[ RadSafe ] Survey Instrument Question

[Ted de Castro]

I would wonder about the aspect ratio of the geometry of the tube.  Not 
so much the diameter but the length as this affects the active center of 
the tube and thus the effective distance from the source and a source of 
XRF leakage COULD be very divergent/ high gradient and thus fall off 
rapidly and thus geometry would affect the reading.  When the tube was 
calibrated - surely it was a low gradient field and calibrated at the 
active center of the probe.

You quoted sensitivity at 50 keV and an XRF operating at 50 keV. Well - 
the xrays are a bremstraulung  spectrum with a characteristic but WAY 
mostly low energy brem and a very small percentage of 50 keV.

Also - keep in mind that for surveys around analytical x-ray equipment a 
GM detector is an excellent and sensitive leakage DETECTOR but a very 
poor MEASUREMENT instrument due to a very non dose equivalent energy 
response in that region.  But fortunately almost always the point of the 
survey is to DETECT the leak and deal with it.  MEASURING the level of 
the leakage is pretty much an academic exercise with little practical 
use.  Leaks are so easily eliminated to the level of non detection with 
the GM that trying to survey their level for pass/fail with respect to 
some appointed limit level usually is pretty impractical.


On 7/29/2016 12:41 PM, Bob Westerdale wrote:
> Hi All!
>        We recently had two survey meters calibrated ( same manufacturer, different models)-  both are Geiger instruments, one has a 45mm dia tube, other has a
> very small ( ie about 10 mm dia) tube, both with comparable thickness  Mica windows.   The Energy dependence curves are fairly similar and within 10%  of each other
> at 50 keV,  and both are fundamentally intended to be used at less than 100 mR/hr.   We survey Analytical XRF equipment,   Max.  Tube voltage is 50 kV.
>       Our tech was using both meters during a routine survey, and noticed the smaller device was reading only about 1/3 of what the larger one was seeing.
> We're below 1 mR/hr so pulse pileup or detector time constant issues should not be a problem.  ( well within the published countrate specs)
> The tech asked me to investigate,  so we contacted the Manufacturer ( who also did the calibration)  who responded that:
> " , the size of the window and overall size of the tube, is what gives each unit a different reading"
>
> He maintained that both units were working correctly.
>
> I am aware of the measurement constraints and correction factors necessary when the radiation being measured illuminates only a portion of the active area of the detector,  we're dealing here
> with a fairly uniform radiation field that would fully cover both of the detector's windows.      I always thought that a calibration should include review ( and tweaking if needed) of the  CPS / mR  factor
> that would be used to provide a reasonably accurate ( maybe +/- 15%) determination of the doserate regardless of the detector's dimensions.
> I'm also aware of the limitations of using a Geiger counter for low energy measurements;  I use an ionization chamber  ( with appropriate corrections) when
> an accurate doserate must be determined.
>
> Is the comment from the manufacturer valid?
> Thanks in advance!
> Bob Westerdale
> EDAX, Inc.
> RSO
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