[ RadSafe ] measurement of Kr-85 using Overhoff 400 portable tritium moni...

[JGinniver at aol.com]

Hi,
thanks for responding to my query.  Just to clarify, I'm not concerned  about 
the photon emissions from Kr-85, and I am aware of the two compensation  
detectors in the instrument.  Rather it is the difficulty of estimating the  
number of ion pairs formed per Bq from the betas from Kr-85 compared with the  
number of ion pairs formed per Bq from tritium.  The difficult bit is  estimating 
the average energy deposited in each of the measuring chambers in the  
Overhoff from the Kr-85 betas.  This is difficult because the detectors are  designed 
so that all of the energy of a typical Tritium beta is deposited in the  
detector, whereas the Kr-85 have much more energy and will only deposit part of  
their energy within the detector.
 
For standard Ion chamber instruments it is usually assumed that the  
radiation field exists outside the detector and that any losses from within the  
detector are counterbalanced by further photons/particles entering the  detector.
 
Unfortunately in this case the gas is inside the detector as it  was sampling 
from and returning to a duct (i.e. it was not immersed in a  cloud) any betas 
that leave the detector are lost and are not balanced by  further betas 
entering the detector.
 
What I'm hoping is that someone in Radsafe land may either have undertaken  
claibration/testing of an Overhoff 400 with a know quantity of Kr-85 and so 
have  a conversion factor, or alternatively may have done some montecarlo or 
other  calculation to estimate the Mean Free Path of Kr-85 betas in a given 
detector  size or volume.
 
Here's hoping,
                Julian
 
For a better summary of the problem I would recommend Page 147 of Radiation  
Detection and Measurement (3rd edition), Glen F Knoll, Wiley, ISBN  
0-471-07338-5