[ RadSafe ] HPGe efficiency calibration source for soil samples

[Philip Simpson]

Carl,

If you have a well shielded HPGe this technique will work for you.

Obtain a known mass of potassium chloride, the "hottest" natural soil sample you can find and a blank such as sugar. Individually encapsulate all three in the same configuration as the Japanese soil you will be counting. Ideally you want the same density as the Japanese soils you want to count, but the sample counting geometry is the most important factor here. Individually count all three (blank, KCl, and soil) using the same geometry that you will be using for the Japanese soil samples and long enough to differentiate the natural soil and KCl from the blank. This will probably take at least 24 hrs each if you are using only a few ounces. I suggest you count the blank two times or more  to see how stable your background is. Use the known gamma abundance and relative background subtracted peak areas of the major Bi-214 gammas in your "hot" soil sample to develop the shape of your efficiency curve.  Pay particular attention to the 609.3 and 1764.5 keV lines.  Note that a HPGe's detector efficiency  can be well approximated as a negatively sloped straight line on a log(eff) vs. energy plot at energies above about 400 keV.  Calculate the K-40 activity in your KCl sample and use the background subtracted K-40 peak area in the KCl to get the absolute efficiency at 1460.5 keV.  Move your shape curve up or down to until it intersects the measured K-40 efficiency and you will have a pretty good efficiency calibration. 

I've not tried this method myself but have done a lot of soil counting with HPGe detectors the last seven years and know it will work.  I'm guessing the efficiency curve generated by this method would be within 10% to 20% of what you would measure with a calibrated source at energies above 400 keV.

Phil Simpson
Assistant Manager (Ret.)
University of  Michigan
Ford Nuclear Reactor


On Sep 20, 2011, at 5:15 PM, Carl Willis <carl.willis at gmail.com> wrote:

> Dear Radsafers,
> 
> This inquiry is directed toward those of you familiar with
> radionuclide analysis of soil or similar samples.  A friend in Japan
> has mailed me (in the US) a bunch of soil specimens collected in the
> environs of Tokyo in order to determine anthropogenic radionuclide
> (e.g. Cs-137, Cs-134) concentrations therein with my HPGe detector.
> We both understand this is a strictly avocational and uncompensated
> pursuit and that I have no accreditation, nor are my capabilities
> likely to satisfy ANSI N42.14---It's basically a hobby.  That said
> though, I would like my effort to be as credible as possible without
> breaking the bank.  Can anyone recommend a multi-energy source I can
> buy off the shelf, license exempt, for calibrating efficiency and
> energy (0.2-2 MeV or so), whose physical attributes are a good match
> for a few ounces of soil in a bottle?  Any perspectives on how much
> such a source costs?  Any creative ideas or tricks for "making" one
> using a more-common encapsulated cal source?  Thanks for any ideas.
> 
> -Carl Willis
> 505-412-3277
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