[ RadSafe ] Thorium in gas lamps -gamma cal for Ge detectors
farbersa at optonline.net
Fri Jun 23 12:21:19 CDT 2006
In terms of using a Thorium lantern mantle as a check source, you can also
use them to do a quick two point energy cal on a germanium gamma detector.
In the absence of other known energy check sources which might not be
available to you at the time, there are many easily seen gamma peaks to the
Th series present in a lantern mantle. Just select the 583 keV peak from
Tl-208 [30.9% abund] and the 2.615 MeV peak from Tl-208 [35.8% abund] center
your cursor on each of these two major peaka, set the peak energy to the
known major peak, and voila your detector is ready to identify unknown
peaks by energy on subsequent counts.
I used this technique to cal a portable Ge[Li] detector and MCA I had been
sent [without check sources] to try out back in the early 1980s. Did an
energy cal using the lantern mantle, took a bucket of [cold] wood ash out of
my Hearthstone free standing woodstove [a GREAT! unit] after returning home
from a trip, and counted the wood ash [wrapped the detector in plastic
and inserted it into a bucket of ash to get good geometry] while I went out
for pizza on a Friday evening. Came home to find dozens of peaks from the U
and Th decay series and ONE MAJOR PEAK that dwarfed all other peaks and had
exceeded the counts full scale.
After expanding counts full scale from 512 counts to 32K counts full scale
[and all other peaks moved to the baseline in doing this], I moved the
cursor on the MCA display over to the one remaining large peak, and found
the large peak had an energy of 661.6 keV. Thus it was obvious woodash
contained major concentrations of fallout related Cs-137 which I later
verified to be typically 10,000 to 20,000 pCi Cs-137 per kg of ash in New
England, 25,000 pCi/kg in northern Florida, and about 300 pCi/kg in
California. Years later after having some quantitative gamma spec
measurements made of the first three wood ash samples from New England, I
authored a Feature Article to the HPS Newsletter:
"Preliminary Study of Cs-137 in Wood Ash and Its Implications for BRC, Waste
Disposal, and Dosimetry", Health Physics Society Newsletter, Vol. 18;
Feature Article, Pages 1-5, 1990
This article called for scientists all over the US to make wood ash Cs-137
measurements in calibrated systems since the first quantitative sample from
my fireplace in Northern Vermont had Cs-137 measured at about 20,000 pCi/kg.
A mesurement near Vermont Yankee in southern VT over 100 miles distant from
my home showed Cs-137 at about 2,000 pCi/kg and this kind of variability of
Cs-137 in wood ash needed to be documented to understand spatial variability
and avoid false claims of nuclear facility impacts. These subsequent
measurments made by a dozen or more labs and research centers, with data
sent to me, resulted in a paper I prepared and presented "Cesium-137 in
Wood Ash -Results of Nationwide Survey, Presented at the Annual Meeting of
the Health Physics Society, Washington, DC, 1991.
But of relevance to the original thread, all these subsequent measurements
of Cs-137 in wood ash started with one measurment using a portable,
hand-held Ge[Li] detector with a basic energy cal done with a Thorium
Stewart Farber, MS Public Health
----- Original Message -----
From: <LNMolino at aol.com>
To: <radsafe at radlab.nl>
Sent: Friday, June 23, 2006 11:30 AM
Subject: Re: [ RadSafe ] Thorium in gas lamps
> In a message dated 6/23/2006 1:59:24 A.M. Central Daylight Time,
> ksparth at yahoo.co.uk writes:
> Do they serve as check sources? Do you sell them?
> Almost every instructor Know that teaches in the area of radiation
> for First Responders uses them as they are great to hid in a room or even
> the pocket of a "victim" for dirty bomb drills.
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