[ RadSafe ] Survey Instrument Question

Joseph Shonka jjshonka at shonka.com
Fri Jul 29 18:37:14 CDT 2016

when using an open window GM tube, some individuals place a plastic cap on the open window when they want to read units of mR per hour.  The calibration was likely performed with the radiation field perpendicular to the axis of the tube. If you are encountering a flux of radiation that enters the open window, One might expect 3 to 1 ratio for those two tube sizes. when surveying analytical x-ray equipment, a GM tube is an excellent choice for discovering penetrations or holes through the lead shielding. 

Joe Shonka

Sent from my iPhone

> On Jul 29, 2016, at 3:41 PM, Bob Westerdale <Bob.Westerdale at ametek.com> 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.
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