Biteau, J.

Deligny, O.

Ghia, P.

Lhenry-Yvon, I.

Schmidt, David

English

Pierre Auger Collaboration

KITopen

Scintillator Surface Detector simulations for AugerPrimeDavid Schmidt1,∗ for the Pierre Auger Collaboration2,∗∗,∗∗∗1Karlsruhe Institute of Technology (KIT)2Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, ArgentinaAbstract. Knowledge of the mass composition of ultra-high-energy cosmic rays is understood to be a salientcomponent in answering the open questions in the field. The AugerPrime upgrade of the Pierre Auger Observa-tory aims to enhance its surface detector with the hardware necessary to reconstruct primary mass for individualevents. This involves placing a scintillation-based detector with an active area of 3.8 m2 on top of each existingwater-Cherenkov detector in its surface detector array. Here, we present the methods for simulating this Scin-tillator Surface Detector. These simulations have and will continue to aid in the interpretation of measurementswith AugerPrime as well as the development and improvement of event reconstruction algorithms includingprimary mass.1 IntroductionThe Pierre Auger Observatory (Auger) [1], with its un-precedented exposure of over 60 000 km2 sr yr acquiredduring more than decade of data collection, has led toprogress in the field of Ultra-High-Energy Cosmic Rays(UHECR) physics in a number of ways. To name a cou-ple, the suppression in the energy spectrum above approxi-mately 40 EeV has been confirmed to high precision [2, 3],and a large-scale dipole anisotropy above 8 EeV has beenclearly observed [4]. With these advances, however, Augerhas also uncovered additional and unexpected complexity.Perhaps most notable is the trend towards heavier com-position at the highest energies, which reopens questionsregarding the origins of the flux suppression. Additionally,the long standing mystery as to the origins of UHECRs re-mains unsolved, and the additional complication of massnow presents further challenges. For this reason, the ob-servatory is embarking on its next phase by equipping itsSurface Detector (SD) array with the hardware necessaryto estimate the mass of UHECRs on an event-by-event ba-sis.This upgrade, known as AugerPrime [5, 6], capitalizeson the fact that the magnitude of the muonic componentof Extensive Air Showers (EASs) produced by UHECRsscales with primary mass. AugerPrime aims to disentan-gle the contributions of the electromagnetic and muonicshower components in SD measurements, thereby obtain-ing a mass-sensitive estimator. This is being accomplishedthrough the installation of an Scintillator Surface Detec-tor (SSD) on top of each of the existing Water-CherenkovDetectors (WCDs) that make up the SD array. The rela-tively higher sensitivity of the SSDs (WCDs) to particles∗e-mail: david.schmidt@kit.edu∗∗e-mail: auger_spokespersons@fnal.gov∗∗∗Authors list: http://www.auger.org/archive/authors_2018_10.htmlof the electromagnetic (muonic) air shower componentsallows for this disentanglement.A prototype array of SSDs has been operating in thefield since September of 2016, and deployment of pro-duction design SSDs for the complete array is currentlyunderway. In order to aid in understanding and interpret-ing measurements of the upgraded SD, as well as developreconstruction algorithms, we have developed comprehen-sive SSD simulations. The methods used in these simula-tions are described in this proceeding and build upon [7].2 Scintillator Surface DetectorThe active area of an SSD is comprised of 48 polystyrenescintillator bars measuring 1600 mm by 50 mm with athickness of 10 mm. This makes for an active area of justover 3.8 m2, which is split between two wings of 24 barseach (see Fig. 1). 1 mm diameter Wave-Length Shift-ing (WLS) fibers of 5.8 m length are routed through twobean shaped holes inside each bar with a U-bend of 10 cmdiameter at the ends furthest from the central axis of theSSD such that both fiber ends may be routed into a cookiesituated between the two scintillator wings. There are ap-proximately 1.25 m of fiber between the central ends ofthe scintillator bars and the cookie, and the lengths of thefibers are uniform for all scintillator bars through whichthey are routed. The bars and fibers are housed within analuminum casing whose bottom consists of a compositepanel with 22 mm of Extruded Polystyrene (XPS) situatedbetween two 1 mm sheets of aluminum. The top of thehousing consists of a 1 mm sheet of aluminum. The spacewithin the SSD housing not occupied by the bars and fibersis mostly filled with Expanded Polystyrene (EPS), result-ing in a volume of air inside the detector of less than 10 L.For a detailed description of the detector materials, geom-etry, and construction, see [8].© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 210, 06010 (2019) https://doi.org/10.1051/epjconf/201921006010UHECR 2018160 cm120 cmWing 1 Wing 2PMTFor schematic purposes onlyNeither to scale nor representative in all geometric aspectsFigure 1. Top-down schematic depiction of the arrangement ofbars and fibers within the SSD casing. The written dimensionsdescribe the active surfaces of the detectors, namely the scintil-lator bars.3 Simulation ApplicationThe SSD simulation application is housed within Auger’sofficial simulation and reconstruction framework, Offline[9], which was upgraded to accommodate for the demandsof the large-scale detector upgrade that is AugerPrime [7].The SSD simulations are based in the field-standard sim-ulation software Geant4 [10] (version 4.10), but, whereit is possible, make use of measurements of SSDs per-formed with a centimeter-precision muon telescope for en-richment and tuning. The combined use of Geant4 and themuon telescope measurements has made highly detailedsimulation of the detector possible. The simulations ac-count for the energy-loss processes of traversing particles,the decays occurring from excited states of atoms withinthe scintillator bars and WLS fibers, the attenuation ofphotons along the fibers, the Photoelectrons (PEs) gen-erated at the photo-cathode of the Photomultiplier Tube(PMT), the current at the base of the PMT, and the re-sponse of the AugerPrime station electronics [11]. Thefollowing sections step through the construction of the de-tector in Geant4 and each stage of the simulation.Figure 2. Visualization of both the SSD and WCD volumes de-fined in the Geant4-based simulation application with a close-upof one corner of the SSD3.1 Detector ConstructionSSDs are constructed within the same Geant4 world vol-ume as their partner WCDs. The volumes for both de-tectors may be observed in Fig. 2. Regarding the SSD,volumes for the aluminum casing, XPS, EPS, and scintil-lator bars themselves are defined according to the dimen-sions and material properties of the production SSD de-sign, described in Sec. 2. This is important to accuratelyreproduce the shielding above and below the active areaof the scintillator bars. Precise definition of the positionof the SSD relative to the WCD is important for preserv-ing signal correlations, as these depend on the fraction ofparticles that intersect both detectors.3.2 PhotoelectronsFor each particle that intersects or is generated within theactive scintillator volumes, the energy deposited within thebars is extracted from Geant4 and converted into a numberof photoelectrons at the photo-cathode of the PMT. Thisconversion must take into account the attenuation of pho-tons within the bars and along the fibers as well as theexcitations and decays corresponding to scintillation andwavelength-shifting processes. The conversion takes theform ofN¯(x) = NrefEdepEreffatt(x) (1)where Edep is the energy deposit in the scintillator bars,Eref and Nref are the reference energy and PE number de-rived from simulations, and fatt(x) describes the degree ofphoton attenuation as a function of particle crossing posi-tion x. Eref is the peak in the distribution of energy de-posited in the SSD by an ensemble of vertical, through-going muons simulated for the volumes described in Sec.3.1. Nref is a constant derived such that average value ofEqn. 1 when integrated over all possible values of x insidethe scintillator bars is equal to the the average number ofPEs observed in real measurements of the SSDs with theaforementioned muon telescope. For a more detailed de-scription of how these parameters were obtained, see [7].With this construction, the mean number of PEs for a sin-gle, through-going muon is guaranteed to match real mea-surements. The attenuation function fatt(x) is written asfatt(x) = A(x)L(x) , (2)where A(x) describes the attenuation along the WLS fibers.This attenuation is of the formA(x) = e−`c(x)/λf + e−`f (x)/λf , (3)where the two exponential terms correspond to the twopaths photons may travel to reach the PMT due to the U-bend routing of the fibers. `c (`f) corresponds to the shorter(longer) travel path and is calculated for a given crossingposition x. λf is the effective attenuation length of the fiberobtained through analysis of muon telescope SSD mea-surements. The term L(x) corresponds to a decreased yieldat crossing positions very close the edges of the scintilla-tor bars. Thus, for any energy deposit and particle cross-ing position, an expected number of PEs corresponding toeach of the two paths photons may travel to reach the PMTmay be calculated. A Poisson randomization is then per-formed on each of these expectations. For each PE comingout of these randomizations, two draws, one from each of2EPJ Web of Conferences 210, 06010 (2019) https://doi.org/10.1051/epjconf/201921006010UHECR 20180 10 20 30 40 500510152025Charge / a.u.0 10 20 30 40 500510152025Charge / a.u.0 10 20 30 40 50051015202530t [ns]Charge / a.u.0 10 20 30 40 50051015202530t/nsCharge / a.u.Figure 3. Depiction of the time evolution of the signal from single, through-going muons in an SSD. Muon telescope measurementsare shown in the plots on the left and the parameterization of these measurements are shown in those on the right. Middle: Densityplot of the signal evolution over one scintillator wing of the SSD. The bottoms of the histograms correspond to a particle crossing atthe non-central edges of the scintillator bars, where photons corresponding to the two legs of the signal traverse approximately equaldistance and therefore produce a unimodal distribution. The tops of the histograms correspond to the central edges of the scintillatorbars, where the photons corresponding to the two legs of signal traverse significantly different distances and therefore result in a bimodaldistribution. Slices in the horizontal axis have been normalized to the same maximum. Top and bottom: Signal shapes at the centraland non-central ends of the scintillator bars.two exponential distributions are performed. This modelof two exponential decays, which may arbitrarily be at-tributed to the excitation associated with scintillation inthe scintillator bars and the wave-length shifting processin the WLS fibers, was found to provide a sufficient de-scription of the data [7]. The production time for each PEis then obtained viatPE = tglobal + tkin + tbar + tfiber (4)where tglobal is the global time obtained from Geant4, tkin isthe kinematic delay of the photon as it traverses its respec-tive path to the PMT, and tbar and tfiber are the times aris-ing from the two aforementioned exponential decays. Theaverage measured and parameterized time distributions forcrossing positions along the complete active surface of onewing of the SSD, which compound Eqn. 1, Eqn. 4, andthe convolution of the two exponential decays, may be ob-served in Fig. 3.3EPJ Web of Conferences 210, 06010 (2019) https://doi.org/10.1051/epjconf/201921006010UHECR 20180 10 20 30 40 50 60 70θ/◦0.60.81.01.21.41.61.82.02.2q/a.u.Muon telescope dataSim resultssec(θ)Figure 4. Left: Average charge measured for single, through-going muons as a function of the crossing position for one wing of anSSD. Measurements with the muon telescope are shown by the square, red markers, whereas simulated data is represented by thecircular, black markers. Larger values on the horizontal axis correspond to crossing positions further from the central edge of thescintillator, as depicted by the semi-transparent schematic in the backdrop. Right: Average charge as a function of zenith angle. Oncemore, muon telescope measurements are given by the red markers, whereas simulations are shown in black. Both data sets have beenscaled such that they equal one at an angle of 35◦ to compare their evolution with tracklength (1/cosθ).3.3 PMT and electronicsWith a trace of PE production times in hand, the PMT issimulated as follows. For each PE, the average pulse shapeat the PMT base corresponding to an Single Photoelectron(SPE) is scaled by a random draw from the correspondingcharge distribution. Both this pulse shape and the chargedistribution were measured for SPE events for the PMTin question, as described in [7]. Performing this processfor each PE results in a time distribution of the electricalcurrent at the base of the PMT, which is the input to theupgraded SD station electronics [11].The effect of the electronics on the time trace is simu-lated by convolving the base current distribution with themeasured electronics transfer function. This results in ananalog voltage trace, which is then scaled, sampled, anddigitized. The scaling is performed such that the resultingpeak in the charge distribution for a simulated ensembleof through-going muons equals that which has been mea-sured by prototype SSDs deployed in an Engineering Ar-ray (EA) [12] at the observatory site near Malargüe since2016. The sampling and digitization are performed in ac-cordance with the 120 MHz 12 bit design of the Auger-Prime electronics.4 ValidationValidation of the output of the simulations was per-formed on a per-particle basis by comparing the position-dependent charge and time distributions of simulatedmuons to those measured by the muon telescope. In Fig.4, the dependencies of charge on crossing position andzenith angle of the particles are compared between mea-surements and simulations. In both cases, the simulationsprovide a reasonable description of the data. A full valida-tion closely examining the shapes of the charge and timedistributions is currently underway as are additional com-parisons with measurements of SSDs deployed in the EA.100 150 200 250 300 350 400 450 500arbitrary time/ns01020304050NumberofPhotoelectrons0.00.20.40.60.81.0BaseCurrent/a.u.02004006008001000120014001600ADC051015202530Signal/MIPPeakFigure 5. Simulated SSD measurement. Time traces for PE pro-duction, current at the base of the PMT, post-electronics digitizedsignals, and calibrated to the signal of one Vertical MinimumIonizing Particle (VMIP) are shown.5 Example ApplicationFor a given simulated extensive air shower, the particlesreaching ground near each SD station are injected into acylindrical volume that completely encases both the SSDand WCD. Propagation of these incident particles into theshared volume and subsequent interactions and generationof new particles is then left to the Geant4 software, andsignals are simulated as described in Sec. 3. A depictionof time distributions of photoelectrons, current at the baseof the PMT, and digital signals after electronics processingare shown in Fig. 5 for an example station simulation. Anexample event, where SSD and WCD pairs sample the lat-eral distribution of particles at ground at various locations,based on Auger’s hexagonal grid, is depicted in Fig. 6.4EPJ Web of Conferences 210, 06010 (2019) https://doi.org/10.1051/epjconf/201921006010UHECR 20181000100101102103104Signal/orWater Cherenkov DetectorScintillator Surface Detector500 5000Distance to shower axis / metersMIPVEM-Figure 6. Simulated signals of SSDs alongside partner WCDsfor various samples of the lateral distribution of the particlesreaching ground in a simulated EAS.6 ConclusionsDetector simulations for the SSD of the AugerPrime up-grade have been developed and the methods, making useof Geant4 and real detector measurements, have been de-scribed here. Initial validation has shown general agree-ment between the detector simulations and centimeter-precision muon telescope measurements for the chargeand time distributions produced by single, through-goingmuons. Additional, more detailed, validation is currentlybeing performed. The combined SSD and WCD simula-tion application, housed in the Offline detector simulationand reconstruction software framework of Auger, has al-ready and will continue to aid in the interpretation of mea-surements performed with AugerPrime as well as in thedevelopment and improvement of event reconstruction al-gorithms which include primary mass.References[1] The Pierre Auger Collaboration, Nucl. Instrum.Meth. A 798, 172-213 (2015)[2] The Pierre Auger Collaboration, Phys. Rev. Lett.101, 061101 (2008)[3] The Pierre Auger Collaboration, Phys. Lett. B 685,239-246 (2010)[4] The Pierre Auger Collaboration, Science 357, 1266-1270 (2017)[5] The Pierre Auger Collaboration, arXiv:1604.03637(2016)[6] Daniele Martello for the Pierre Auger Collaboration,Proc. 35th Int. Cosmic Ray Conf. PoS(ICRC2017)486, (2017)[7] D. Schmidt for the Pierre Auger Collaboration, PoSICRC2017, 353 (2017)[8] R. Smida for the Pierre Auger Collaboration, PoSICRC2017, 390 (2017)[9] S. Argiro et al., Nucl. Instrum. Meth. A 580, 1485-1496 (2007)[10] S. Agostinelli et al., Nucl. Instrum. Meth. A 506,250-303 (2003)[11] T. Suomijärvi for the Pierre Auger Collaboration,PoS ICRC2017, 450 (2017)[12] Z. Zong for the Pierre Auger Collaboration, PoSICRC2017, 449 (2017)5EPJ Web of Conferences 210, 06010 (2019) https://doi.org/10.1051/epjconf/201921006010UHECR 2018

Scintillator Surface Detector simulations for AugerPrime

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Scintillator Surface Detector simulations for AugerPrime

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