1. Test of Silicon Photomultipliers (SiPM) at Liquid Nitrogen Temperature Yura Efremenko, Vince Cianciolo nEDM CalTech Meeting 02/14/2007

2. SiPM – Principle of Operation Topology Electric field distribution in epitaxial layer The pixel size – 32x32 μm 2 Total number of pixels – 576 for 1x1 mm photodetector Real topology is patented and different from it

3. SiPM We use standard 42V green sensitive 1_mm SiPM without any extra modification for low temperature operation SiPM was connected to the readout electronics via following preamplifier SiPM 1mm

4. Experimental Setup Cryostat with SiPM Blue LED Optical fiber to SiPM Control Computer CAMAC DAQ Signal from SiPM Pulse generator Note: the LED and the SiPM wavelengths were not optimized, but we were looking for a relative measurement.

5. Protocol All equipment was setup and debugged at room temperature Liquid nitrogen (-195 0 C) was added into cryostat SiPM actually was not inside liquid nitrogen but in its vapor With remote probe it was verified that SiPM temperature is actually at –195 0 C System let alone for extended period of time After a day all nitrogen boiled out and temperature stat to come back to the room temperature.

6. Result for Light sensitivity Horizontal axes on both plots are signal amplitude in.25 pC. Upper: -195 O C Lower: room temperature One can see strong improvement in both light sensitivity and single photon detection resolution at colder temperatures On upper plot (-195 O C) up to 11 individual photons can be seen The mean increases twice as much as the gain, suggesting an increase in the quantum efficiency of two times. Gain increases by x1.8 Room temperature Liquid Nitrogen Mean increases by x3.6

7. Transition from Cold to Room Temperature Horizontal axis time, hours. Upper plot: dark rate Hz Single photon noise rate ~ 10 kHz. Lower plot: signal charge, in 0.25 pC. One can clearly see that noise was lower and gain was higher at low temperature. Between 15 and 23 hour, SiPM was illuminated by ambient light to test its recovery back to single photon regime after exposure to strong light. Its performed as expected

8. One possibility for nEDM Light Readout w/SiPMs Inside of mst. cell TPB impregnated acrylic WLS fibers Tyvek coating Front View Top View WLS fibers Clear fibers out to SiPMs

9. Estimate of #photons Initially ~ 8350 prompt EUV photons from neutron capture. –5470/250 KeV β (764KeV/250 KeV) * 50% [1] dTPB conversion efficiency (in matrix): 30% Blue-to-Green conversion efficiency in WLS fibers ~35% –Assume 8% of area covered w/ WLS fibers, 88% reflectivity for TYVEK –8% is ~ four 1mm fiber loops/side (32 channels/cell) –Could gain by adding more fiber coverage with resulting increase in # of channels. Capture inside WLS fibers: 7% –Readout both ends. Attenuation in WLS fibers: 85% –3m attenuation length, assume 25 cm average length + few % loss at clear/WLS fiber joint. Sensor efficiency: 70% –Guesstimate based on room temperature quantum efficiency of 35% and observed x2 increase at LN2 temperature. Total # photons/event: 37 Paul H. suggested the possibility of multi-clad fibers coated w/ TPB inside the acrylic. –Could gain ~ x4 in TPB efficiency. –Activation issues? [1] D.N. McKinsey et al., NIM A516 (2004) 475. Realistic Pessimistic Optimistic

10. Other Advantages Very small power consumption Small service penetration requirements –Exactly how much depends on location of preamplifiers. Not expected to be affected by magnetic fields. –Should be tested.

11. Conclusion & To Do SiPM performed extremely well ant liquid nitrogen temperature. According to all tested parameters its performance did not show any degradation but rather improvement. Future tests at even lower temperatures are possible. Assembling a setup to make absolute measurement of quantum efficiency.