05/19/2011 n's detector threshold calibration source/(w/source)

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Below are my counts rate measurements as function of threshold voltage for n's detector. The source was CF252. The distance between source and n's PMT was about 50 cm. The counts rate was calculated as:

[math]\mbox{Rate} = \frac{N_{\mbox{source}} - N_{\mbox{w/source}}}{\mbox{Time}}[/math]

First I did my calibration for power supply voltage -1250 V:

Second I did my calibration for power supply voltage -1500 V:

And bellow I overlay both cases (-1250 V and -1500 V)

There are no any plateaus, so what is the threshold should be?

Let's recalculate data using the formula:

[math]\mbox{Ratio} = \frac{R_s - R_{w/s}}{R_{w/s}}[/math]

My data and calculation looks like:

• -1500 V

• -1250 V

And below I overlay my results in one plot for linear ratio scale:

and for log ratio scale:

Now there are two important conclusion at least for this type of PMT was used:

1. The desirable threshold is about -50 mV
2. The power supply voltage is as expected -1500 V (there are no plateau for -1250 V)