A self-monitoring precision calibration light source for large-volume neutrino telescopes
F. Henningsen, M. Boehmer, A. Gärtner, L. Geilen, R. Gernhäuser, H. Heggen, K. Holzapfel, C. Fruck, L. Papp, I. C. Rea, E. Resconi, F. Schmuckermaier, C. Spannfellner, M. Traxler
PPrepared for submission to JINST
A self-monitoring precision calibration light source forlarge-volume neutrino telescopes
F. Henningsen, a , b , M. Boehmer, a A. Gärtner, a , d L. Geilen, a R. GernhÃďuser, a H. Heggen, c K. Holzapfel, a C. Fruck, a L. Papp, a I. C. Rea, a E. Resconi, a F. Schmuckermaier, a C. Spannfellner, a M. Traxler. c a Physik-Department, Technische Universität München, D-85748 Garching, Germany b Max-Planck-Insitut für Physik, D-80805 Munich, Germany c GSI Helmholtz Centre for Heavy Ion Research GmbH, Darmstadt, Germany d Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2E1
E-mail: [email protected]
Abstract: With the rise of neutrino astronomy using large-volume detector arrays, calibration im-provements of optical media and photosensors have emerged as significant means to reduce detectorsystematics. To improve understanding of the detector volume and its instrumentation, we devel-oped an absolutely-calibrated, self-monitoring, isotropic, nanosecond, high-intensity calibrationlight source called "Precision Optical Calibration Module" (POCAM). This, now third iteration,of the instrument was developed for an application in the IceCube Upgrade but, with a modularinstrument communications and synchronization backend, can provide a calibration light sourcestandard for any large-volume photodetector array. This work summarizes the functional principleof the POCAM and all related device characteristics as well as its precision calibration procedure.The latter provides fingerprint-characterized instruments with knowledge on absolute and relativebehavior of the emitted light pulses as well as their temperature dependencies.Keywords: Neutrino detectors, detector alignment and calibration methods, Instrument optimisa-tion, large detector systems for particle and astroparticle physics, photoemission, IceCube Corresponding author. a r X i v : . [ a s t r o - ph . I M ] M a y ontents Neutrinos have proven to be highly valuable cosmic probes for fundamental physics research andthe newly emerging field of multi-messenger astronomy. Extremely light and electrically neutralleptons only interact by gravitation and the weak force and can therefore travel long distancesthrough the cosmos without attenuation. Unlike cosmic rays, which are electrically charged and canbe deflected by magnetic fields, neutrinos point back to their origin. Sources accelerating cosmicrays are also believed to produce high energy neutrinos in an energy range between GeV to EeV [1].Neutrinos are the unique interaction and decay products of hadronic cosmic rays and are thus oneof the most promising routes to discover the origins of high-energy cosmic rays. At energies fromMeV to GeV, neutrinos emitted from the sun and supernovae [2] offer insights into the evolution ofstars, enable measurements of flavor oscillations [3], and with that establish evidence for physicsbeyond the Standard Model [4].To detect these elusive particles, scientists employ large-volume neutrino telescopes. Thesetypically consist of a three-dimensional array of photomultiplier tubes (PMTs) surrounded by atransparent dielectric medium (e.g. water or ice). Neutrinos traversing the detector interact insidethe detector medium with a small probability and produce highly energetic charged particles.If these charged particles are travelling fast enough through the detector medium, Cherenkovlight is produced. Measurements of the Cherenkov radiation by the PMT array then allows thereconstruction of the neutrino’s energy and direction [5].– 1 –he primary high-energy neutrino interaction channel is deep-inelastic scattering with nucleiin the detector medium. In general, neutrinos can undergo a charged current (CC) interaction in theform of ν l + X → l − + Y or a neutral current (NC) interaction in the form of ν l + X → ν l + Y , where thesame interaction is also possible with ¯ ν l and l + respectively and l = e , µ, τ . Depending on the type ofinteraction and flavor of the incoming neutrino, different event signatures arise in the detector. NCinteractions of all flavors and CC interactions of electron neutrinos produce cascade events caused bythe initial hadronic shower and in the latter case also the electromagnetic shower resulting from theoutgoing electrons. Cascade events appear as isotropic point like sources of Cherenkov light. Dueto the short distances traveled by leptons produced in cascade events, they deposit all of their emittedenergy in the detector volume and allow for a comparatively accurate energy reconstruction, whereasthe directional reconstruction is based on slight light intensity and timing asymmetries and is lessaccurate. This also makes the distinction between NC and CC interactions exceedingly difficult.In contrast,outgoing muons from muon neutrino CC interactions can travel for several kilometersthrough the detector medium, producing Cherenkov light on their way. These track events onlymake a rough energy reconstruction possible, since only a fraction of the whole track is contained inthe detector volume. On the other hand, the elongated geometry enables a more precise directionalreconstruction. CC interactions caused by tau neutrinos produce a cascade at the first interaction ver-tex, a track during the short propagation of the produced tau lepton and then a second cascade causedby the tau decay. The resulting signature is called ’double bang’, but is nearly indistinguishablefrom regular cascade events due to the short distance of the two cascades. Most of the backgroundin neutrino detectors is caused by muons and neutrinos originating from cosmic-ray air showers. Figure 1 . The IceCube Neutrino Observatory with thein-ice array, its sub-array DeepCore, and the cosmic-ray air shower array IceTop [6].
Neutrino detectors can use the Earth to shieldmost of the muon background when searchingfor neutrinos from the opposite hemisphere.However, the Earth increasingly absorbs neu-trinos with energies above several tens of TeV,which means high-energy neutrinos must be ob-served by looking in the same hemisphere asthe detector. To remove the large muon back-grounds from this direction, the analysis canfocus exclusively on very high energies (abovea few PeV) or select events with their primaryvertex inside the detector. The latter approachnaturally excludes background muons and canbe used down to energies as low as 10 TeV. Inaddition the coincidence of a starting event anda muon can be used to reject atmospheric neu-trinos origination from the same air shower asthe muon [7].Since both the cross section of neutrino interactions and the flux of astrophysical neutrinos atEarth is small, large volumes of detector medium in the scale of km are required to observe morethan a handful of astrophysical events per year. Due to these requirements, PMT arrays are embeddedin natural sites containing large volumes of glacial ice, lake water or sea water. The first-ever neutrino– 2 –elescope in ice, AMANDA [8], was deployed between in 1993/94 at the geographic South Pole andwas followed by several further upgrades and extensions. The development of drilling techniques,characterization of the optical properties of the glacial ice and improvement of reconstruction meth-ods among others laid the groundwork for the major successor experiment: The IceCube NeutrinoObservatory [6], the first cubic-kilometer neutrino detector. IceCube was completed in 2010 and itsin-ice array consists of 5160 digital optical modules (DOMs) on 86 strings, deployed between a depthof 1450 m and 2450 m. Part of the in-ice array includes the DeepCore sub-array, which consistsof 8 strings with closely-spaced DOMs to extend the sensitivity of IceCube down to 10 GeV [9]. Figure 2 . Schematic view of theANTARES detector [10].
The next stage of the IceCube project will be the deployment ofthe IceCube Upgrade [11] during the Antarctic Summer season2022/23. Consisting of seven new strings, each carrying newlydeveloped DOMs and calibration devices, the Upgrade will extendthe detector’s science capabilities in the low energy region and en-able a re-calibration of the detector. New calibrations are madepossible by deploying a variety of novel calibration devices, includ-ing the Precision Optical Calibration Module (POCAM) describedhere. The improved understanding of the optical properties of theSouth Pole ice and the detector response will result in an enhancedcalibration of the detector and a re-analysis of archival data.The first neutrino telescopes to use water as a detection mediumwere realized in the Mediterranean sea with the ANTARES detec-tor [10] and in Lake Baikal with the BAIKAL neutrino telescopeNT200 [12]. These pearly experiments are being followed up withlarger-scale detectors both in the Mediterranean Sea with the cubic-kilometer neutrino telescope (KM3NeT) [13] as well as the GigatonVolume Detector (GVD) in Lake Baikal [14]. Plans for an additional cubic-kilometer scale detectorin the Northern Pacific Ocean, the Pacific Ocean Neutrino Explorer (P-ONE), are currently ongo-ing and the pathfinder mission STRAW indicates this is a promising location [15]. The generalgeometries of neutrino interactions are similar for both water- and ice-based detectors except fordifferences in the optical properties and the systematic uncertainties faced by the different envi-ronments. Understanding the optical properties of the detector medium is crucial because thereconstruction of events is typically based on the intensity and arrival times of secondary photonswhich have undergone single or multiple scattering since the initial Cherenkov emission. Thefollowing section gives a qualitative summary of how systematic uncertainties can be estimated andimproved with the aid of artificial calibration light sources.
The first main source of systematic uncertainties are the response and behavior of the opticalmodules. The latter typically consist of PMTs embedded in a gel-coated glass sphere. Both, thequantum efficiency of the PMT alone and the efficiency of a fully integrated module can be accuratelydetermined in the laboratory. However, different conditions on the experiment site and ageing effectsof the PMT components can cause deviations from the laboratory findings over time. Recurring in-situ measurements of the detection efficiency are therefore advantageous. Furthermore, the– 3 –aturation of the PMTs at high light levels can have an impact on event reconstruction. For a lownumber of arriving photons, the charge output of a PMT is proportional to the number of measuredphotons. For high photoelectron rates this linear behavior can break down and the PMT saturates.In case of the IceCube DOMs, this saturation limit is reached at around 31 photoelectrons pernanosecond [16]. For most neutrino events, the majority of PMTs will measure low light levels andtheir response will be linear. However, some high-energy neutrino interactions can deposit enoughenergy to saturate close-by PMTs. Absent a model of the nonlinear behavior of PMTs near theirsaturation limits, saturated PMTs must be excluded from event reconstruction.A second major systematic uncertainty is the geometry of the detector array. In this regard,detectors embedded in glacial ice benefit from a static geometry. Once deployed, the positions ofindividual modules can be estimated from the deployment data and then be verified by trilateration.In water based detectors, the strings are flexible structures fixed at the seabed or lake bottomand held under tension by buoys. Hence, currents in the water are able to move and rotate thestrings. Especially in sea water, these currents can change the geometry significantly and a frequentredetermination of the detector geometry via optical or acoustic systems is essential.The third major source of uncertainties are the optical properties of the detector medium [17].In general, propagation of light in transparent media is determined by the effects of scatteringand absorption. The former results in a directional change, the latter in a loss of the photon.
Figure 3 . Observed median angular error andstatistical limit of fully contained high energycascade directional reconstruction in IceCube asa function of reconstructed deposited energy [11].
The typical parametrization of these effects isachieved via the definition of a scattering lengthand an absorption length, which marks the distanceafter which the probability of the photon not be-ing influenced by the respective effect drops to 1 / e .For detectors embedded in glacial ice, the scatteringlength is considerably shorter than the absorptionlength [18], whereas for water-based detectors, ab-sorption is the prevailing effect [19]. An accuratedepth-dependent estimation of both scattering andabsorption forms the basis of the optical character-ization of the detector medium and, in the case ofwater based detectors, these properties need to bemonitored with in-situ instruments frequently to ac-count for changes in the environment. In the case ofIceCube, an additional anisotropic scattering effect[20] has been observed. The optical anisotropy ofthe ice produces an azimuthal dependency in photon propagation. Furthermore, the refrozen icearound the DOMs in the drill hole, commonly referred to as ’Hole Ice’ [18], exhibits differentoptical behavior compared to bulk ice. The main difference is a much shorter scattering length,making it possible for photons approaching the back-side of the DOM to still be detected whenscattered around the DOM towards the the PMT, and for photons heading towards the PMT-sideof the DOM to be scattered away more frequently. Together, these systematic uncertainties cansignificantly restrict the accuracy of event reconstruction, especially the directional reconstructionof high energy cascade events [11]. Figure 3 shows estimates of the angular uncertainty of high– 4 –nergy cascade events in IceCube as a function of reconstructed deposited energy. The dashed linerepresents the angular error with perfect knowledge of the detector response and optical propertiesof the ice, hence only the statistical uncertainty. The estimated total median angular errors, however,are far above the statistical limit, indicating a significant restriction on the reconstruction caused bya lack of knowledge about the contributing systematic uncertainties. Artificial light sources are the most common approach to ensuring accurate characterization ofdetector systematic uncertainties. This section provides an overview of the contemporary techniquesand methods used for the calibration of neutrino telescopes and discusses potential improvements,using the POCAM as an absolutely calibrated and isotropic light source. The main artificial lightsource present in IceCube is the LED flasher board [6]. It contains 12 LEDs mounted circularlyaround the board covering six different azimuth angles (with 60 ◦ spacing) and two different zenithangles. LEDs can be flashed with a rate of up to to 610 Hz and emit a spectrum centered around399 nm, with an intensity range of 10 − photons.The estimation of the bulk ice parameters were performed using data sets created by the LEDflasher boards [18]. The ice is parameterized by six global parameters (see [14] for details), thedepth-dependent temperature, and the scattering and absorption length at 400 nm, averaged overlayers of 10 m thickness. A photon propagator was then used to simulate the propagation of lightemitted by the flasher boards under varying ice parameters. Using a maximum likelihood analysis,a global fit for all parameters to the real flasher data can then be performed, which results in a tableof estimates of depth-dependent values for the scattering and absorption length. For the detectorvolume below depths of 2100 m, the resulting values of the absorption length are ranging fromabout 100 m - 200 m, while the scattering lengths range from around 30 m - 80 m, making scatteringthe dominant optical effect [18]. The same data set is also utilized to fit the parameterizationof the observed anisotropy in the bulk ice [20]. The effect is currently characterized by threefree parameters defining a diagonal matrix, which modifies the scattering behavior. However, theaccuracy of the model is limited by a depth-dependence of the anisotropy and the rather uncertainemission profiles and pointings of the flasher LEDs. Due to the isotropic light emission of thePOCAM, we expect that there is potential for an improvement of the anisotropy fit, since no initiallight emission profile has to be assumed.The hole ice effect is described as a modification of the angular acceptance curve of the DOM.Only for certain low energy event reconstructions, where the photon origin is close to the receivingDOM and no straight wavefront can be assumed, is the hole ice modeled as an actual ice columnwith optical properties deviating from the bulk ice [21]. Since the POCAMs will be deployed atseveral depths, they offer the possibility of an improved in-situ acceptance measurement affectedby hole ice [22]. The optical efficiency model of the DOMs is supplemented after deploymentwith in-situ measurements of down going, minimal ionizing muons, since they are abundant andhave a fairly known light emission [6]. The flasher board LEDs are not suitable here due to theirunknown emission intensity. The absolution calibration of the POCAM means it is not affectedby the normalization uncertainty of the LED flashers, and thus will provide an independent in situ measurement of DOM efficiencies. Additionally, the POCAMs can contribute to the investigationof the PMT saturation. Precise knowledge about the intensity and an extensive dynamical range– 5 –f the emitting light source enables an in-situ characterization of the non-linear PMT response.This allows the inclusion of a higher number of otherwise neglected DOMs in high energy eventreconstructions, which increases the overall detected charge and hence decreases the statisticaluncertainty. Moreover, the efficiency and linearity behavior of modules can drift over time due toaging of the DOM hardware. A light source with absolute calibration is thus valuable for detectinglong-term drifts in detector response.Neutrino detectors deployed in lakes and seawater have developed a variety of similar calibrationtools for studying the time-varying optical properties of the water. The KM3NeT detector will belocated at several sites in the Mediterranean Sea. One of them, situated about 10 km west of theANTARES telescope, was already intensively studied by the ANTARES collaboration [23]. Adedicated detector line containing two separated spheres (light source and PMT) was deployed tomeasure absorption and scattering lengths with blue light ( λ = 473 nm) and UV light ( λ = 375 nm).The absorption length is around 60 m for blue and 25 m for UV light. In contrast, the scatteringlength is about 260 m for blue and 120 m for UV, illustrating an optical behavior opposite to glacialice. Similar setups, developed by the NESTOR and NEMO collaborations, have been utilized tocharacterize the remaining KM3NeT sites [19]. Additional light sources for the calibration of thetime offset between DOMs were implemented in ANTARES in the form of the Optical BeaconSystem [24]. It consists of pulsed LEDs and lasers located throughout the detector enabling asub-nanosecond timing calibration. The developed system in KM3NeT, the Nanobeacon [25], iscomposed of LEDs installed in the upper parts of each DOM. The optical water properties in LakeBaikal were determined with the aid of lasers [26] and later measurements with a deployed POCAMprototype confirmed the findings [27].It becomes apparent that any large-scale neutrino telescope has a comparable approach to itscalibration and hence a similar demand in regards of calibration devices. The next generation of up-coming neutrino telescopes (Baikal-GVD, KM3NET, IceCube-Gen2 & P-ONE) as well as currentlyoperating telescopes benefit from precise and well-tested light sources. Especially for IceCube, sig-nificantly limiting systematic uncertainties indicate the need for an improved calibration, in orderto fully exploit the detector’s scientific capability. Due to its absolute intensity calibration, largedynamic range, short nanosecond pulses and self-monitoring ability, the POCAM has potential tofunction as a standard calibration light source for any large-scale optical neutrino telescopes.– 6 – Precision Optical Calibration Module
The need for higher precision calibration in neutrino telescope arrays has been evident for a numberof years. In this scope, we developed a novel calibration light source providing isotropic self-monitored light pulses with large dynamic range [15, 22, 28]. This Precision Optical CalibrationModule (POCAM) pursues the goal of providing a detector-independent calibration light sourcestandard for large-volume photosensor arrays used for high- and low-energy neutrino detection.In general, this calibration instrument consists of diffuse isotropic light flashers, photosensors forself-monitoring as well as necessary electronics for readout and data aquisition (DAQ). All of thesecomponents are located in a pressure housing enabling deployment in deep water or other harshenvironments. A cut-view of the POCAM is shown in fig. 4a with a detailed view of the flangeassembly in fig. 4b. The following subsections will summarize the development results of all devicesub-components and will go into detail about their performance. (a)
Full instrument
BK-7 glass
DiffuserTi flange
Photosensors
Aperture diskAnalog board
Distribution board
Digital boardIceCube interface (b)
Flange assembly cut-view with all sub-components.
Figure 4 . Full instrument (a) and flange assembly (b) cut-view of the POCAM instrument. In the flangeassembly all the sub-components of one hemisphere are visible and annotated. For details on the latter referto the text.
The POCAM developments started in late 2014 with conceptual studies and simulations [29, 30].After around two years of investigating the baseline instrument performance requirements, a firstprototype module was developed. To provide proof-of-concept of the device, we collaboratedwith the GVD telescope located in Lake Baikal to deploy the first prototype in 2017. This firstdeployment has proven successful and the POCAM was able to reproduce [27] the measured opticalproperties of the Baikal Lake water [26, 31]. An event view of a POCAM light pulse recorded in theGVD cluster is shown in fig. 5 and illustrates the detector coverage achieved with this instrumentiteration. Following the successful application in Russia, three second-iteration instruments havebeen deployed in the STRAW experiment in 2018 [15]. This detector uses the POCAM to probethe optical properties of the North-East Pacific deep sea and. During this deployment we were– 7 –ble to significantly extend and improve the POCAM performance and self-calibration. Furtherchanges and improvements of the instrument where then done in the scope of the IceCube Upgradedevelopments [11, 28] during 2019. A total of 30 POCAM instruments will be produced with 21being planned for the Upgrade detector volume [28]. This current instrument design baseline andits performance is the main focus of this work. x [ m ] − − − − − − − − y [ m ] − −
20 0 20 40 60 80100 z [ m ] T i m e [ n s ] Figure 5 . First POCAM prototype event view within a GVD cluster. With size representing integratedcharge and color arrival time of the light peak at observing, down-facing photosensors, the POCAM locationon the back left string can be inferred by eye. Data provided by the GVD collaboration.
As previously discussed, neutrino telescopes are typically located deep in water or ice to providenot only a Cherenkov medium but also shielding against atmospheric background. Thus, a potentialcalibration device has to be able to withstand high pressure (up to 250 atm) and cold temperatures(down to -40C). In partnership with the German marine housing specialist Nautilus Marine ServiceGmbH, we developed a titanium pressure housing providing close to 4 π light emission field-of-view(FOV), a theoretical pressure resistance up to around 1 ,
000 atm, a temperature resistance down to − ◦ C and a vibration- and shock resistance according to ISO 13628-6. The cylindrical design– 8 –hown in fig. 4a allows electronics and necessary cable penetrators for supply and communication tobe placed outside of its light emission FOV. Especially important was the design of the two flangesas they pose the major influence on the eventual emission profile of the instrument. This will bediscussed in detail in section 2.3. In general, the flanges are used to complement the pressurehousing with optically-enhanced borosilicate glass [32], attached via deep-sea epoxy resin [33].Furthermore, they also provide means of mounting electronics internally to each of the instrumentsides. The instrument measures around 40 cm in length and 13 cm in diameter, excluding penetators.We have applied several âĂŸstress testsâĂŹ to verify the performance of the POCAM underenvironmental conditions. For pressure testing, the sealed housing was cycled repeatedly in apressure chamber to 700 bar (690 atm) at a facility provided by Nautilus. For temperature testing,it was placed in a freezer at − ◦ C for a total period of five weeks. In both cases internal pressure,temperature and humidity were monitored throughout the testing period and were found to remainstable. Vibration and shock tests were performed and passed at the German test facility IABG.
In total the POCAM can host six light emitters which in turn can be controlled by two differentlight pulsers each. This concept is visualized in fig. 6. The typical choice of optical emitters arelight emitting diodes (LEDs) as well as laser diodes (LDs). Due to the support for two pulse drivertypes, the POCAM generally makes use of two distinct pulse drivers: the Kapustinsky circuit [34]and a LD-type LD driver [35]. By using switching components and circuitry, we allow the usage ofa specific emitter with a different number of pulse drivers. This not only allows for redundancy inthe field but also enables us to include various emitter wavelengths as they can then be selectivelydriven by different pulsers. λ λ λ λ λ λ LED driver
Figure 6 . POCAM LED and pulse driver layout. Four diodes are driven with an LD-type driver, two LEDswith an optimized Kapustinsky driver. Both driver types exist on the board twice for redundancy and can beselectively enabled for a specific emitter wavelength λ i . For details refer to the text. The Kapustinsky circuit is optimized for LEDs and makes use of a controlled capacitor dischargewhich is artificially cut short by an inductance parallel to the emitter. This pulser has the greatadvantage of linear light output adjustable with bias voltage and a relatively simple circuitry whichis shown in fig. 7. Pulse properties can be pre-selected by adjusting the values of the dischargedcapacitor C and the used inductance L , and the pulse width scales qualitatively with √ LC . In– 9 –ddition to being easy to set up, it is further used in related experiments [15, 25] and so comeswith the benefit of experience and reliability. In the current configuration, the Kapustinsky circuitdrives 405 nm and 465 nm LEDs per optimization for its IceCube application. For the POCAMapplication in the IceCube Upgrade we will host two distinct and switchable Kapustinsky driversper LED with respective ( L , C ) values of (
22 nH ,
100 pF ) for the fast and (
22 nH , . ) for thedefault pulse configuration. Figure 7 . Kapustinsky flasher schematic for the POCAM. The circuit is operated on negative bias voltageand produces a pulse when triggered with a square pulse signal. The light pulse is mainly shaped by thecapacitor C and the inductance L as well as the LED itself. The LD-type driver is an industry standard frequently used for LIDAR applications [35] and makesuse of high-current ultra-fast switching of Gallium-Nitride field effect transistors (GaN-FETs). TheGaN-FETs allow picosecond switching of voltages up to 100 V and currents up to 40 A which aretypically supplied by a bank of capacitors and discharged through the LD. This typically results invery short and intense light pulses if used with appropriate emitters. The general circuitry conceptis shown in fig. 8. As for emitters, for this circuit we are limited to available LD wavelengths withsufficient performance. However, typical wavelengths between 400 nm and 600 nm with matchingcharacteristics are readily available. It is also useful to drive UV LEDs with these circuits asthey typically require much larger currents to produce significant light outputs. In the currentconfiguration the emitters include an LED at 365 nm as well as LDs at 405 nm, 455 nm and 520 nm.A summary of selected emitters is given in table 1 together with available drivers.Both circuits perform in different regions of the dynamic range of the POCAM. With the fastKapustinsky driver being the dimmest followed by the default Kapustinsky driver, the LD drivergenerally outperforms in terms of intensity but at the marginal cost of longer pulse widths. Thenormalized intensity behavior of all circuits and an emitter wavelength of 405 nm is shown infig. 9, and their corresponding timing performance in fig. 10. While the former has been measuredwith an external photodiode, the latter was done using time-correlated single photon counting withan avalanche photodiode (APD) to sample the pulse time profile. It is evident that the intensitybehavior is linear for all circuits over the major parts of their dynamic range with estimated total– 10 – igure 8 . LD driver flasher schematic for the POCAM. The circuit is operated on positive bias voltage andproduces a pulse proportional to the input signal. The light pulse is mainly shaped by the input pulse and thebias voltage as well as the LED itself.
Emitter Wavelength [nm] Kapustinsky LD-type
XSL-365-5E [36] 365 – 2xXRL-400-5E [37] 405 fast / default –RLT405500MG [38] 405 – 2xPL-TB450B [39] 450 – 2xNSPB300B [40] 465 fast / default –LD-520-50MG [41] 520 – 2x
Table 1 . POCAM emitter selection with corresponding wavelengths and available drivers. Each LED drivenby a Kapustinsky can select a fast or default pulser configuration with different pulse widths and light yields.Each LD-type driven diode can select one of the two redundant LD-type drivers. number of photons for each pulser at maximum brightness given in table 2. Furthermore, as seen infig. 10, the timing behavior of the Kapustinsky circuits has been fine-tuned by selection of respective( L , C ) values to FWHMs of around 1 − − . −
35 ns in FWHM input pulse widths for the LDs. A summary oftypical FWHMs achieved with different pulsers is also given in table 2. It should be noted that thecircuit performance is heavily influenced by the selected emitter and it is generally necessary toperform appropriate selection procedures to identify suitable choices. This can even be dependenton production year with varying semiconductor batches for LEDs and LDs. These emitters furthergive need for appropriate calibration procedures, especially with respect to ambient temperature.The experimental setups and prototype results from calibrations are explained in section 2.6. Lastly,both circuits support flashing frequencies up to at least a few tens of kHz but were optimized for amaximum flashing frequency of 1 kHz for the IceCube Upgrade application.– 11 –
10 15 20 25 30Bias voltage [V]0 . . . . . L i g h t y i e l d [ a . u .] Kapustinsky (fast)Kapustinsky (default)LD-type (12ns)
Figure 9 . POCAM light pulser linearity. The figure shows the normalized intensity of all light pulse driverswith respect to applied bias voltage for the default 405 nm LED/LD at room temperature. . . . Kapustinsky (fast)Kapustinsky (default)LD-type (12ns) . . . N o r m . c o un t s Figure 10 . POCAM light pulser time profiles. The figure shows the normalized time profiles of all lightpulse drivers with respect to minimal-working (top) and maximal (bottom) applied bias voltage for the default405 nm LED/LD at room temperature.
One primary aspect of the POCAM is its isotropical emission profile. This not only guaranteesintrinsic independence of its orientation but further allows parallel self-monitoring of its lightoutput. Thus the POCAM is independent of the observing detector, and hence suffers reducedloop-back uncertainties resulting from imperfect detector understanding. The general concept heremakes use of the aforementioned light pulsers for generation of light pulses which are then diffusedby an integrating sphere made from teflon. Here, a two-part geometry ensures that the light pulseis pre-diffused by a teflon plug before being integrated in the remainder of the sphere, ensuring ahigher grade of isotropy. The latter is further made from optical teflon [42] which has been shownto drastically improve the isotropy. And, as will be discussed in section 2.4, self-monitoring is then– 12 – ulser Emitter [nm] Low intensity High intensity
Photons FWHM [ns] Photons FWHM [ns]
Kapustinsky (fast)
405 6 . × . × . × . × Kapustinsky (default)
405 8 . × . × . × . × LD-type (5ns)
365 1 . × . × . × . × . × . × . × . × LD-type (25ns)
365 3 . × . × . × . × . × . × . × . × Table 2 . Exemplary bare light pulser performance for all wavelengths at low and high intensity settings atroom temperature. The LD driver is further shown for two input pulse widths as it scales the brightness butalso time profile of the emitted pulses. The photon number was measured using a photodiode and representsa lower limit on the emission with a systematic uncertainty of approximately 10 %. done by integrated photosensors which are provided fixed solid angles by the aperture mounting.The latter comprise of two half-disks and a mounting ring, all from stainless steel, to mount thediffusing spheres and provide fixed solid angles for the sensors. Stainless steel is the material ofchoice as it provides a similar thermal expansion coefficient to the titanium in the pressure housing.It should be noted that low-reflective coating of all internal components is required in orderto avoid adding a cosine-like reflective component to the emission profile which would skew theachieved isotropy. This is realized using MagicBlack coating [43] which is applied to both theapperture disks as well as the internal flange surfaces. A hemisphere assembly is shown in fig. 11together with a view during assembly of sub-components. The flange design poses a significantinfluence on the emission profile and hence its design was critical. Especially since the glasshemispheres are attached to the flange via epoxy, we required the manufacturer to provide wellcontrolled and small edges of the latter around the waistband. This was shown necessary bymeasurements using a dedicated calibration setup in order to avoid significant deviations fromisotropy around the waistband as well as azimuthal deviations. The setup itself is explained indetail in section 2.6. To investigate optimizations of the system, a GEANT4 [44] simulationframework of the POCAM was setup which was able to reproduce the measured emission profileto within 1 %. This is shown in fig. 12 together with measured emission profile data of a singlehemisphere assembly. The simulation further showed that a small remaining glue edge couldbe counteracted by offsetting the integrating sphere upwards with respect to the equator of thehemisphere assembly, which was further confirmed by measurements. However, this simulationframework is also necessary to relate the calibration measurements done in air to the actual emittedlight profile into water or ice. Measurements of the emission profile in water as well as a complete4 π scan are currently underway and are expected to confirm the simulated results.– 13 – igure 11 . POCAM hemisphere prototype during assembly (left) and assembled (right). For details on thecomponents of the visible prototype analog board refer to fig. 4b and the text. . . . . . . N o r m .i n t e n s i t y [ a . u .] data (no offset, air)data (offset 4 mm, air)simulation (no offset, air)simulation (no offset, air), totalsimulation (offset 4 mm, air)simulation (offset 4 mm, air), totalsimulation (offset 5 . . − . . . I d − I s I d no offset, air offset, air Figure 12 . Measured and simulated emission profile of a POCAM hemisphere assembly. The top part ofthe figure shows the measured and simulated intensity per solid angle, given as a function of zenith anglewith respect to the POCAM cylinder axis. Both simulations and measurements are shown for the singlehemisphere alone (solid) and for a virtual sum of two mirrored hemispheres at infinite distance (dashed)including respective data. The bottom plot shows the normalized difference between data and simulation withmaximum deviations of around 1 %. The data errorbars represent the 1 σ errors from azimuthal deviations. – 14 – .4 Self-monitoring In order to provide independent monitoring from the telescope, the POCAM includes self-monitoringphotosensors. These sensors monitor its per-pulse light output and give a handle for correction ofintensity fluctuations over the course of its deployment period. The photosensors are two-fold: weuse a Silicon-photomultiplier (SiPM) for low light intensity and a photodiode (PD) for high lightintensity. The chosen SiPM is the
Ketek 3315-WB [45] and the PD is the
Hamamatsu S2281-01 [46].As discussed in section 2.3, the internal mounting of the diffusing spheres is done using aperturedisks from stainless steel. These disks provide the solid angles for the photosensors and furthermeans of mounting for the PD. The SiPM measures low charges as well as the pulse on-set timeand its approximate duration at 350 ps binning, the PD measures integrated charge over a large dy-namic range, pre-dominantly aimed at higher intensities. Both sensors use dedicated charge readoutschemes to provide intensity information with relevant parts of the circuit are shown in fig. 13. (a)
Transimpedance charge readout for the pho-todiode with two gain stages. (b)
GSI-developed charge and time readout of theSiPM fed into an FPGA-based discriminator.
Figure 13 . Conceptual views of the self-monitoring sensor readouts for the PD and the SiPM used in thePOCAM. For details on their functionality refer to the text.
For the SiPM, we make use of a time-to-digital-converter (TDC) circuitry developed together withthe GSI Helmholtz Centre for Heavy Ion Research GmbH in Darmstadt, Germany and the TRB-Collaboration (trb.gsi.de). This circuit enables the measurement of both on-set time and chargeinformation by correlated pulse splitting and controlled discharge of a capacitor. Both signals arethen fed into an FPGA-based discriminator and eventually the TDC for digitization with chargeproportional to the signal time-over-threshold (ToT). For the PD we make use of a traditionaltransimpedance amplifier (TIA) circuit using an extremely low-noise amplifier AD549S [47] whichprovides a voltage amplitude proportional to the measured charge of the PD. The TIA is thenfollowed by two additional low- and high-gain amplifier stages which are fed into an analog-to-digital converter (ADC). These are necessary in order adjust to the DAQ input voltage rangelimits and to tune the signals and their offsets appropriately. The goal of these circuits is toeventually provide a dynamic range of self-monitoring which is able to cover the full range ofemission intensities with good precision and their performance has been optimized in that respect,with response behavior shown in fig. 14. While the PD was expected to be linear across all itsmeasurable range [48], the SiPM shows the expected saturation behavior but can easily be fit withappropriate exponential functions [49, 50]. Aging of the sensors, while not expected for either the– 15 –iPM [51, 52] nor the PD due to low light levels [e.g. 53], will be tested in long-term test stands. π POCAM photons / pulse10 − − − T D CT o T [ a . u .] Fast dischargeSlow dischargeSiPM fitFit ± σ Kapustinsky (fast)Kapustinsky (default)LD-type (25ns) (a)
SiPM readout responses and fits for both fast and slow channels of the TDC circuit in anassembled POCAM hemipshere using all pulser types at 405 nm and a SiPM over-voltage of 5 V. π POCAM photons / pulse10 − − T I A a m p li t ud e [ a . u .] High gainLow gainLinear fitFit ± σ Kapustinsky (fast)Kapustinsky (default)LD-type (25ns) (b)
Normalized photodiode readout responses and linear fits for both gain channels of the TIAcircuit in an assembled POCAM hemipshere and using all pulser types at 405 nm. The high-gainchannel shows saturation at highest intensities but is compensated by the low-gain channel.
Figure 14 . Self-monitoring sensor response for both the SiPM (top) as well as the photodiode (bottom) asa function of total POCAM light emission including respective fits for all pulser types at 405 nm and roomtemperature. The errorbars represent the 1 σ pulse-to-pulse spread of the measured sensor responses and thegiven photon number is subject to a systematic uncertainty of approximately 10% in the used setup. The POCAM electronics generally consist of a mirrored system for both hemispheres with ananalog and digital front-end board. Those contain the components necessary for light pulsing,self-monitoring as well as their power supplies and control by means of an FPGA. In additionto these mirrored systems, this POCAM iteration hosts three IceCube-specific boards. The wirepair originating from the IceCube network is first handled by the
IceCube communications module – 16 –ICM) [54] which interfaces the system to the IceCube data-, clock- and power stream and takescare of providing the proper communication protocols, synchronization signals and power. TheICM signal chain is then fed to a secondary board which provides both hardware and softwaresimilar to the DAQ of IceCube Upgrade sensor instruments with a micro-controller unit (MCU)in its core. A third distribution board distributes data, clock, and communication streams as wellas power to each hemisphere using an FPGA. The first two interface boards are aimed specificallyfor an application in the IceCube Upgrade but can be exchanged to fit any telescope-specific backend. In this configuration the POCAM was designed to be powered with 96 V and a maximumpower consumption of 8 W. As for functionality, the FPGA in each hemisphere receives slow-controlcommands and clocks from the MCU via the distribution board. The slow control prepares theFPGA registers according to the measurement run to be carried out and sets up the configurationparameters for the flashers as well as the self-monitoring sensors and their DAQ. Once the flash isinitiated, the self-monitoring sensors monitor the per-pulse intensity as well as its timing behaviorand write this data to the FPGA internal storage. After the flashing procedure has finished, the MCUcan access this data and eventually transfer it via the IceCube data stream for offline processing.While the major part of the POCAM-specific digital electronics were already successfully testedand optimized in GVD and STRAW, the back end interfaces are a new addition for the IceCubeUpgrade and are currently in advanced prototyping.
In order to streamline the calibration process for the production phase of around 30 POCAMs,we developed two dedicated setups to characterize the flashers and the emission profile of eachhemisphere, respectively. The former is a relative flasher characterization setup with the goal toprovide knowledge of the flasher’s relative intensity, time profile and spectrum variation as a functionof configuration parameters and temperature. The latter is an emission profile characterization setupwhich aims to characterize the emitted light pattern of each POCAM hemisphere relatively.
Relative flasher characterization
The flasher characterization setup, shown in fig. 15, consistsof four sensors: a photodiode [46], a PMT [55], an avalanche photodiode (APD) [56] and aspectrometer [57]. These sensors are located in a dark box and each is coupled to one end of a4-to-1 fan-out fiber, the single end of which is coupled to a flasher assembly including diffusersand apperture disks. This assembly itself is further located in a freezer together with the remainderof the POCAM electronics and can be cooled down to − ◦ C. While the PD and PMT record theintensity, the APD uses time-correlated single photon counting to measure the pulse time profile andthe spectrometer directly records the output spectrum. The APD further uses a controllable neutraldensity filter wheel to achieve low occupancies of below 10 % which is necessary to provide propersingle-photon sampling of the time profile. The raw PMT pulses are recorded with the help of adigital oscilloscope, the PD is read out with the
Keithley 6485 picoammeter, the APD is fed into ahigh-precision TDC and the spectrometer outputs the spectrum via serial command. Together withnecessary power supplies and other peripheral electronics, all of the sub-components are controlledby a dedicated computer running all the necessary software. The procedure starts by mounting aspecific POCAM inside the freezer and, using a dedicated stainless steel structure, coupling the fan-out fiber to the diffuser. Then, the automated system scans temperature and configuration parameters– 17 –nd measures the pulse properties as well as the self-monitoring sensor responses. This data canthen be used offline to provide individual relative characterization of a specific POCAM hemisphereassembly and results in fingerprint-characterized or golden
POCAMs. To further remove systematicuncertainties resulting from potential fiber coupling changes over the course of cooling and heating,we flush the freezer with nitrogen as well as monitor a temperature-stabilized reference halogenlight source coupled into the diffuser using the same type of fiber.
Dark boxAPDHV supply FreezerTriggerFilterPhotodiodeSpectrometerPMTComputerPicoscopePicoampOdroidPSU Arduino PSU Analog ConnectionDigitalConnectionFiberConnectionMeasurementDeviceLaboratoryDeviceSupportingElectronicsFlasher assembly + fiber adapterReferenceTDC
Figure 15 . Schematical workflow diagram of the light pulser calibration station. For details on the sub-components and their functions, refer to the text.
Relative emission profile characterization
The emission profile setup consists of a two-axisrotation stage assembly which allows mounting a POCAM hemisphere including flange, diffusers,apperture disk and sensors in a dark box of around 140 cm inner length. The rotation stages usedprovide sub-degree precision and two of them are used to create a custom two-axis rotation stage.On the opposite side, a photodiode [46] is mounted and light baffles in between further reduce straylight from reflections off of inner surfaces. The hemisphere is then mounted to the rotation stageswith a dedicated illumination board which provides identical layout to the analog board but uses theLEDs in switchable forward mode. A dedicated measurement PC then controls the characterizationscan for a set of azimuth and zenith angles as well as LEDs and measures the intensity data of thePD. The PD current is monitored at each angle step with a
Keithley 6485 picoammeter for bothevery LED switched on individually and every LED switched off. This data is then eventuallywritten to file. Due to the rotation of the hemisphere, this provides a relative characterization ofits emission profile and can further be used to calculate the total hemispherical light yield. Theworkflow of this setup is shown in fig. 16 including peripheral electronics.
Absolute calibration
After all POCAMs have undergone relative characterization, the last stepis absolute calibration of the light yield. For this, the instruments are placed in the emission profilesetup and the regular PD is exchanged for one precision-calibrated by the National Institute forStandards and Technology (NIST). Then, iterating through all emitters and pulse drivers as wellas a range of configuration parameters and measuring the photocurrent at the PD, this provides– 18 – ark boxPhotodiode ComputerPicoamp Arduino Analog ConnectionDigitalConnectionMeasurementDeviceLaboratoryDeviceSupportingElectronicsHemisphere assembly+ illumination boardLight baffles Rotationstages
Figure 16 . Schematical workflow diagram of the emission profile calibration station. For details on thesub-components and their functions, refer to the text. absolute intensity scales to all previous relative calibrations. These results are then the referencefor self-monitoring data over the course of the operational period of the instrument and can beused for correcting the instrument emission in-situ using the integrated photosensors. The intrinsicsystematic uncertainties of this calibration chain are given in table 3 including the total anticipatedsystematic uncertainty on the POCAM light yield of 4 . Prototype calibration
Preliminary calibration of a full prototype assembly was carried out beforegoing into production. The general procedure first consists of a relative flasher characterizationversus configuration parameters and temperature and second a relative emission profile characteriza-tion. As for the light emission, shown for the 405 nm emitter in fig. 17, the pulsers show decreasingintensities for decreasing temperatures due to most likely increased series resistance [58]. However,it should be noted that the slope of temperature dependence varies with emitter type. The timeprofile generally does not show any significant temperature dependence for any driver at higherintensities but the LD-type driver pulses show a marginally longer tail at lower intensities and lowertemperatures. However, since all the drivers will be fingerprint-characterized and the LD-type willbe predominantly used for high intensities, this does not pose an issue. The emission spectrumshowed only small dependence on both temperature and configuration parameters for a small sampleof emitters but will be also be characterized for each POCAM.The measured emission profile of a POCAM hemisphere prototype and the virtual sum of twohemispheres emulating a complete POCAM, is visualized in Mollweide projection in fig. 18a andfig. 18b, respectively. The virtual emission profile shows only marginal deviations from idealisotropy as was expected from simulation.For its 1 σ deviations we find 0 . ± .
014 and 1 . ± .
015 over the full zenith range of θ ∈ [ ◦ , ◦ ] and 0 . ± .
009 and 1 . ± .
004 within θ ∈ [ , ◦ ]∨[ ◦ , ◦ ] by averaging allLEDs and all azimuthal angles and using the resulting standard deviation as errors. The hemisphere-to-hemisphere spread is expected to be small due to the precision of production but will nonethelessbe characterized. This profile was then further used to roughly estimate the total 4 π POCAM lightyield using the 405 nm emitter at maximum intensity but yet without NIST-calibration. We findrespectively ( . ± . )× photons / pulse for the fast Kapustinsky, ( . ± . )× photons / pulsefor the default Kapustinsky and ( . ± . ) × photons / pulse for the LD-type driver at 25ns– 19 – ystematic effect Affected quantities Estimated impact Temperature-dependent fiber coupling Relative light yield 2 . . ≤ . . . . . < . < . < . ≤ . ≤ . Total
Spectrum 1 . . . . . Table 3 . Summary of estimated systematic uncertainties of the POCAM calibration chain. The top part ofthe table summarizes the dominating systematic effects in the calibration setups, the bottom part shows thetotal estimated systematic uncertainties on different calibration parameters. width. A dedicated test stand to confirm the 4 π emission profile is currently in planning. This work summarizes the developments of the third and final POCAM iteration in the scope ofthe IceCube Upgrade. In comparison to previous deployments in GVD and STRAW, we haveoptimized several features of the POCAM including total light yield and subsequent dynamicrange, spectral composition of emitters, self-monitoring precision, isotropy and internal structure.Additionally we have developed two dedicated experimental setups which allow a streamlinedfingerprint-characterization of individual POCAMs versus temperature, light pulser configurationand orientation. Further we provide an absolute intensity scale using a NIST-calibrated photodi-ode to provide an absolute scale for all characterization measurements and hence maximize theknowledge of the instrument emission and hence provide what we call golden POCAMs. Lastly,we introduced a modular interface to the detector back end including data stream and synchroniza-tion. This can be adjusted to the telescope in which the POCAM is supposed to be deployed inand currently provides an interface to the IceCube detector. With the production of the IceCubeinstruments starting this year, we aim to provide a large-volume calibration light source standard for– 20 – . . . . . L i g h t y i e l d [ a . u .] T − . ◦ CT − . ◦ CT − . ◦ CT − . ◦ CT − . ◦ C T − . ◦ CT 0 . ◦ CT 10 . ◦ CT 20 . ◦ C (a) Default Kapustinsky driver intensity as a function of bias voltage and temperature at 405 nm. . . . . . L i g h t y i e l d [ a . u .] T − . ◦ CT − . ◦ CT − . ◦ CT − . ◦ CT − . ◦ C T − . ◦ CT 0 . ◦ CT 10 . ◦ CT 20 . ◦ C (b) LD-type (25ns) driver intensity as a function of bias voltage and temperature at 405 nm.
Figure 17 . Temperature dependence of the pulser intensity measured for both the default Kapustinsky (a)and LD-type (b) driver using respective 405 nm emitters. The fast Kapustinsky shows a similar behavior tothe default driver and thus has been omitted in this figure. large-volume photosensor arrays which can be used in a multitude of scenarios and which providesprecise and independent calibration capabilities.
Acknowledgements
We would like to express our gratitude to the German Federal Ministry for Education and Research(BMBF) for supporting the development related to IceCube through grant 05A17WO5 "Verbund-projekt IceCube: Astroteilchenphysik mit dem IceCube-Observatorium". We thank the cluster ofexcellence "Origin and Structure of the Universe" (DFG) and the SFB1258 "Neutrino and DarkMatter in Astro and Particle Physics" (DFG) for the support related to the GVD and P-ONE ap-plications. We further thank the GVD and STRAW collaborations for deploying our instrument– 21 – ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ φ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ θ I n t e n s i t y [ a . u .] (a) Single hemisphere prototype emission ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ φ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ θ I n t e n s i t y [ a . u .] (b) Virtual dual-hemisphere emission
Figure 18 . POCAM hemisphere prototype emission profile in Mollweide projection for a single hemisphere(a) and a virtual complete POCAM (b) in air. In the bottom figure we mirrored and randomly rotated theemission of the hemisphere to create a virtual complete POCAM emission pattern. The pixels represent allmeasured angular steps with color normalized to maximum (top) and average (bottom) intensity. prototypes and sharing of related detector data as well as the IceCube collaboration for commentsand feedback on this manuscript.
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