Tuning PHA Settings

[Probeman]

Tuning our PHA settings for our WDS spectrometer electronics has always seemed, to me at least, a bit of a "dark art".   

It doesn't help that on JEOL instruments one cannot make fine adjustments to the PHA gain (they are fixed values at multiple of 2, i.e., 16x, 32x, 64x, etc.), so that one must set the PHA gain to a rough value and "tune" the PHA by adjusting the high voltage bias.

While on Cameca instruments, one adjusts on the detector bias to obtain a proportional response from the detector (or even better just leave the bias voltage at nominal values of ~1300v for low pressure flow detectors and ~1850v for high pressure detectors), and "tune" the electronics by making fine adjustments to the PHA gain.

Traditionally we are taught to simply adjust the bias or gain (as the case may be) to produce a PHA peak of around ~4v or so on JEOL instruments and ~2 to 2.5v on Cameca instruments.  The idea being that this should keep most of the PHA peak above the baseline level, but still not encroach on the window level.

But as SEM Geologist has pointed out, we generally should not be using the DIFFERENTIAL PHA mode (where both the baseline and window levels are actively filtering pulses), and instead simply utilize INTEGRAL PHA mode where only the baseline level filtering is active.  As SG correctly points out, the count rates utilized in the dead time correction will not be accurate if we have excluded a significant number of pulses from our dead time corrections, i.e., from higher order reflections being filtered by the window level.

In fact, to obtain a linear response from the spectrometer it is better practice to keep our PHA tunings as "open" as possible, so that we do not lose any pulses when we perform analyses at different count rates, thereby causing pulse height depression effects as Anette demonstrated here:

https://probesoftware.com/smf/index.php?topic=1466.msg11271;topicseen#msg11271

That generally means leaving our baseline values at the lowest possible settings, e.g., ~0.2v for Cameca and ~0.5v for JEOL. And if one absolutely must utilize DIFFERENTIAL mode, then leave the window levels at ~4.5v for Cameca and ~9.5v for JEOL.  The idea being to obtain a linear response of intensity with concentration (and/or beam current) as much as possible, and then have the quantitative spectral interference correction do its job!  Remember, for 1st order spectral interferences, the window level filtering has no effect at all, except for a few stray cosmic rays!   

But again, there is generally no reason to utilize PHA DIFFERENTIAL mode at all, except possibly in rare cases where a high order (<1) spectral interference is present, and no suitable interference standard is available. Remember, the quantitative interference correction requires a standard with a known concentration of the interfering element, and *none* of the interfered element (nor any other interfering elements).  I can only think of the interference situation of Na Ka 2nd order interference on O ka, and that is because it is difficult to find a standard material that contains sodium, but no oxygen.  I've tried natural cryolite for this interference correction, but it seems to contain some oxygen, maybe from surface reactions.

I recently was asked to explain how I approach PHA tuning of the baseline and window levels, and based on recent work with several colleagues in our soon to be published new dead time correction calibration method and dead time correction expression, here is how I would explain things:

Here are some places to start for PHA settings in the EPMA user forum:

https://probesoftware.com/smf/index.php?topic=1466.msg11416#msg11416

https://probesoftware.com/smf/index.php?topic=1466.msg11450#msg11450

https://probesoftware.com/smf/index.php?topic=1466.msg11636#msg11636

In summary, one should almost never use PHA differential mode (so that removes question about the PHA window setting!), and instead use only integral mode. The key point is not to set the center of the PHA peak at a specific position, but only to ensure that the tail (and escape peak if present), are fully *above* the base line level. If the right side of the PHA peak is cut off on the right side of the PHA plot, that will not matter when one is in integral mode. Anette and I have tested this question on both JEOL and Cameca instruments.

The worse thing one can do is set one's PHA settings so that at different concentrations or beam currents the PHA peak shifts downwards (to the left) so that is gets cut off by the baseline at lower count rates.  That is why one should always tune one's PHA setting at the highest expected concentration and at the highest expected beam current for that particular probe run.  That way, at lower concentrations and/or beam currents, the PHA peak will only shift to the right, and in integral mode all the photons will still be counted even though the PHA peak may appear to be cut off on the right side of the plot.

We realize that this is not the way we were taught to tune PHAs but that is what we have learned since then!