Thoriated Welding Rods and Glass

[dj_strom@ccmail.pnl.gov]

Gamma dose rate from thorium-bearing materials has a funny 
time dependence for the first few decades after purification 
of the thorium.  Freshly-purified thorium experiences a drop 
in hard gamma dose rate for about decade as the unsupported 
1.9 year Th-228 decays away, only to be re-supported by the 
ingrowth of 5.75 year Ra-228.  With large volumes of 
thoriated materials, the dose rate comes from the amazingly 
hot 2.6 MeV photon of decay product Tl-208.  I've measured 
gamma dose rates of about 1 uSv/hour (that is, 10 times 
background) next to pallets of zirconium sand (used in the 
steel industry), which contains primordial thorium (as 
opposed to freshly-purified thorium).  I concluded that dose 
rates of up to 2.5 uSv/hour would be possible in a warehouse 
full of "zirc sand."  I've also encountered a thoriated 
glass lens that read 40 uSv/hour with a beta cap on an ion 
chamber.  Spectroscopy showed a large proportion of the 2.6 
MeV Tl-208 photon.  Look for that peak as the highest energy 
peak in a 3X3 NaI background spectrum (along with the 1.46 
MeV K-40 line, useful for calibration).  It is seen in many 
building materials, tundish nozzles, some welding rods, fire 
brick, and zirc sand where the thallium has grown in.  One 
final item: while working on ultra low-level detection 
systems for the scrap metal industry, we encountered a 
flatbed semi-trailer that had one "hot" structural member 
that had been welded on to repair a damaged one.  It was 
shown to be contaminated with thorium.  That strut triggered 
the gamma sensors every time it came into the yard.