Answer to Question #1825 Submitted to "Ask the Experts"

Q

While checking my son's rock collection with my Gammascout radiation detector, one of his rocks registered a maximum of 0.59 microsievert (µSv)/hr. at a distance of about 1/2 to 1 inch from the detector. The rock is about 1/4 to 1/2 cubic inches in size. Is there any hazard in keeping this rock or holding it or the dust on the outside of it? During the test, the rock was held for about 5 to 10 minutes. There is a chance that the rock could have uranium in it because my son had an educational rock display that listed uranium as one of the rocks included in the set from which he had removed all the rocks that he put with his collection some time ago.

A

It is likely that the rock in question does contain elevated levels of uranium, but keep in mind that all the rocks in your son's collection contain some uranium. One other possibility: if not uranium, the elevated measurements might indicate thorium. For those of us who have trouble with the new units, 0.59 µSv/hr translates to 59 µrem/hr—approximately six times background. And that is at one inch. Beyond one foot, the radiation levels will be indistinguishable from background. It is true that the exposure rate to the hand will be higher when holding the material but, even then, any resulting exposure will be extremely low. Even the accidental ingestion of the small quantities of dust associated with the rock would pose no hazard. Nevertheless, it is always prudent to wash one's hands after handling material like this. The bottom line is that having this rock in the collection is substantially safer than many other activities he could, and probably does, engage in, for example, riding a bicycle or playing baseball. Paul Frame, PhD, CHP