研究目的
To develop a new model of low radioactivity photomultiplier tubes (PMTs), R10789, for the XMASS-I detector to meet the low background requirements necessary for direct detection of dark matter.
研究成果
The R10789 PMT was successfully developed, achieving significant radioactivity reductions compared to the previous R8778 model. It meets the requirements for XMASS-I and provides a foundation for future ultra-low-background PMTs, with key improvements from material substitutions. Further reductions are possible by addressing high-radioactivity components and using Co-free alloys.
研究不足
The developed PMT exceeded target values for 226Ra and 228Ra by factors of 1.3 and 2, respectively. Limitations include high radioactivity in components like glass beads, difficulties in reducing 40K from photocathode materials, and the need for higher sensitivity measurements for further reductions. The use of Kovar alloy containing cobalt limits 60Co reduction.
1：Experimental Design and Method Selection:
The development involved screening numerous component materials for radioactivity to reduce background. Methods included HPGe detector measurements and mass spectrometry (ICPMS and GDMS) for specific isotopes.
2：Sample Selection and Data Sources:
Components of the PMT and voltage divider circuit were selected based on their potential radioactivity, with samples weighed and measured in quantities larger than per-PMT usage.
3：List of Experimental Equipment and Materials:
HPGe detectors (CANBERRA models), materials like Kovar alloy, quartz, glass beads, resistors (KTR10EZPF), capacitors (ECWU-JC9, C4520X7R3D), circuit boards, cables, etc.
4：Experimental Procedures and Operational Workflow:
Samples were measured in HPGe detectors under shielded conditions with Rn-free air flushing; mass spectrometry was used for 238U and 232Th in inner components.
5：Data Analysis Methods:
Gamma-ray spectra were analyzed to identify radioisotopes; upper limits were calculated for non-detections; systematic errors were estimated.
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