特定の種類の光検出器、例えばMPPCⓇ (Multi-Pixel Photon Counter)のようなデバイスは、単一光子を検出できるほど高い感度を備えています。これは、単一光子吸収事象の検出が求められるHigh-Energy Physics実験において非常に有用です。他の一般的な光検出器とは異なり、MPPCには温度の影響を受ける特有の特性があり、その1つがDark Count Rate (DCR:ダークカウントレート)です。
MPPCの動作中には、光子によって生成されたキャリアだけでなく、熱的に生成されたキャリアによってもパルスが発生します。後者の効果によって生じるパルスは「ダークパルス」と呼ばれ、ランダムに出現し、光子による出力パルスとは無関係に発生します。これらのダークパルスは信号パルスと同時に観測されるため、測定精度の低下を招く要因となります。
アプリケーションにおけるDCRの測定精度への影響は、測定パラメータの設定に依存します。例えば、ある測定において時間窓が10 nsに設定されている場合、DCRが1,000 kcps (1秒あたり100万カウント)であっても、その時間内に単一のダークカウントが発生する確率は約0.01回と非常に低くなります。このような条件下では、MPPCを用いることで高い統計的信頼性をもって単一光子の検出が可能となります。
一方で、上記のような厳密なタイミング条件を満たすことが難しい場合には、MPPCの熱電冷却によってDCRを低減することが可能です。温度を約10°C低下させるごとに、DCRがほぼ半減する傾向があり (図1)、より高い測定精度を実現できます。
図1:熱電冷却によるMPPCのDCR低減
複数光子が関与する光イベントを扱うアプリケーションでは、ディスクリミネータレベルを上げることでDCRを大幅に低減できます。一般的な傾向として、ディスクリミネータのしきい値を単一光電子 (1 p.e.)出力パルス高さの1段分上げるごとに、DCRの読み値は約1桁減少します (図2)。
図2:ディスクリミネータしきい値の増加によるDCRの低減
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