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We were lucky enough to run into Bob Shannon on the Radsafe forum. Bob is a radiochemist and was gracious enough to go over the ARS Houston Bordeaux lab results and give us his personal opinion on the report. I removed much of the technical info and left in what many homeowners would find interesting.

Much of this info has to be quite technical, but a quick review of this report would be that Bob found that the radium 226 reported might be a little bit high, that based on the radon present, the radium 226 most likely present in the sample was in the range of 400 to 900 pCi/g. He also pointed out that the results actually indicate that Cs-137 and Co-60 show that are not detected in the sample and that Sc-46 is an anomalous reading, an artifact of the radon. Now for a more detailed version of Bob’s findings on this ARS lab report.

First of all, Bob points out that the lab noted in the report that the Ra-226 results could be a bit high. This is because Ra-226 has a photopeak at the same energy as U-235 (gamma spectrometry uses ‘photopeaks’ to measure radionuclides). Uranium is the parent product of radium. Given enough time (that is geological timescales), the uranium undergoes decay and results in radium. Since uranium 235 is present naturally in most geological products, the levels of radium 226 reported by the lab may show a high bias.

Bob also points out that it is possible to estimate the amount of radon in the sample.

From looking at the bismuth-214 and lead-214 results (decay products of radon 222), we get an estimate of the amount of radon trapped within the sample at the time of the analysis. In order to do the gamma spectrometry measurements on the stone, the lab crushes and grinds the sample, so some of the radon in the original stone is lost before the analysis. Counteracting this, however, once the stone is ground and sealed in a container, the radon increases in concentration over several weeks time until its activity nearly equals that of Ra-226. That is, in fact, one of the best ways to measure Ra-226 activity by gamma spectrometry as it is not susceptible to uranium 235’s interfering gamma rays. Of course, one must be willing to wait the several weeks to get better results.

In our case, without more info on how the sample was prepared or how long it was stored before the analysis, it is pretty reasonable to assume that less than half of the radon originally present in the stone was lost prior to the analysis (in reality the losses are probably much less than this but we want to be conservative when we make assumptions). In other words, it is reasonable to assume that the lead 214 and bismuth 214 only account for something ranging between one-half and all of the radium 226 activity present.

In all, one can try and make some conclusions about the first sample (the one with the highest activity) in the ARS lab report:

· It is reasonable to assume that the losses of radon 222 in this material are less than 50%. From looking at the lead 214 and bismuth 214 results as indicators of the radon 222 in the sample, it is seems reasonable to conclude that radium 226 in this sample would be somewhere in the 400 to 900 pCi/gram range.

· If you estimate Ra-226 concentration using the lead 210, the radium 226 activity would probably be roughly in the 400 to 500 pCi/gram range. (Bob notes that radium 226 and lead 210 are often present in similar activities in geological materials and that measuring lead 210 by gamma spec in materials that are very dense can be difficult and the result uncertainty may be higher than expected and certainly higher than one would see for lead 214 and bismuth 214 measurements)

· If you attempt to estimate Ra-226 using typical concentrations of uranium-235 you might conclude that there is somewhere around 800 pCi/gram of Ra-226 (Bob notes that U-235 is often present at roughly 1/20 the activity of radium 226 in geological materials).

· In the end, without allowing 3 weeks for radon ‘ingrowth’ or without specifically testing for Ra-226, these conclusions are not much more than educated guesses.

As far as some of the other results go, the results for Cs-137 and Co-60 are below the detectable concentrations, so these results should be understood to indicate that the two radionuclides are not present in the sample. The lab flagged the Sc-46 as anomalous; it is a spectral artifact resulting from radon decay products in the sample. Finally there are also indications of the presence of thorium chain activity in the ~100 pCi/isotope/g range (once again in the first and highest sample).

Bob had found it interesting so see that the results of the three tests varied so much, reflecting the difference between your average American granite and some of the ones being imported from around the world.

This information was heavily edited for the audience expected in this blog, so any inaccuracies would be mine. Bob works as a consultant and any stone company interested in talking to labs about setting up the right testing or deciphering their own lab reports can contact him. Bob can be reached at the number below or at his email address,

Bob Shannon Tel: 303-432-1137
Quality Radioanalytical Support, LLC (QRS)
BobShannon “at” earthlink.net
(remove the “at” and replace with @ keeps the spam bots away)