Hello probers!
Over the past couple days Michael Jercinovic, John Donovan, and I were discussing over the range of values used for the SCA/MCA (resp. PHA) settings on Cameca instruments vs JEOL. To summarise and simplify the discussion and it's output, it appears that:
1) On Cameca, the bias is fixed (around 1300 V [1500 V for PCx] for low pressure counter, and around 1820 V for high pressure counter, variable...). Then, the gain is adjusted with an arbitrary value from 0 to 4095 (?) to reach a certain “recommended” value for the pulse (usually around 2 to 3 V in a scale of 0 to 5.6 V), with differences for P-10 low pressure vs high pressure (and if not, then the bias is lowered/increased to reach an adequate gain).
2) On JEOL, this is the opposite: the gain is fixed [4, 8, 16, 32, 64 or 128 - apparently higher for LDEx monochromator and for Xe-detector] and the bias is adjusted to reach an optimum pulse around 4 V in a 0-10 V range. The recommended bias should ideally be around 1700 V. If the gain increases, the optimum bias decreases.
The question is then about the setting of the base line (& window)... In each case, the operator is setting a base line (and a window if using differential). On JEOL, it is clear that the electronic noise is present when the BL is set to 0 (zero). However, according to Mike, it appears that Cameca has an hardware Baseline correction set to 560 mV, and therefore you never see the electronic noise...
[Quoting Mike J] the User Guide says that the PHA preamplifier delivers pulses from 0 to 5.6V. In integral mode, the baseline is set with a zener diode at 560mV, and window at 10V. So the low is automatically set. Anything you enter as baseline will be added to this 560mV, but it does not show the 560mV in the PHA display. That makes sense as you do not see the noise at all when you run a PHA scan. They don't mention ranges other than that specifically, but I do remember we had to put an external power supply on to get to nearly 3kV for the high pressure p10 tests (up to almost 4 bars), so I think in general you are limited to 2kV bias.
Feel free to add more details notably on the PHA / SCA / MCA setting you are using on your machine! And be aware that when you set your baseline on Cameca to 0, it is NOT really zero =)
Cheers,
Julien Allaz