[ RadSafe ] Survey Instrument Question

[Bob Westerdale]

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