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Carl, The first
thing to do is state in detail the question you wish to answer by doing a gamma
scan over a large area. By your last sentence, it appears you want to measure
the extent of residual contamination that produces a cancer risk of 1 in
1,000,000. This requires dose modeling, including ingestion of vegetables grown
on the land, drinking water from wells, drinking cow’s milk, breathing airborne
dust, as well as direct gamma exposure. But this is quite easy using commonly
available environmental dose codes. Then just
equate radiation dose to cancer risk using the Linear Non-Threshold theory. (Please,
no flames.) The big
problem, as I see it, is determining the depth over which the residual
contamination is spread. For
example, does it go down 4 inches of 4 feet? And how is it distributed by depth?
The only way I know to determine this is to drill holes and analyze the core
samples for radionuclide concentration and also doing down-hole gamma readings.
This gets expensive and requires a lot of drill holes. Regards, Wes Wesley R. Van
Pelt, PhD, CIH, CHP Wesley R. Van Pelt Associates, Inc. Consulting in Radiation Safety and
Environmental Radioactivity. http://home.att.net/~wesvanpelt/Radiation.html -----Original
Message----- I am in
the beginning stages of starting my own business of performing radiation
scanning surveys of large land areas. If I was to perform gamma surveys
for license termination in California, what would be the "best"
technology available to perform these surveys? I know sampling will have
to be done and can be counted on a HPGe, REGe or the like, but what about land
area scanning? Typically in the past 2 x 2 NaI detectors were used, and
large area plastic scintillator (LAPS) detectors are comparable, but the
"Best" is . . . .? I have thought about getting a 4 x 4 x 4 NaI
but then a 4 x 4 x 16 NaI would be better. . .then four 4x4x16 NaI
detectors would be better still. But then this large of a detector array
would mean I would be averaging a point source over the field of view of the
detector system . . . I run into the same line of thinking when I try to figure
out how fast to scan. I typically take a moving one second count
with a LAPS using a GPS, but wouldn't a one minute static count be better
before moving on? What detector and scanning method is out there that can
measure gamma emitting radioisotopes to a 1 in a 1,000,000 cancer
risk? Carl Speer Real-time
Radiological Services, Inc Las Vegas, NV 702-639-0066 |