Performance of the MCP-PMT for the Belle II TOP counter Kodai Matsuoka (KMI, Nagoya Univ.) S. Hirose, T. Iijima, K. Inami, Y. Kato, Y. Maeda, R. Mizuno, Y. Sato, K. Suzuki (Nagoya Univ.)
TOP (Time Of Propagation) counter 2 A novel ring imaging Cherenkov detector A key device of PID in Belle II to extend the physics reach toward New Physics. TOP 2 cm K or p TOF (~1 m) e + Mirror Cherenkov photons generated in the quartz bar travel in the bar as they are totally reflected on the quartz/air boundaries. p K C Air (n=1) Quartz (n=1.47) @ 400 nm 1 TOP cos C e n Measure (TOF + TOP) with a resolution better than 50 ps for single photon detection. (p efficiency > 95% and K fake rate < 5% for < 3 GeV/c) Air (n=1)
400 mm MCP-PMT (Micro Channel Plate PMT) 3 Square shape multi-anode MCP-PMT with a large photocoverage Developed at Nagoya in collaboration with HAMAMATSU Photonics K.K. e Photon (Cross-section) Photocathode (NaKSbCs) MCP x 2 4 x 4 anodes 23 mm 13 10 mm 5.275 mm Micro channel e ~1 kv / 400 mm 10 mv 1 ns 1.9 x 10 6 gain Fast signal KT0117 ch0 2480 V The best time resolution of photon sensors
Mass production and testing 4 Belle II TOP uses 512 MCP-PMTs in total. Mass production started in March 2011 and finished in March 2014. The first half of the PMTs uses a conventional MCP. The latter uses an ALD (Atomic Layer Deposition) MCP to extend the lifetime of the photocathode. Both of them are installed in Belle II. The following performances were measured for all the 16 channels of every PMT: Quantum efficiency (QE) Gain Transit time spread (TTS) Relative collection efficiency (CE) All the measurements are fully automated and the performances can be systematically studied.
QE measurement setup Measure the photocathode current with a picoammeter: I QE MCP = MCP IPD QE PD A 200 V 5 Photocathode MCP1 MCP2 MCP-PMT Light spot < 1 mm f Photodiode Slit Variable ND filter Sharp cut filters Monochromator Xe lamp Moving stage
QE measurement 6 Scan the photocathode at 18 x 18 points x 20 l. QE at l = 360 nm JT0629_20130320 Photocathode QE peaks around 360 nm
QE 7 On average, 28.1% of 283 conventional-mcp-pmts and 29.1% of 231 ALD-MCP-PMTs at 360 nm. (Requirement: 28%) 360 nm Conventional ALD Peak QE at 420 nm
Laser measurement setup 8 Single photon irradiation to each channel one by one. Dark box Reference PMT Moving stage ND filters MCP-PMT Fiber Slit Slit Light spot 1 mm f Laser MCP-PMT Pico-second pulse laser (l = 400 nm) Variable amp +19.5~35 db ATT Amp 10 db +33 db Discriminator Threshold: 20 mv ADC TDC
Gain measurement 9 Define the gain as the mean of the output charge distribution. ALD-MCP-PMT KT0449_20140717 ch4 2750 V 2650 V 2550 V gain = exp(a HV + b)
SE yield Gain 10 HV for 1 x 10 6 gain Conventional ALD Conventional ALD ~2 Rough drawing ALD Conventional ~65 ~85 ~200 HV (V) The ALD-MCP has a large gain at a lower HV or a sharper gain slope than the conventional one Higher secondary electron yield
TTS measurement 11 Fit double Gaussian to the TDC distribution after time-walk correction. Define the TTS as s of the primary Gaussian. gain (x10 6 ) 0.5 1.0 2.0 JT0886_20150302 ch5 3340 V Photo electron recoil on the MCP1 surface Photocathode MCP1 MCP2 TTS does not depend on HV.
TTS TTS less than 50 ps for every PMT 12 Requirement for the Belle II TOP counter
Relative CE measurement 13 Count the number of TDC hits. Correct the laser intensity variation with the reference PMT. Conventional ALD Normalized to the number of incident photons ALD Conventional (same gain of 2 x 10 6 ) JT0763_20140626 ch6 3460 V KT0162_20140612 ch6 2550 V Higher CE of the ALD-MCP by ~15% than the conventional one. Increase of CE for the recoil photo electrons due to a higher secondary electron yield of the ALD-MCP
Lifetime (C/cm 2 ) Aging of the photocathode 14 Gas out of the MCPs damages the photocathode. QE drop The amount of outgassing depends on the output charge. Define the lifetime as the total output charge where QE decreases to 80%. 10 1 0.1 Conventional MCP Added ceramic block ALD MCP Introduce some methods of process Belle II beam bkgd MC (5 x 10 5 gain, 50 ab 1 ) 0.01 Square shape with Al layer Mass production 2011 2013 2015 year Tried six methods of process to improve the lifetime.
Lifetime measurement setup 15 Tested several samples of each method. Load the output charge of the MCP-PMTs by the LED. The output charge is measured by a CAMAC ADC. Monitor the hit rate ( QE) by the laser single photons. Pulse laser (400 nm) MCP-PMTs LED (100 khz) Reference PMT
Extended lifetime Method A Method B 16 YH0148 YH0149 YH0160 YH0163 YH0168 YH0170 YH0171 YH0173 Method A+B+C YH0203 YH0205 YH0206 Three methods of process were found to be promising. 20 C/cm 2 or longer lifetime can be expected with Method A+B+C.
Summary 17 We succeeded in development and mass production of the MCP-PMT for the Belle II TOP counter. We measured the performance of all the MCP-PMTs. QE: >28% (l = 360 nm) on average Gain as a function of HV TTS: ~30 ps above 5 x 10 5 gain Higher CE of the ALD-MCP by ~15% than the conventional one Those meet our requirements for the TOP counter The lifetime of the ALD-MCP-PMT can be extended by applying the new methods of process. Expected lifetime: >20 C/cm 2 (fully survive in Belle II environment) The conventional-mcp-pmts will be replaced with the lifeextended ALD-MCP-PMTs after a few years of Belle II operation.