Re: Computer-Simulated HPGe Gamma-Ray Spectra-Fission gases

[fbronson@canberra.com]

The answer is quite specific to the age of the fission products [i.e. the Xenon 
composition and the Iodine composition], the detector type, and the sample 
geometry.   So, "real spectra" but from other systems, may not represent the 
situation on your system.  But, based upon TMI experiences measuring similar 
ratios, I suspect you can see I-131, even in the presence of 1000x the 
concentration of Xe133/135.  You may not be happy with the MDA, however.  

There is a simple code, SYNTH, that generates spectra for a somewhat generic Ge 
detector and for a simple cylindrical source geometry. I think I have one of 
the few legitimate copies of it.  I don't know if it is available any more.  
But, if you will fully describe the sample-detector geometry, the Ge detector 
efficiency, diameter, and thickness, the radionuclides that are present, and 
the concentrations of each of those nuclides, I will be happy to generate the 
spectrum for you.  But, while it makes  a pretty picture, there is little data 
validating the accuracy of the results.  It may or may not be sufficient;  you 
will have to decide.

We have much experience designing complex Ge spectroscopy systems with more 
accurate computer codes.  We normally use MCNP to perform primary efficiency 
calibrations of complex sample-detector geometries, such as this.  We have much 
data validating this process.  We also use MCNP to generate the primary 
detector response characteristics for our ISOCS product, which the user uses to 
do efficiency calibrations on devices such as this;  but ISOCS doesn't generate 
spectra.  The input for the MCNP model requires an exact knowledge of the 
source, the detector internals, and everything in-between the source and the 
detector.  To generate full and accurate spectra, including the low energy 
scatter from higher energy nuclides, requires much more modeling.  Everything 
that causes scatter [e.g. things behind the sample] must be included in the 
model, and the computer time is much longer, as some of the computer tricks to 
reduce computation time are no longer valid.   So, we must charge for that 
service.    

Please contact me if you want more details.

Frazier Bronson  CHP
Canberra
Meriden, CT
203-639-2345


YOU WROTE:

The Illinois Department of Nuclear Safety, would like to obtain HPGe 
gamma-ray spectras of fission product gases with different fractions of 
iodines and noble gases.  We are interested in learning, whether or not 
iodines can be resolved in the presents of fission product nobles gases, 
1000 times the quantity of the iodines.

If you are willing to contribute a real spectra or know of a 
computer code that can model HPGe gamma-ray spectra, your help would be 
appreciated.

Melanie Caspary
Illinois Department of Nuclear Safety
caspary@idns.state.il.us
tel: 217-786-6366 
fax: 217-786-7223