RE: GM calibration question

[Fred_Haywood@radian.com]

To All:

In about 1960, Sam Hurst and Brian Wagner, ORNL Health Physics Division,
solved this problem by using a thin graded tin and lead filter around the
sensitive area of the tube. This reduced the peaked low-energy response and
resulted in a response protortional to roentgens over a wide energy range.
The initial application was for rather elevated radiaiton fields, so the
first GM tube to be used was a small Phillips halogen quenched tube. Later,
John Thorngate and Phil Perdue (also ORNL) extended the filter to larger
tubes and we used it for many applications. At least a few companies offer
this compensated tube in today's catalogs, for example the Eberline HP-270,
HP-290, and the HP-300 are commercially available. I am sure the other
leading instrumentation firms also have this type tube.

Fred Haywood

> -----Original Message-----
> From:	Lester Slaback [SMTP:Lester.Slaback@NIST.GOV]
> Sent:	December 03, 1999 2:37 PM
> To:	Multiple recipients of list
> Subject:	GM calibration question
> 
> 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
> ************************************************************************
> The RADSAFE Frequently Asked Questions list, archives and subscription
> information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html
************************************************************************
The RADSAFE Frequently Asked Questions list, archives and subscription
information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html