Unstable cps on standard samples

[Rom]

I thought everything is easier. Pressure changing leads to changing of number of gas molecules in the multiplier. It causes the optimal bias voltage is changing and distribution centroid shifts to high side (pressure grows) or low side (pressure decreases) from "standard" 4V. It is in the agreement with results of the good work  https://probesoftware.com/smf/index.php?topic=1109.msg10889#msg10889.
As result the number of counted cps (not real x-ray intensity) decreases in any pressure changing direction relatively the pressure at the time of PHA parameters calibration.
Could you correct me.
Thank you!

[Probeman]

I suspect it has nothing to do with atmospheric gases back flowing into the detector, because even at low flow rates, the pressure (and flow) is always positive. But, as the barometric pressure increases so does the (absolute) pressure inside the detector...

Instead I suspect it has more to do with the number of atoms in the detector that need to reach a charge potential state that can induce proportional response.  At higher pressures we simply require more bias voltage to achieve the same detector response because more atoms are in the detector.

For example, in the Cameca instrument, the 2 atmosphere P-10 detectors require ~1850v to reach proportional response, while the 1 atmosphere pressure P-10 detectors only require ~1300v to reach proportional response.

QED   

[JonF]

Not claiming to know what causes the effects we see on the detectors, but if we use the proposed simple relationship between increasing ambient pressure = increasing detector pressure without allowance for back-streaming of atmospheric gases in to the detectors, how does this hypothesis explain the variation in detector response (e.g. bias voltage difference in Brian Joy's data set) for changes in pH2O for a constant overall atmospheric pressure (i.e moving vertically on the plot below)?



I'm also not sure how it would explain the difference in response between the inlet and exhaust GFPC, both in terms of the scale of the vertical difference with pH20 and the overall difference in bias voltage.

As with everthing to do with physics, I suspect the mechanism that is at work on the detector response to atmospheric pressure is anything but straightforward!

[Probeman]

Not claiming to know what causes the effects we see on the detectors, but if we use the proposed simple relationship between increasing ambient pressure = increasing detector pressure without allowance for back-streaming of atmospheric gases in to the detectors, how does this hypothesis explain the variation in detector response (e.g. bias voltage difference in Brian Joy's data set) for changes in pH2O for a constant overall atmospheric pressure (i.e moving vertically on the plot below)?

No question we've got more than one variable here, but the most striking aspect of Brian Joy's plot above are the sloping trends on bias as a function of barometric pressure.  In other words, he saw that as the barometric pressure increases, one requires more bias voltage to maintain a PHA peak at 4 v.  That is perfectly consistent with the hypothesis that more atoms (the higher pressure in 2 atm detectors vs 1 atm detectors) requires more bias voltage to operate.

I'm also not sure how it would explain the difference in response between the inlet and exhaust GFPC, both in terms of the scale of the vertical difference with pH20 and the overall difference in bias voltage.

Good question. I wish he had recorded the temperature of his P-10 bottle instead, but the relationship between the dew point and relative humidity is:

Td = T - ((100 - RH)/5.)

where T is temperature and RH is relative humidity. Note that the dew temperature can never be higher than the actual temperature. Given that his instrument is in Canada, maybe we can assume that low dew temperatures are due to winter (colder temperatures and drier air) and higher dew temperatures are seen in summer when the humidity is higher. In any event, for a constant relative humidity the dew temperature is essentially the actual temperature.  So where is his P-10 bottle located?

The above plot seems to reveal that the higher the dew temperature (higher relative humidity), the higher the voltage necessary to maintain the PHA peak at 4v.  So if H2O can indeed infiltrate the P-10 system, perhaps the water is acting as a quench gas?  Or maybe the humidity is affecting the high voltage capacitors in the spectrometer pre-amps? 

Though I suspect though this response is more likely some kind of plain temperature dependency in the P-10 gas or lab temperature.

As with everthing to do with physics, I suspect the mechanism that is at work on the detector response to atmospheric pressure is anything but straightforward!

Yes, indeed.

[Rom]

Just from curiosity, is it possible to use 1.04-1.05 bar (absolute) in the multiplier or we can destroy it because the difference between inner and atmospheric pressure in this case potentially could reach 100mm Hg.

[Probeman]

Just from curiosity, is it possible to use 1.04-1.05 bar (absolute) in the multiplier or we can destroy it because the difference between inner and atmospheric pressure in this case potentially could reach 100mm Hg.

Have the detectors on your JEOL 8200 ever been replaced?  I am guessing that the cps instability issues you are seeing are due to contaminated/old detectors.

[Rom]

Just from curiosity, is it possible to use 1.04-1.05 bar (absolute) in the multiplier or we can destroy it because the difference between inner and atmospheric pressure in this case potentially could reach 100mm Hg.

Have the detectors on your JEOL 8200 ever been replaced?  I am guessing that the cps instability issues you are seeing are due to contaminated/old detectors.

Sorry for forcing You to repeat the question. I didn't receive the answer yet from the lab manager.  I search our probe issue and trying to divide it like You suggested and I promised.

But there was a question just for my curiosity and education: is it possible or not to use extra pressure in flow type multiplier.

[Probeman]

Just from curiosity, is it possible to use 1.04-1.05 bar (absolute) in the multiplier or we can destroy it because the difference between inner and atmospheric pressure in this case potentially could reach 100mm Hg.

Have the detectors on your JEOL 8200 ever been replaced?  I am guessing that the cps instability issues you are seeing are due to contaminated/old detectors.

Sorry for forcing You to repeat the question. I didn't receive the answer yet from the lab manager.  I search our probe issue and trying to divide it like You suggested and I promised.

But there was a question just for my curiosity and education: is it possible or not to use extra pressure in flow type multiplier.

No worries.  But what exactly do you mean by a "multiplier"?

[Rom]

It means "detector".

[Probeman]

Just from curiosity, is it possible to use 1.04-1.05 bar (absolute) in the multiplier or we can destroy it because the difference between inner and atmospheric pressure in this case potentially could reach 100mm Hg.

Have the detectors on your JEOL 8200 ever been replaced?  I am guessing that the cps instability issues you are seeing are due to contaminated/old detectors.

Sorry for forcing You to repeat the question. I didn't receive the answer yet from the lab manager.  I search our probe issue and trying to divide it like You suggested and I promised.

But there was a question just for my curiosity and education: is it possible or not to use extra pressure in flow type multiplier.

Of course it is possible to increase the flow rate and/or pressure in these flow detectors, but there are several downsides to doing so and little benefit I can think of.

I do not know exactly how the JEOL P-10 system works exactly, but on my Cameca instrument one can increase the pressure of the bottle regulator, but that only increases the flow rate, not the pressure inside the detectors. But any flow faster than what is necessary to keep the detectors clean is simply a waste of P-10 in my humble opinion.

One could also increase the pressure in the detectors by increasing the back pressure in the system as Cameca does with their 2 atmosphere pressure detectors for LIF spectrometers.  But modifying the 1 atmosphere detectors by increasing the back pressure could cause the polypropylene detector windows to fail.  So I would be very careful modifying the back pressure in your detectors.

And besides, a higher pressure inside the detectors would only be a slight decrease in the absorption path length, when in fact, for low energy x-rays we would usually prefer to decrease the pressure in these detectors by lowering the pressure using a vacuum pump on the outlet:

https://probesoftware.com/smf/index.php?topic=197.msg4536#msg4536

Why would you want to increase the pressure inside your flow detectors?  It is true that sealed detectors can get pumped out over time and become noisy and unstable, but flow detectors do not. They can only become contaminated (perhaps by a bad bottle of P-10?) which is I think the problem you are seeing in your ancient instrument (assuming that your flow rates are nominal).