GM calibration question

[Lester Slaback <Lester.Slaback@NIST.GOV>]

While one can hypothetically address the possibility of calibrating a
detector that does not have an energy independent response relative to the
parameter to be measured, e.g., dose equivalent, I would assert that it is
poor practice to take this approach if adequate detectors are available.
For neutrons one usually must do spectral adjustments.  For photons plenty
of good detectors are available.  Bare GM detectors, e.g., pancake tubes,
and non-energy compensated detectors, e.g., side window GM's in a steel
housing, are not among these choices.

While all of us can identify situations where these bare GM detectors can
be used to make a good dose measurement lacking specifics about the desired
measurement to be made, as a matter of good practice I would direct the
questioner to another, more energy-independent detector.

Note also that when a previous responder stated that you had to match the
calibration spectrum to that being measured that statement includes all
components of the field (e.g., scatter), not just the isotope identity.
For a detector like a bare GM that over-responds to low energies it is
particularly sensitive to those components of the field.  That is why minor
changes in source collimation for some source designs can significantly
affect a calibration for such detectors.


Disclaimer:  the above are the personal musings of the author, and do not
represent any past, present, or future position of NIST, the U.S. government,
or anyone else who might think that they are in a position of authority. 
Lester Slaback, Jr.  [Lester.Slaback@NIST.GOV] 
NBSR Health Physics 
Center for Neutron Research 
NIST
100 Bureau Dr.  STOP 3543 
Gaithersburg, MD  20899-3543 
301 975-5810 voice
301 921-9847 fax
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