Suppression of crosstalk in SiPMs for the next generation gamma-ray observatory, CTA

Suppression of crosstalk in SiPMs for the next generation gamma-ray observatory, CTA 16aS41-3 Hiro Tajima, Akira Okumura, Yuki Nakamura, Anatolii Zenin, Nagoya University (for the CTA-Japan Consortium) Japan Physical Society Meeting Shinshu University, September 16, 218

Cherenkov Telescope Array Observations of gamma rays in 2 GeV 3 TeV band Cherenkov light from electromagnetic shower produced by interaction of gamma rays with atmosphere Large collection area by placing many telescopes 1 better sensitivity than current instruments Wide energy band coverage by three different sizes of telescopes Large-sized telescope (LST): Φ = 23 m, 2 GeV 1 TeV, 4 telescopes Medium-sized telescope (MST): Φ = 1 12 m,.1 1 TeV, ~2 telescopes Small-sized telescope (SST): Φ = 4 m, 1 >3 TeV, 5 7 telescopes all SSTs are placed at south site LST 23 m MST 1 12 m SST 1M ASTRI GCT G. Peŕez, IAC, SMM 2/14

Requirements for Photon Sensors in CTA Properties of Cherenkov photons from gamma-ray air shower ~5 photons/m 2 for 1 TeV gamma-ray shower Several photons per pixel Cherenkov photons peaks around ~35 nm Blue to near UV sensitivity is important Cherenkov photons arrives within a few to few tens of ns ns-timing is important Night sky background (NSB) is the dominant background Rate is >25 MHz/pixel Dark count rate is not important [NSB] x [Optical crosstalk (OCT)] can cause false triggers due to accidental coincidences Low OCT rate is important NSB peaks above 55 nm Low red sensitivity is preferred Cherenkov spectrum NSB spectrum 3 4 5 6 7 3/14

Photon Sensor Silicon Photomultiplier is chosen as a photon sensor for SST Cost per channel Photon detection efficiency Tolerance against high rate environment (> 25 MHz per pixel) Reliability Major drawback of SiPM Optical crosstalk (OCT) High rate night sky background (NSB) + OCT can cause false triggers due to accidental coincidences Gain dependence on the temperature High sensitivities for red light (NSB wavelength) Main objective of CTA SiPM development Suppress OCT while retaining photon detection efficiency (PDE) credit: HPK website credit: KETEK website 4/14

Summary of Past Studies Thicker coating or no coating give lower crosstalk 25 Optical Crosstalk Rate (%) 2 15 1 LCT5-35 Epoxy 1 µm #1 LCT5-35 Epoxy 1 µm #2 LCT5-35 Epoxy 3 µm #665 LCT5-35 Epoxy 3 µm #666 LCT5-35 Silicone 45 µm #965 LCT5-35 Silicone 45 µm #966 LVR2-65 No coating #15 LVR2-65 No coating #14 LVR2-75 No coating #11 1 µm 3 µm 45 µm No coating 5 2 4 6 8 1 Over Voltage (V) 5/14

Propagation of Crosstalk Photons No coating (or very thin coating) Reflected photons come back to the original cell Intermediate thickness Photons reflected by the air interface may produce avalanches in other cells Very thick coating Photons reflected by the air interface may get out of the device Smaller device may have lower crosstalk rate How about the crosstalk from the backside? 6/14

OCT Dependence on Device/Cell Sizes We have systematically investigated the OCT rate with varying device size, cell size, and with and without coating OCT rate is expected to be proportional to the number of electron-holes pairs (=charge) produced in an avalanche proportional to a product of [cell capacitance] and [over voltage] Find out propagation properties of crosstalk photons Product ID Device size Cell size Coating Fill factor S1452-35VS 3 mm 5 µm 3 µm 74% S1452-35VN 3 mm 5 µm None 74% S1452-375VS 3 mm 75 µm 3 µm 82% S1452-375VN 3 mm 75 µm None 82% S1452-65VS 6 mm 5 µm 3 µm 74% S1452-65VN 6 mm 5 µm None 74% S1452-675VS 6 mm 75 µm 3 µm 82% S1452-675VN 6 mm 75 µm None 82% 7/14

SiPM Measurement Setup at Nagoya Take waveform data by digital oscilloscope 8 Offline data analysis 7 Digital filter to minimize 6 the effect of pile ups 5 Pulse analysis 4 Light output is monitored 3 2 Wavelength is fixed at 45 nm 1 for this measurement V (mv) Raw data Filtered data Pulse Generator Tektronix AFG 3251 Oscilloscope Tektronix MSO454B 1 5 1 15 2 25 3 35 4 45 5 t (ns) LED ND filter fiber diffuser SiPM (Monitor) ZFL-5LN-S+ collimator SiPM (DUI or Reference) climate chamber (25 C) 8/14

We measure number of photons for short LED (or laser) pulses Current measurement does not provide accurate PDE due to optical crosstalk, delayed cross talk and after pulse Number of photo electrons (p.e.) does not follow Poisson distribution due to optical crosstalk Probability of p.e. is used to obtain the average to avoid effects of optical crosstalk Effect of dark count still need to be taken into account P (n) =e µ µ n /n! P () = e µ µ = ln(p ()) P true () = P ON ()/P OFF () PDE Measurements p.e. 1 p.e. 2 p.e. 3 p.e. 9/14

PDE vs Relative Over Voltage PDEs were measured for 2 devices for each type PDEs were measured twice for one device Measured PDEs were very consistent, which indicates varying light intensity is properly compensated by the monitor SiPM Average p.e..36.35.34.33.32.31 Averege p.e. for Monitor SiPM (Photon Detection Efficiency)/(Fill Factor) (%) 6 5 4 3 2 1 3 mm 6 mm 5 µm 75 µm no coating.3 2 4 6 8 1 12 14 16 18 2 22 Measurement number.5.1.15.2.25 (Over Voltage)/(Breakdown Voltage) 1/14

<latexit sha1_base64="ctkujlq36imz8hpcthlvx+ecqb4=">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</latexit> <latexit sha1_base64="ctkujlq36imz8hpcthlvx+ecqb4=">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</latexit> <latexit sha1_base64="ctkujlq36imz8hpcthlvx+ecqb4=">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</latexit> <latexit sha1_base64="ctkujlq36imz8hpcthlvx+ecqb4=">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</latexit> Optical Crosstalk Measurements Assume 1 p.e. peak of dark signal is dominated by dark count 2 p.e. peak consists of optical crosstalk from 1 p.e. and chance coincidence of dark counts within tps (~3 ns in our setup) N( 1.5 p.e.) N(.5 p.e.) =1 (1 R OCT)e f DC t PS N( 1.5 p.e.) R OCT =1 1 /e f DC t PS N(.5 p.e.) ƒdc: Dark count rate tps: Double-pulse separation time Probability to have no optical crosstalk Poisson probability to have no overlapping dark count within tps N(.5 p.e.) N( 1.5 p.e.) p.e. 1 p.e. 2 p.e. 3 p.e. 11/14

Optical Crosstalk Rate (%) Crosstalk Rate vs. Over Voltage Dark count pileup correction works well Factor out cell capacitance dependence of crosstalk rate by scaling it with cell area and depth (assuming cell depth break down voltage) 3 mm device gives slightly lower OCT than 6 mm device OCT rate scales very well with cell capacitance with coating Not so without coating Differences among individual SiPMs are small 45 Before DCR pileup correction 4 35 3 25 Optical Crosstalk Rate (%) 4 35 3 25 After DCR pileup correction 3 mm 6 mm 5 µm 75 µm no coating 2 15 1 5 2 4 6 8 1 Over Voltage (V) 2 15 1 5 2 4 6 8 1 Over Voltage (V) 12/14

Optical Crosstalk Rate (%) Crosstalk Rate vs. Over Voltage Dark count pileup correction works well Factor out cell capacitance dependence of crosstalk rate by scaling it with cell area and depth 4 35 3 25 2 15 1 (assuming cell depth break down voltage) 3 mm device gives slightly lower OCT than 6 mm device OCT rate scales very well with cell capacitance with coating Not so without coating Differences among individual SiPMs are small Before scaling by cell capacitance Scaled Optical Crosstalk Rate (%) 35 3 25 2 15 1 After scaling by cell capacitance 3 mm 6 mm 5 µm 75 µm no coating 5 2 4 6 8 1 Over Voltage (V) 5 2 4 6 8 1 Over Voltage (V) 12/14

<latexit sha1_base64="ol/4xzgnvxoahhwxecao1ya224k=">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</latexit> <latexit sha1_base64="ol/4xzgnvxoahhwxecao1ya224k=">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</latexit> <latexit sha1_base64="ol/4xzgnvxoahhwxecao1ya224k=">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</latexit> <latexit sha1_base64="ol/4xzgnvxoahhwxecao1ya224k=">aaacn3icbvfdb9mwfhxc1ygmuveid4ykab9wkmkasahpwiq2cctkthabmla57s1mzykj+wzrovwtfghv/buctjcycsvlr+ccx1+fg+dsgps8x4574+at23dw7rbu3v998lc9tj4yqtachlxjpu9izkckdiyoumjjroglsytj+cko9eovoi1q2rhocohsdpajrhcglpqfxxowpthuu7lg+cooi/eadbhhyqspuhes+kdn8mjkpsva+pbvbrrnyhqvdl89xob5vdw4rorfssoxgtotouvxhct3y8eby8fbghanib3alepn3x+t686hxglghlgswaf8mp4oxkwtijtnm7hs5rixtklieqhuwbnlgl9gzjc3mwaomkuf5vvs5zayudqedomc/fdgyvjjzmlsnfwq5qpwk//txgumb6jszhmbkphfqkhkspal4tohqaocmyb41rywsk/z9rgb1fasih4v798hyw2+77x9z9vdxa2l3gskmfkgeksn7wmo2sfdmiqcoejs+t8cd66t9995m7wfhdz3nnewmue/ob5w7oqg==</latexit> Fit to Otte Function Optical crosstalk rate should proportional to the charge produced in the avalanche and avalanche trigger probability R OCT = C OCT (Fill factor) (Cell size) 2 /V BR V OV (1 exp[ C OtteV OV /V BR ]) COCT is smaller for 75 µm cells (smaller crosstalk efficiency) COtte is smaller without coating Avalanche seed is produced in the region where it is harder to trigger avalanche Product ID COCT COtte S1452-35VS S1452-65VS S1452-375VS S1452-675VS S1452-35VN S1452-65VN S1452-375VN S1452-675VN.1 5.9 1.6 18.7 17.9 2.7 3.3 5.3 5 cell capacitor Optical Crosstalk Rate (%) 4 35 3 25 2 15 1 5 3 mm 6 mm 5 µm 75 µm no coating Avalanche probability PDE saturating voltage pile up rate due to 25 MHz night sky background 2 4 6 8 1 Over Voltage (V) 13/14

Summary Optical crosstalk rate is significantly affected by protection coating Smaller device size and thicker coating reduce OCT rate Larger cell size increase OCT rate, but not proportional to the cell area No coating significantly reduces OCT rate OCT rate does not scale with the cell size OCT seeds are produced in the region where avalanche trigger probability is low Prospects 6 mm device with 75 µm cell without coating may be the best choice for CTA Without protective coating, damages on the SiPM surface are of major concerns We put a piece of protective film to avoid damages during the assembly handling Damages during observations can be avoided by the UV transparent shield on the camera front This shield will also act as a filter for red lights.) Further reduction of OCT by suppressing crosstalk due to photons reflected at the backside of SiPM Photon Detection Efficiency (%) 5 4 3 2 1 3 mm 6 mm 5 µm 75 µm no coating 5 1 15 2 25 3 35 4 Optical Crosstalk Rate (%) 14/14