Answer to Question #5271 Submitted to "Ask the Experts"

At some nuclear facilities lead aprons/lead vests are used in radiation fields from ⁶⁰Co. The attenuation is quite low because of the high-gamma energy (probably normally just 1-2 percent) but it gives some personnel a secure feeling knowing that actions are taken and looking from a collective dose point of view there will probably be a small gain. To consider using lead aprons in some applications I would like to know how to wear the TLD badge, inside/outside the lead apron. Should we do multiple badging? Is secondary gamma photon emission of significance to determine/measure the whole-body dose? Can the interaction of gamma photons/lead (Compton effect) result in no gain or even worse? What are the other aspects of using thin shields of high-density material near the body in 0.6-1.3 MeV radiation fields (⁶⁰Co, ¹³⁷Cs)?

Let's start with the question of TLD measurements under the apron. Depending on the thickness of plastic between the apron and the TLD, it is very likely that the TLD will read higher under the apron than without the apron, particularly because of the increased electron fluence from the high atomic number components of the apron. This should not be seen as "erroneously high" because if the person's skin is there, he/she will actually receive that dose, which leads to the observation that wearing the apron may reduce the deep dose by 1 percent (or not, depending on the lead equivalent) but increases the surface dose. Since the aprons are heavy, wearing them will also likely make the wearers move more slowly, INCREASING their exposure due to increased time. Thus, for the illusion of reducing a person's exposure, wearing a lead apron may make the exposure worse.

As to where to wear a badge, multiple badges would be best until you determine how your badges compare under the apron to over. The outside badge may give a better indication of the deep dose if the under badge is affected by the increased fluence from the lead (again, that depends on the construction of your badges). But you also may want to know the surface dose under the apron. The badges may, or not, tell you that, again based on their construction. Almost no badge is intended to record the dose from high-energy photons under a high-atomic number apron, so the under badge may give funny readings. The outside badge may also be affected by increased backscatter from the apron, so it should be worn somewhere other than directly on the apron.

Life gets complicated, doesn't it?

Bruce Thomadsen, CHP