Unexplained Changes in Spectrometer Intensities

[Mike Jercinovic]

I guess I am just being more confusing than usual.
P-10 is 90% Ar
"Xe" in Xe sealed counters appears to be about 10% Xe (via the Geller and Herrington paper)
Both are operating at about 1 bar.

So JEOL can use a bias voltage for either counter type at around 1700v to efficiently avalanche ionizations given the physical dimensions of their counters.  Despite the large difference in the gas mixtures, Xe is very efficient (54 electrons) compared to Ar (18 electrons) at the same pressure.
Cameca operates the low pressure P-10 counters at a lower bias voltage to optimize low energy photon detection given the physical dimensions of their counters, then requires a higher voltage for an efficient proportional response at higher pressure for higher energy photon detection. 

[Probeman]

Hi Mike,
No I'm just being dense.  I thought that is what you meant when you said:

This should make sense if the total pressure is about 760 torr given the absorption of moderate energy x-rays through about a 5 or 10% Xe in methane, at least with some crude calculations I did awhile back. 

I'm surprised that they would use so little Xe in their sealed detector gas mixture.  But in any event you think it's the higher pressure in the high energy Cameca detectors that requires higher bias voltage.   

Thanks for explaining all this.

[Probeman]

This article here:

https://science.mcmaster.ca/radgrad/images/6R06CourseResources/4R6Notes3_GasFilled_Detectors.pdf

Mentions proportional counters using Xe or Kr:

The  type  of  fill  gas  used  is  dependent  on  the  function  the  counter  is  to  perform.  Commonly  used  gases  for  [beta] measurements  are  the  noble  gases.  These  often  require  a  quench gas however. Cost dictates that argon is commonly used, usually as a mixture of 90% argon with 10% methane. This is called P-10 gas. For better [gamma]-ray detection the fill gas is switched to krypton or xenon.

This Wiki article says something similar:

https://en.wikipedia.org/wiki/Proportional_counter

Usually the detector is filled with a noble gas; they have the lowest ionization voltages and do not degrade chemically. Typically neon, argon, krypton or xenon are used. Low-energy x-rays are best detected with lighter nuclei (neon), which are less sensitive to higher-energy photons. Krypton or xenon are chosen when for higher-energy x-rays or for higher desired efficiency.

Often the main gas is mixed with a quenching additive. A popular mixture is P10 (10% methane, 90% argon).

Typical working pressure is 1 atmosphere (about 100 kPa)

Neither mentions noble gas percentages except for P-10 of course.