The Large PMT Quantum Efficiency Test Platform

Feng GAO1,2,3, Sen QIAN1,2, Zhe NING1,2, Yichao MA4, Zhigang WANG1,2, Na ZHU4, Lishuang MA5, Pengyu CHEN5, Shiyu YIN5, Zhile WANG6, Yao ZHU6

1 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

2 State Key Laboratory of Particle Detection and Electronics, Beijing 100049, China

3 University of Chinese Academy of Sciences, Beijing 100049, China

4 Shaanxi University of Science & Technology, Shaanxi 710021, China

5 North China University of Technology,Beijing,100144, China

6 Harbin Institute of Technology, Harbin 150001, China

* Corresponding author Sen QIAN, qians@ihep.ac.cn

Abstract. Photomultiplier tubes are widely used in physics experiment, industrial production, medical equipment and other fields. The performance of the photocathode is very important for the large area PMT. A platform for the photocathode studying has been built in our lab. These performance parameters such as quantum efficiency at specific wavelength, QE spectral response from 200 nm to1000 nm and photocathode uniformity could be measured in this platform. For the different size and shape of the large area photocathode, four types of photocathode uniformity scanning system were setup, one for 2 inch plant PMTs, another for 8 inch PMTs, and two for the spherical surface 20 inch PMTs to test the uniformity of the whole photocathode.

Keywords: PMT, QE, photocathode uniformity.

Introduction

Photomultiplier tubes (PMTs) are widely used in nuclear and particle physics experiments. Especially in the neutrino observatory experiments [1–2], such as DayaBay[3] and Jiangmen Underground Neutrino Observatory (JUNO)[4], of which hundreds or even thousands of large area PMTs are employed to detect scintillator photons or water Cherenkov light.

With the increasingly common use of large area PMTs for nuclear and particle physics experiments, information on the uniformity of photocathode is important to accurate particle identification. Especially using the non-transfer cathode system to produce the photocathode, the position of the antimony ball arrangement and photocathode preparation technology always contribute non-uniformity of large area PMTs.

A platform for the PMT photocathode performance studying has been built in our lab. These performance parameters such as quantum efficiency (QE) at specific wavelength, QE spectral response from 200 nm to1000 nm and the photocathode uniformity could be measured in this platform.

Quantum Efficiency Test Platform

Cathode spectral response

A tungsten-halogen lamp and a monochromator were utilized as the light source in the photocathode spectral response measurements. The monochromator controlled by a computer is used to select every specific monochromatic light within 3 nm FWHM from 200-1000 nm. PMT photocathode spectral response has been tested by this platform [5]. As shown in Table 1, all the QE of the photocathode of the different types of vacuum tube, or the detection efficiency (DE) of the Si-based photoelectron sensor.

Table 1. The spectral response of photo-detectors.

Photo-detectors	Type No.	Spectral response range /nm	Peak sensitivity wavelength /nm
PMT	R5912	300-650	420
MPPC	S13360-1325CS	270-900	450
APD	S12051	400-1000	800
PD	S1087	320-730	560

Photocathode current versus voltage (I-V) curve

The first step QE test is to monitor the Photocathode current versus voltage (I-V) curve, which shows all photoelectrons emitted from the photocathode could be collected by the dynode in the vacuum tube. The light intensity and electrical potential should be adjusted to appropriate levels to get correct photo current values. Take a 2 inch PMT as an example, the relationship between the measured current and loading voltage at different light levels are shown in Fig.1.a.

Fig. 1. Photocathode current versus voltage curve: a) at different light intensity @400nm;

b) at different wavelength

Firstly, the current increases with the voltage and then reaches to a plateau, which should be explained as the maximal collection for photoelectrons. With stronger incident light, larger electrical potential is needed for thorough collection. Definitely, the photo current used for QE calculation should be the plateau value, under moderate light and potential level.

The spectral response curves were obtained by combining the QE values at different wavelengths light. The I-V curve with different wavelength lights produced by a monochromator is showed in Fig.1 b). The light intensity should be adjusted by selecting fibers with different thickness to get correct photocurrent values.

Photocathode Uniformity Scanning Platform

PMTs are always required good photocathode uniformity property in practical applications to achieve globally high photo detection, especially in cases that employ large area PMTs. On the other hand, during the production process of the vacuum tube, the operation situation of the bialkali metal source (the position, the evaporation) and the temperature will directly affected the thickness of the photocathode, and the uniformity of the photocathode would be worse as shown in Fig. 2.

Fig. 2 a) Affected by bialkali evaporation; b) Affected by temperature

Because the size and shape of photocathode were different within different types of PMTs, four photocathode uniformity-scanning systems were setup in our lab in these years. A Two-Dimensional X-Y scanning system for the 2 inch plant PMTs in 2013, A Three-Dimensional rotting system for the 8 inch spherical surface PMTs in 2014[6]. The light coupling scanning system for the 20 inch spherical surface PMTs 2014, and the electric coupling scanning system for the 20 inch spherical surface PMTs in 2015. The test system consists of hardware and software and is highly automatic and user controllable by remote automation monitoring.

Light coupling and electric coupling scanning platform.

Two specialized setups were designed and assembled for the uniformity scanning for 20-inch PMTs (see Fig. 3). In each of the setup, the PMT can be fixed in a support, which is installed on a rotatable platform. A track with the same curvature as the bulb of a given PMT is used to fix a sequence of optical fiber heads. The difference between the two setups is different light switch modules are equipped to enable light emitted from LEDs enter one of the optical fibers and come out from the head on the track side.

One of the modules is based on an electronics design and the other based on a mechanical design. In the electronics module, circuits are designed to lighten one of a series of LEDs, which are coupled to equivalent optical fibers one by one, as requirement (9 LEDs to 9 fibers). And in the mechanical module, only one LED is moved by a stepper motor to couple with expected nine fibers mounted on different positions (1 LED to 9 fibers).

For both setups, with the illumination from different fibers, scanning covering most photocathode area can be achieved while rotating the tube along its central axis meanwhile. Since the curvature of the light track is same as the measured PMT, the incident light is always perpendicular to the cathode, which means no angle effect is involved in the scanning results.

Fig.3. Scanning platform: a) Light coupling; b)Electric coupling.

Summary

The QE test platform has run steadily with convenient operation for a long time. The various types PMT can be researched and tested carefully by using a series of light sources and automated instruments in this platform. Not only can the system test the cathode spectrum response and QE, also review the cathode uniformity property for most existing large area PMT.

Acknowledgments

The MCP-PMT development project has been partially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA10010400) and the National Natural Science Foundation of China (Grant No.11175198 and No.11475209 and No.11675205 and No.11675196).

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