Answer to Question #1543 Submitted to "Ask the Experts"

Q

While performing a recent radiological survey for a building demolition, I found some elevated readings using an Alpha-Beta Probe (ABP-100, Nuclear Research Corporation manufacturer). The highest reading observed while on the "Alpha+Beta" setting was 10.14 cps. This reading was taken 5 cm above the concrete floor. The mean background reading (on A+B setting also) taken outside the building was 5.04 cps. Swipe tests have confirmed that there is no surface contamination present. Could these readings be the result of some kind of naturally occurring radiation found in the concrete? Are these readings high enough to be concerned about? The concrete floor is to be demolished and taken to the local landfill so there is a concern about public safety.

A

In that Aptec-NRC has just been acquired by Canberra, I can't seem to find specific information online about their ABP-100 alpha/beta probe. Nevertheless, your background measurement outside the building may not be appropriate, that is, if the probe responds to gamma radiation (for example, if it is a "pancake" style Geiger-Mueller tube). Concrete can have some low level of natural uranium series radionuclides in the cement/sand matrix, which will emit alpha, beta, and gamma radiation. However, at 5 cm above the floor, you would not likely see any alpha radiation. You will however detect betas emitted from the surface and, again, if the probe detects gamma rays, you'll have a potential for an increased background due to natural uranium series in the concrete. In fact, this may be the greater contributor to the reading, in that the gamma will be more penetrating and form a larger source term. I'd suggest evaluating the probe instrument readings inside the building with the probe directed away from the concrete surface and/or shielding the face of the detector probe with a 2.54 cm thick layer of plastic. This should help you determine if you have a gamma response or a beta emission from the concrete. Not knowing what the contaminant radionuclide you're looking for and probe area, I cannot comment on the significance of the elevated readings. For example, if this was a plutonium or ⁹⁰Sr processing facility, and the material was fixed on the surface, a twice background reading may be of concern, compared with normal release criteria. Lastly, the disposal of any potentially contaminated concrete going to a local landfill should be discussed with the operator and regulator of that landfill. There may be permit conditions that prohibit even free released materials from entering the facility. Additionally, the landfill may have radiation alarms set such that a small hot spot or the aggregate radioactivity may trip an alarm. David J. Allard, CHP