[ RadSafe ] Notes from the workshop: The search for tramp DU, Towed Gamma Scintillator Arrays, Radiometric mapping.
GEOelectronics at netscape.com
Sat Dec 4 13:35:25 CST 2010
A petro-geologist friend of mine developed a program that melds GPS data and radiometric spectroscopy data
into a map. Actually I have two friends who have done this, each with different goals and very different end results and displays.
Shown in the link above is a radiometric map of the type used in oil exploration. Although I have been asked to keep
the details a secret, the "halo" effect is well known. Basically, oil ( or even water) domes beneath the
surface displace naturally occurring radiation
patterns, making a "halo" around or deficit where the deposit exists. Such data is
easily discerned from the positive readings caused by radioactive ore deposits.
If only gross counts ( overall radiation form all sources) is needed, a large area plastic scintillator is deployed, along with rather rapid
grid tracing. If gamma spec data is required, something like a sodium iodide probe or array of probes is used and the electronics can scan for one or more lines simultaneously.Probe dimensions vary but a standard airborne array might comprise of 4 ea 4" X 4" X 16" NaI(Tl)
Shown here is a vintage GAD-6 which is a 4 channel analyzer.
One channel monitors the 1764 Bi-214 line ( U channel),
another the Tl-208 line at 2620 keV (Th channel)
another the K-40 line at 1460 ( K channel) and finally the TC channel monitors the Total Counts, which is essentially what
we call "background radiation".
Temperature compensated sodium iodide probes are used, with on board electronics to generate high voltage, preamp and
wave shaping as well as line drivers, as the cord between probe and metering unit might be very long ( as in down-hole monitoring).
A fifth channel, located far from the regions of interest, called the "stabilizer channel" monitors a phantom energy line signal generated
within the probe by either an imbedded isotope
or a light source. This "stabilizer channel" will drift with changing temperature, but this drift is recognized by software, causing the
high voltage to be adjusted until the signal is once again in the correct "window". All stabilized analyzers use some such feedback
loop trickery to compensate for varying field conditions. Slick
Early scintillometers from the 1950's sometimes contained as many as 5 meters, each reading radiation or magnetic flux in
a different way. One meter was set to be zeroed against normal background radiation, then when that level was reduced, as
over a slat dome, the meter gave a positive deflection. Clever, and effective.
Today's computer and GPS assisted methodology makes surveying a large area much easier, precise, and even fun!
New London Nucleonics Lab
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