Answer to Question #2258 Submitted to "Ask the Experts"

Q

I work daily in the lab with ³²P (five days a week). We have and wear adequate protection (plexiglass of 1 cm and gloves and lab coat) but I do not have a wrist or ring dosimeter (I have a badge). My exposure is external. I am exposed for approximately 15-20 minutes to a concentrated form (stock of ³²P is 1 mCi) and my manipulations take approximately two hours with a diluted form. I use 25.9 MBq as a working concentration a day. I would like to know how would I go about calculating my exposure to ³²P. What is my dose rate or exposure rate? How do I convert from MBq to mSv/hr?

A

The question of what dose one receives from an external beta source has no simple answer.

The dose pattern in tissue irradiated by an external beta source is very non-uniform—it can vary significantly from point to point. There are several reasons for this. Beta particles are emitted from a source with all energies up to a maximum value. The particles travel in tortuous, rather than straight, paths through matter. Like other radiations, their intensity falls off geometrically with distance from the source. As a result, the dose delivered by an external beta source depends upon how the source is distributed and where one looks in the tissue. The dose is largest near the surface of the tissue and drops off to zero as one moves in any direction beyond the range of the particles. Any relative motion between the source and the irradiated tissue also affects the dose pattern. Under federal regulations for protection purposes, within ALARA the occupational exposure of an individual to external beta radiation is controlled by limiting the annual shallow dose equivalent (defined as the dose equivalent at a depth of 0.007 cm) to the skin or to any extremity to 50 rem and the annual dose equivalent to the lens of the eye (at a depth of 0.3 cm) to 15 rem. Accreditation procedures exist for dosimeters that can be calibrated to perform these tasks. The regulations also specify how dose-equivalent averaging over different areas of the exposed skin is to be carried out for control purposes.

There is no gamma radiation from ³²P, and the beta particles would not be able to penetrate the 1 cm plexiglass you mention.

In response to your specific question—"How do I convert from MBq to mSv/h?"—I offer the following. The answer (which would probably be a pretty rough estimate) would depend upon where in the tissue you wanted to know the dose and upon the specific conditions of the exposure time and distances. You might, alternatively, be interested in an average dose, say over the first 2 mm of depth in a typical exposure situation. However, any such average would depend numerically on the conditions that define it. Rather than dealing with the complexities of beta dose, the two operational controls mentioned above are deemed adequate to prevent deterministic effects from occurring with exposure of the skin and lens of the eye.

James E. Turner, CHP, PhD