Alpha and Beta Doses

DOE-STD-1136-2004

Guide of Good Practices for Occupatio nal Radiation Protection in Uranium Facilities

Figure 6-3.

Changes in Beta Energy Spectra and Shallow Dose Rate From a Natural Uranium Metal

Slab Source Caused by Protective Apparel (Note the bremsstrahlung peak in the low-

energy ranges.)

Uranium is an alpha-emitter and is of concern if inhaled or ingested into the body. However, the

skin is an effective barrier to alpha particles, and alpha radiation emitted from external contamination is only

a problem if there is a wound or break in the skin.

Beta radiation fields are usually the dominant external radiation hazard in facilities requiring

contact work with unshielded forms of uranium. Figure 6-1 gives the estimated beta dose rates from a

semi -infinite slab of uranium metal of various enrichments. For uranium enrichments up to 30%, the beta

radiation field is dominated by contributions from 238U decay products. Thus, for uranium of these

enrichments, one is dealing essentially with 2.29-MeV (Emax) beta particles from 234mPa, the most energetic

contributor to the beta exposure.

Beta doses to the skin, extremities, and (sometimes) the lens of the eye can be limiting in facilities

that process unshielded depleted, natural, or low-enrichment uranium. Absorbed dose rates as a function of

depth were measured by P. Plato (Plato 1979) with an extrapolation chamber in a tissue equivalent medium

(Mylar) (See Figure 6-2). Skin doses at less than 4 mg/cm2 resulting from alpha particles are of no concern

from an external radiation exposure standpoint. Potentially significant skin exposure from uranium occurs

primarily from the 234mPa betas at tissue depths of 4 mg/cm2 and greater.

Processes that separate and sometimes concentrate beta -emitting uranium daughters are not

uncommon in DOE uranium facilities. Surface beta dose rates on the order of 1 to 20 rad per hour have

been observed in such circumstances. Exposure control is complicated by the fact that considerable

contact work takes place in facilities that process uranium metal. Beta particles are shielded by rubber

gloves or other protective devices or are usually absorbed within the dead layer of skin. The actual beta

dose to live tissue would depend on the energy of the beta particles and the thickness and types of

intervening shielding.

6-7