Granite radioactivity
In my opinion, "Radiation from granite"
would be best measured in terms of GAMMA RAYs, and RADON contribution ( if
any).
Gamma Ray
quantification:
Even with a Model 19, the sodium iodide
detector vastly over estimates low energy gamma rays and X-Rays as found in
lower Uranium daughters. See:
The only instruments that can give accurate
readings with mixed sources, such as decay chains of U and Th with K-40 mixed in
are called energy compensated. A short list of these would be the Bicron
MicroRem, Ludlum Model 9, Victoreen 440RF/D and most of the so-called "cutie
pie" gun style ion chamber detectors, i.e. Fluke 470A, Eberline RO-3C, RO-5A
etc.
Geiger Muller probes (GM) can be energy
compensated and are available in a very few probes in that configuration, namely
the Ludlum Model 44-38 and Eberline Model 270.
Pancake probes are excellent for detecting
surface contamination, primarily alpha and beta. Their gamma efficiency is less
than 1% and even then is it highly energy dependent.
Alpha and beta particles are pretty much
universally considered to be of no external concern whatsoever. Once the beta
contribution is removed from the granite material's radioactivity profile, I
expect that the true gamma contribution is quite negligible, to the point that
it can probably be accurately measured only with the most sensitive of the above
mentioned meter systems, the Bicron MicroRem. My most "radioactive" granite
sample was gleaned from a granite yard's scrap bin, easily detected by my field
pancake detector. Back in the lab, it does not even register in real time on my
MicroRem Low Energy meter on the 20 MICRO Rem scale.
In my Gamma Spectrum lab, spectrum
analysis is carried out with super sensitive detectors ( 3" X 3" NaI(Tl) ) which
must be encased in thick virgin lead graded shield and have long timed
counts:
Yes granite does contain naturally
radioactive mineral inclusions. So do all rocks, including those used in
construction, concrete, fireplaces, brick ovens firebrick, tile,
ceramic, etc. Of all the building materials listed, most are radioactive
only in the scientific curiosity sense of the word. I do have a small concern
that the next generation firebrick, which will be made from coal fired electric
plant waste fly ash might pose a bit of a rad concern. It is well known that
coal contains a measurable amount of uranium daughters and those will be present
to some extent or another. Not to raise an alarm, but we should and will be
monitoring the material as it become available.
Geo
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