Author Topic: Flow Proportional Counter Backflow Gas Regulation  (Read 2048 times)

Karsten Goemann

  • Global Moderator
  • Professor
  • *****
  • Posts: 171
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #30 on: October 23, 2018, 05:07:50 pm »
Well the P10 flow is very low (around 1ml/min). One could try using a longer exhaust tube and see if the issue becomes less apparent. But if water vapour can diffuse against the flow due to the concentration gradient the same should be true for air...

I was wondering as well if the humidity could affect the detector electronics. But then the effect should also be visible for the Xe detectors, right?

Cheers,
Karsten

Brian Joy

  • Professor
  • ****
  • Posts: 136
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #31 on: October 24, 2018, 07:32:18 am »
One could try using a longer exhaust tube and see if the issue becomes less apparent.

I have ~6.7 m of spare tubing.  I'm going to attach it to the channel 4 P-10 exhaust and resume making measurements.  Currently the length of tubing attached to the exhaust is only 30 or 40 cm (not including the length inside the spectrometer).
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Brian Joy

  • Professor
  • ****
  • Posts: 136
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #32 on: November 24, 2018, 09:57:07 am »
Below is a new plot of GFPC bias as a function of atmospheric pressure with ~22 feet of tubing added to the P-10 exhaust.  I've only had a chance to make a handful of measurements in the past few weeks, but it seems clear that the addition of the tubing makes little or no difference.  The cool-weather regression lines do not include the new measurements.



EDIT 2018-11-28:  I may have made a conclusion based on too little data.  Measurements that I've made during the past few days suggest that bias actually has shifted to lower values (to get a distribution centered at 4 V at a count rate of 5000 s-1).  I'll post a new plot at some point during the winter.
« Last Edit: November 28, 2018, 01:15:25 pm by Brian Joy »
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Ben Buse

  • Professor
  • ****
  • Posts: 390
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #33 on: November 29, 2018, 09:58:11 am »
I’ve continued to add data to my plot of GFPC bias versus atmospheric pressure.  Lately we’ve had some larger oscillations in pressure as is typical of the non-summer months.  I’m curious to know if other JEOL users who have gas flow counters arranged in series see the same pattern that I do:  the spectrometer that serves as the P-10 exhaust 1) requires higher counter anode bias and 2) shows a smaller slope on the plot of bias versus pressure than the one into which the P-10 enters.



Hi Brian,

This is really interesting what you've done, I'm hoping to repeat the measurements. I've just taken some measurements today and yes the exhaust spectrometer has a higher voltage. Pressure 997.8 mb, inlet spec 1710, outlet spec 1718

Ben

DavidAdams

  • Post Doc
  • ***
  • Posts: 14
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #34 on: December 18, 2018, 07:36:59 am »
It's very interesting to see these plots! I'm glad I'm not the only one experiencing this.

What are everyone's P-10 pressures set to? The JEOL engineer here has set our 8530F gas pressure to 0.068 MPa. Does anyone know what the maximum pressure specification for the flow counters is?

Thanks!
« Last Edit: December 18, 2018, 09:01:42 am by DavidAdams »

Ben Buse

  • Professor
  • ****
  • Posts: 390
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #35 on: January 10, 2019, 06:39:33 am »
Hi,

What type of curve should you fit to a PHA spectra (bias scan) - e.g. Gaussian, etc. So far split pearson 7 seems to work.

Ben
« Last Edit: January 10, 2019, 08:06:18 am by Ben Buse »

Brian Joy

  • Professor
  • ****
  • Posts: 136
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #36 on: January 11, 2019, 05:03:39 pm »
Hi,

What type of curve should you fit to a PHA spectra (bias scan) - e.g. Gaussian, etc. So far split pearson 7 seems to work.

Ben

Hi Ben,

The distribution is usually described as "quasi-Gaussian," with σ < √N, where N is the average number of primary ion pairs produced by an X-ray photon.  Fano (1947, Physical Review 72:26-29) showed that σ = √FN = √FE/ε, where E is the photon energy and ε is the mean ionization energy of the counter gas.  For argon, the "effective" F is usually set at 0.8 and takes into account widening of the distribution due to secondary ionizations produced during avalanching.  See Reed's "Electron Microprobe Analysis," 2nd ed., p. 86-87.

Brian
« Last Edit: January 11, 2019, 07:29:58 pm by Brian Joy »
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230