[ RadSafe ] Survey Instrument Question

Bob Westerdale Bob.Westerdale at ametek.com
Fri Jul 29 14:41:22 CDT 2016

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.

More information about the RadSafe mailing list