New method for calibration of dead times (and picoammeter)

[BenjaminWade]

Below are some quick scans at different gain and corresponding movement of the baseline. At the max 4V I still cant get it above the baseline. However looking at it now it is a lot better than the Gain I used. I might redo it and see if I can get to higher beam current before the response falls over.

[John Donovan]

Below are some quick scans at different gain and corresponding movement of the baseline. At the max 4V I still cant get it above the baseline. However looking at it now it is a lot better than the Gain I used. I might redo it and see if I can get to higher beam current before the response falls over.

Hi Ben,
I think setting the gain to 4000 or so, and trying the constant ratio dead time calibration method again, is worth trying.  You should be able to get to higher count rates before it starts clipping at the baseline.

By the way gain values are unitless (not volts).

[Probeman]

Now for spectrometer 2 using a LTAP crystal (~370 kcps on SiO2) at 200 nA:



Again, we do not care that the peak is being "cutoff" on the right side of the plot because in INTEGRAL mode the PHA system still counts those pulses as previously demonstrated using a gain test acquisition on Ti Ka on Ti meta:



See here (and subsequent posts) for more details on PHA tuning:

https://smf.probesoftware.com/index.php?topic=1475.msg11330#msg11330

According to SG, the Cameca PHA display only goes to 5 volts, but the counting electronics will perform energy discrimination up to 10 volts.  Pulses above 10 volts are set to 10 volts and still counted.

Speaking with John Glass recently I asked this same question for JEOL PHA electronics and he said that the JEOL energy discrimination was limited by the 15 volt "rail" voltage so the JEOL graphics only displays up to 10 volts, but the counting electronics provides energy discrimination up to 15 volts. But similar to the Cameca electronics it will set pulses greater than 15 volts to 15 volts and still count them.

On the question of whether one should increase the gain or the bias voltage to ensure that the PHA peak is fully above the baseline at the highest expected count rate (usually one's primary standard), he said that both methods can introduce some noise into the counting system, but that the main concern is to check that the zero volt noise peak doesn't rise above the baseline level when at lower count rates.