Re: Radiation Detector Calib

["Mike Bohan" <mike.bohan@yale.edu>]

        Reply to:   RE>Radiation Detector Calibrations

Hi Eric,

We calibrate our Ludlum Model 3's, which are used for therapeutic survey work,
(pancake GM's - Model 44-9) with our Cs-137 brachytherapy tubes because they are
traceableto NBS/NIST.  We calibrate our Nuc. Med. GM's with a Tc-99m source
calibrated in a dose calibrator which meets all linearity, accuracy, geometry
and constancy requirements.  We do this to account for the GM's over-response at
140 keV compared to the response to Cs-137's 660 keV.  Typical over-response is
approx. a factor of 2 high.  

This allows the GM survey meter to serve both as a detection and measurements
device, but it is only appropriate when used as intended.  You can't take the
Tc-99m calibrated Nuc. Med. GM to do Cs-137 measurements without correction
factors.  Otherwise, in Nuc. Med. you must use a detector with a flat energy
response (e.g. Ion Chamber survey meter) for accurate measurement use.

We also develop correction factors for I-125 (30 keV) for our therapy GM's which
usually over-respond by a factor of 4 when calibrated with Cs-137.  

You can also develop cpm correction factors for GM's if your going to use them
for beta emitter survey work.  The pulse generators only check the pulse meter
counting accuracy not it's radiation measurement calibration and the NRC  (and I
imagine agreement states) will not consider it an appropriate calibration method
for survey measurements required by license or regulation.

We use a spreadsheet to calculate the distances from the source to the desired
radiation calibration point.  However, for Tc-99m you must account for the decay
of the source during the calibration process, so we create a series of
calibration ranges corrected for decay every 30 minutes (~ 5% total error).

Mike Bohan

<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><
Michael J. Bohan, RSO   |  e-mail: mike.bohan@yale.edu
Yale-New Haven Hospital |    Tele: (203) 785-2950
Radiological Physics    |     FAX: (203) 737-4252
20 York St. - WWW 204   |    As usual, everything I say may be plausibly
New Haven, CT    06504  |    denied at my employer's convenience ...
><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>   

--------------------------------------
Date: 26/06/96 12:49 PM
To: Mike Bohan
From: radsafe@romulus.ehs.uiuc.edu
Time for a new topic.  

I am trying to determine the "best" way to calibrate a survey instrument (i.e.
Ludlum 
III.)  There have been two methods proposed: A pulse generator or a Cs-137
Calibration 
source. Any information would be appreciated. 
-- 
Eric A. Goldman, B.S.,CNMT	
Baylor University Medical Center
Department of Medical Physics/Radiation Safety
Dallas, Texas  75246
(214) 820-3870

"Gotta run, the cat's caught in the printer."