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什么是 MPPC (SiPM)？ | 多像素光子计数器 (MPPC/SiPM)
什么是多像素光子计数器？
MPPC 是多像素光子计数器的缩写，这种探测器也称为硅光电倍增器 (SiPM)。它是一种固态光电检测器，使用在盖革模式下运行的多个雪崩光电二极管 (APD) 像素。更多详细信息和不同光电检测器的比较如下。
结构
SPAD 和 MPPC 的结构如下所示。
SPAD 配置有一个像素，其中盖革模式 APD 和粹灭阻抗结合为一组。MPPC 配置有多个像素，其中排列多个上述 SPAD 并采用并联电气连接。
结构
MPPC 光子计数图像
基本操作
MPPC 中的每个像素会在检测到光子时输出相同振幅的脉冲。会输出由多个像素产生的脉冲，同时彼此叠加。例如，如果三个光子入射到不同像素上并且被同时检测到，则 MPPC 输出振幅等于三个叠加脉冲的高度的信号。每个像素仅输出一个脉冲，这不会随入射光子数而变化。因此，无论一个光子还是两个或两个以上光子同时进入某个像素，输出脉冲的数量始终为一个。这意味着，当更多光子入射到 MPPC 上时（例如当两个或两个以上光子进入一个像素时），MPPC 输出线性度变差。因此，选择具有足够像素以匹配入射光子数的 MPPC 变得至关重要。以下两种方法用于估计 MPPC 检测到的光子数。
(1) 观察脉冲
当光在特定时间进入 MPPC 时，其输出脉冲高度会根据检测到的光子数量而变化。该图显示了在光子计数水平下用脉冲光照射 MPPC，然后用线性放大器放大并在示波器上观察时，从 MPPC 获得的输出脉冲。从图中可以看出，根据检测到的光子数量（例如一个、二个、三个光子等）将脉冲彼此分离。通过测量每个脉冲的高度，可估计检测到的光子数量。
使用线性放大器时的脉冲波形
(2) 集成输出电荷
可通过使用电荷放大器或类似装置测量 MPPC 输出电荷来估计在特定时段期间检测到的光子数目分布情况。图中显示了通过区分累积电荷量所获得的分布情况。左起每个峰对应于基座、一个光子、两个光子、三个光子等。由于 MPPC 的高增益足以产生大量输出电荷，因此分布可以根据检测到的光子数量显示分立峰。
使用电荷放大器时的脉冲高度频谱
如何使用
取决于工作电压和环境温度，MPPC 的特性千差万别。通常，提高工作电压会增加 MPPC 内部的电场，从而改善增益、光子检测效率和时间分辨率。另一方面，这也增加了不希望出现的组成部分，例如暗计数、后脉冲和串扰，从而降低 S/N。必须仔细设置工作电压，以获得所需的特性。
可根据用途通过各种方法使用 MPPC。在这里，我们介绍一种观察脉冲光的典型方法。使用宽频带放大器和示波器可简化测量。图中显示了与宽频带放大器的连接示例。电源部分上的 1 kΩ 电阻和 0.1 μF 电容器用作低通滤波器，可消除电源的高频噪声。1 kΩ 电阻也是防止电流过大的保护电阻。
然而，MPPC 本身是弱光线探测器，在大量光进入 MPPC 的情况下（例如当其耦合到闪烁体以检测辐射时），大电流会流入 MPPC。这可能导致保护电阻两端出现显著的电压降，因此必须根据用途仔细选择保护电阻的阻值。放大器应尽可能靠近 MPPC 进行连接。
特性
| PD | APD | MPPC | PMT | |
|---|---|---|---|---|
| 增益率 | 1 | 102 | 至 106 | 至 107 |
| 量子效率 | 最高 | 高 | 中 | 低 |
| 工作电压 | 5 V | 100 至 500 V | 30 至 60 V | 800 至 1000 V |
| 大面积 | 否 | 否 | 中 | 是 |
| 多通道 且间隙窄 |
是 | 是 | 是 | 否 |
| 读出电路 | 复杂 | 复杂 | 简单 | 简单 |
| 噪声 | 低 | 中等 | 中等 | 低 |
| 均匀性 | 优秀 | 良好 | 优秀 | 良好 |
| 能量分辨率 | 高 | 中 | 高 | 高 |
| 温度灵敏度 | 低 | 高 | 中 | 低 |
| 环境光抗扰度 | 是 | 是 | 是 | 否 |
| 磁阻 | 是 | 是 | 是 | 否 |
| 紧凑和重量 | 是 | 是 | 是 | 否 |
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