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 2.GQ Geiger Muller Counter
 dose estimation with known radiation source

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wergor Posted - 03/18/2018 : 23:52:04

I bought a GMC500+ a few months back and have been using it to record data during 2 medical examinations and am now trying to estimate the dose. The GMC500+ is unmodified.
Tube parameters:
  • voltage: 68kV
  • current: 1.47mA
  • filter: 0.2mm Cu + 1mm Al

for simplicity, assume the geiger counter was subjected to primary radiation.

I know the GMC calculates dose based on CPM, but I don't know which standards are used to calibrate it and how useful the dose calculation is for low energy x-ray radiation.

Can anyone help me with this, or direct me to where I can find more information about the tube and the effect of the filter?
4   L A T E S T    R E P L I E S    (Newest First)
wergor Posted - 03/25/2018 : 22:50:44
thanks for your links, I'll check them out.
the filters are used to filter low energy photons that would otherwise be (almost) completely absorbed by the target and therefore do not contribute to the imaging process, but add to the dose the target (patient) receives. as far as I know, electrons cannot pass the glass of the x-ray tube.
ullix Posted - 03/24/2018 : 07:40:45
My guess is that GQ never ever 'calibrated' their counters, but only used some numbers published for different tubes to slap on the GQ devices. I have published my findings in this forum post:

While this may actually be justified, there is no documentation anywhere to support this conclusion. As we don't know better, let's pretend the GQ tubes behave just as the SBM20 tubes.

So, GQs calibration is: 0.0065 (Sv/h) / CPM

The SBM20 have:
for Ra226: 0.0058 (Sv/h) / CPM
for Co60: 0.0076 (Sv/h) / CPM

Your X-rays at 68kV are therefore closer to the Ra226 radiation pattern thanCo60, but it remains difficult to credibly extend the Ra226 curve to the low end of energy.

I don't know what the filters do, but I guess they are supposed to catch any electrons resulting as an unwanted byproduct in the X-ray gun. The 2 layers of a heavier and a light atom suggest it. See in the discussion of Bremsstrahlung in my Potty Training paper ( ). At 1.2mm they will have only a minor effect on the X-ray yield.

At the end it remains impossible to answer what amount of Sieverts you have received. My guess is the 0.0058 from the Ra226 will give you an upper estimate.

wergor Posted - 03/21/2018 : 06:13:58
I do not have a standard to calibrate the GMC with.
I do not think the dose reading is a good approximate because neither the factory calibration standard nor the quantum yield of the geiger tube as a function of energy is known. E.g. if I assume the quantum yield to be constant across the detectable energy range and the calibration relates to a 60Co source (1.17MeV and 1.33MeV gamma), my readings (max 0.07MeV) would be 16-20 times too high (for a 137Cs (0.66MeV gamma) source: ~10 times too high) - not a good approximation
ZLM Posted - 03/20/2018 : 04:37:27
I think you can use the GMC-500+ reading as an approximate reading. Unless you have a standard source or meter to calibrate it. You can change the conversion rate from the device menu under Init Setup -> Calibrate.

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