Author Topic: Off-peak background intensities and Mean Atomic Number  (Read 28 times)

Hwayoung Kim

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Off-peak background intensities and Mean Atomic Number
« on: October 20, 2019, 11:08:38 pm »
Hi all,

I would like to ask for a help or comment about sudden increase of off-peak X-ray background intensities from samples of similar mean atomic number range under same analytical conditions.

The issue is.. we have been measuring trace elements from olivines of varying compositions through 4 different sessions with single conditions and settings:
  - 13 elements : Si, Mg (EDS)  /  Na, Al, K, Ca, P, Fe, Mn, Zn, Ti, Cr, Ni (WDS)
  - 15kV, 500nA, 3um beam
  - 360s for peak, 180s for upper and lower background (off-peak)
  - exponential modeling for background, +-2mm background positions

and these are measured and interpolated background intensities at Al Ka position with TAP crystal:

(Time interval of 3 weeks between 1st and 2nd sessions, 2days between 2nd and 3rd sessions, and 6 hours between 3rd and 4th sessions)

You can see the background level at 2nd & 4th sessions are higher than those of 1st & 3rd sessions. (Beam, spectrometer and detector settings are *exactly same* for all sessions, except minor change of peak position). And this increase in seen for all WDS elements, not only for Al.

And this is background X-ray intensities vs. calculated mean atomic number (Z-BAR) of olivines:


Mean atomic number (MAN) range from 10.5 to 13.5 is due to different olivine compositions. And background level between 1.2 and 1.4 (cps/nA) of 1st and 3rd session seems normal, because they sat on a range of past background measurements using our probe from other matrices of similar mean atomic numbers. Why are there two different correlation lines of background vs MAN even though we used same analytical settings?

All olivine grains were in a single mount and vacuum conditions were constantly good through entire sessions. There are daily temperature variations in our lab but the 3rd session took 3 days, so we can rule out possible problems in coating, vacuum and temperature.

So far, we can not find any reasons for this dichotomy. Any comments would be helpful.
Thank you.
« Last Edit: Today at 01:28:18 am by Hwayoung Kim »
Hwayoung Kim
Korea Polar Research Institute (KOPRI), South Korea
JEOL JXA-8530F

Probeman

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    • John Donovan
Re: Off-peak background intensities and Mean Atomic Number
« Reply #1 on: Today at 11:48:03 am »
Hi Hwayoung,
Are you only seeing this "dichotomy" in intensities on spectrometer 2?
john
The only stupid question is the one not asked!

Hwayoung Kim

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Re: Off-peak background intensities and Mean Atomic Number
« Reply #2 on: Today at 06:24:05 pm »
Hi John,

No, we are seeing this in all five spectrometers.




Hwayoung
« Last Edit: Today at 07:12:41 pm by Hwayoung Kim »
Hwayoung Kim
Korea Polar Research Institute (KOPRI), South Korea
JEOL JXA-8530F

Probeman

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    • John Donovan
Re: Off-peak background intensities and Mean Atomic Number
« Reply #3 on: Today at 06:30:53 pm »
OK, that has to be a high voltage power supply for the bias voltage for the flow detectors.

Or maybe a bad P-10 bottle?

Are the intensities changing minute by minute?  It's only around 30-50% more counts at background level. Probably you wouldn't notice it when counting peak intensities I assume?

Weird.
« Last Edit: Today at 06:33:42 pm by Probeman »
The only stupid question is the one not asked!

Hwayoung Kim

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Re: Off-peak background intensities and Mean Atomic Number
« Reply #4 on: Today at 07:38:15 pm »
We are not seeing short-term changes, it changes session by session.

One session took more than 1-2 days and the background level is constant within a session. (Checked by chart recorder in JEOL software)

In a session when background intensity increased peak intensity also increased, so background increase does not affect net intensities.

That's why we're thinking this is related to detectors (probably bias voltage or gain?). Our 3, 4, 5 spectrometer has sealed Xe counter so P-10 may not be a problem, I think.
Hwayoung Kim
Korea Polar Research Institute (KOPRI), South Korea
JEOL JXA-8530F

Probeman

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    • John Donovan
Re: Off-peak background intensities and Mean Atomic Number
« Reply #5 on: Today at 07:45:59 pm »
Do the different sessions use different bias voltages on the detectors?

What happens if you change the bias voltage manually?  Do the counts visibly change? 

Can you monitor the detector bias power supply?
john
The only stupid question is the one not asked!