Liquid Xenon Scintillation Detector with UV-SiPM Readout for MEG Upgrade

Liquid Xenon Scintillation Detector with UV-SiPM Readout for MEG Upgrade W. Ootani on behalf of MEG collaboration (ICEPP, Univ. of Tokyo) 13th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD2013) Oct. 7-10 2013 Siena, Italy 1

Contents MEG Experiment and Its Upgrade Plan Concept of LXe Detector Upgrade Development of UV-enhanced MPPC Expected Detector Performance Summary and Prospects 2

MEG Experiment MEG searches for lepton flavor violating decay µ + e + γ as an unambiguous evidence of BSM physics, by utilizing World s most intense µ + beam at Paul Scherrer Institute (PSI) (3 10 7 µ + /sec for MEG) 900l LXe γ-ray detector with PMT readout Positron spectrometer (low-mass drift chamber system +fast timing counter in gradient B-field) MEG Detector 3

MEG Experiment MEG set the most stringent upper limit on µ eγ decay rate in Mar. 2013. (Phys. Rev. Lett. 110, 201801(2013)) B <5.7 10-13 (90% C.L.) with 2009-2011 dataset Already in the branching ratio range predicted by many BSM physics. Could be just around the corner... MEG has finished DAQ in end-aug 2013. 50% of full data still to be analyzed. Normalization-factor/10 12 5 4 3 2 1 0 Data statistics Published Expected 2008 2009 2010 2011 2012 2013 4

MEG Upgrade MEG 1. 5. 6. 2. 7. 4. MEG 7. 5 3. Sensitivity goal: ~5 10-14 ( 10 improvement over current MEG goal) MEG upgrade plan approved at PSI in Jan. 2013. (arxiv 1301.7225) Upgrade items Higher µ intensity (1) pgraded MEG 6. 7. MEG Upgrade 7. Thinner target /active target (2) Poster by E. Ripiccini LXe with SiPM readout (6) 1. 5. 2. 4. Upgrade timeline 3. Drift chamber with stereo angle wire configuration (3) Pixelated timing counter with SiPM readout (5) Talk by M. De Gerone (Thursday) Active BG suppression (optional) 5

Present MEG LXe γ-detector World s largest 900l-LXe scintillation detector to measure 52.8MeV-γ from µ eγ Scintillation light (VUV λ=175±5nm) collected by 846 UV-sensitive PMTs immersed in LXe Hamamatsu R9869 Operational in LXe (T=165K, P<0.3MPa) Synthetic quartz window Photocathode: K-Cs-Sb (QE~15% at 165K) Limited resolutions for events near γ- entrance face due to non-uniform PMT coverage UV-sensitive PMT 52.8MeV! 6

Concept of Upgrade Upgrade to overcome weak points of present detector Highly granular readout of LXe scintillation light Replace 216 PMTs (2-inch) on γ-entrance face with ~4000 smaller photo-sensors (SiPM ~12 12mm 2 ). Improve response at acceptance edge Extend γ-entrance face along z ( beam) reduce energy leakage Modify PMT layout at lateral faces uniform response 7

Concept of Upgrade Present Upgrade Upgrade to overcome weak points of present detector Highly granular readout of LXe scintillation light Replace 216 PMTs (2-inch) on γ-entrance face with ~4000 smaller photo-sensors (SiPM ~12 12mm2). Improve response at acceptance edge Extend γ-entrance face along z ( beam) reduce energy leakage Modify PMT layout at lateral faces uniform response 7

UV-enhanced MPPC Commercial SiPM is NOT sensitive to LXe scintillation light in VUV range (~175nm). UV-enhanced MPPC is under development in collaboration with Hamamatsu Photonics. Requirements Photon detection efficiency (PDE) (>10%) Large sensitive area (~12 12mm 2 ) Single photon counting capability Fast signal (fall time < 50ns) Improving sensitivity to VUV light Remove protection coating Thinner contact layer Protection coating Contact layer Optimize optical matching bw/ LXe and Si (refractive index, AR coating) p- n++ Deep UV photon E field 8

UV-MPPC Performance Development of UV-MPPC is in good shape. LXe scintillation light is successfully detected by prototypes of UV-enhanced MPPC. Best prototypes already show PDE~15% and gain>5 10 5, which more or less fulfill our requirement. PDE Gain 1000 10 900 800 700 600 500 400 300 200 100 3 Gain 50μm pixel, x2 segmented sensor 50μm pixel, x4 segmented sensor 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Over voltage [V] 9

UV-MPPC Performance Full size prototype successfully tested in LXe. Active area: 12 12mm 2 ( 3 3mm 2 for commercial MPPC) 50µm pixel pitch, 57600 pixels. Single photoelectron peak is clearly resolved. Dark count rate is quite low (~750Hz) at LXe temp. World s largest VUV-sensitive SiPM with single photon counting capability! Long signal tail (~200ns) due to large sensor capacitance would be an issue ( pileup in high rate environment). pedestal Scintillation signal from 5.5MeV alpha 1 p.e. 12mm 12mm 2 p.e. 3 p.e. 100ns 10

Sensor Capacitance Issue Large sensor capacitance due to large sensor area (~12 12mm 2 ) causes long signal tail (~200ns). Solution Sensor is segmented and all segments are connected in series to reduce overall capacitance. Drawback: reduced gain Series connection of multiple SiPMs has been proven to be useful in PSI µsr detector and new timing counter developed for MEG upgrade A. Stoykov et al., NIMA 695(2012)202 W. Ootani et al., DOI 10.1016/j.nima.2013.07.043 MPPC Sensor segmentation ( 4) PCB 6mm 12mm Coaxial cable +HV 11

Sensor Capacitance Issue Tested the scheme using 4 MPPCs (6 6mm 2 each) 4 segmented: all 4 MPPCs connected in series 2 segmented: Two sets of two MPPCs connected in parallel are connected in series. Signal fall time reduced down to 30-50ns! Still reasonably high gain (>5 10 5 ) Single photoelectron signal 100ns 2 segmented Nonsegmented 100ns 2 segmented 4 segmented 4 segmented Fall time 200ns 45ns 25ns Gain 1000 10 900 800 700 600 500 400 300 200 100 3 x2 segmented x4 segmented 0 0 0.5 1 1.5 2 2.5 3 3.5 4 Over voltage [V] Better single photoelectron resolution ( 4 segmented) 12

Series Connection Two options for series connection Simple Coaxial cable Bias Gain uniformity Potential diff. bw/ adjacent segments External circuit Simple 280V ( 4 segmented) Automatic gain equalization Hybrid 70V (common) Required ~70V 0V No Required Both work! But hybrid is more advantageous. MPPC PCB +HV 6mm 12mm Hybrid (signal: series, bias: parallel) Coaxial cable MPPC PCB +HV 13

MPPC with New Technologies Recent improvements in Hamamatsu MPPC technology (T.Nagano et al. @IEEE2012) Metal quench resistor smaller temp. coeff. than that of poly-si Much less after-pulsing less pileup Operational at higher over-voltage higher PDE/gain We tested UV-MPPC prototype with new technologies in LXe. Confirmed large suppression of after-pulsing Resistance increase from room temp. to LXe temp ~20% as expected. (~100% increase in case of old MPPC with poly-si quench resistor) Prob#(crosstalk#+#a6erpulse) 60%# 50%# 40%# 30%# 20%# 10%# 0%# Reduc&on)of)A,er)pulse) Old MPPC New MPPC new#technology##1# new#technology##2# previous#3mm# previous#12mm##1# previous#12mm##2# 0# 1# 2# 3# 4# Over#voltage#[V] 14

Signal Transmission Cryostat MPPC MPPC signals in the final detector are supposed to be transmitted over long coaxial cable (~12m) without any amplification. Effect of long cable is measured. No significant deterioration of signal observed. Planned electronics chain Vacuum feedthru PCB coax cable (3-5m) coax cable (7m) DAQ board Decay Constant [s] 60 50 40 30 20 Gain Cable Length [m] -9 10 Signal fall time x2 segmented x4 segmented 10 0 0 2 4 6 8 10 12 Cable Length [m] 15

Vacuum Feedthrough PCB-based vacuum feedthrough is under development. PCB with coaxial-like signal line structure 50Ω impedance, good shielding, high bandwidth, small crosstalk (<0.3%) High density 72ch in each PCB 6 PCBs on each flange (DN160) 10 flanges in total 16

Readout Electronics All LXe channels (4000 MPPCs, 600 PMTs) will be readout by waveform digitizer (DRS) on a newly designed DAQ board (WaveDREAM, PSI in-house developed) High density and compact Bias voltage circuit for MPPC included Two selectable gains Improved inter-channel time jitter (~5ps) x0.1 x0.1 HV HV x1... x10 (resistor) Buffer... 16 x... x1... x10 (resistor) Splitter previous channel DAC calibration x10 Buffer Splitter x10 DAC calibration DAC Buffer PLL (LMK) Buffer DAC C Comp DRS4 DRS4 Comp C Trigger RAM Trigger ADC ADC 32 FPGA SPI +5V +3.3V +2.5V +1.2V -2.5V -5V PHY DC power on PROM Temp DAC DC PoE 24 V RJ45 BUS DC in 24 V 70 V to piggy back 70 V 2x2 LVDS Crosspoint Switch CLK IN CLK OUT 17

Expected Detector Performance Energy resolution Uniform coverage with MPPCs events near entrance face Modified PMT layout deep events Low energy tail reduced because of smaller energy leakage Position resolution Higher granularity with MPPC Efficiency Upgrade (MC) Present 10% improvement (MPPC is much thinner than PMT) depth 2cm Upgrade (MC) Present depth 2cm Present Upgrade (MC) 18

Expected Detector Performance Present Upgrade Energy (%) (depth<2cm / depgh 2cm) Position (mm) (u/v/w) Timing (ps) Efficiency (%) resolutions in sigma 2.4/1.7 1.1/1.0 * 5/5/6 2.6/2.2/5 67 76 ** 63 69 * 0.7% fluctuation added to MC σ ** Preliminary estimate 19

Summary and Prospects Significant performance improvement of LXe detector is expected with highly granular readout using MPPCs. Development of UV-sensitive MPPC in collaboration with Hamamatsu is in good shape. Prototypes already show promising performance. Large sensor capacitance issue solved with segmented sensor scheme. Prospects Construction of prototype detector with ~600 UV- MPPCs and 70l-LXe to perform beam test in 2014 Mass production of UV-MPPC for full-scale detector in late 2014 Commissioning in 2015 preparing for startup of MEG upgrade in 2016 20

Thank You for Your Attention! MEG collaboration ~60 physicists from 12 institutes from 5 countries 21

Backup 22

Radiation Hardness Neutron γ Modest radiation hardness is a kind of weak point of SiPM (MPPC). Possible effects Increase of dark noise Gain degradation Expected radiation in MEG upgrade MEG upgrade (3 years) Threshold 7 10 7 n/cm 2 10 9 n/cm 2 0.3Gy 200Gy Radiation hardness of MPPC should NOT be an issue in MEG upgrade. 23

MPPC Package and Assembly Package design of MPPC Sensor chip mounted on ceramic base Thin quartz window for protection Assembly Sensor is plugged in socket pins on assembly PCB. (44 MPPCs on each PCB strip (~15 800mm 2 )) Assembly 93 PCB strips assembled on inner wall of cryostat PCB has coaxial-like signal line structure Package design PCB MPPC PCB cross section Quartz window Ceramic Sensor chip 24

MEG Upgrade Upgraded MEG is expected to search for µ eγ down to B~5 10-14 in three years! 10 improvement w.r.t. current MEG More details in arxiv:1301.7225 Expected performance Branching ratio 10-12 Projected sensitivity 90% C.L. MEG 2011 90% C.L. MEG 2013 5σ Discovery 3σ Discovery 90% C.L. Exclusion -13 10 Upgraded MEG in 3 years MEG upgrade timeline 10-14 0 20 40 60 80 100 weeks 25

What Is SiPM? Silicon photomultiplier (SiPM) is a silicon-based new photosensor with multi-pixelated Geiger-mode avalanche photodiode. Many variants (SiPM, MPPC (HPK), gapd, MRS-APD,...) Each pixel operated in Geiger-mode above breakdown voltage works as a binary device. Sum of all fired pixel outputs is proportional to number of impinging photons Already largely used in many other experiments as replacement of PMT technology 1pe 2pe 3pe 4pe 26

SiPM in LXe Detector Pros Scintillation readout with higher granularity and uniformity Reduction of material on γ-entrance face (much thinner than PMT) Operational in high B-field Easier gain calibration using single photoelectron signals Low bias voltage (<100V) Low power consumption (crucial for operation at low temp.) High suppression of dark count at low temp. Cons Low photon detection efficiency for VUV light Too small sensor size Temperature dependence Correlated noise (optical crosstalk, after-pulsing) Radiation hardness Non-linear response 27

Temperature Dependence Temperature dependence of MPPC properties Dark count is highly suppressed at LXe temperature (165K). Resistance of quench resistor (poly-silicon) increased at LXe temp. ( 2) Temperature coeff. of breakdown voltage LXe temp stability <0.15K gain variation <0.3% gain+pde variation <0.6% (~ expected best energy resolution) dark count rate [Hz] 7 10 6 10 5 10 4 10 3 10 2 10 10 MPPC Dark Count Rate room temperature pulse height [mv] Temperature dependence of MPPC gain 6 5 10-5 4 205K 3 Slope~2%/K 165K 2 1 56.2V 56.1V 56.0V 55.9V 1 0 0.5 1 1.5 2 over voltage [V] 0 165 170 175 temperature [K] 28

Cryogenics Heat load in upgrade detector Only one cable necessary for each MPPC ( two cables for PMT (HV and signal)) Extremely low power consumption of MPPC (~5mW for 4000ch) Only 20W increase in heat load compared to present detector It can be covered by more powerful refrigerator or dual refrigerator Power consumption Cable Total Photosensor-related heat load Present 40W 50W 90W Upgrade 30W 80W 110W 29

Time Resolution The same time resolution is basically expected in upgrade detector. However effect of the electronic noise could be enhanced by the longer rise time of MPPC signal for LXe scintillation. Possible solutions Optimize MPPC parameters for faster rise time (quench resistance, pixel size) Part of MPPC channels dedicated for timing measurement with higher gain (one in every ~10 MPPCs) Time Resolution [ps] 100 90 80 70 60 50 40 30 20 Preliminary 10 0 0 2 4 6 8 10 Depth [cm] 30

Higher Beam Intensity BG-γ spectra were measured with present LXe detector at different beam intensities. No effect up to 8 10 7 µ/s after pileup elimination (7 10 7 µ/s planned for upgrade) 31