[ RadSafe ] Thorium in gas lamps -gamma cal for Ge detectors

Stewart Farber farbersa at optonline.net
Fri Jun 23 12:21:19 CDT 2006

Hi all,
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[4]; 
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 
lantern mantle!!

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 
> response
> for First Responders uses them as they are great to hid in a room or  even 
> in
> the pocket of a "victim" for dirty bomb drills.

No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.394 / Virus Database: 268.9.3/374 - Release Date: 6/23/2006

More information about the RadSafe mailing list