Author Topic: Flow Proportional Counter Backflow Gas Regulation  (Read 4598 times)

Karsten Goemann

  • Global Moderator
  • Professor
  • *****
  • Posts: 194
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: 147
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: 147
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: 413
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

  • Professor
  • ****
  • Posts: 27
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: 413
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: 147
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

Ben Buse

  • Professor
  • ****
  • Posts: 413
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #37 on: January 22, 2019, 08:19:13 am »
Here's some initial results

Gas in at Sp3, out at Sp1.



Also shown are the maximum height and the FWHM of the bias scan

« Last Edit: January 22, 2019, 08:23:24 am by Ben Buse »

Probeman

  • Emeritus
  • *****
  • Posts: 1947
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #38 on: January 22, 2019, 08:48:43 am »
Here's a dumb question:  I know that the JEOL gas flow spectrometers are generally(?) connected in series as Ben mentions above (gas in at Sp3, out at Sp1), but I suspect that the Cameca (gas flow) spectrometers are connected in parallel.

From a recent visit to PNNL, I noticed that their P-10 bottle is tiny and yet they claim it lasts around a year. However on the Cameca there are 5(!) bubblers all running at the same time.  So even though we use a full size cylinder, our P-10 cylinder only lasts about 4 to 6 months.

Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?
john

Edit by John: Edgar Chavez confirms the Cameca WDS spectrometer P-10 gas flow is in parallel.
« Last Edit: January 22, 2019, 11:05:07 am by Probeman »
The only stupid question is the one not asked!

Karsten Goemann

  • Global Moderator
  • Professor
  • *****
  • Posts: 194
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #39 on: January 22, 2019, 02:27:02 pm »
Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?

I don't think doing in series for Cameca is possible as they run at different pressures ("low" pressure similar to the JEOL P10 channels, and "high" pressure for higher kV where JEOL uses Xe). You would need at least two "chains" one low, one high pressure?

That's probably also the reason that the JEOL P10 bottles last that long? Their overall P10 usage would be equivalent to 1 low pressure spectro on a Cameca?

Cheers, Karsten

Probeman

  • Emeritus
  • *****
  • Posts: 1947
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #40 on: January 22, 2019, 06:47:05 pm »
Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?

I don't think doing in series for Cameca is possible as they run at different pressures ("low" pressure similar to the JEOL P10 channels, and "high" pressure for higher kV where JEOL uses Xe). You would need at least two "chains" one low, one high pressure?

That's probably also the reason that the JEOL P10 bottles last that long? Their overall P10 usage would be equivalent to 1 low pressure spectro on a Cameca?

Cheers, Karsten

That's a good point. One would need two systems, one for the 1 atm and one for the 2 atm detectors.  But still one would be using (theoretically) half the gas flow for the two high pressure detectors and 1/3 the gas flown for the others.

Based on the bubble rate I don't think the high pressure detectors flow any more gas than the low pressure detectors.  They're both about one bubble per second.
john
The only stupid question is the one not asked!

Mike Matthews

  • Professor
  • ****
  • Posts: 110
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #41 on: January 23, 2019, 09:58:16 am »
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

Probeman

  • Emeritus
  • *****
  • Posts: 1947
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #42 on: January 23, 2019, 11:04:28 am »
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

That is a very good point.

That said, the 2 atm detector windows very rarely leak as they are 1.5 um(?) Be, and on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

https://en.wikipedia.org/wiki/Monty_Hall_problem
The only stupid question is the one not asked!

Mike Matthews

  • Professor
  • ****
  • Posts: 110
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #43 on: January 24, 2019, 10:24:05 am »
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

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

Hmm, 3 doors and only one is leaking. All you need is three trolls that you’re only allowed to ask one question of and you’ve got a proper riddle.

Probeman

  • Emeritus
  • *****
  • Posts: 1947
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #44 on: January 24, 2019, 02:34:57 pm »
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

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

Hmm, 3 doors and only one is leaking. All you need is three trolls that you’re only allowed to ask one question of and you’ve got a proper riddle.

Hi Mike,
Well seriously, it is an interesting dilemma because it would be nice to see which spectrometer has a leaky detector window.  That said, we haven't had a bad detector window since we added these "soft start" values to our roughing pumps about 5 years ago:

https://probesoftware.com/smf/index.php?topic=120.msg487#msg487

And I sure wouldn't mind if our P-10 gas lasted several times longer!

I'll run it by our instrument engineer and see what he thinks...
The only stupid question is the one not asked!