多通道定標平均器是什麼 ?
多通道定標平均器是用來累積或平均離子或電子數目的儀器，它通常被用於某些涉及到偵測微弱的離子或電子訊號的實驗。以本實驗的研究課題為例，當離子被加速並打在微通道片(microchannel
plate)上，離子的數目即被放大約4
x 107倍且轉換成電流，利用50
Ohm的電纜線配上BNC接頭即可輸入多通道定標平均器，若電壓大於所設定的discrimination
level(例如-10 mV)，則可被計數。實驗進行中可設定取樣時間(例如30
seconds)，由多通道定標平均器可得知單位時間內所偵測到的總離子數目，或平均離子數目。以Stanford
Research Systems Model SR 430 Multichannel Scaler/Averager為例，它具有下列特性:
Singal Input
| Bandwidth | dc to 250 MHz |
| Input impedance | 50 Ohms |
| Linear range | +/- 300 mV |
| Input protection | +/- 5 Vdc, 50 V for 1 microsecond |
Discriminator
| Discriminator range | -300 mV to +300 mV |
| Discriminator slope | Rising or falling |
| Resolution | 0.2 mV |
| Input offset voltage | 1 mV |
| Minimum pulse input | 10 mV |
| Pulse pair resolution | 5 ns |
| DISC output | NIM levels into 50 Ohms |
Trigger Input
| Impedance | 10 ohms |
| Threshold | -2 V to 2 V |
| Slope | Rising or Falling |
| Protection | 15 VDC |
Internal Time Bins
| Bin width | 5, 40, 80, 160, ... ns, 10.486 ms |
| Accuracy | 1 ns |
| Jitter | 100 ps + 10 ppm of delay |
| Accuracy | 2 ns +1% |
Counter/Accumulation
| Bins per record | 1k to 16k in 1k increment |
| Max count rate | 100 MHz |
| Max count | 32,767 per bin per trigger |
| Records per accumulation | 1 to 64k (or free run) |
Trigger
Rate
Minimum period between
triggers
Tp = record time + accumulation time + overhead
= (no. of bins x bin width) + (no. of bins x 250
ns)
+ 150 microseconds
Outputs
DISC = Nim level into 50 ohms
如何使用多通道定標平均器?
External trigger <--- 以NSC接頭和50
Ohm的電纜線連接
自pulse/delay generator或光電二極體(photodiode)
的輸出端(output)
Input
<--- 以NSC接頭和50
Ohm的電纜線連接
自MCP的輸出端(output)
設定的discrimination level，取樣時間，bin
width，number of channel等。
