Electron beam studies of light collection in a scintillating counter with embedded fibers
M. Lau?, P. Achenbach, S. Aulenbacher, M. Ball, I. Beltschikow, M. Biroth, P. Brand, S. Caiazza, M. Christmann, O. Corell, A. Denig, L. Doria, P. Drexler, J. Geimer, P. Gülker, M. Kohl, T. Kolar, W. Lauth, M. Littich, M. Lupberger, S. Lunkenheimer, D. Markus, M. Mauch, H. Merkel, M. Mihovilovi?, J. Müller, B. S. Schlimme, C. Sfienti, S. ?irca, E. Stephan, S. Stengel, C. Szyszka, S. Vestrick, A. Wilczek
EElectron beam studies of light collectionin a scintillating counterwith embedded wavelength-shifting fibers
M. Lauß a,1 , P. Achenbach a,b,c, ∗ , S. Aulenbacher a , M. Ball d , I. Beltschikow a ,M. Biroth a , P. Brand e , S. Caiazza a , M. Christmann a,b , O. Corell a ,A. Denig a,b,c , L. Doria a,c , P. Drexler a , J. Geimer a , P. G¨ulker a , M. Kohl f ,T. Kolar g , W. Lauth a , M. Littich a , M. Lupberger i , S. Lunkenheimer a ,D. Markus a , M. Mauch b , H. Merkel a,c , M. Mihoviloviˇc g,h , J. M¨uller a ,B. S. Schlimme a , C. Sfienti a,c , S. ˇSirca g,h , S. Stengel a , C. Szyszka a ,S. Vestrick e , for the MAGIX Collaboration a Institut f¨ur Kernphysik, Johannes Gutenberg-Universit¨at, 55099 Mainz, Germany b Helmholtz Institute Mainz, GSI Helmholtzzentrum f¨ur Schwerionenforschung, Darmstadt,Johannes Gutenberg-Universit¨at, 55099 Mainz, Germany c PRISMA + Cluster of Excellence, Johannes Gutenberg-Universit¨at, 55099 Mainz, Germany d Helmholtz-Institut f¨ur Strahlen- und Kernphysik, RheinischeFriedrich-Wilhelms-Universit¨at Bonn, 53115 Bonn, Germany e Institut f¨ur Kernphysik, Westf¨alische Wilhelms-Universit¨at M¨unster, 48149 M¨unster,Germany f Department of Physics, Hampton University, Hampton, Virginia 23668, USA g Joˇzef Stefan Institute, 1000 Ljubljana, Slovenia h Faculty of Mathematics and Physics, University of Ljubljana, 1000 Ljubljana, Slovenia i Physikalisches Institut, Rheinische Friedrich-Wilhelms-Universit¨at, 53115 Bonn, Germany
Abstract
The light collection of several wavelength-shifting fiber configurations embed-ded in a box-shaped plastic scintillating counter was studied by scanning withminimum ionizing electrons. The light was read out by silicon photomultipliersat both ends. The light yield produced by the 855-MeV beam of the Mainz Mi-crotron showed a strong dependence on the transverse distance from its positionto the fibers. The observations were modeled by attributing the total light yieldto the collection of diffuse light inside the counter and of direct light reachinga fiber. The light collection with fibers was compared to that of a scintillating ∗ Corresponding author at: Institut f¨ur Kernphysik, Johannes Gutenberg-Universit¨at,55099 Mainz, Germany.
Email address: [email protected] (P. Achenbach) Part of master thesis.
Preprint submitted to Nucl. Instrum. Methods Phys. Res. A January 27, 2021 a r X i v : . [ phy s i c s . i n s - d e t ] J a n ounter without fibers. These studies were carried out within the developmentof plastic scintillating detectors as an active veto system for the DarkMESAelectron beam-dump experiment that will search for light dark matter particlesin the MeV mass range. Keywords:
Plastic scintillating counter, Wavelength-shifting fiber, Lightyield, Silicon photomultiplier (SiPM), Electron beam tests
1. Introduction
The Johannes Gutenberg University Mainz is currently constructing the newcontinuous-wave multi-turn electron linac MESA (Mainz Energy Recovering Su-perconducting Accelerator) on the Gutenberg Campus [1]. For the DarkMESAexperiment, the high-power beam dump of the accelerator will be used as atarget for the possible production of dark sector particles in the MeV massrange [2, 3]. Once discovered, these could provide information on the structureof dark matter, which makes up a large proportion of our universe [4].The detector concept of the DarkMESA experiment will implement electro-magnetic calorimeters surrounded by active veto counters. These calorimeterswill detect the transferred energy in elastic scattering of the dark sector par-ticles off atomic-shell electrons [5], where the energy range is defined by the150 MeV energy of the electron beam. The detector site will be heavily shieldedfrom the beam so that practically all beam-related Standard Model particleswill be blocked. It will be crucial for this experiment that cosmogenic parti-cles leading to background events are vetoed with a high detection efficiencyand homogeneity. The planned veto detector system will consist on the orderof 80 plastic box-shaped scintillating counters each of 2 cm thickness and ap-proximately 5000 cm in area, arranged in two layers, and read out by siliconphotomultipliers (SiPMs), the latter possibly connected to wavelength-shifting(WLS) fibers for an enhanced light collection. Sheets of lead, between an in-ner and an outer veto layer, will prevent low-energy γ -rays from reaching thecalorimeter. This design for a veto system in the search for dark matter at ac-2elerators follows the approach of the BDX Experiment at the Thomas JeffersonNational Accelerator Facility (JLab) in the USA [6, 7].This paper describes studies of prototype counters for the DarkMESA vetosystem in the 855-MeV electron beam of the Mainz Microtron (MAMI). A scin-tillation counter, in which different configurations of fibers were embedded, isdescribed in Section 2, the electron beam tests are presented in Section 3, thelight collection is discussed and modeled as a function of the transverse distancefrom the beam position to the fibers in Section 4, and the conclusions are givenin Section 5.
2. Description of the scintillation counters
50 mm 100 mm 150 mm 200 mm
Figure 1: Photograph of one of the two polished read-out ends of the studied scintillationcounter with WLS fibers. Configurations from left to right: round fiber of 1-mm diameter,round fiber of 1 . × Two identical scintillation counters of type EJ-200 from Eljen Technology [8]with dimensions of 50 × × were studied. The opposite ends of thecounters were each read out by four independent 6 × SiPMs of type J-Series 60035 from SensL [9]. Parallel grooves of 2 . × . cross sectionwere milled into the surface of one counter, so that fibers could be placed intothese grooves, which were then filled with optical cement of type EJ-500 fromEljen Technology. The protruding ends of the fibers were cut off and the tworeadout sides of the counter were polished. Four different fiber configurationswere realized: Ch0
Round fiber of 1-mm diameter of type BCF-92 from Saint Gobain Crys-3 -Readout board with SiPMs SiPMs γγ γ
Wavelength-shifting fiberPhotons trapped by total reflection Photons leaving scintillatorPhotons reflected at boundary Figure 2: Schematic view of how a WLS fiber is influencing the collection of scintillation lightthat is produced by a minimum ionizing electron beam penetrating the active volume of acounter. The scintillation counters in this study had dimensions of 50 × × . The lightwas read out using four SiPMs mounted on a readout board on each of the two opposing ends. tals [10] Ch1
Round fiber of 1 . Ch2
Square fiber of 2-mm edge length of type BCF-20 from Saint GobainCrystals [10]
Ch3 × iPMSiPM SiPM SiPM (a) Top layer PreAmp OpAmp DAC (b) Bottom layer
PreAmpDAC SiPMOpAmp (c) Electronic circuit diagramFigure 3: Photographs of the top and bottom layer of the readout board and the electroniccircuit diagram. The board has a cross section of 1 ×
25 cm . (a) Top layer. Four SiPMs with6 × active area, connected with the fast output. (b) Bottom layer. PreAmp (one perSiPM): Signal preamplifier based on the gain block AD8354 with a transimpedance gain of Z = 500 Ω and high analog bandwidth; OpAmp (one per SiPM): Non-inverting high-voltageoperational amplifier circuit with current-limiting resistor for generating the bias voltage froman adjustable reference voltage; DAC (one per board): Digital-to-analog converter for settingthe individual values for the reference voltages and thus the bias voltages. (c) Electroniccircuit diagram.
25 mm
Left side SiPMs
Ø 1 mmØ 1.5 mm□ 2 mmØ 1 mm × Ch0Ch1Ch2Ch3 L L L L [mm] Figure 4: Schematic view of the electron beam positions (black crosses) on the 250-mm widescintillation counter with fibers separated by 50 mm. Near each fiber nine positions with apitch interval of 1 mm were scanned at a distance of 225 mm from the read-out side. The twodashed, perpendicular lines indicate the symmetry axes of the counter. The SiPMs of the fourchannels Ch0 to Ch3 at the left side are labeled L to L . On the right, the relative scanningpositions with respect to the fiber are shown in the enlarged view. Left side SiPMs Right side SiPMs
Ch0Ch1Ch2Ch3 L L L L R R R R [mm] Figure 5: Schematic view of the electron beam positions (black crosses) on the 250-mm widescintillation counter without fibers for the reference measurements. Three positions with apitch interval of 100 mm were scanned at a distance of 225 mm from the read-out side. Sevenadditional measurements were taken along the central longitudinal axis. The two dashed,perpendicular lines indicate the symmetry axes of the counter. The SiPMs of the four channelsCh0 to Ch3 at the left and the right side are labeled L to L , respectively R to R . . Electron beam tests of the scintillation counters In separate beam tests, electrons of 855 MeV energy from the Mainz Mi-crotron MAMI were precisely pointed to a set of positions on the top of one ofthe scintillation counters. The detector was placed in a dark box to shield itfrom external light sources and the whole setup was supported by a remotelysteerable x - y table. The beam position relative to the counter was determined bya small, separate scintillation detector located in the center position. To studythe light collection of the different configurations as a function of the transversedistance from the electron beam to a fiber, a scan parallel to the read-out sideof the counter was performed as depicted in Fig. 4. For reference, correspondingmeasurements were performed with the scintillation counter without embeddedfibers. Three positions of the electron beam at the same distance from theread-out side were scanned as seen in Fig. 5.All SiPMs were operated at a bias voltage of V bias = 27 . V OV ∼ .
25 pC per ADC channel. The trigger signal for the dataacquisition was realized by forming the analog sum of the non-amplified signalsfrom all the SiPM channels. The ADC pedestals in the charge spectra weredetermined in separate measurements.
4. Analysis and modeling of the light yield
To convert the ADC values into the light yield expressed as an absolutenumber of photoelectrons (pe), each SiPM was exposed to short LED lightpulses, which statistically guaranteed a Poisson distributed number of photonsper pulse which was sufficiently large to be in the Gaussian limit. Consequently,the resulting charge spectra showed symmetric peaks. If one assumes that thewidth of such a peak is caused by statistical fluctuations only, it follows that7 /ˆ n = √ λ/λ = 1 / √ λ , where ˆ n is representing the position of the peak max-imum, σ the peak width, and λ being the mean and variance of the Poissondistribution for the number of pe, i.e. the light yield. Including the subtractionof the measured pedestals in the charge spectra leads to the relation: (cid:113) σ − σ ˆ n − ˆ n ped = 1 √ λ , (1)where ˆ n ped and σ ped are the position and width of a fit to the pedestal peakwith a Gaussian distribution.The conversion factors c i of calibrated ADC channels ( V bias = 27 . n calib = (ˆ n − ˆ n ped ) · κ i . The damping factors κ i needed to be included for eachone of the eight SiPMs to account for signal losses through the cable pathways.They were determined by sending a well-defined amount of charge in pulses of ahigh precision frequency generator of type 81160A from Keysights Technologiesthrough the signal pathways to the ADC. Finally, the mean number of pe fromthe charge spectra of interest is given by λ = (ˆ n peak − ˆ n ped ) · κ i /c i .Figure 6 shows a typical ADC spectrum of a single SiPM, recorded when thescintillation light was produced by the electron beam penetrating the counter.The observed asymmetric peak shape was similar in both the fiber and thereference measurements. It could be explained by an asymmetric energy-lossdistribution, or by signal pile-up with dark counts and afterpulses, especially asthe probability for afterpulses in SiPMs increases with intensity. The peak couldbe well described by a modified Gaussian distribution whose width parameter σ increased linearly above the maximum position ˆ n . For a counter without fibers, the measured light yield was approximatelyconstant for beam positions along the transverse axis: the four inner SiPMs(Ch1 and Ch2, left and right) showed a variation of less than 1 %, while thefour outer SiPMs (Ch0 and Ch3, left and right) showed a decrease or increase ofnot more than 3 %. The mean value of λ ref = (24 . ± .
3) pe was then used as a8 harge [ N u m be r o f E v en t s / ndf χ Σ Area 3.032e+02 ± dns-Param 0.0007 ± Charge [ N u m be r o f E v en t s / ndf χ Σ Area 3.032e+02 ± Position n Width σ 72 ±± 0.1 14.57 ± 0.05 dns-Param 0.0007 ± Figure 6: Typical asymmetric ADC spectrum ( (cid:98) = 0 .
25 pC) for a single SiPM recordedwhen the minimum ionizing electron beam penetrated the counter in a distance of 225 mmfrom the read-out side. A scintillation counter without embedded fibers was used and the lightyield was λ (cid:39)
25 p e . The peak could be well described by a modified Gaussian distributionwhose width parameter σ increased linearly above the maximum position ˆ n . reference value for the light yield from such a counter. These observations couldbe explained by light being produced in a thin counter that will get distributedalmost homogeneously over the volume due to the many internal reflections.The observations also motivate the following expectation for the counter withfibers: A fiber collects some of this diffuse light, so that one contribution to thelight yield from a fiber should be a constant or varying only slowly with respectto the transverse direction.The attenuation of the light along the central longitudinal axis was deter-mined by the eight measurements indicated in Fig. 5. For beam positions atdistances of more than 20 cm from the read-out side, no significant differencebetween the four channels of either side was found. The observed attenua-tion was less than 5 % / cm, being consistent with the light attenuation lengthΛ att = 260 cm provided by the manufacturer of the scintillating material [8].9
50 100 150 200 250Beam position on vertical axis [mm]1520253035404550 M e a n i n t e n s i t y [ p e ] L ( 1mm) L ( 1.5mm) L ( 2mm) L ( 1mm × 4) Wide-range distrib . (a) Left end SiPMs M e a n i n t e n s i t y [ p e ] R ( 1mm) R ( 1.5mm) R ( 2mm) R ( 1mm × 4) Wide-range distrib. (b) Right end SiPMsFigure 7: Mean intensity in units of pe for each SiPM connected to a fiber as a functionof the transverse position of the beam. The light yield was reduced by approximately 10 %when the beam was located at the grooves in the scintillation counter and was increased byapproximately 20 to 40 % when the beam was located close to a fiber. The curves show themodel description for the light yield (full line), that includes a wide distribution (dashed line).(a) Left end SiPMs. (b) Right end SiPMs. .3. Light yield from a counter with fibers Figure 7 shows the light yield from a counter with fibers as a function of thetransverse position of the beam. The uncertainties include the statistical errorsand a 2 % systematic uncertainty from the fitting and calibration procedures.For all channels, the mean intensity across the whole transverse width of thecounter showed a broad peak on top of a wide distribution, with the maximumposition of the peak at the respective fiber position. As each fiber was placedin a groove of 2 . χ for a reasonably low number of fittedparameters, except for the 1-mm fiber. The best parameter values and the χ / n . d . f . as a goodness of the fit are listed in Table 1, where the number ofdegrees of freedom ( n.d.f. = 21) equals the number of scanned beam positionsminus the number of fitted parameters. The nominal position of each fiber wasdetermined by taking the mean value of all eight extremal positions from bothends, left and right, of the counter.Within the context of the model, the light yield of the counter with fiberscan be interpreted as composed of two contributions:11 able 1: Mean intensities from the model description for each pair of SiPMs from the left andthe right side connected to a fiber in comparison with the reference value from the counterwithout fibers. Within the context of the model, the far intensity quantifies the collectionof diffuse light and the peak intensity quantifies the direct light reaching a fiber. The nearintensity is the sum of these two contributions and thereby is a measure of the light yieldwhen light is being produced in closest proximity of a fiber position. Channel Far (pe) Peak (pe) Near (pe) χ / n.d.f. Left RightCh0 ( ∅ . ± . . ± . . ± . . . ∅ . . ± . . ± . . ± . . . (cid:1) . ± . . ± . . ± . . . ∅ ×
4) 21 . ± . . ± . . ± . . . . ± . . ± .
31. One contribution to the collected light has a very weak dependence on itspoint of origin.2. Another contribution to the collected light has a strong and peaking de-pendence on its point of origin.The first contribution could be explained in analogy to the case of the counterwithout fibers. A fiber collects a certain fraction of the diffuse light, so thatthis contribution to the light yield would stay approximately constant along thetransverse length of the counter for each SiPM. The second contribution couldbe explained by light directly emitted into the solid angle covered by a fiber.This contribution increases as the position of the light production gets closer tothe fiber.As can be seen from the comparison in Table 2, the fiber with a diameterof 1 . . able 2: Contrast and relative intensity differences between the scintillation counter withfibers and the reference counter without fibers, when the correction for missing scintillatingmaterial was taken into account. The far and peak intensities are explained in the text. Thecontrast is defined as peak intensity divided by far intensity. Channel Contrast (%) ∆ Far (%) ∆ Peak (%)Ch0 ( ∅ ± − ± ± ∅ . ± ± ± (cid:1) ± − ± ± ∅ ×
4) 45 ± − ± ± − . /
20 mm / mm ≈
88 %, yielding λ ref = (21 . ± .
3) pe, see last line in Table 1. The relative difference was largestfor the 1 .
5. Conclusions
In nuclear and particle physics it is well known that the combination of WLSfibers with a SiPM readout is a viable option for the operation of a scintillation13ounter [12, 13]. This work has shown that, e.g. , a WLS fiber with a roundgeometry and a diameter of 1 . i.e. , a strong dependence ofthe light yield on the point of origin of the scintillation light, leads to complica-tions in the interpretation of the SiPM output signals. For the BDX Experimentat JLab, a detailed description of the counter geometry and the photoelectronresponse needed to be implemented in a simulation framework to account forthese complications [7]. On the other hand, the position sensitivity could have apositive effect, for instance to determine the position within the scintillator withan increased resolution when considering signal intensities of multiple SiPMs.To avoid the high contrast and other issues that surround WLS fiber con-figurations, the planned veto counters for the DarkMESA experiment will beconstructed without embedded fibers. The design of the readout board wasoptimized for this application and incorporates now nine instead of four SiPMs,thereby increasing the total light yield, improving the uniformity at the read-outends, and retaining the relative ease of construction of the veto system.14 RediT authorship contribution statementM. Lauß:
Conceptualization, Formal analysis, Investigation, Methodology,Review & Editing, Software, Visualization, Writing – Original Draft.
P. Achen-bach:
Conceptualization, Formal analysis, Funding acquisition, Investigation,Methodology, Project administration, Review & Editing, Supervision, Writ-ing – Original Draft.
S. Aulenbacher:
Review & Editing.
M. Ball:
Re-view & Editing.
I. Beltschikow:
Investigation, Review & Editing, Software.
M. Biroth:
Conceptualization, Formal analysis, Investigation, Methodology,Review & Editing & Editing, Visualization.
P. Brand:
Review & Editing.
S. Caiazza:
Review & Editing.
M. Christmann:
Conceptualization, Inves-tigation, Methodology, Review & Editing.
O. Corell:
Resources, Review &Editing.
A. Denig:
Funding acquisition, Project administration, Review &Editing.
L. Doria:
Funding acquisition, Project administration, Review &Editing.
P. Drexler:
Investigation, Review & Editing, Software.
J. Geimer:
Review & Editing.
P. G¨ulker:
Investigation, Review & Editing.
M. Kohl:
Review & Editing.
T. Kolar:
Review & Editing.
W. Lauth:
Conceptual-ization, Investigation, Resources, Review & Editing.
M. Littich:
Review &Editing.
M. Lupberger:
Review & Editing.
S. Lunkenheimer:
Review &Editing.
D. Markus:
Review & Editing.
M. Mauch:
Review & Editing.
H. Merkel:
Funding acquisition, Project administration, Resources, Review& Editing.
M. Mihoviloviˇc:
Review & Editing.
J. M¨uller:
Review & Edit-ing.
B. S. Schlimme:
Funding acquisition, Project administration, Review &Editing.
C. Sfienti:
Funding acquisition, Review & Editing.
S. ˇSirca:
Review& Editing.
S. Stengel:
Review & Editing.
C. Szyszka:
Review & Editing.
S. Vestrick:
Review & Editing.
Acknowledgments
The authors would like to thank the MAMI operators, technical staff, andthe accelerator group for their excellent work. We also thank P. L. Cole forlanguage editing the manuscript. 15his work was supported by the PRISMA + Cluster of Excellence “Pre-cision Physics, Fundamental Interactions and Structure of Matter”, and bythe Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF) with a HGF-Exzellenznetzwerk.
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