Developement of Hybrid Photo-detectors for the Hyper-Kamiokande Experiment

Seiko Hirotaa, Yasuhiro Nishimurab, Shoei Nakayamab, Isao Kametanib, Masato Shiozawab, Yoichiro Suzukib,Hiroyuki Sekiyab, Masayuki Nakahatab, Yoshinari Hayatob, Yuto Hagab, Makoto Miurab, Atsuko Ichikawaa,

Motoyasu Ikedaa, Tsuyoshi Nakayaa, Akihiro Minaminoa, Keiji Tateishia, Hiroaki Aiharac, Yusuke Sudac, MasashiYokoyamac, Takayuki Omurad, Yoshihiko Kawaid, Masatoshi Suzukid, Mark Robert Vagins1

aKyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto-city, Kyoto, JapanbICRR, Kashiwanoha 5-1-5, Kashiwa-sity, Chiba, Japan

cUniversity of Tokyo, Hongo 7-3-1, Bunkyo-Ku, Tokyo, JapandHamamatsu K.K., Sunayama-cho, 325-6, Naka-ku, Hamamatsu-city, Shizuoka, Japan

eKavli IPMU, Kashiwanoha 5-1-5, Kashiwa-sity, Chiba, Japan

Abstract

We are developing a hybrid photo detector (HPD) for the Hyper-Kamiokande Project. Eight-inch HPDs wereprepared to evaluate their performance. Based on the results from these measurements, HPDs achieve a better perfor-mance such as single photon separation than conventional PMTs. A verification study lasting a few years in a watertank is planned in 2013 to check their feasibility.

Keywords: Hyper-Kamiokande, Photo Detector, Neutrino, Cherenkov Detector

1. Hyper-Kamiokande

Hyper-Kamiokande is a next-generation undergroundwater Cherenkov detector designed to study a widerange of topics in physics and astronomy. Its to-tal (fiducial) volume is 1.0 (0.56) million metric tons,which is 20 (25) times larger than that of Super-Kamiokande. Accordingly, in its baseline design theHyper-Kamiokande detector requires 100,000 20-inchand 25,000 8-inch PMTs, almost 10 times as many asSuper-Kamiokande [1].

The excellent performance and low production costare crucial for the Hyper-Kamiokande detector. There-fore the photosensor for Hyper-Kamiokande is re-quiered to have a large photo-cathode area to realizewide photo-coverage with a small number of detectors.Moreover single photoelectron must be clearly sepa-rated for a long run over tens years.

2. Principle of a hybrid photo-detector

The HPD is a hybridization of a photomultiplier tubeand an avalanche diode (AD) that can possibly achieve

both of low cost and excellent performance. The ampli-fication system of HPD is different from a normal PMTusing dynode. Figure 1 shows that HPD has a photocathode like a standard PMT and a avalanche diode.Therefore amplification consists of two steps; bombard-ment from the photocathode to the AD surface underhigher voltage (∼8 kV) and then an avalanche at AD.Because of the large bombardment gain (about 400) anda fast electron propagation, the HPD is expected to showbetter photon counting efficiency and timing resolutionthan conventional PMTs. In addition, the mass produc-tion cost is expected to be lower thanks to the simplestructure of AD compared to the dynode of a standardPMT.

3. Research and development of HPD

There is something to consider concerning usingHPDs in Hyper-Kamiokande; HPDs have never beenused in water and for neutrino detection though theymust be applied higher voltage. Therefore we need toconfirm the operation of HPDs and their feasibility in awater Cherenkov detector over long periods by measur-

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 208–209

0920-5632/© 2014 Published by Elsevier B.V.

www.elsevier.com/locate/npbps

http://dx.doi.org/10.1016/j.nuclphysbps.2014.09.052

��

~8kV

Avalanche Diode (AD)�

� ~400

~260V

× ~100 ������� ������������

ΔV ~ -260V

����

��������������������� ���� � �����

� ���������× ~!�"#!

"������

��#����

Figure 1: Left: Schematic diagram of HPD, Center: Set Up for measurements, Right: Wave form

ing performance of eight 8-inch HPDs relative to 232PMTs in the tank. We plan to perform such studies ina 200-ton water tank starting spring 2013 and a few-years operation is planning. First, we have started usinga smaller HPD (8-inch), and then we will promote thedevelopment of 20-inch HPDs based on the results frommeasurements of 8-inch HPDs.

3.1. Basic performance measurements of 8-inch HPDs

An 8-inch HPD has been developed by HamamatsuPhotonics K.K. We measured several characteristics ofthem. The measurement uses a preamp and a 70m cableas shown in Figure 1.

Figure 1 shows the signal produced by an HPD when1 or 2 photons from a laser diode (OPG-1000-NIM,405nm) are detected. We can distinguish single photonwaveform from others better than with standard PMTs.

Figure 2 shows the separation of the first and secondphotoelectrons by pulse height. The peak to valley ratio,which contributes to the trigger performance, is about6. The performance of the HPD is better than that of20-inch PMT which typical value is 1.4. It provides aclearer criteria to determine the HPD threshold at valley,which is related to data taking trigger rate.

The expected depending of the gain on high voltageand bias of HPDs are got as shown in Figure 3.

������������ ���

������������

�����

�������

����� �

������������������ �

������������������� �

������������� ������������

Figure 2: Pulse Height Distribution

Based on these features, HPDs can be a stronger can-didate of photo detectors in Hyper-Kamiokande thanconventional PMTs.

����������� � �� ���������

Gain =1.04×104 × 1

1− V377⎛

⎝⎜

⎞

⎠⎟0.0015

Figure 3: Gain depends on both HV and Bias. ,Left: On HV, Right: On Bias, Red lines: Fittingresults

3.2. Confirmation of stable operation of HPDs beforelong term verification study in 200-ton tank

Before the implementation of the verification studyin 200-ton tank, we want to confirm stable operation ofHPDs over long periods both in and out water.

We monitored a fluctuation of pulse height and ob-served charge with LED, to check an HPD’s gain sta-bility for a few days. It was stable within 1 %. Nowwe are going to test its more detailed and longer termstability for months. Also we have applied power cycleto HPDs and check durability of turing them on and offmany times to avoid breakdown of HPDs after installa-tion into a tank where we can not replace them to newone easily.

After a short term operation check of an HPD in wa-ter, the installation of HPDs and PMTs in 200-ton tanklocated in the Kamioka mine will start in spring 2013.

4. Summary

Development of HPDs for Hyper-Kamiokande is ongoing. Better performances such as single photon sep-aration are confirmed. The verification study in aCherenkov detector will start in 2013.

References

[1] K.Abe et al. (Hyper Kamiokande working group): Letter of In-tent: The Hyper-Kamiokande Experiment -Detector Design andPhysics Potential- , arXiv:1109.3262v1

S. Hirota et al. / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 208–209 209