Cold PM test at Indiana final measurements, September 9 24, 2007 PM under test: Hamamatsu R7725 serial # ZK3692 (tube with Pt underlayer) Hans-Otto Meyer Indiana U this is a glimpse of the results. A detailed report is forthcoming R&D1 8/20/07 #2 Office of Nuclear Physics Cold PMTs The Motivation to operate the nEDM PMTs at 4 K is to avoid the light pipe gaps between 4 K and 50 K and between 50 K and 300 K, thus gaining up to a factor of two in photoelectrons Indiana University +HV signal cold warm R&D1 8/20/07 #3 Office of Nuclear Physics Cold PMT Evaluation First cold run, August 2 4, 2007Tube in vacuum, cooled by radiation PM under test:Maximum cooling rate ~40 K/hr Hamamatsu R7725, serial # ZK3378Tube survives the change (this is a normal tube (no Pt underlayer)) R&D1 8/20/07 #4 Office of Nuclear Physics Gain The relative gain is determined by the position of the 1 pe peak in the spectrum (normalized to 1 at room temperature). The tube was operated at 1800 V, never changed. The PM stopped working below 150 K. The gain increases by ~30% between 300 K and 200 K, in rough agreement with Hamatsus report of a temperature dependence of the anode current of 0.35 %/K between 260 and 330 K (purple line) PRELIMINARY Light pulses (10 ns) from a blue LED are transported by optical fiber with half the light is split off in front of the PM and the rest is piped back out for monitoring Measured: few-electron response. Deduced: quantum efficiency, gain, dark rate, after-pulsing rate R&D1 8/20/07 #5 Office of Nuclear Physics Quantum Efficiency The relative quantum efficiency is determined by the average number of pes, deduced from the shape of the few-electron spectrum (or, alternatively, by the counts in the pedestal), divided by the intensity of the light pulses (from the return light seen by monitor). This is normalized to 25% at room temperature. The pulse rate was constant (3.3 kHz), but the amplitude of the pulse was varied by an order of magnitude. There is no dependence on the pulse amplitude, i.e., on the current extracted from the cathode. This result is puzzling, since the PM failure at low- temperature supposedly is due to the resistivity increase of the cathode. PRELIMINARY R&D1 8/20/07 #6 Office of Nuclear Physics Dark Rate The dark rate decreases by an order of magnitude in the first 60 K below room temperature as expected, but it is in direct contradiction with the recent Yale data (arXiv:astro-ph/ v1). The number of after-pulses is roughly independent of T. The shape of the after-pulse spectrum did not reproduce earlier work and needs study. A test with a coated Pt photocathode will commence in the next two weeks. PRELIMINARY 3+ days at 77 K 2+ days at 4 K wall of container glass near photocathode temperature history T = 8.8 K HV = 1970 V ~ 1.0 pe/pulse 1pe 2pe (0.25 pC/channel) Pulse height spectrum at T = 8.8 K the quantum efficiency decreases by about 10%, i.e., is affected only weakly. the gain decreases by about a factor of 0.7. This loss of gain may be recovered by raising the tube voltage by about 150 V. the dark rate (~300 Hz for this tube) initially decreases like thermionic emission, reaches a minimum near 200 K, then rises monotonically to ~100 Hz at 4 K. the number of afterpulses is proportional to the light intensity, increases strongly with tube voltage, and is practically independent of temperature (no adsorption of residual gas). Most afterpulses occur within 4 s of the primary event. Increasing the tube voltage shifts their time distribution towards smaller times. the cold (~8 K) PM has been stored for 12 hours in a dense He atmosphere. The afterpulse spectrum of the warm tube before and after this test showed no change. Thus, there is no evidence against exposing a cold PM to He. When cooling the modified PM from room temperature to liquid- helium temperature