Answer to Question #9011 Submitted to "Ask the Experts"

In my new house, the floor of one room was recently paved with a cement mixture of grit and some fibers of glass (the fibers are white, about 5–8 cm long, and apparently used as a binder), while a nearby room remains unpaved. In the paved room, an "Inspector XP+" instrument registers 0.45 µSv h^-1 in contact with the paved floor. In the unpaved room, the maximum level is 0.11 µSv h^-1 (background for my country is 0.25 µSv h^-1). What values should I expect from this instrument?

The Inspector instrument uses a thin-window, pancake-style Geiger-Mueller (GM) detector. If it is used without a covering over the window, the detector may exhibit some appreciable energy dependence in its photon response when exposed to photon energies different from those used in the calibration. It is likely that the detector was calibrated with 662 keV photons from ¹³⁷Cs. Photons emitted from the paving most likely include different energies, and the readings may not accurately represent the true dose rates, but the relative readings (between the paved and unpaved floors) should still be legitimate.

I suspect that the excess radiation from the cement/grout that was used on your floors comes from naturally occurring radionuclides such as uranium and thorium and their possible decay products, as well as ⁴⁰K that frequently is present in materials that come from the earth. I would not expect the glass fibers to include appreciable radioactivity, so it would not add much to the measured dose rates.

The gamma radiation emitted by the natural radionuclides in the paving mix presumably produced the higher readings, but if the detector was used with no added covering over the thin window, it is possible that part of the response could be due to low-penetrating particulate radiation, especially beta radiation from uranium decay products. I would recommend that you make a measurement about 1 cm above the paved floor surface with the thin-window detector uncovered, then make a measurement at the same distance with about 1 cm of plastic or wood between the floor surface and the detector. If the measurement with the plastic or wood in place is much lower than the measurement with no plastic or wood, it is likely that much of the original reading is associated with low-penetrating particulate radiation. The reading with the plastic or wood is more indicative of the gamma radiation dose rate near the paving surface and provides an overall better indication of the penetrating dose close to the surface. If you take a reading about 1 meter above the surface, you will get a better idea of the dose rate at a height that is more meaningful for purposes of estimating effective dose to adults.

I don't know what country you are in, but the value of 0.25 µSv h^-1 that you cite as typical background for your country is about two to three times higher than the typical external radiation background level in the United States. The value of 0.11 µSv h^-1 that you cite for the unpaved room seems reasonable, although it is two times less than what you cite as the typical background in your country.

I think you will gain some more informative insights by making the measurements suggested above. In any event, even if the readings you have provided turn out to be realistic estimates of gamma dose rate, I would not recommend undertaking any difficult remedial changes to the rooms that have already been paved. The levels that you cite are not high enough to represent a significant increased risk in any room. There are several locations in the world where natural background levels are much greater than the level you measured directly above the paved floor, and such locations have not been associated with any deleterious health effects. If the readings are realistic, and you had not already paved the floors, and you had an option not to pave the floors, I would recommend that you consider an alternative floor treatment since the elevated radiation level, although small, does represent an unnecessary and avoidable exposure.

I hope you enjoy your new house.

George Chabot, PhD, CHP