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)

Let's recalculate data using the formula:

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

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