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Abstract
A new large area photomultiplier tube based on the microchannel plates (MCP-PMT) with high collection efficiency (CE) and good time performance is proposed in this paper. A novel focusing system with two cylindrical and a conical barrels is designed to generate the accelerating and focusing electric field. A three-dimensional model is developed by CST Studio Suite to validate its feasibility. Finite Integral Technique and Monte Carlo method are combined to simulate the process. Results predict that CE of the novel MCP-PMT is expected to be 100%. TTS of the photoelectrons from the whole photocathode achieves 1.2 ns. Differ from other large area PMTs, it performs well in the geomagnetic field.
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1. Introduction
Jiangmen Underground Neutrino Observatory (JUNO) [1] was proposed as the Daya Bay II in 2012. The energy resolution was required to be , which means that the detection efficiency (DE) of the large area photomultiplier tube (PMT) should be above 25%. At that time, DE of R3600, Hamamatsu 20-in. PMT applied in Daya Bay experiment, was just 15.4%. None of the 20-in. Dynode chain PMT product could meet this criterion. In this situation, a 20-in. large area PMT based on the microchannel plates (MCP-PMT) [2] was proposed and developed by the MCP-PMT collaboration formed by the scientists from Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences, Northern Night Vision Technology Co., LTD (NNVT) and Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences. Performance parameters of a high-performance dynode chain PMT (R3600-02 from Hamamatsu) and MCP-PMT (P6201 from NNVT) products are listed in Table 1 [3,4]. Low cost, high DE, high quantum efficiency (QE) and high collection efficiency (CE) are critical parameters for it.
Table 1. Performance parameters of the dynode chain PMT and MCP-PMT
In the MCP-PMT multiplication system, traditional dynode chain is replaced by a pair of MCPs. Typical MCP designs use a well-known alveolate structure. For a high CE performance, a high secondary electron yield (SEY) [5] material is plated on the MCP flat inter-channel area [6]. A great deal of secondaries can be excited by the photoelectrons impacting on the area and be collected by MCP channels in an appropriate-directed electric field. By this approach, CE of P6201 is greatly improved to 100%. Even so, as listed in Table 1, compared with R3600-02, the time performance especially transit time spread (TTS) of P6201 is not good enough.
This paper presents a new design of the large area MCP-PMT aiming at achieving high CE and small TTS.
2. Concept design and simulation details
A novel large area MCP-PMT is proposed in this paper. The schematic diagram is exhibited in Fig. 1 (right side). Compared to the traditional MCP-PMT (left side), the novel structure has been greatly changed. The shape is designed as a cylinder covered with a curved surface which is coated by the photocathode. The cathode diameter is Ф 460 mm as other large area PMT design listed in Table 1. A new focusing system including a conical and two cylindrical barrels is designed to generate the accelerating and focusing electric field which is benefit for CE and time performance. Electrode III is also designed to shield the supporting structures and the electrode wires of the multiplication subassembly. The applied voltage distribution ratio is listed in Table 2.
Fig. 1. Schematic diagram of the large area traditional (left side) and novel (right side) MCP-PMTs.
Table 2. Voltage distribution ratio
Simulations are conducted to validate the feasibility and effectiveness. The novel MCP-PMT model is built to systematically study its performances including CE and transit time distribution. CST Studio Suite [7] is adopted to build the model and calculate the electric fields, electron trajectories, energies and velocities based on the Finite Integral Technique and Monte Carlo method. The feasibility and effectiveness of this simulation approach has already been validated [6], [8].
3. Simulation results and discussions
3.1 Electric field distribution
Electric field distribution in the novel MCP-PMT is simulated. The accelerating and focusing electric field with symmetrical property in the PMT cavity is observed in Fig. 2. The field distribution is benefit for the CE and time performance. Field intensity upon the Electrode III is stronger than that nearby the photocathode.
3.2 Collection efficiency
Previous study shows that plating a high secondary electron yield (SEY) material is an effective approach to improve CE of the large area MCP-PMT. If all the photoelectrons land on the active area of the MCP input facet including the channel and the flat inter-channel area, CE could be 100%.
Photoelectron trajectories from the photocathode to MCP are well simulated. The initial energy, elevation, azimuth and position of photoelectrons obey certain statistical distributions. Initial energy of photoelectrons obeys β distribution. Initial elevation follows Lambert cosine distribution from 0° to 90°. The azimuth subject to 0-2π uniform distribution. The initial position obeys uniform distribution. 2000 electrons sampled by Monte Carlo (M-C) method are emitted from the photocathode. Simulated photoelectron trajectories exhibited in Fig. 3 show that all the photoelectrons from the whole photocathode land on the MCP active area, which means that CEp is 100%. CE of the novel MCP-PMT is expected to achieve 100%.
3.3 Transit time distribution
Considering the symmetry of electric field in the PMT cavity, the transit time distributions of the photoelectrons from the latitude of 0° to 61.5° (corresponding to Ф 460 mm photocathode diameter) are evaluated, respectively.
Owing to the short transit distance and high potential difference, the electron transit time (TT) from the first MCP to anode is around several hundred picoseconds and TTS is just tens of picoseconds which thus are negligible. In our simulation, the transit time distribution between the photocathode and the first MCP is evaluated.
Figure 4 shows the transit time distribution of photoelectrons from the whole photocathode. The simulated TT and TTS are 42.7 ns and 1.2 ns which are greatly improved than those of the PMTs with Ф 460 photocathode diameter listed in Table 1 and in Ref. [9,10]. Besides, compared with the optimized 20-inch MCP-PMT model reported in Ref. [8], TT and TTS are shortened to a half. Simulation result shows that the new design has an outstanding time performance.
Fig. 4. Transit time distribution of the photoelectrons emitted from the whole photocathode. The FWHM is TTS.
TTS versus the illuminated latitude result is exhibited in Fig. 5. It is obvious that TTS increases with the increasing latitude. TTSs of photoelectrons from various latitudes are all less than that of photoelectrons from the whole photocathode (1.2 ns). With the enlargement of the photoelectron emission region, the uniformity of photoelectron travel paths and movements deteriorates, and thus the TTS. The lowest TTS is 1.03 ns at latitude 0°. The TTS difference of all the calculated latitude is no more than 0.1 ns, which implies a good uniformity.
3.4 Cathode transit time difference
Cathode transit time difference (CTTD) of each position is represented as the transit time difference with respect to the transit time of photoelectrons emitted from the photocathode center [11]. It is an important factor that affects the time performance especially TTS.
CTTD for various points is calculated. Affected by the symmetry distribution of the electric field in the PMT cavity, four points from latitude 0° (photocathode center, the reference point), 30°, 45°, 61.5° with the same longitude value are employed in the simulation. Result is exhibited in Fig. 6. It is shown that CTTD difference of all the calculated points is no more than 2 ns. At latitude 45°, the maximum CTTD (1.6 ns) is observed. The travel path lengths of photoelectrons from the photocathode to MCP and the electric field distributions along the paths are two primary factors which greatly affect the CTTD.
3.5 Geomagnetic field effects
Geomagnetic field has a significant impact on the large area PMTs, owing to the long travel path of the photoelectrons from photocathode to the dynode. Effects of the geomagnetic field on CE and TTS of the novel MCP-PMT are studied. Simulations are conducted under both shielded (no magnetic field applied with the simulation model) and unshielded (55000 nT geomagnetic field applied) conditions. PMT is rotated in the X-Y plane as shown in Fig. 7. The top point of the photocathode is illuminated.
CE and TTS of the novel MCP-PMT in 55000 nT geomagnetic field are well evaluated. Results are exhibited in Fig. 8 and Fig. 9. In the best case, CE and TTS are 100% and 1.05 ns at θ=90° and 270°. In this situation, photoelectron trajectories are basically parallel to the magnetic field direction, thus the effect is relatively small. CE and TTS are at least 88% and 1.1 ns, even in the worst case at θ=0° and 180°. The simulated result shows a good agreement with the experimental data in Ref. [12]. The novel MCP-PMT shows a good performance in the geomagnetic field.
4. Conclusion
This work proposes a new type of large area MCP-PMT. A novel focusing system with three electrodes is designed to obtain a high CE and a good time performance. A three-dimensional model is developed in CST Studio Suite to validate its feasibility. Results show that CE of the novel MCP-PMT is expected to be 100%. TTS of photoelectrons from the whole cathode achieves 1.2 ns. Compared with the traditional large area PMTs, it shows a good performance in the geomagnetic field. It will be a good candidate for the physical experiments with both high CE (or DE) and high time resolution requirements.
Funding
National Natural Science Foundation of China (12005083); Ph.D. Project supported by the Jinling Institute of Technology (jit-b-201837).
Acknowledgments
Lin Chen thanks the National Natural Science Foundation of China (Grant No. 12005083) and the Ph.D. Project supported by the Jinling Institute of Technology (Grant No. jit-b-201837).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. Y. Wang, “Neutrino Detectors: Present and Future,” Phys. Procedia 37, 22–33 (2012). [CrossRef]
2. Y. Wang, S. Qian, T. Zhao, J. Tian, H. Li, J. Cao, X. Xu, X. Wang, S. Liu, H. Liu, S. Liu, D. Liu, Y. Heng, X. Cao, and J. Shentu, “A new design of large area MCP-PMT for the next generation neutrino experiment,” Nucl. Instrum. Methods Phys. Res. A Accel. Spectrom. Detect. Assoc. Equip. 695, 113–117 (2012). [CrossRef]
3. NNVT, “Photomultiplier tubes,” http://www.nvt.com.cn/art/2020/3/23/art_1235_25349.html.
4. HAMAMATSU, “Large Photocathode Area Photomultiplier Tubes,” https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/etd/R14747_TPMH1375E.pdf.
5. I. M. Bronshtein and S. S. Denisov, “Secondary electron emission in the scanning electron microscope,” J. Appl. Phys. 54(11), R1–R18 (1983). [CrossRef]
6. L. Chen, J. Tian, T. Zhao, C. Liu, H. Liu, Y. Wei, X. Sai, P. Chen, X. Wang, Y. Lu, and D. Hui, “Simulation of the electron collection efficiency of a PMT based on the MCP coated with high secondary yield material,” Nucl. Instrum. Methods Phys. Res. A Accel. Spectrom. Detect. Assoc. Equip. 835, 94–98 (2016). [CrossRef]
7. CST Studio Suite, “Computer Simulation Technology,” (2014). www.cst.com.
8. P. Chen, J. Tian, Y. Wei, H. Liu, X. Sai, J. He, L. Chen, X. Wang, and Y. Liu, “Optimization design of a 20-in. elliptical MCP-PMT,” Nucl. Instrum. Methods Phys. Res. A Accel. Spectrom. Detect. Assoc. Equip. 841, 104–108 (2017). [CrossRef]
9. S. Qian and S. Liu, “The 20 inch MCP-PMT R & D in China,” in Proceedings of the 38th International Conference on High Energy Physics, Chicago, 3–10 August 2016.
10. T. Brugière, “The Jiangmen underground neutrino observatory experiment,” Nucl. Instrum. Methods Phys. Res. A Accel. Spectrom. Detect. Assoc. Equip. 845, 326–329 (2017). [CrossRef]
11. Hamamatsu, Photomultiplier Tubes: Basics and Applications, (Hamamatsu2006).
12. D. Liao, H. Liu, Y. Zhou, F. Luo, Z. Wang, A. Yang, M. Xu, W. Xie, and Z. Qin, “Study of TTS for a 20-inch dynode PMT,” Chinese Phys. C 41(7), 076001 (2017). [CrossRef]
