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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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