Re: GM Calibration Question

["Eli A. Port" <port@iit.edu>]

At 03:26 AM 12/3/99 -0600, Malek Chatila <mc02@aub.edu.lb> wrote:
>
>Dear All,
>
>I have few related questions concerning the calibration of GM probes that I
>hope you might clarify for me.
>
>1. When calibrating an open-end window GM probe, the probe is placed
>perpendicular to the beam axis, i.e. the window is not directly facing the
>radiation beam, is this correct? 
>2. When calibrating a pancake GM probe, the pancake window is placed
>directly facing the beam, is this correct?
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Both of the above questions raise a more fundamental question about the appropriateness of using either a thin end window or pancake probe for exposure or dose rate measurements.  RSSI operates a commercial calibration service.  we discourage customers from attempting to use thin end window, pancake or scintillation probes for expose or dose rate measurements.  The same applies to microR meters.  These probes are energy dependant and the GM types are also highly geometry dependant.  Geometry dependance introduces unexpected challenges when measuring fields from diffuse or multiple sources. It then becomes impossible to orient a detector as it was when calibrated.  On the occasions when we calibrate an energy or geometry dependant instruments for exposure rate, the orientation and energy spectrum limitations are clearly stated on the calibration certificate.

Other conceptual problems arise in calibrating instruments for dose rate. This problem arises most frequently with instruments made outside the US.  Most manufacturers' instructions are to expose their instruments to a known exposure rate and to apply a calibration factor so the instrument reads in units of dose or dose equivalent rate.  For most commonly used detectors, this only creates the illusion of dose rate measurements.  The detector has actually been calibrated for adjusted exposure rate.  The detectors are frequently not tissue equivalent and the manufacturer does not state whether the hypothesized dose rate is in air, soft tissue, bone, or other material.  

Incorporation of WR and WT probably are beyond the capability of portable survey instruments.  Some exceptions exist.  The HPI 1010 is described by its manufacturer as tissue equivalent and reads in dose rate, but not dose equivalent rate.  Physical and spectrum based logic energy compensation are available for some non-GM detectors.

If anyone wishes to further discuss problems in calibration and interpretation of readings, please call me at 847-965-1999.


Eli Port, CHP, CIH, P.E.
RSSI
<mailto:consult@rssinc.org>
VOICE: 847.965.1999 24X7
FAX:   847.965.1991
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