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。
MPPCは高い内部ゲインと画素間のゲイン均一性により、単一光子の分解能に優れています。しかし一部のアプリケーションでは、検出器からの電気的出力信号の速度が、光信号の時間的ダイナミクスを十分な精度で再現するために必要な要件を満たさない場合があります。
理想的な状況では、図1に示すように、各光信号に対するMPPCの電気的出力がカウンタ上で完全に分離して観測されます。これらのパルスは、ゲインのばらつきやホワイトノイズに起因するわずかなランダム性を除けば、完全に離散的なパルス高さを示します。
図1:パイルアップのないMPPC
MPPCが特定のアプリケーションで端子容量の増加により立ち下がり時間が長くなる場合、パルスの蓄積が発生することがあります。これは、高い光子レートが存在する状況で顕著に見られます。さらに、パイルアップの発生リスクは、ダークカウントレートが高い場合や、遅延クロストークおよびアフターパルスの発生確率が高い状況では一層大きくなります。これらの例は、下記の図2に示されています。
図2:パイルアップが発生しているMPPC
パイルアップが発生すると、MPPCの出力パルス数が実際よりも少なくカウントされてしまいます。この結果、ディスクリミネータしきい値に対するパルス数のプロットにおいて、各プラトーに対応するカウント数が正確に判断できなくなります。
さらに、パルス群が「1つの大きなパルス」としてまとめて認識される場合もあり、とくに1光電子 (1 p.e.)未満のパルスではその傾向が顕著です。このため、低いしきい値におけるパルス数は、実際よりも少なく見えることがあります。
より広い観点では、これらの効果によって、実際の値よりも高いプロンプトクロストーク確率が生じることになります。具体的には、0.5 p.e.を超えるパルス数は減少し、1.5 p.e.を超えるパルス数は逆に増加します。
大気チェレンコフ実験など、バックグラウンド光を伴うアプリケーションでは、パイルアップの効果的な抑制が極めて重要です。
浜松ホトニクスでは、図3に示すように、パイルアップを抑制したMPPCを提供することが可能です。
図3:パイルアップ抑制の一例
当社MPPCの性能について、ぜひ詳細をご確認ください。お客様のアプリケーション要件に最適なデバイスを特定するため、より詳しいご相談が必要な場合はお気軽にお問い合わせください。
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