/ √ N frames . When integrating a region of interest of 9 × i.e. N pixel = N bias = σ bias = (cid:115) σ · N pixel N bias . (3)For the standard deviation of a single pixel ( σ pixel ) a typical value of 40 ADU can be used to estimate σ bias .The resulting σ bias = e.g. the case where 5 bias frames are used. An important question when operating gaseous detectors with optical readout is which gas mixtureswill yield the most light from the interactions of interest. For this measurement light from a single
Am source in the overlap region of the top two cameras was used. Since the alpha particles from thedecays travel only a few cm at the pressures considered ( cf. the beginning of the section), a small regionaround the source location was considered for the light yield measurement. One of the cameras (topleft, Fig. 9b) had a lower conversion gain than the other three so only the top right camera was used forthis analysis. The trialled gas mixes were: pure argon (3 and 4 bar absolute), argon with carbon dioxide(4 bar absolute, Ar/CO (99/1) and (99.25/0.75)), argon with nitrogen (3 bar absolute, Ar/N (98/2)) andargon with nitrogen and carbon dioxide (4 bar absolute Ar/CO /N (98.75/0.75/0.50) and 4.9 bar absoluteAr/CO /N (96/2/2)).To determine the light gain the calibration procedures in Sec. 5.1 and Sec. 5.2 are applied to the relevantdata runs. A 20 ×
20 pixels ( ∼ × ) region of interest around the source position is examined inthe bias subtracted and calibrated exposure frames. All light recorded in the region of interest is integrated.The results of this study can be seen in Fig. 13. For each result presented in the two plots of Fig. 13 onedata taking run has been used. All data is normalised to the same integrated exposure time.Two different comparisons were made, one at fixed anode voltages (Fig. 13a) and one at the maximumanode voltages reached during stable operation (Fig. 13b). The voltage settings for both data sets areshown in Tab. 1. In both cases the light yield from the Am source was found to be highest in pure argon. ersion 15th February 2021 submitted to
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20 of 42 per 2s - Background)/ stat (98/2)4 Bar Ar CO (99/1)4 bar A r CO /N (98.75 0.75 0.5) Light Yields for Like Voltage Settings /// / (a) per 2s - Background)/ stat (99.25/0.75)3 Bar Ar/N (98/2)4.9 Bar Ar/CO /N (96/2/2) Light Yields for Max Voltage Settings (b)Figure 13.
Light yield measured for an
Am source with different gas mixtures (a) at near constantanode and cathode voltages and (b) the maximal light yield achieved. The voltages used during thesemeasurements are listed in Tab. 1. ersion 15th February 2021 submitted to
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21 of 42 (a) mixture or gas
P V a1 [ V ] V a2 [ V ] V a3 [ V ] V c [ V ] Ar/CO /N (98.75/0.75/0.5) 4 bar 1000 2000 4000 -7000Ar/CO (99/1) 4 bar 1200 2400 4000 -7000Ar/N (98/2) 3 bar 1200 2800 4000 -7000Ar 3 bar 1500 2100 4500 -5250 (b) mixture or gas P V a1 [ V ] V a2 [ V ] V a3 [ V ] V c [ V ] Ar/CO /N (96/2/2) 4.9 bar 3000 5900 7600 -8500Ar/N (98/2) 3 bar 1550 3300 5000 -5000Ar/CO (99.25/0.75) 4 bar 1200 2500 4800 -7000Ar 4 bar 1000 1750 2800 -5700Ar 3 bar 1500 2100 4500 -5250 Table 1.
Voltage settings for the result plot shown in Fig. 13: (a) shows the voltages used for the settingsshown in Fig. 13a, while (b) the settings used for the data in Fig. 13b. The absolute pressure is quoted.
The pure argon results shows also that a high relative light gain can be achieved with lower voltages ascompared to the gas mixtures with a quencher. The high light gain of the 4 bar pure argon measurement inFig. 13b is surprising since intuitively a lower light gain than for the 3 bar gas mixture would have beenexpected. Even more so as the voltages (normalised by pressure) applied during the 4 bar measurementare lower than in the 3 bar case. A saturation of the light gain at a given voltage setting can explainsuch findings. Furthermore the fields during the 4 bar measurements could allow for the incoming andamplified electrons to create more excitations and thus photons on the cost of ionisations, as compared tothe 3 bar case.
Having identified pure argon as the brightest gas among the mixtures set out in Sec. 2, we nowexamine how the optical gain in this gas is affected by the operational settings of our amplification stage.Doing so we use pure argon at a pressure of 3 bar absolute, following the findings in the previous section.Precisely how each of the multiple anodes contributes to the gain depends – among other parametersas e.g. the voltage settings – on the anode’s relative alignment, which for meshes is difficult to modelanalytically and numerically. The aim of the optical gain measurements in this section is to understandhow the light gain of the TPC is affected by a) the absolute voltage of the three anode meshes V a , V a and V a when the potential difference between meshes is kept constant; b) the potential difference betweenanode meshes 2 and 3 ( ∆ V a ); and c) the potential difference between anode meshes 2 and 1 ( ∆ V a ). Todo this we chose three voltage schemes which are as follows:• Scheme A - Vary V a , V a and V a by the same amount each step thereby keeping ∆ V a and ∆ V a constant• Scheme B - Vary ∆ V a and keep ∆ V a constant by varying V a • Scheme C - Vary ∆ V a and keep ∆ V a constant by varying V a and V a by the same amount each stepTo conduct a light measurement a single Am source is used, positioned so that it can be imaged bycamera 2, the bottom left camera. The source has a activity of 10 ± Am sources with known decay rates. Anexposure time of 2 seconds per frame was chosen to balance reduction of readout noise with reduction indead time due to lost frames from sparking. ersion 15th February 2021 submitted to
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22 of 42 − − − − Pixel Intensity (ADU) C oun t s Figure 14.
Intensity distribution of pixels within the source box for a single event. x and y projections of the ADU distribution offrames within a scheme. Two super bias frames were selected resulting in Schemes B and C sharing thesame super bias frame.Then a region of interest around the source is defined (referred to as source box ). The source box’s size isoptimised to contain as few pixels as possible whilst not rejecting any signal. The analysis found a nineby nine pixels (16.56 × ) source box to be optimal. After a loose pixel ADU cut, a Gaussianis fitted to the ADU values of the N pixel =
81 pixels in the box for a given frame (as shown in Fig. 14).The integrated ADU per frame is then calculated by: I frame = ˆ µ pixel · N pixel , where ˆ µ pixel is the mean ofthe fitted Gaussian. The integrated ADU measurement for a run ( I run ) is calculated by fitting a Gaussianto the distribution of I frame values in that run. I run is given by the mean of the fit and its uncertainty( σ I run ) by the standard deviation on that mean. The final step takes the I run values of the 10 to 15 runs ineach configuration and calculates their weighted mean ( ¯ I w ) and weighted standard error ( σ w ). The meanand standard error for each voltage configuration are weighted by w i = σ i where σ i is the standarddeviation per run ( σ I run ) of the i th run in the configuration. The weighted mean and weighted standarderror are calculated as follows: ¯ I w = ∑ N run i = w i I i ∑ N run i = w i (4) σ m w = σ w √ N run = (cid:118)(cid:117)(cid:117)(cid:116) ∑ N run i = ( I i − ¯ I w ) ( N run − ) ∑ N run i = w i (5) ersion 15th February 2021 submitted to Instruments
23 of 42 where I i is the integrated ADU value per run ( I run ) of the i th run in the configuration, N run is the totalnumber of runs in the configuration and σ w is the weighted standard deviation. ¯ I w and σ m w give the finalintegrated ADU value of the voltage configuration ( I config ) and its uncertainty ( σ I config ). In theory onecould calculate σ I config without the intermediate step of calculating I run . However, examining I run ensuresthat run to run instabilities are accounted for in the the uncertainty of the final measurement.5.4.2. Light gain as function of voltageThe final results of the light gain measurements can be seen in Fig. 15. In Scheme A, showingFig. 15a, the voltages are varied in 200 V steps to maintain a fixed potential difference between the threeanodes meshes so that always ∆ V a = ∆ V a = ∆ V a and ∆ V a are fixed at1200 V. This suggests the amplification is driven by the voltage differences between the anode meshes.Across all Scheme B voltage configurations, shown in Fig. 15b, V a and V a are equal to 1200 V and 2400 Vrespectively, while V a and thus ∆ V a are varied. Scheme B shows a correlated linear dependency of lightgain on ∆ V a over the range 0 V to 2500 V with a gradient of 0.074 ± V a and ∆ V a are equal to 1200 V, whilst ∆ V a and thus V a and V a arevaried. The results of Scheme C suggest the light gain has a positive linear dependence on ∆ V a up to ∼
800 V where the light gain plateaus to a value of 343.0 ± ± ± ∼
700 V observed in Scheme C. One hypothesis is that theplateau occurs when the electric field between anodes 1 and 2 ( E a ) equals that between anodes 2 and 3( E a ). When E a > E a less electrons will be able to move from the gap between anode 1 and anode 2into the gap between anode 2 and 3 and thus there are less electrons available for amplification and/orexcitation. The analysis of the circuit response and the inferred capacitances (Sec. 6.3.1) suggest thatthe distance between anode 1 and 2 is 1.20 ± ± E a = ± − and E a = ± − .As both value agree with each other, we find E a = E a where the plateau occurs. The fact that therise in light gain stops when E a = E a , could thus be related to a change in electron transparency ofanode 2. Observing a plateau and not a simple drop in the light gain’s gradient with increasing voltageis however surprising, because for a plateau to arise the hypothesised electron loss needs to be exactlycompensated by an increased light yield from the electrons in the anode 1 and 2 gap. During the Scheme Bmeasurements E a was held at a value of 10.00 ± − , using the distances discussed before. E a was scanned from 0 to 13.0 ± − . E a > E a is fulfilled from a ∆ V a of 2000 ±
200 V onwardsand a plateau should be visible as in the case of Scheme C. The data in Fig. 15b is not sufficient to concludethat the trend reaches a plateau at said value nor the opposite as the plateau’s expected position is tooclose to the end of the ∆ V a voltage scan. With the maximal E a in its error-bars, a ∆ V a of 2800 V wouldbe required to reach the cross over between the rising and the plateau region.The conclusion drawn form this study is that the light gain in the amplification region depends moststrongly on the potential differences between the meshes, rather than the absolute voltage on the meshwires.5.4.3. Number of Photons in Amplification Region Per Primary ElectronIn order to calculate how many photons are produced in the amplification region per primary electronin the drift volume it is necessary to make use of additional measurements and some assumptions. In ersion 15th February 2021 submitted to Instruments
24 of 42 I n t eg r a t ed A DU ( A DU ) Scheme A (a) I n t eg r a t ed A DU ( A DU ) Scheme B (b)
200 400 600 800 1000 1200 1400 1600 1800Potential Difference Between Anode 2 and Anode 1 (V)100150200250300350 I n t eg r a t ed A DU ( A DU ) Scheme C (c)Figure 15.
Light gain measurements of Integrated ADU from
Am source (a) vs anode 1 voltage wherethe voltage difference between anode 1 and 2 (anode 2 and 3) is kept constant at ∆ V a = ∆ V a = ersion 15th February 2021 submitted to Instruments
25 of 42 this work we do not attempt to calculate the relationship between primary electrons in the drift volumeand the number of electrons in the amplification region directly (by considering diffusion, mesh transitand charge gain) as we cannot externally constrain all the variables. Instead we calculate the number ofphotons per second in the amplification region ( N γ ) from the Am source using the observed ADU inthe CCD as follows: N γ = ADU obs ( Conversion Gain ) × QE ∗ ( ε ) × Ω × (cid:2) T window × T lens × T cathode × T (cid:3) ,where ADU obs is the observed ADU from the CCD per second in the region around the source. In ourmeasurements the maximum value reached was 225 ±
10 ADU/s, cf . Fig. 15b divided by the exposure timeof 2 s. The conversion gain provided by the manufacturer is 1.5 ADU per electron. The quantum efficiency QE ∗ is a function of the incident photon energy, for light in the near infrared the manufacturer specifies60 ±
10 % of photons converted into electrons. In principle we could be seeing light in the visible, infraredand ultraviolet from the argon scintillation. However, since both the quantum efficiency of the CCD and thetransmission probability through the quartz windows has a rapid drop off below 350 nm we assume in thiscalculation that we are not sensitive to the UV light. Some photons will not make it from the amplificationregion to the CCD. The geometric acceptance of the system, Ω was calculated to be ( ± ) × − . T lens = ±
10 % and T window = + − % are the transmission probabilities through the lens and quartzwindow respectively. In addition, all of the photons imaged from the amplification must pass through atleast one cathode and one anode mesh, with the majority of them passing through two anode meshes. Thetransmission probabilities through the cathode and anode meshes are T cathode = T anode = N γ = ( ± ) × photons per second in the amplification region.We then make a calculation of the expected primary electrons in the drift volume per second, N e , based onthe measured activity of our Am source as follows: N e = N α × (cid:104) ε α (cid:105) W where N α = ± (cid:104) ε α (cid:105) = W = ± N e = ( ± ) × electrons per second in the imagedpart of the drift volume. Combining these two results we expect there to be a total of ( ± ) × − photons in the amplification region per primary electron in the drift volume.
6. Charge readout analysis and performance
In this section, we discuss the raw data obtained from the charge readout and the analysis whichturns this raw data into physical quantities. We explain the calibration of the TPC charge readout withradioactive sources and cosmic radiation, and report the charge gain obtained with different high voltagesettings.
Signals from the three anode meshes are decoupled from their respective HV line as described inSec. 4.5, fed into a pre-amplifier, and digitised. Figure 16 shows an example of a digitised waveform, asa trace of voltage versus time. We define the quantities V i and t i to be the digitised voltage and time,respectively, at the i th time sample.A waveform is comprised of three characteristic regions in time, shown on the sample waveform: theperiod before the digitiser has triggered ( pre-trigger ), the time at which the digitiser triggered and the ersion 15th February 2021 submitted to Instruments
26 of 42 − s) µ time ( − Anode2Waveform6_Event4Anode2Waveform6_Event4 a m p li t ude ( m V ) Baseline10 %90 %Maximal ValuePre-triggerregion t f , (a) − − s) µ time ( − Anode2Waveform6_Event4
Baseline10 % 90 %Maximal Value t r , t r , t f , (b)Figure 16. Example for a charge signal, a waveform – (a) and (b) zoom – with some of its defining featuresindicated. See the text for more explanations. The first vertical line in Figure (a) shows the approximateposition of all the vertical lines in the zoomed plot in (b). period after ( post-trigger ). The pre-trigger region – that is, sample 1 to sample N pre-trig corresponding to t = Baseline in Fig. 16) and baseline RMS for a waveform.The pre-amplifiers are charge-integrating; thus the maximum voltage of the charge waveform isproportional to the total charge collected on an anode. The amplitude (negative amplitude) of a waveformis taken to be the largest (smallest) V i value of the waveform, max ( V i = N ) (min ( V i = N ) ), subtracted bythe mean baseline. We distinguish properties of negative polarity pulses from positive ones by adding a negative where appropriate to the respective property’s name.The start-time ( t r ,10 in Fig. 16b) of a pulse is found by looking backwards in time (examining samples withdecreasing sample number i ) from the sample with the maximum (minimum) V i value to the point intime where the waveform reaches 10 % of its amplitude value. The point at which the waveform reaches90 % ( t r ,90 in Fig. 16b) of its amplitude is identified in the same manner. From the maximum voltage, thewaveform decays exponentially with a time-constant depending on the pre-amplifier chip used. Likewise,the t f ,10 and t f ,90 points on the tail of the waveform are found by finding the point after the maximum(minimum) where the amplitude first falls below 90 % and 10 % of the peak value. A pulse’s rise time(fall-time) is calculated as the difference between t r ,10 and t r ,90 ( t f ,90 and t f ,10 ).In addition to the above the Integral of a charge waveform is calculated by summing the voltage of thewaveform relative to the baseline from t r ,10 to t f ,10 , i.e. : Integral = k ( t = t f ,10 ) ∑ i = m ( t = t r ,10 ) ( V i − Baseline ) , (6)and the RMS of the full Waveform as WaveformRMS = (cid:118)(cid:117)(cid:117)(cid:116) N j ∑ i = V i (7) ersion 15th February 2021 submitted to Instruments
27 of 42 - - - - Baseline mean (mV) C oun t s ( / . m V ) R2083056-R2083065_BaselineAnode1 (a) - - - - Baseline mean (mV) C oun t s ( / . m V ) R2083056-R2083065_BaselineAnode1 (b)Figure 17.
Anode 1
Baseline spectrum (a) before cleaning, and (b) after cleaning. Waveforms with large
Baseline values are cut, which removes spark events. where m , k , and j are sample numbers – the latter being the number of the last sample. The BaselineRMS iscalculated in a similar manner, but only taking V i in the pre-trigger region into account. In order to get the most accurate values of the parameters described above, we apply a series ofcleaning steps to the waveforms before calculating the parameters. Cuts are made to select waveformsbased on their
Baseline and
BaselineRMS . The mean of the
Baseline values of all waveforms in a run iscalculated. If the baseline mean of a waveform is not within a 5 RMS interval of the mean of all
Baseline values, the waveform is rejected. Similarly, if the
BaselineRMS of waveform is not within a 5 RMS intervalof the mean of all
BaselineRMS values, the waveform is cut. This cut allows to remove all waveforms withanomalous fluctuations of the baseline, as occur e.g. during sparks. Figure 17 shows a spectrum of theanode 1
Baseline values before and after applying these cuts.Waveforms with a
Baseline above the trigger threshold are cut. Furthermore, a set of simultaneouslyrecorded waveforms is rejected when the maximum V i value of the anode 3 waveform is below the triggerthreshold. This is because we trigger the simultaneous readout of all three anodes with the anode 3 signal.In cases where only the anode 3 waveform’s maximal V i is above its trigger threshold, the correspondinganode 1 and 2 waveforms can still be used in an amplitude measurement.Checks are made to identify events containing sparks and such events with a damaged pre-amplifier.An “event” contains all waveforms recorded during the exposure time of the simultaneously taken CCDframes. In case of sparks the pre-amplifiers’ baselines moves substantially and it takes time for thepre-amplifier to return to the pre-spark status. Thus an event is flagged as spark event when it containsmore than 5 waveforms with a Baseline above the trigger threshold. When a pre-amplifier gets damagedthe result is flat waveform. So we flag events where the maximum value is very close to the baseline ( max ( V i ) < Baseline × ) and ( max ( V i ) > Baseline × ) as having been taken with a damagedpreamp.Finally, waveforms are accepted or rejected based on their rise time and Peak Time . The
Peak Time is thetime value t j in a waveform for which V j = max ( V i ) , i ∈ N . We calculate the rise time as the timedifference between: t r,90 and t r,10 . For anodes 2 and 3, waveforms with a rise time above 5 µs or a Peak ersion 15th February 2021 submitted to
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Cut surviving signalssingle cut cuts applied subsequentlyNo Cuts 100 % 100 %
Baseline < Trigger-threshold 99.97 % 99.97 %
Baseline within mean interval 99.97 % 99.97 %
BaselineRMS within interval 99.99 % 99.97 %max ( V i ) > Trigger threshold 11.25 % 11.23 %rise time < Peak Time < Table 2.
Fraction of analysed waveforms rejected for each data cleaning cut for a run where no sparkingwas observed. Cut surviving signalssingle cut cuts applied subsequentlyNo Cuts 100 % 100 %
Baseline < Trigger-threshold 53.26 % 53.26 %
Baseline within mean interval 26.85 % 26.85 %
BaselineRMS within interval 68.29 % 26.85 %max ( V i ) > Trigger threshold 5.64 % 5.22 %rise time < Peak Time < Table 3.
Fraction of analysed waveforms rejected for each data cleaning cut for a run containing sparkevents.
Time which is not within a 5 µs interval around t = Peak Time outside of t < ± In this section the charge gain of the three anode amplification stage is calculated from the amplitudespectra discussed above. Features in the spectra have to be related to a known energy deposition insidethe HPTPC. A known energy deposit can be realised using a radioactive source e.g.
Am ( cf . Sec. 5).Primary ionisation electrons ( Q e ) from converted γ -rays or α particles drift towards the anode meshes andare amplified there. The amplification factor, the charge gain of the amplification region G amp , depends onthe meshes’ configuration such as inter mesh distance and HV settings. After charge signals are decoupledfrom the HV line, they are amplified by the pre-amplifiers ( G preamp ). The amplitude A of a waveform thusrelates to Q e as A [ mV ] = f · G preamp (cid:104) mV pC − (cid:105) · G amp · Q e [ pC ] . (8)The factor f is another dimensionless factor which we introduce in order to describe (attenuating) effectsof the readout circuit on the signal height. G preamp and f are determined with dedicated measurements tocalibrate the readout circuit. ersion 15th February 2021 submitted to Instruments
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Measurement taken Capacitance’sbetweenAnode 1/2 [nF] χ / N dof ofAnode 1/2 CapacitancesbetweenAnode 2/3 [nF] χ / N dof ofAnode 2/3 CapacitancesbetweenAnode 1/3 [nF]fit 7.3 ± ± ± ± ± Table 4.
Mesh capacitances determined by a fit [24] and by a direct measurement with a multimeter. V input , and a low frequency and long width as compared to the pre-amplifiers decay timeof a few 100 µs. These pulses are recorded with the HPTPC’s data acquisition system and analysed withthe analysis chain descried above, but without applying cleaning since no noise signals are present whennot applying HV to the detector. For a given test pulse height, the resulting amplitude spectrum featuresone peak. The ratio of the peak’s mean amplitude to the input pulse height gives G preamp when takingthe pre-amplifiers input capacitance into account. Testing several chips, the average gain of the CR-112chips is measured to be G CR − = ± − , and the average gain of the CR-113 chips wasmeasured to be G CR − = ± − . These values are consistent with the values provided bythe supplier of 13 mV pC − and 1.3 mV pC − , respectively.A detailed description of the HPTPC’s circuit response to test pulses can be found in [24]. To performthese tests one of the three pre-amplifiers is disconnected from its bias box ( S anode i in Fig. 6) and test pulsesare injected where it is usually connected. Doing so induces signals on the other two anode meshes, whichare read out. Table 4 shows the inter-mesh capacitances measured with a digital multimeter as well asthe results from a fit to the data obtained during the test-pulse campaign. The capacitance determined bymeasuring pulse amplitudes and by multimeter measurement differ by 17 %. This difference is likely dueto the fact that the multimeter measurement is performed close to the detector, i.e. no long cables and otherparasitic capacitances are present. The distance between the mesh planes can be determined knowing thecapacitances: C = (cid:101) o Ad , (9)where C is the capacitance, (cid:101) the vacuum permittivity, A the area of the mesh planes and d the distancebetween two mesh planes. This assumes that the meshes can be approximated as a parallel plate capacitor.Inserting our mesh geometry into the calculations in [25] shows that such an approximation overestimates(underestimates) the actual capacitance (mesh distance) by less than 10 %. Furthermore we use (cid:101) Ar = (cid:104) [26], hence (cid:101) Ar (cid:101) = (cid:101) . We can calculate that anode 1 and 2are 1.20 ± ± cf. Sec. 4.3).The values determined here have the right order of magnitude and are close to the design values. Thedifference can be due to the fact that the exact thickness of the glue layers in the amplification region is notknown, therefore the design values are most likely a lower limit.Figure 18 shows an example where a test pulse is coupled into S anode1 at the anode 1 bias box while theanode 2 and anode 3 signals are amplified and digitised. In the figure the anode 2 signal is saturated,whilst the decay of the anode 3 signal shows a change of decay constant around ∼
20 µs. A systematicstudy of the amplified signals’ peak height ( V ampoutput ) revealed that as soon as one pre-amplifier is saturated, ersion 15th February 2021 submitted to Instruments
30 of 42 − s) µ time ( − a m p li t ude ( m V ) Anode1Waveform212_Event0
Test pulseAnode 2 signalAnode 3 signal
Anode1Waveform212_Event0
Figure 18.
Waveform of a test pulse, coupled into the anode 1 mesh and the resulting amplified pulses(CR-112) as digitised by the HPTPC’s data acquisition system. the signal on the other pre-amplifier shows a modified decay similar to what is visible in Fig. 18 [24].This behaviour affects the measured amplitude as shown in Fig. 19: The points for anode 2 feature twodistinct regions: An initial region of linear increase up until an output voltage of 3330 ±
20 mV where thepre-amplifier saturates and the region after that. The saturation value is in line with the manufacturer’stechnical specification for output swing of ± V input and the circuit elements shown in Fig. 6. The slope of the anode 2 data before saturation in theplot gives the pre-amplifier gain ( G CR − = ± − ) multiplied by f a , which describes signalattenuation and losses in the circuit ( cf . Eq. (8)). Fitting of a polynomial of order one to the data points,corresponding to the anode 2 line in Fig. 19a, yields f a · G CR − = ± − , the correspondingvalue for f a is shown in Tab. 5.For anode 3, however, three regions can be identified in Fig. 19a. There are two regions of distinct linearincrease but with different gradients. The first region – up to a V input of 150 mV – ends at the point whenthe anode 2 pre-amplifier saturates. From this point onwards two decay constants are observed in anode3 waveforms similar to what is shown in Fig. 18. In the second region the rise is still linear, but with adifferent slope than in the first region and the third region covers the saturation of the anode 3 pre-amplifier.When the anode 2 pre-amplifier saturates, the AC signal current can no longer simply flow through itsinput and feedback capacitor and the signal sharing is modified. This feedback is then seen in the detectoras more charge being measured by the anode 3 pre-amplifier than expected. It has been confirmed thatthis behaviour is indeed due to the anode 2 pre-amplifier saturating. Removing this pre-amplifier from thecircuit results in anode 3 signals with only one decay constant and no change in gradient – similar to what isshown for anode 2 in Fig. 19a. A fit of a polynomial of order one yields f a · G CR − = ± − for anode 3 before the anode 2 saturation and f a · G CR − = ± − after the saturation.Table 5 shows the circuit response f obtained by comparing the measurements of f · G preamp to the bare G CR − measurements at the beginning of this section.Finding f = f values measured here show a substantial attenuation which can be corrected for since f is constant asa function of amplitude. Theses losses may occur through the resistive elements shown in Fig. 6. Thechange in f on anode 3 when the anode 2 pre-amplifier is saturated makes this correction slightly more ersion 15th February 2021 submitted to Instruments
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Peak Amplitude on Anode 1 ( mV ) P e a k A m p li t u d e o n A n o d e s ( m V ) Measured voltage on anode 2Measured voltage on anode 3 :presaturation of anode 2 preampMeasured voltage on anode 3 : postsaturation of anode 2 preamp (a) pC )0500100015002000250030003500 P e a k A m li t u d e o n A n o d e ( m V ) Measured voltage on anode 2Measured voltage on anode 3 :presaturation of anode 2 preampMeasured voltage on anode 3 : postsaturation of anode 2 preamp (b)Figure 19.
Peak height ( V ampoutput ) measured by the anode 2 and anode 3 readout channel (with pre-amplifier)for test pulses injected into the amplification region via the anode 1 mesh. Both plots show the same datawith different units on the horizontal axis: (a) V ampoutput as function of input test pules signal height ( V input )and (b) as a function of the charge seen at the pre-amplifier input. One polynomial of order ( P
1) one isfitted to the anode 2 (blue) measurement and two separate P
1s are fitted to the different regions on anode 3.One in the pre-saturation region of the anode 2 pre-amplifier (red) and one in the post-saturation region ofanode 2 pre-amplifier (orange).Anode f · G preamp [ mV/pC ] Modification factor f anode 2 9.8 ± ± ± ± ± ± Table 5.
Using the measured pre-amplifier without the circuit response ( G CR − = ± − )and the measurements of the pre-amplifiers connected to the detector f · G preamp , the circuit responsemodification-factor f is determined [24]. ersion 15th February 2021 submitted to Instruments
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Amplitude (mV) C oun t s ( / . m V ) Waveform Amplitude Spectrum Anode 1Waveform Amplitude Spectrum Anode 1 (a)
Amplitude (mV) C oun t s ( / m V ) Waveform Amplitude Spectrum Anode 2Waveform Amplitude Spectrum Anode 2 (b)
Amplitude (mV) C oun t s ( / m V ) Waveform Amplitude Spectrum Anode 3Waveform Amplitude Spectrum Anode 3 (c)Figure 20.
Waveform amplitude spectra for anodes (a) 1, (b) 2, and (c) 3. On the vertical axis counts areshown, normalised to the time of one CCD exposure, i.e. V a = V a = V a = V c = − complicated. The capacitances of the amplification region and the available pulse generator did not allowto drive the anode 3 pre-amplifier into saturation to examine whether a similar feed-back occurs on anode2. In general, events with either pre-amplifier being saturated occur only rarely, as do events where thesignal amplitude on anode 2 is higher than on anode 3 due to the way the amplification region is biased.6.3.2. Charge gain of the amplification regionThis section discusses the analysis of charge waveform data taken simultaneously as the CCD framesused for the light analysis described in Sec. 5.4. All data was taken in the same TPC fill of pure argon at3 bar absolute pressure. The three voltage schemes – A, B, and C – are described in detail in the previoussection. In short: In Scheme A we investigate the dependence of the gas amplification factor on the valuesof V a , V a , and V a , whilst keeping ∆ V a and ∆ V a constant; Scheme B investigates the gain’s dependenceon ∆ V a by varying V a keeping V a and V a constant, and Scheme C investigates the gain’s dependenceon ∆ V a by varying V a and V a for constant ∆ V a . Determining the Am-241 alpha decay peak amplitude
After data cleaning (Sec. 6.2) and taking into account the calibration discussed in Sec. 6.3.1 we createamplitude spectra for each anode per voltage setting as shown in Fig. 20. The number of entries in theamplitude spectra for each voltage configuration varies between 100 and 2500, with an average of 910entries per configuration. This variation is caused by the fact that at higher voltage settings a greaternumber of waveforms are recorded and survive the cleaning cuts. From the results presented in Sec. 5.4(and Sec. 5.3), we are confident that we should see the α particles from the Am decay in the waveformamplitude spectra. Whilst the qualitative example spectrum in Fig. 8 does neither account for the gasgain and the electronic noise, the measured amplitude spectra should show some resemblance to thissimulation. The measured amplitude spectra (Fig. 20) appear as an exponentially falling background, witha clear peak. This peak corresponds to the deposit of the ∼ α particles from the Am decay. The ersion 15th February 2021 submitted to
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33 of 42 exponential background is a mix of the expected cosmic radiation background, of the
Am x-ray signalsand noise triggers. The amplitude spectra are fitted with the function s ( amplitude ) = exp { p + p · amplitude } + p · exp (cid:40) − · (cid:18) amplitude − p p (cid:19) (cid:41) + p · exp (cid:40) − · (cid:18) amplitude − p p (cid:19) (cid:41) , (10)where the first term is an exponential function to fit the noise, and x-ray and γ -ray background, and thesecond term is a Gaussian function to fit the α -peak. The third term is a second Gaussian function whichfits the higher amplitude entries of the spectra, where the spectra are shaped by cosmic muons. Examplesof these fits are shown in Fig. 20.The mean of the Gaussian fitting the α -peak from the Am decay, p , is extracted and taken as a measurefor the mean energy deposit of the α particles. In Fig. 21 the α -peak position is plotted against the variedvoltage in the respective voltage scheme. The peak position uncertainty shown in the plots are the fituncertainties on the mean of the Gaussian, scaled by the χ / N dof of the fit, for fits where χ / N dof > Gas gain against voltage for the three voltage schemes
Before calculating the gas gain for the three voltage schemes A, B and C from the values in theamplitude spectra, gas quality degradation needs to be considered. Degrading gas quality can have asignificant effect on the gain measurements, and so we took data at identical gas, pressure and bias voltagesettings every 24 hours to obtain calibration correction as the data used in this analysis was taken overthree days.We reconstruct the peak position in the amplitude spectra of these calibration runs. After the gas qualitycalibration is fit to these data points vs. the measurement time. The correction function is y ( time ) = m · time + b , where the values of ( m , b ) are (0.8 ± ± ± ±
42 mV)and (27 ± ±
141 mV) for anode 1, anode 2, and anode 3 spectra respectively, and thecalibration is normalized such that the non-calibrated data and the calibrated data have the same value atthe beginning of Scheme C. We observe a drift in the peak position as can be seen from the ( m , b ) pairs,however, the drift is such that no change could been observed when examining the amplitude spectra foreach run in a voltage setting individually.A systematic uncertainty contribution is assessed to account for this effect, represented by the dotted errorbars in Fig. 21. This contribution takes the expected peak position shift over the measurement time in eachvoltage scheme into account and is calculated as the standard deviation of the measured peak positionswith respect to the peak position after correction.Equation (8) allows now to calculate the gas amplification factor of the amplification region, G amp , using G amp = Af · G preamp · Q e where (11) Q e = (cid:104) ε α (cid:105) W · × − C . (12) ersion 15th February 2021 submitted to
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Anode1 Voltage (V) A node P ea k P o s i t i on ( m V ) Anode 1Anode 1 (a)
Anode 1, Scheme A
Anode1 Voltage (V) A node P ea k P o s i t i on ( m V ) Anode 2Anode 2 (b)
Anode 2, Scheme A
Anode1 Voltage (V) A node P ea k P o s i t i on ( m V ) Anode 3Anode 3 (c)
Anode 3, Scheme A
Potential Difference Anode 3, 2 (V) A node P ea k P o s i t i on ( m V ) Anode 1Anode 1 (d)
Anode 1, Scheme B
Potential Difference Anode 3, 2 (V) A node P ea k P o s i t i on ( m V ) Anode 2Anode 2 (e)
Anode 2, Scheme B
Potential Difference Anode 3, 2 (V) A node P ea k P o s i t i on ( m V ) Anode 3Anode 3 (f)
Anode 3, Scheme B
Potential Difference Anode 2, 1 (V)
500 1000 1500 A node P ea k P o s i t i on ( m V ) Anode 1Anode 1 (g)
Anode 1, Scheme C
Potential Difference Anode 2, 1 (V)
500 1000 1500 A node P ea k P o s i t i on ( m V ) Anode 2Anode 2 (h)
Anode 2, Scheme C
Potential Difference Anode 2, 1 (V)
500 1000 1500 A node P ea k P o s i t i on ( m V ) Anode 3Anode 3 (i)
Anode 3, Scheme C
Figure 21.
Plots of the position of the α -peak in the respective amplitude spectra. In the first row ((a), (b),and (c)) the peak position is plotted vs anode 1 voltage (Scheme A). During Scheme A, the voltages of allthree anodes are increased in steps of 200 V while the potential difference between anodes is kept constant.In the second row (Scheme B: (d), (e), and (f)) the peak position is plotted vs the potential difference betweenanodes 2 and 3 ( ∆ V a ). During the measurement V a , V a and ∆ V a are kept constant. Third row (SchemeC: (g), (h), and (i)): Peak position vs the potential difference between anodes 1 and 2 ( V a ) while V a and ∆ V a are kept constant. All measurements have been made in the same gas fill of 3 bar absolute of pureargon. ersion 15th February 2021 submitted to Instruments
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Scheme Voltage setting (A1 / A2 / A3) [V] Gas gain at anode 3 at voltage settingLowest Highest at lowest setting at highest settingA 1200 / 2400 / 3600 2000 / 3200 / 4400 (2.61 ± × (3.14 ± × B 1200 / 2400 / 2400 1200 / 3400 / 5000 (1.44 ± × (3.18 ± × C 1200 / 1400 / 2600 1200 / 3000 / 4200 (1.61 ± × (2.63 ± × Table 6.
The charge gain measured at the highest at lowest voltage settings of each voltage scheme.
We calculate G amp for the amplitude spectra measured at each mesh. In the calculation we use the best-fitpeak position of the α -peak in the amplitude spectra to be A , corrected by the calibration proceduredescribed above. Figure 22 shows the gas gain at each anode plotted against the respective voltage in thethree voltage schemes. The goal of this analysis is to determine the dependence of the gain on the absolutevoltages of the anodes ( V a , V a , and V a ) and on the potential differences between the anodes ( ∆ V a and ∆ V a ). The results of the charge gain measurement for schemes A, B and C are shown in Fig. 22 in thefirst, second and third row, respectively, and the gas gains measured at the highest and lowest voltagesettings for each scheme are presented in Tab. 6. The voltage range covered during the three schemes hasbeen optimised for the light analysis, to the end that i) all voltage settings of the three schemes could betaken in one gas fill without the degradation of the gas fill, ii) to avoid to reach a voltage regime wheresparking occurs, and iii) to have sufficient overlap between the three voltage schemes. As a result of thisour study of the charge gain of the amplification region covers only a small gain range (Tab. 6 and Fig. 22).For all three voltage schemes the measured gas gain increases from anode 1, to anode 2, to anode 3, asis expected from a cascade of amplification stages. The gas amplification factor in Scheme C is overallthe lowest. Examining the multiplication factor between different meshes we find G mesh2amp ∼ · G mesh1amp ( G mesh2amp ∼ · G mesh1amp ) and G mesh3amp ∼ · G mesh2amp ( G mesh3amp ∼ · G mesh2amp ) in scheme A and Scheme B (SchemeC). The highest contribution to the combined gas gain G mesh1amp · G mesh2amp · G mesh3amp is thus the contribution ofthe anode 1 mesh. The dependence of the gain on the various voltages shows a similar functional shape asthe light gain reported in Sec. 5.4.2, Fig. 15. Due to relatively large uncertainties, the results in Scheme Aare consistent with either a slight dependence or no dependence of the gain on the absolute voltages ofthe anodes while ∆ V a and ∆ V a are fixed at 1200 V and are therefore consistent with the conclusions ofthe light gain analysis. The results of schemes B and C are consistent with a positive correlation of gainon ∆ V a and ∆ V a , this is again consistent with the conclusions drawn from the light gain analysis. Theresults of the charge gain analysis supports the conclusions of the light gain analysis, that the amplificationis primarily driven by the electric field between the anodes.
7. Combined Optical and Charge Readout Analysis
In this section we present the results of the combined optical and charge gain analysis. The opticaland charge gain analyses described in Sec. 5 and Sec. 6 were performed on data taken simultaneouslywith both readout systems. We investigate the correlation between optical gain and charge gain in Fig. 15and Fig. 22. Plots of the optical gain against the charge gain for Schemes A, B and C are shown in Fig. 23.Figure 24 shows the ratio of the charge gain to the measured light intensity in ADU as a function of therelevant voltage in the respective voltage scheme. The larger of the two charge gain error bars (showndotted in Fig. 23) has been propagated through to produce the error bars seen in Fig. 24.To measure the correlation factor between the two gain measurements we use the Pearson correlationcoefficient, which takes values between -1 and 1 for fully negative and positive correlated data. Thecoefficient is zero for uncorrelated data. In order to take the uncertainties of our measurements intoaccount we take every measured value as the centre of a normal distribution and its uncertainty as thedistribution’s standard deviation. From these distributions a 1000 random data series are drawn for each ersion 15th February 2021 submitted to
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Anode1 Voltage (V) G a s G a i n A node Anode 1Anode 1 (a)
Anode 1, Scheme A
Anode1 Voltage (V) G a s G a i n A node Anode 2Anode 2 (b)
Anode 2, Scheme A
Anode1 Voltage (V) G a s G a i n A node Anode 3Anode 3 (c)
Anode 3, Scheme A
Potential Difference Anode 3, 2 (V) G a s G a i n A node Anode 1Anode 1 (d)
Anode 1, Scheme B
Potential Difference Anode 3, 2 (V) G a s G a i n A node Anode 2Anode 2 (e)
Anode 2, Scheme B
Potential Difference Anode 3, 2 (V) G a s G a i n A node Anode 3Anode 3 (f)
Anode 3, Scheme B
Potential Difference Anode 2, 1 (V)
500 1000 1500 G a s G a i n A node Anode 1Anode 1 (g)
Anode 1, Scheme C
Potential Difference Anode 2, 1 (V)
500 1000 1500 G a s G a i n A node Anode 2Anode 2 (h)
Anode 2, Scheme C
Potential Difference Anode 2, 1 (V)
500 1000 1500 G a s G a i n A node Anode 3Anode 3 (i)
Anode 3, Scheme C
Figure 22.
Plots of the calculated gas gain vs either anode voltage or inter-anode voltage difference. The gainis calculated from the data shown in the respective plot in Fig. 21. First row ((a), (b), and (c)): Scheme A, gainvs anode 1 voltage ( V a ), V a , V a and V a are increased by the same amount whilst ∆ V a = ∆ V a = ∆ V a ), V a and ∆ V a are increased whilst V a = V a = ∆ V a ), V a , ∆ V a and V a are increased whilst keeping V a and ∆ V a constant. All data has been taken in the same gas fill of 3 bar absolute of pure argon. ersion 15th February 2021 submitted to Instruments
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350 355 360 365 370 375 380 385 390Integrated ADU (ADU)240026002800300032003400 G a s G a i n Scheme AScheme A (a)
250 300 350 400 450Integrated ADU (ADU)15002000250030003500 G a s G a i n Scheme BScheme B (b)
100 150 200 250 300 350Integrated ADU (ADU)140016001800200022002400260028003000 G a s G a i n Scheme CScheme C (c)Figure 23.
Measured light intensity (Integrated ADU) (Fig. 15) plotted against the gas gain measured in thecharge readout on anode 3 (Fig. 22, right column) for Schemes A, B and C. ersion 15th February 2021 submitted to
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38 of 42 G a s G a i n / I n t eg r a t ed A DU Scheme AScheme A (a) G a s G a i n / I n t eg r a t ed A DU Scheme B (b)
200 400 600 800 1000 1200 1400 1600 1800Potential Difference Between Anode 2 and Anode 1 (V)6810121416182022 G a s G a i n / I n t eg r a t ed A DU Scheme CScheme C (c)Figure 24.
Ratio of gas gain measured in the amplification region at anode 3 (Fig. 22, right column) to themeasured intensity (integrated ADU) (Fig. 15) vs (a) anode 1 voltage ( V a ) where the voltage differencesbetween the meshes is always ∆ V a = ∆ V a = V a ) while the anode 1 and 2 voltages are kept constant (c) potential difference between anodes 1 and 2( ∆ V a ) while V a is kept constant and ∆ V a is maintained at 1200 V ersion 15th February 2021 submitted to Instruments
39 of 42 voltage scheme with the same number of points as in the original data series in Fig. 24a, Fig. 24b andFig. 24c and the correlation factor is calculated for each of them. The correlation factors quoted in thefollowing are the mean of these 1000 correlation factors and their standard deviation.Both the optical and charge analysis found Scheme A to be consistent with no change in gain. Thefigures in this section show continued support for this case as expected. The correlation factor of thedata sample in Fig. 23a is 0.50 ± ± ± ∆ V a =
800 V (Fig. 15c). This effect is not clearly visible inthe charge gain analysis (Fig. 22i). However, given the size of the gas gain error bars it is not possible toconfidently exclude this as a possibility.Figure 24 shows a largely consistent ratio of gas gain to the measured light gain in integrated ADUof around 8 for all schemes. The only deviation from this ratio occurs at the lowest voltage settings forSchemes B and C. In Sec. 5.4.3 we found that there are ( ± ) × − photons in the amplificationregion per primary electron in the drift volume, when analysing the voltage setting with the highestlight yield. The authors of [15] examine pure argon at a pressure of 3 bar absolute, too. For this gas theymeasure, albeit with a much smaller detector and a two mesh amplification region with 4 mm distance, amaximal value of ∼
8. Summary
In order to reduce neutrino interaction related systematic uncertainties in future neutrino oscillationexperiments, a key measurement is proton-nucleus scattering. Hadronic interactions as particles producedin neutrino interactions exit the nucleus and obfuscate the secondary particle multiplicity and kinematics,causing event migrations between data samples and introducing biases in neutrino event reconstruction.Measurements of protons interacting with nuclei can constrain these hadronic interactions and therebyreduce these biases. A HPTPC prototype detector with a three mesh amplification region has beenconstructed and operated at RHUL and CERN as a first step in the development of a HPTPC capable ofperforming these measurements.In this work, for the first time, we demonstrate the successful combined optical and charge readout of ahybrid high pressure gaseous TPC with an active volume of ∼ . The optical readout utilises CCDcameras, most sensitive in the visible part of the wavelength spectrum. In a series of pilot measurementswe identified pure argon at an absolute pressure of 3 bar as the gas best suited to perform in depth testsof the optical readout performance with a high pressure gas. Our measurements were done using the α ersion 15th February 2021 submitted to Instruments
40 of 42 particles emitted by an Am source. When using the HPTPC with argon at 3 bar we were not able toimage tracks on an event by event basis with the optical readout. This may be due to the large diffusion inpure argon. Integrating over many exposures we measure an increasing light yield when increasing theelectric field between mesh 2 and mesh 3 ( E a ), where the mesh number increases for anodes further awayfrom drift region. An increase in light yield is also measured when increasing the field between mesh 1and mesh 2 ( E a ). In this case the light yield reaches a plateau when E a ∼ E a . At the maximum lightyield measured, we find that there are ( ± ) × − photons in the amplification region per primaryelectron in the drift volume.The analysis of the charge signals signals reveals that light gain and charge gain are correlated and that thegas gain at the voltage settings of the maximal light yield is 3000. The first mesh in the cascade contributesthe largest fraction of the amplification stages gain of ∼
70 whilst the following meshes contribute anotherfactor of about 8 and 5.5, respectively.
Author Contributions:
Conceptualization, Anastasia Basharina-Freshville, Steven Boyd, Dominic Brailsford, LindaCremonesi, Patrick Dunne, Jennifer Haigh, Asher Kaboth, Jocelyn Monroe, Ryan Nichol, Jaroslaw Nowak, RubenSaakyan, Nicola Serra, Yuri Shitov and Morgan Wascko;data curation, Edward Atkin, Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi, Alexander Deisting,Adriana Dias, Patrick Dunne, Jennifer Haigh, Philip Hamacher-Baumann, Sebastian Jones, Asher Kaboth, AlexanderKorzenev, Maria Mironova, Jocelyn Monroe, Ryan Nichol, Toby Nonnenmacher, Jaroslaw Nowak, William Parker,Harrison Ritchie-Yates, Stefan Roth, Adam Tarrant, Melissa Uchida, Sammy Valder, Abigail Victoria Waldron, MarkWard and Morgan Wascko;formal analysis, Edward Atkin, Dominic Brailsford, Zachary Chen-Wishart, Alexander Deisting, Adriana Dias,Patrick Dunne, Jennifer Haigh, Philip Hamacher-Baumann, Sebastian Jones, Maria Mironova, Toby Nonnenmacher,William Parker, Harrison Ritchie-Yates, Adam Tarrant, Abigail Victoria Waldron and Mark Ward;funding acquisition, Gary Barker, Anastasia Basharina-Freshville, Steven Boyd, Asher Kaboth, Jocelyn Monroe,Ryan Nichol, Jaroslaw Nowak,Stefan Roth, Ruben Saakyan and Morgan Wascko;investigation, Edward Atkin, Gary Barker, Anastasia Basharina-Freshville, Christopher Betancourt, Steven Boyd,Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi, Alexander Deisting, Adriana Dias, Patrick Dunne,Jennifer Haigh, Philip Hamacher-Baumann, Sebastian Jones, Asher Kaboth, Alexander Korzenev, William Ma, PhilippeMermod, Maria Mironova, Jocelyn Monroe, Ryan Nichol, Toby Nonnenmacher, Jaroslaw Nowak, William Parker,Harrison Ritchie-Yates, Stefan Roth, Ruben Saakyan, Nicola Serra, Yuri Shitov, Jochen Steinmann, Adam Tarrant,Melissa Uchida, Sammy Valder, Abigail Victoria Waldron, Mark Ward and Morgan Wascko;methodology, Edward Atkin, Gary Barker, Anastasia Basharina-Freshville, Christopher Betancourt, Steven Boyd,Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi, Alexander Deisting, Adriana Dias, Patrick Dunne,Jennifer Haigh, Philip Hamacher-Baumann, Sebastian Jones, Asher Kaboth, Alexander Korzenev, William Ma, PhilippeMermod, Maria Mironova, Jocelyn Monroe, Ryan Nichol, Toby Nonnenmacher, Jaroslaw Nowak, William Parker,Harrison Ritchie-Yates, Stefan Roth, Ruben Saakyan, Nicola Serra, Yuri Shitov, Jochen Steinmann, Adam Tarrant,Melissa Uchida, Sammy Valder, Abigail Victoria Waldron, Mark Ward and Morgan Wascko;project administration, Gary Barker, Steven Boyd, Linda Cremonesi, Alexander Deisting, Asher Kaboth, JocelynMonroe, Ryan Nichol, Jaroslaw Nowak and Morgan Wascko;resources, Steven Boyd, Linda Cremonesi, Asher Kaboth, Jocelyn Monroe, Ryan Nichol, Jaroslaw Nowak, StefanRoth and Morgan Wascko;Software, Edward Atkin, Dominic Brailsford, Zachary Chen-Wishart, Alexander Deisting, Patrick Dunne, JenniferHaigh, Philip Hamacher-Baumann, Sebastian Jones, Toby Nonnenmacher, William Parker, Harrison Ritchie-Yates,Adam Tarrant, Sammy Valder and Abigail Victoria Waldronsupervision, Gary Barker, Anastasia Basharina-Freshville, Steven Boyd, Dominic Brailsford, Linda Cremonesi,Alexander Deisting, Patrick Dunne, Jennifer Haigh, Asher Kaboth, Jocelyn Monroe, Ryan Nichol, Jaroslaw Nowak,Ruben Saakyan, Abigail Victoria Waldron, and Morgan Wascko;validation, Edward Atkin, Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi, Alexander Deisting,Adriana Dias, Patrick Dunne, Sebastian Jones, Asher Kaboth, Maria Mironova, Jocelyn Monroe, Ryan Nichol, TobyNonnenmacher, William Parker, Harrison Ritchie-Yates, Adam Tarrant and Abigail Victoria Waldron;visualization, Edward Atkin, Zachary Chen-Wishart, Alexander Deisting, Adriana Dias, Patrick Dunne, PhilipHamacher-Baumann, William Parker, Harrison Ritchie-Yates, Adam Tarrant and Abigail Victoria Waldron;writing—original draft, Zachary Chen-Wishart, Alexander Deisting, Patrick Dunne, Asher Kaboth, JocelynMonroe, Harrison Ritchie-Yates, Abigail Victoria Waldron and Morgan Wascko;writing—review and editing, Edward Atkin, Dominic Brailsford, Zachary Chen-Wishart, Linda Cremonesi,Alexander Deisting, Adriana Dias, Patrick Dunne, Philip Hamacher-Baumann, Asher Kaboth, Jocelyn Monroe,Jaroslaw Nowak, Harrison Ritchie-Yates, Abigail Victoria Waldron and Morgan Wascko; ersion 15th February 2021 submitted to
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Funding:
This research was funded in part by Science and Technology Facilities Council grant number ST/N003233/.
Acknowledgments:
We wish to acknowledge support for summer students from the Ogden Trust and St. AndrewsUniversity, and outstanding support during the beam test from Johannes Bernhard of CERN as well as RebeccaConybeare, Nicole Cullen, Kate Gould, Veera Mikola, Christopher Thorpe, and Simon Williams.
Conflicts of Interest:
The authors declare no conflict of interest.
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