Hi Owen,
the sealed counters seem to be fine (gain/bias in good values). Different samples show it, so it does not seem to be sample related.
There are is a test to see if the detector window is deteriorated or charging up. I did not get a good read on charging but otherwise only get a very broad distribution rather than a defined peak for carbon (other elements are fine), which makes me suspect that the window/detector has a problem.
How to check your detector window (from the JEOL manual) in case you are interested:
Using the CKa line, carry out the pulse-height analysis to check the window.
The measurement conditions are as follows:
Analyzing crystal: STE (I use LDE's obviously)
Specimen: Carbon graphite
Accelerating voltage: 10kV
- Perform peak search and set the spectrometer to the peak position
- Open the Full Scale window from the Ratemeter window and the Full Scale to 10 kcps. With the PROBE CURRENT knob on the 1st control panel set the ratemeter bar (counting rate) to read 7,000 to 8,000 cps and leave the instrument for 10 minutes.
- Measure the pulse height distribution using the SCA Scan window
- Using the HV Scan window, a subwindow of the SCA scan window, perform HV scan with Base set to 3.0 V and Window at 0.2 V and measure the voltage that makes the counting rate maximum.
- Open the SCA control window and set High V to this voltage. With this operation the pulse height of CKa is set to around 3 V.
- Open the Base L Scan window, a subwindow of the SCA scan window, and perform Base L Scan measurement with Base set at 0.2V, Window at 0.2V, Step at 0.2V and S time at 1000 msec to check the pulse height distribution of the CKa signal pulse
Inspect the window for deterioration or damage in accordance with the pulse height distribution obtained in the preceding step (see attached Figures).
As the conductivity of the window surface deteriorates (window charging), the X-ray intensity will gradually decrease even if the probe current is kept at a fixed value. Check the window charging as follows:
From the Ratemeter window, open the spectrometer window that shows the settings of the specified spectrometer whose proportional counter window is to be checked for charging. Using the SCA Control window, a subwindow of the Spectrometer window, increase the voltage applied to the proportional counter by 50 to 100 V, then decrease it to the original value. If this operation lowers the X-ray intensity below the value before raising the voltage, proportional counter window is charged.