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.)
2 TOP (Time Of Propagation) counter A novel “ ring imaging ” Cherenkov detector A key device of PID in Belle II to extend the physics reach toward New Physics. Cherenkov photons K or p generated in the quartz bar travel in the bar as they are totally reflected on the Mirror quartz/air boundaries. TOP Air (n=1) TOF (~1 m) p C Quartz (n=1.47) K 2 cm e + @ 400 nm Air (n=1) 1 TOP cos e – C 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)
3 MCP-PMT (Micro Channel Plate PMT) Square shape multi-anode MCP-PMT with a large photocoverage Developed at Nagoya in collaboration with HAMAMATSU Photonics K.K. Photon (Cross-section) Photocathode (NaKSbCs) MCP x 2 e – 4 x 4 anodes 23 mm 1 ns Micro channel 5.275 mm e – ~1 kV / 400 m m 10 mV Fast signal 400 m m 1.9 x 10 6 gain KT0117 ch0 2480 V 13 ° 10 m m The best time resolution of photon sensors
4 Mass production and testing 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.
5 QE measurement setup Measure the photocathode current with a picoammeter: Photocathode A MCP1 𝐽 𝑁𝐷𝑄 𝐽 𝑄𝐸 ∙ 𝑅𝐹 𝑄𝐸 𝑅𝐹 𝑁𝐷𝑄 = 200 V MCP2 Light spot < 1 mm f MCP-PMT Photodiode Slit Xe lamp Variable Sharp Monochro- ND filter cut filters mator Moving stage
6 QE measurement Scan the photocathode at 18 x 18 points x 20 l . QE at l = 360 nm JT0629_20130320 Photocathode QE peaks around 360 nm
7 QE 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
8 Laser measurement setup Single photon irradiation to each channel one by one. Dark box Moving stage Reference PMT ND filters MCP-PMT Laser Slit Slit Light spot Fiber ≈ 1 mm f MCP-PMT Variable amp Pico-second ADC pulse laser ( l = 400 nm) Discrim- ATT Amp TDC inator – 10 dB +33 dB Threshold: – 20 mV +19.5~35 dB
9 Gain measurement Define the gain as the mean of the output charge distribution. ALD-MCP-PMT KT0449_20140717 ch4 2750 V gain = exp(a ∙ HV + b) 2650 V 2550 V
10 Gain HV for 1 x 10 6 gain Conventional Conventional ALD ALD The ALD-MCP has Rough drawing SE yield a large gain at a lower HV or ALD a sharper gain slope than the conventional one ~2 Conventional Higher secondary electron yield ~65 ~85 ~200 HV (V)
11 TTS measurement 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 Photocathode TTS does not depend on HV. recoil on the MCP1 MCP1 surface MCP2
12 TTS TTS less than 50 ps for every PMT Requirement for the Belle II TOP counter
13 Relative CE measurement Count the number of TDC hits. Correct the laser intensity variation with the reference PMT. Normalized to the number of incident photons Conventional ALD ALD Conventional (same gain of 2 x 10 6 ) JT0763_20140626 ch6 3460 V KT0162_20140612 ch6 2550 V Increase of CE for the recoil photo electrons due to a higher secondary electron Higher CE of the ALD-MCP by yield of the ALD-MCP ~ 15% than the conventional one.
14 Aging of the photocathode 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%. Introduce some ALD MCP Lifetime (C/cm 2 ) methods of process 10 Conventional MCP 1 Belle II beam bkgd MC (5 x 10 5 gain, 50 ab – 1 ) Added 0.1 ceramic block 0.01 Mass production Square shape with Al layer 2011 2013 2015 year Tried six methods of process to improve the lifetime.
15 Lifetime measurement setup 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. MCP-PMTs Pulse laser (400 nm) LED (100 kHz) Reference PMT
16 Extended lifetime Method A Method B YH0148 YH0168 YH0149 YH0170 YH0160 YH0171 YH0163 YH0173 Method A+B+C Three methods of process were found to be promising. 20 C/cm 2 or longer lifetime can be YH0203 expected with YH0205 YH0206 Method A+B+C.
17 Summary 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 life- extended ALD-MCP-PMTs after a few years of Belle II operation.
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