光電子増倍管は完全な暗中にあるときでも微弱な電流を出力しています。このノイズは検出限界を左右する重要な要素で、グラフでは、マイクロPMTの標準的な暗電流を示しています。
光電子増倍管は高速の時間特性を有しており、極めて短い事象を捉えることができます。グラフはマイクロPMTに、パルス幅70 psの光パルスを入射した時の出力波形例です。
入射光が光電面に達してから陽極電流がピークに達するまでの時間を電子走行時間といいます。T.T.S.は光電面を単一光子により全面照射した時の走行時間のゆらぎを、そのヒストグラムの半値幅で表したものです。
光電面に強い光が入った時、ダイノード後段で大電流が流れることにより、飽和減少が現れます。その結果、理想的な直線性からズレが発生します。グラフはマイクロPMTのリニアリティ特性を示しています。
内蔵電源の発振回路起因のノイズです。出力信号をオシロスコープに入力し、入射光がない状態で電圧レンジを下げてベースラインを観測すると、ノイズ信号が確認できます。
光電面上の入射位置による感度の均一性です。グラフは直径1 mmのスポット光を0.1 mmピッチで光電面上のX軸・Y軸それぞれの方向に走査した時の陽極出力例です。
光電子増倍管の陽極感度は周囲温度の影響を受けます。また、その温度特性には波長依存性があり、一般的には長波長側の限界波長付近で温度計数がマイナスからプラスに変化します。グラフは各光電面の波長対温度係数データです。
光電子は外部磁場によって通常の軌道から外れ、ゲインが減少します。減少の度合いは磁場の向きにより異なります。グラフはマイクロPMTの磁場による影響を表し、Z軸方向の磁場に最も大きな影響を受けることが分かります。
光電子増倍管は、光入射がない状態でもいくらかの暗電流パルスが生じます。暗電流によるパルス出力を1秒間当たりのカウント数(s-1)で表したものがダークカウントで、信号検出限界レベルの目安となります。
入射光がランダムな事象の場合、光量を増やしていくとパルスの重なりが発生しカウント値が光量に比例しなくなります。カウントリニアリティを大きく超えた光量が入射すると、過大光の入射を示す信号が出力されます。
計測カウントが106 s-1以上になるとパルスの重なりによる係数誤差が発生するようになります。カウントリニアリティ改善方法の1つに補正式を用いて近似する方法があります。グラフは補正式を用いた特性改善を示しています。
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