Optical Calibration System for the LUX-ZEPLIN (LZ) Outer Detector
W. Turner, A. Baxter, H. J. Birch, B. Boxer, S. Burdin, E. Fraser, A. Greenall, S. Powel, P. Sutcliffe
OOptical Calibration System for the LUX-ZEPLIN (LZ) Outer Detector
W. Turner a, ∗ , A. Baxter a , H. J. Birch a,b , B. Boxer a,c , S. Burdin a , E. Fraser a , A. Greenall a , S. Powell a ,P. Sutcliffe a a University of Liverpool, Oliver Lodge Laboratory, Oxford Street, Liverpool, L69 7ZE, UK b Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA c STFC Rutherford Appleton Laboratory (RAL), Didcot, OX11 0QX, UK
Abstract
The LUX-ZEPLIN experiment will search for dark matter particle interactions with a detector containinga total of 10 tonnes of liquid xenon. Surrounding the liquid xenon cryostat is an outer detector veto systemwith the primary aim of vetoing neutron single-scatter events in the liquid xenon that could mimic a weaklyinteracting massive particle (WIMP) dark matter signal. The outer detector consists of approximately17 tonnes of gadolinium-loaded liquid scintillator confined to 10 acrylic tanks surrounding the cryostatand 228 ,
000 litres of water as the outermost layer. It will be monitored by 120 inward-facing 8-inch pho-tomultiplier tubes. An optical calibration system has been designed and built to calibrate and monitorthese photomultiplier tubes allowing the veto system to reach its required efficiency and thus ensuring thatLUX-ZEPLIN meets its target sensitivity.
Keywords:
Low-background, Optical Calibration, Water, Neutron Veto, LED
1. Introduction
LUX-ZEPLIN (LZ) is a dark matter direct de-tection experiment with liquid xenon (LXe) as thetarget material. Its projected sensitivity (90% C.L.)for WIMP-nucleon spin-independent interactionsreaches 1 . × − cm for 40 GeV/c WIMPs [1][2]. The core of LZ is a dual-phase xenon time pro-jection chamber (TPC) with an active mass of 7tonnes which will be the largest detector of its kind.The TPC is surrounded by an outer detector (OD)which is fundamental in reaching the desired sensi-tivity. In the TPC, signals produced by neutronsare indistinguishable from those of WIMPs on anevent-by-event basis; the purpose of the OD is toveto these neutrons [3]. LZ is in the final stages ofcommissioning ∼ ∼ ∗ Corresponding author
Email address: [email protected] (W. Turner) as shown in Fig. 1. Schematic of the LZ detectortaken from LZ TDR [3]. The LXe ‘skin’ detectoris located between the TPC and the inner cryo-stat wall and is principally used to veto MeV-scalegamma rays. The OD forms a near hermetic sealaround the cryostat and is composed of a segmentedacrylic tank housing 17 tonnes of linear alkylben-zene based gadolinium-loaded liquid scintillator andis surrounded by 228 ,
000 litres of deionised water.The segmented acrylic tank and housed scintillatorare refereed to collectively as the scintillator tanks.The primary function of the scintillator tanks isto veto neutrons that have interacted in the TPC,predominantly via neutron capture on gadolinium[4]. Light generated in the OD is subsequentlycollected by 120 inward facing 8-inch HamamatsuR5912 photomultiplier tubes (PMTs) [5]. To max-imise light collection efficiency, there is a Tyvek ® curtain behind, above and below the PMTs, and alayer of Tyvek ® surrounding the cryostat [3].The main requirement to the OD is to provide aveto efficiency to the neutrons scattered in the TPCgreater than 95% with less than 5% dead time at theeffective threshold of 200 keV (to be above the endof the C beta spectrum.) [1]. This requires under-
Preprint submitted to Nuclear Instruments and Methods in Physics Research Section A February 17, 2021 a r X i v : . [ phy s i c s . i n s - d e t ] F e b igure 1: Cutaway drawing of the LZ detector systems. The LXe TPC in the centre is surrounded by the scintillator tanks(green) and light collection system (white), all housed in a large water tank (blue-grey). The positions of some of the injectionpoints for the OD OCS are highlighted by red circles. The route the produced light takes to reach the water tank and themonitoring PMT housed in the dark box is shown as red dashed line. Schematic of the LZ detector is taken from LZ TDR [3]. standing the OD optical model and PMT stabilityat the ∼
1% level. An Optical Calibration System(OCS) has been designed and built to validate andmonitor OD optical properties and calibrate the ODPMTs at the required level. Though the expectedOD threshold is 200 keV, the OCS was designed tocalibrate the OD down to 150 keV.Section 2 of this article describes the designchoices made to ensure the OCS meets the givenrequirements, while Sec. 3 outlines the hardwaredeveloped to meet these requirements as well as thehardware selection criteria. Section 6 presents thecalibration procedure and Sec. 5 shows the final sys-tem performance. Additionally, Sec. 4 presents theradioassaying results of the OCS components andhow the observed rates compare to that of the outerdetector.
2. System Requirements
The OCS must be capable of executing a numberof routines to monitor the OD. The requirementsunderpinning these routines informed the design of the OCS. These requirements are summarised inTab. 1 and described below.Verification of the OD PMTs response is pre-requisite to all other monitoring techniques. Fullgeometry simulations were carried out using aGEANT4 based simulation framework developedby the LZ collaboration, BACCARAT [6] [7] [8].This demonstrated that the response of OD PMTscan be monitored via the repeated injection of1000 photons from the central row of fibres. Theseinjections yield a high proportion of 1-photon pulsescollected by each of the 120 PMTs. The BAC-CARAT simulations show that the average pulsearea resulting from 1 photoelectron escaping thephotocathode of a PMT can be determined with anuncertainty of ∼ ∼ able 1: Requirements for the OD OCS with the reason for each requirement. Based on LZ TDR requirements for the OD [3]. Requirement ReasonAbility to inject 700 – 50 ,
000 photons per chan-nel. To be able to test the threshold energy of 150 keV (1350 photons)as well as being able to match the signals of the calibrationsources which can go up to about 2.6 MeV (20 ,
000 photons).Total number of photons: 1M photons globally. To mimic the average muon signal.Variation between injected pulses: (cid:46) (cid:46)
100 photonsat 150 keV. Need to be able to keep the threshold of the OD stable at around150 keV.Light intensity precision: 100 photon precisionfor 700–2000 injected photons; 1000 photonprecision for 5000–10 ,
000 injected photons. To allow scanning over the OD threshold region and simplify thematch to expected background and calibration sources.Pulse width: <
20 ns. Should be shorter than the amplifier shaping time of the DAQelectronics (30 ns).Injected light wavelength: 430 – 450 nm, 450 –460 nm, 365 – 390 nm. Matches the peak wavelength and quantum efficiency of the ODPMTs and the scintillation light from the GdLS.Pulsing frequency: up to 10 kHz. Ensure calibrations use the full readout bandwidth.Alignment: < ◦ misalignment. To limit variation in the number of photons in each PMT. Thisvalue is an acceptable upper limit to satisfy this requirements.Radioactivity: <
5% of the total OD rate. To ensure the background rates in the detector are kept to aminimum. are required to study afterpulsing from energeticcosmic muons; this is the maximum requirementfor the OCS. To achieve this, each channel must becapable of injecting at least 50 ,
000 photons whichwhen pulsed in synchrony with each other will reach1 million photons.When monitoring the light collection efficiencyat the energy scale of the calibration source, vari-ation in the number of photons collected from aninjection needs to be less than the variation in thecalibration source to avoid widening subsequent cal-ibration peaks. Each pulse produced from the OCSis monitored by the rack mounted PMT. This al-lows for pulse-to-pulse calibration. BACCARATsimulations show the resolution value of the peaksresulting from the calibration sources to be ∼ ,
000 photons. Highintensity signals are not connected to the calibra-tion sources and therefore the precision is less of aconcern.The requirement outlining the wavelengths oflight to inject is not only to ensure the injected wavelength matches the peak wavelength and quan-tum efficiency of the OD PMTs but to also ensurescintillator degradation can be detected. The ab-sorption length in the scintillator decreases signif-icantly for wavelengths below 420 nm, this regionshifts to higher wavelengths as the scintillator de-grades [9]. Injections of 435 nm and 450 nm pho-tons from the same injection point directly belowthe scintillator tanks will allow monitoring of thisdegradation. A similar relationship between the ab-sorption of UV light and the degradation of theacrylic housing of the scintillator exists and will bemonitored via the same method using one injectionpoint under the acrylic tank lip with fibres con-nected to 390 nm and 435 nm LEDs.The last three requirements in Tab. 1 are morepractical in nature. To make use of the full 4 kHzbandwidth of the DAQ [3], the OCS should be ableto complete injection at a rate faster than this. Themaximum rate depends on occupancy and it will bedetermined experimentally. A target requirementof 10 kHz was chosen for the OCS electronics asthis would allow collection of suitable statistics ina short amount of time. To ensure that realisticcomparisons can be made with the simulations, themisalignment of fibres in the OD needs to be lessthan 5 ◦ .Calculation of the neutron veto efficiency is re-liant on accurate optical modelling of the OD in3 able 2: Sources used to calibrate the OD with the energy deposited in the OD and the average number of photons producedand detected listed. To go from the energy deposited to photons produced a constant of 9000 photons/MeV have been used,and to calculate the approximate number of photons detected the average light collection efficiency around 7% and mean PMTquantum efficiency around 25% were taken into account [7] [3]. Source Energy Deposited Na 511 keV 4600 64 Na 1275 keV 11 ,
500 160DD 2200 keV 19 ,
800 277
Th 2615 keV 23 ,
500 330Cosmic-rayMuon 0.1 - 1 GeV 1 , , , BACCARAT simulations. This optical model canbe tuned and verified by comparison of light collec-tions in LZ simulation and experimental data usingprocesses which produce predictable light output:cosmic ray muons passing through the OD, calibra-tion sources, and OCS injections. The calibrationsources, shown in Tab. 2, allow absolute calibra-tion of the OD connecting energy depositions inthe liquid scintillator with the number of detectedphotons. These signals will be used as referencepoints for the OCS. Table 2 uses the energy de-posited in the OD from each source to determinethe approximate number of photons emitted using9000 photons/MeV, as well as taking into consid-eration the average light collection efficiency of ap-proximately 7% and mean PMT quantum efficiencyabout 25% to determine the approximate photonsdetected [7][3]. The OCS will also be used for in-jections of light at intermediate and extended in-tensities, and in regions of the OD which are notreachable by the calibration sources. Given that theOCS calibration is much faster than the calibrationwith radioactive sources, the OCS calibrations willbe performed more frequently.
3. System Overview
The OCS will use duplex optical fibres to injectcontrolled pulses of light produced by LEDs intothe OD at 35 locations. Of these, 30 locations aredistributed evenly around the water tank (10 az-imuthal positions at 3 heights) allowing for goodcoverage of the detector. Additionally, four injec-tion points are located beneath the four side scin-tillator tanks in order to probe the scintillator qual-ity, and one injection point beneath one of the sidescintillator tanks to probe the acrylic quality.The OCS electronics system consists of five Op-tical Calibration Cards (OCC). Each card consists of a custom-made Field Programmable Gate Ar-ray (FPGA) motherboard, which houses eight LEDpulser boards as well as two photo-diode boards.Light from each LED is fed to two outputs on thefront panel and to a photo-diode input via a custommade 3-way optical coupler. The components of anOCC can be seen in Figs. 2 and 3.Light is fed from the rack housing the OCS elec-tronics to the patch panel via 40 optical fibres 15 min length. From the patch panel, they are coupledvia SMA-SMA connectors to 21 m long duplex fi-bres that lead to the injection points inside the wa-ter tank. To enter the water tank, the fibres passthrough an air-tight and light-tight feedthroughflange located on top of the water tank. For theinjection points located around the sides of the de-tector, only one of the cores of the duplex fibre willbe used to inject light into the detector. The othercore will be available for potential future upgradesor in case of damage to the first core. The upwardfacing fibres at the bottom of the scintillator tankswill inject light of two different wavelengths via thetwo different cores, as mentioned in Sec. 2 and de-scribed in detail in Sec. 3.1.Precise monitoring of the light intensity is car-ried out in two ways; firstly, via the FPGA boardcontrolled photo-diode boards and, secondly, viaan 8 inch Hamamatsu R5912 PMT installed in arack-mounted dark box close to the OCS electron-ics. This PMT is identical to those used in the OD.The stability of this PMT is also monitored, usinga YAP:Ce pulser unit made by Scionix [10] whichproduces light pulses corresponding to 5k photo-electrons with a rate of 20 counts/s.The OCS will receive the pulse configurationfrom the LZ Slow Control system which will alsostore the required OCS calibration database. Be-tween the Slow Control system and the OCS crateis the OCS controller. The controller will have4 igure 2: A diagram showing an overview of the Optical Calibration System, with eight LED pulsers on one Optical CalibrationCard (OCC) and five Optical Calibration Cards in the VME crate. Lines with arrows show fibre routes with labels representingnumbers of fibres from LEDs with corresponding wavelengths.Figure 3: A picture of an Optical Calibration Card with the key components labelled.
Each FPGA motherboard sends Low VoltageDigital Signals (LVDS) to the LED pulser boardswhich are then converted into a single endedTransistor-Transistor Logic (TTL) signal. This sig-nal then drives the base of a common collector tran-sistor circuit which changes the voltage on the cath-ode of the LED. When the LED is turned off, theanode is held at − The duplex optical fibres used to direct light toinjection points in the water tank are manufac-
Figure 4: A photo of the fibre end showing the two channelswith their polythene jackets after the ice polishing. tured by Mitsubishi and consist of two polymethyl-methacrylate resin cores, with a refractive index of1.49, surrounded by a polyethylene jacket [11]. Thefibre cores measure approximately 980 µ m in diam-eter and the total diameter of each fibre, includingthe jacket, is 2.2 mm with the total width of the du-plex cable being 4.4 mm as shown in Fig. 4. Thesefibres were selected based on the results of test-ing by the SNO+ collaboration as our requirementson light transmission and radiopurity are closelyaligned [12].The fibre injection ends were ice polished atFermi National Accelerator Laboratory to producesmooth fibre ends, therefore reducing back-scatterand ensuring a uniform beam profile. This processinvolves submerging the fibre end in liquid nitrogenin order to freeze it in a layer of ice which, not onlylubricates the polishing process, but also preventsthe cladding and jacket from splitting from the coreduring polishing [13] (see Fig. 4).Light profile data was obtained from each of thewater tank fibres by shining a light from 435 nmLED down one channel and projecting the emerginglight onto a Tyvek ® screen underwater in a darkbox. Images such as that in Fig. 5 of the lightwere taken using an endoscope camera and thenprocessed to extract the light profiles for each ofthe fibres.These datas were used as a quality assurancecheck to ensure that the fibres used have no ma-jor defects and the light emitted from them isdistributed symmetrically to within an acceptablerange. For the fibres to be used for the upwardfacing injection points, the two channels were com-pared to ensure they had similar profiles to allowfair comparisons between the two different wave-6 a) (b)Figure 5: Light from a fibre projected on a screen (a) and (b) the resulting profile plot produced by taking a sample acrossthe middle of the image.The black and blue profiles correspond to the two different channels of the duplex fibre and the slightx-axis offset between them is a result of the 2.2 mm separation between the cores. lengths that will be sent through them. In total49 fibres were tested, and the 35 best ones whichpassed the quality assurance tests were selected foruse in the detector. The potential effects of fibrebending on the light profiles were investigated us-ing the same dark box set up by bending a testfibre around discs of varying diameters, from 2 cmto 5 cm. When comparing the images taken fordifferent diameters, only the smallest disc with adiameter of 2 cm showed a small variation in thedistribution of the light profile. From these resultsit was therefore determined that bending the fibresto a diameter of less than 2 . . A mechanical feedthrough is required to allow thefibres to pass from the electronics rack where theOCS is located into the water tank. Figure 6 showsthe machined flange with 16 ports which are fittedwith rubber bungs to allow the fibres to pass intothe water tank while keeping an air-tight and light-tight seal [14]. There is also a constant nitrogenover-pressure applied above the water level insidethe water tank to ensure air does not leak in andintroduce radioactive contaminants.At each of the 30 injection points on the ODPMT array, the duplex optical fibre will be heldin place using a Fibre Support Structure (FSS).The purpose of the FSS is to ensure each fibre isaligned correctly as it points in towards the scin-tillator tanks and to provide a consistent 50 mmcurvature of the duplex optical fibre. Each FSSconsists of 26 parts: seven machined PTFE struc-tural components, seven passivated bolts, nine pas-sivated inserts, and two PTFE inserts. A photo-graph of a FSS with the lid open can be seen inFig. 7. The support structures which hold the fi-bres under the acrylic tanks consist of a custommade stainless steel bracket, which attaches to theacrylic tank platforms, and a PTFE fibre holderwhich directs the light through holes in the acrylictank platform.Correct alignment of the fibres is of vital impor-tance to allow a realistic comparison between sim-ulations and calibration data. It is for this reason7 a) (b)Figure 6: (a) Water-tank feedthrough flange. (b) Feedthrough bung used to allow fibre to pass into the water tank whilekeeping the water tank light-tight and air-tight.Figure 7: A Fibre Support Structure (FSS) with the lid open.The key components of the FSS are labelled. that the FSS are designed to allow setting the fi-bre direction precisely during installation. The in-ward facing fibres should point perpendicularly tothe water tank wall as well as directing the centreof the beam towards the central axis of the TPC(equivalent to the origin of the water tank coordi-nate system). To achieve this, two red self-levellingcross laser beams will be positioned equidistantlyon either side of the injection point on a custom-made jig with each of the lasers crossing on the faceof the liquid scintillator tanks. The FSSs are thenaligned to this point by attaching a rear-mountedgreen laser that points through the fibre injectionpoint and aligning the green laser to the red crosseson the scintillator tank. Once aligned the FSS istightened to avoid future movement leading to mis-alignment when the fibre is installed.
4. Radioactivity Measurements
After the manufacturing of the FSSs was com-pleted, a random selection of six FSSs, as well asthe complete set of water tank fibres, was sent tothe Boulby Underground Laboratory for radioac-tivity screening. The results from the screening areshown in Tab. 3 along with the estimated activi-ties of the OD components. The activity for theFSSs are calculated by averaging the radioactivityscreening results and multiplying by the number ofFSSs present in the system.These values show that the OCS contributes 1.2%of the total activity in the OD and therefore satisfiesthe requirement on radiopurity (see Tab. 1). TheOCS components are also further from the cryo-stat than the scintillator tanks and therefore havemuch less of an impact on the number of events inthe TPC. The OCS components were thoroughlycleaned before shipping as well as before installa-tion to reduce background contamination further.It is also worth noting that the optical fibres werescreened with the metal SMA connector included;these sit outside of the water tank in the exper-iment so they will not contribute to the detectedbackground events.
5. Full System Performance
After shipping the OCS to SURF and installingthe VME crate in the electronics racks under-ground, a re-calibration process was carried outto validate the performance of the system. This8 able 3: Results from the screening of the OCS components at Boulby Underground Laboratory (shown in bold) in comparisonwith activities of the radionuclides present in the other OD components as described in the LZ radioactivity and cleanlinesscontrol programs article [15], where a comprehensive list of the estimated activities for the rest of the detector can be found.
OD Components Mass (kg) Activity (mBq/kg) OverallContribution (%) U e U l Th e Th l Co KOD Tanks 3200 0.16 0.39 0.02 0.06 0.04 5.36 3.77Liquid Scintillator 17 ,
600 0.01 0.01 0.01 0.01 0.00 0.00 0.14OD PMTs 205 570 470 395 388 0.00 534 94.3OD PMT Supports 770 1.20 0.27 0.33 0.49 1.60 0.40 0.59
Fibre SupportStructures 10.5 31 3.0 6.0 3.0 0.5 330 0.77Optical Fibres(inc. couplers) 5.53 91.5 8 10.5 4.5 1.75 302.5 0.45 on-site testing used the same test stand as usedpreviously at Liverpool and provided a final re-calibration for each channel as well as a cross checkwith the pre-shipment testing. A full system testconsisted of going through each channel one byone, and scanning over the range of injectable pho-tons while taking measurements with a power me-ter (Thorlab PM100USB) and the photomultipliertube (Hamamatsu H10721)[16]. The power meteroutput gives the number of photons detected andthe photomultiplier tube gives the measured pulsewidth. Each measurement taken consists of record-ing the power meter once and recording 4000 sam-ples of photomultiplier tube pulses and LED triggerwidths which are digitised by a DRS4 EvaluationBoard [17]. This measurement is repeated threetimes at each setting. The 4000 samples taken ateach measurement are plotted on a histogram anda Gaussian fit is used to determine the mean valueand spread of the light intensity for each set point.One of the important parameters determined fromthe measurements on each channel is the LED trig-ger pulse width required to produce a certain num-ber of detected photons. The parameters from thisfit can then be used as calibration constants for eachchannel. A schematic of the test stand used for thefull system test is shown in Fig. 8(a). A photographof the installed Optical Calibration System can beseen in Fig. 8(b).
The trigger sent to the LED has 504 differentindexable widths, which corresponds to a triggerpulse width of 20–60 ns. This determines thelight output, ranging from 700–700 ,
000 photons perpulse, while the ability to pulse 1 , ,
000 photons globally is achieved by pulsing multiple channels si-multaneously. The FPGA can change the triggerpulse width with a minimum step of 40 ps, whichis critical to having precise control over the num-ber of photons produced. The trigger width (TW)required for a given number of photons can be cal-culated using Eq. 1, where A , B , C and D are chan-nel specific calibration parameters and N ph corre-sponds to the number of photons which will be in-jected. This equation was determined empirically. T W = A + e λ ,λ = Bx + Cx + Dx ,x = ln( N ph ) (1)An example of the calibration curve is shown inFig. 9. The channel pulse rate was selected to be 4 kHzfor operation in LZ. However the DRS4 EvaluationBoard, which is used to readout the waveforms ofthe electrical trigger and PMT pulses, can digitiseonly 300 Hz with a time resolution of 0 . a)(b)Figure 8: OCS Test Stand: (a) An overview of the test standused at SURF showing the connections made from the OCCto the Thorlab PM100USB Power Meter, PMT, DRS4, Ar-duino Uno and data taking computer [17] [18]. (b) A photo-graph of the installed Optical Calibration System, showingthe VME crate (top) housing the Optical Calibration Cards,each connected by USB to the Optical Calibration Controller(on the shelf). Figure 9: LED trigger width versus the natural logarithm ofnumber of photons per pulse. Equation 1 relates these twovariables. Each channel is fitted with this function and hasa set of parameters for A , B , C and D which are specific tothat channel.Figure 10: Dependence of the number of photons per pulseon the pulse width measured by the test stand PMT. Thisplot shows that the system meets the pulse width require-ment set by the experiment. must be similar to the width of the light pulse pro-duced in the GdLS or water after being processedby the pulse shaper. Optical calibration and monitoring requires pre-cise knowledge of the numbers of injected pho-tons, especially for measurements related to lightcollection efficiency. These requirements are re-flected in Tab. 1 as pulse-to-pulse variations andvariations between calibrations. It is known thatelectronic components have a temperature depen-dence which lead to variations in light output fromLEDs. Though an effort has been made to useprecision components and compensate for these10 igure 11: Distribution of numbers of injected photons perpulse at the level corresponding to 150 keV energy depositionin the OD. The resolution 10%, which is not corrected for thespread due to the PMT measurement, shows that the systemmeets the pulse width requirement set by the experiment.The resolution is better for higher light intensities. temperature dependencies some residual effects re-main. The OCCs have temperature sensors whichwill allow to study these effects further and imple-ment corrections. Ultimately, light output moni-toring has been implemented by integrating overmany pulses (photo-diode per channel) and pulse-by-pulse (monitoring PMT for all channels) meth-ods. The pulse-by-pulse resolution at the light in-tensities corresponding to 150 keV energy deposi-tions in the OD is shown in Fig. 11. The PMT con-tributes to the width of this distribution; thereforethe actual pulse-to-pulse variation is smaller than10%, and it improves at higher light intensities.The variations of average numbers of injectedphotons between calibrations will be monitored andcorrected for using the photo-diode board and mon-itoring PMT. The YAP:Ce pulser provides continu-ous gain calibration for the monitoring PMT. With-out optical coupling compound, it produced a peakcorresponding to ∼ ∼ To validate that each channel could hit key cal-ibration points a routine was carried out to seehow accurately the channels could hit these photonlevels. As described previously, there are certainranges of TWs reachable for each channel. Thistest shows if the key calibration points falls withinan achievable ranges on each channel. The cho- sen calibration points were 700 photons, 1000 pho-tons, 20 ,
000 photons and 50 ,
000 photons. Each ofthe calibration points were entered into Eq. 1 alongwith the parameters for that channel’s calibrationcurve. The resulting trigger width was checkedagainst the reachable range of trigger widths forthat channel, taking the closest achievable TW if itwas not in an allowable range. The resulting num-ber of photons from this achievable TW was plottedon a histogram. Figure 12 shows the results of thisvalidation step where all 40 channels injected lightwith an accuracy of at least 10% at each of the setpoints. These results indicate the precision of theOCS internal calibration, which is used to deter-mine the settings necessary to achieve the requirednumber of photons at each set point.
6. Optical Calibration Procedure
An optical calibration of the OD will be initiatedby the LZ Run Control sending the required calibra-tion routine to the Slow Control system. The SlowControl is a SCADA platform which utilises Igni-tion [3] and communicates with LZ’s subsystemsvia the MODBUS protocol [3]. The Slow Controlsystem will store and manage the calibration con-stants needed for the OCS in a MySQL database.The OCS control commands will pass from the SlowControl to the OCS controller which manages thecommunication with the master OCC. The OCScontroller will also measure OCC temperatures andmonitor photo-diode response.The monitoring PMT will be connected to theLZ data acquisition system. An analysis of OD-PMT data against the monitoring PMT data willbe used to ensure that the OD-PMTs are correctlycalibrated and identify any drift in performance. Aschematic of the OCS communication with the LZRun and Slow Control can be seen in Fig. 13.
7. Conclusions
The ability to perform an accurate calibrationof the OD is critical to ensure the effectivenessof the veto system used by LZ. The OD is essen-tial for understanding and vetoing the signal-likeneutron background as well as numerous gammasoriginating inside and outside of LZ. The OCS dis-cussed in this paper has the capability to preciselyinject a known number of photons ranging from700 photons up to 50 ,
000 photons covering the en-ergy range of the background signals. Light will11 a)(b)(c)(d)Figure 12: Distribution of how accurately each channel ofthe OCS can specifically inject: (a) 700 photons per pulse;(b) 1000 photons per pulse; (c) 20 ,
000 photons per pulse;and, (d) 50 ,
000 photons per pulse. This shows the major-ity of the 40 channels can achieve the desired TW to reachthese arbitrary set points (or within 10%) with the currentcalibration curves. Figure 13: A schematic showing the order in which an ODoptical calibration will be completed. be injected into the OD using an LED-driven sys-tem with 30 duplex optical fibres mounted withinthe array of PMTs and five duplex optical fibresmounted beneath the tanks pointing upwards intoacrylic tanks. The injection points situated withinthe array will maintain the calibration of PMTs andmonitor their performance. The five bottom injec-tion points will monitor the light transmission ofthe acrylic tanks and the degradation of the liquidscintillator over time. A correctly calibrated ODensures that the experiment will be able to meet itsprojected sensitivity.
8. Acknowledgements
We would like to thank our collaborators in theLZ dark matter search experiment for their con-tinuous support, in particular, H. Kraus from Ox-ford, H. M. Ara´ujo from Imperial College Lon-don, M.G.D. van der Grinten and R. Preece fromRutherford Appleton Laboratory, H. Nelson fromU.C. Santa Barbara for useful discussions on theOCS design and development; our colleagues atBrandeis University for tests of the OCS parts;A. Kaboth from Royal Holloway University of Lon-don, R. Mannino from University of WisconsinMadison, A. Kamaha from University at Albany,B. Penning from University of Michigan, B. L´opezParedes from Imperial College London for com-ments on this manuscript. We wish to thank H. Lip-pincott (now at U.C. Santa Barbara) and E. Hahnfrom FNAL for the fibre ice polishing. Thanks aredue to M. Whitley, P. Cook and the staff work-ing in the University of Liverpool Detector Fab-rication Facility and Advanced Materials Labora-tory for hardware manufacture. We acknowledgecontributions from University of Liverpool studentsT. Carter, L. Hawkins, A. Hibbert, B. Philippou.12e thank N. McCauley, J. Rose, A. Pritchard andL. Anthony from the University of Liverpool HyperKamiokande group for their help and design dis-cussions. We acknowledge additional support fromthe Boulby Underground Laboratory for organisingand carrying out the radiation screening requiredfor the OCS components. We acknowledge the LZcollaboration for providing the BACCARAT simu-lation package which allowed the simulations men-tioned in this article to be carried out. With thanksgoing to S. Shaw from U.C. Santa Barbara forhelp writing the simulations for the OCS. The as-sistance of Sanford Underground Research Facilityand its personnel in providing physical access andgeneral logistical and technical support is acknowl-edged. This work was supported by the U.K. Sci-ence & Technology Facilities Council (STFC) underaward numbers ST/S000879/1 and ST/M003639/1,and by PhD studentships ST/N504142/1 (AB),ST/R504920/1 (AB) and ST/S505547/1 (EF).
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