<論文・報告>放射線計測のためのPIN フォトダイオードとアバランシェフォトダイオードの性能評価

Abstract

Scintillation detectors are widely used for measuring radiation. A scintillation detector is composed of a photodetector and a scintillator that emits photons when it absorbs ionizing radiation. A PIN photodiode is a kind of photodetector which has been used conventionally. An avalanche photodiode (APD) is another kind of photodetector that has come into use recently. Since the APD can amplify the photocurrent by applying a high reverse voltage, output signals from the APD are less likely to be affected by background noise than those from the PIN photodiode. We have evaluated the performance of scintillation detectors with either an APD or PIN photodiode based on energy resolution, linearity, and long-term stability. We found that the energy resolution of the APD is about twice as good as that of the PIN photodiode, and that the linearity of both detectors is equally good. The instability of the PIN photodiode was less than 2% after 7 hours, which was a factor of three smaller than that of the APD. We believe that the APD instability is due to temperature variation. In conclusion, scintillation detectors with an APD are useful for high-resolution spectroscopy of ionizing radiation if their temperature is properly controlled.

PINフォトダイオードは従来用いられてきた光検出器の一種である．一方，アバランシェフォトダイオード（APD）は電子と空孔の雪崩効果を利用することで，信号の増幅が可能な光検出器であり，PINフォトダイオードに比べ，ノイズの影響を受けにくくなることが期待される．本研究では，これら2つの光検出器を用いたシンチレーション放射線検出器について，検出器としての性能をエネルギー分解能・直線性・長期安定性の3点から検証した．その結果，エネルギー分解能はAPDの方が約2倍優れており，直線性は共に良く，PINフォトダイオードの不安定性は7時間の観察で2％以下であり，APDより3倍以上変動が小さいことが分かった．信号波高の変動と温度変化には相関が見られるので変動の主な原因はAPDの温度変化であると考えられる．ゆえに，APDの温度を一定に保つ機器を導入すれば，APDは高分解能放射線計測に有効な光検出器となるであると結論される．