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19 March 2010

Answer to Question #161 Submitted to "Ask the Experts"

Category: Radiation Basics — Beta Particles (negative and positive) and Electrons

The following question was answered by an expert in the appropriate field:

I'm having a very hard time finding solid information about the radiation field intensities expected at the surface of a depleted uranium slab. We are hoping to use such a slab to test and calibrate beta detectors to establish their beta dose rate response. Such a technique is often mentioned in the literature, but I find very little said about what sort of beta-dose rate to expect at the surface of such a slab. Obviously the exact alpha, beta, and gamma intensities will depend on the precise makeup of this particular batch of depleted uranium, but it would be nice to find a discussion of the general ranges expected for each as well as some examples of how others perform such calibrations. The few mentions I see of this range in the area of 210 to 235 mR/hr beta-dose rate at the surface and somewhere between 4 and 7 mR/hr for gamma at the surface of the slab. (The alpha will be unimportant since we'll have the slab covered by thin (5 mil) mylar to minimize potential contamination). If someone has a good reference for this, or a site URL which discusses this, I'd really appreciate any information. Thanks.

The 7 mg/cm² (dermal) dose rate at contact with a thick uranium slab in metal form, which includes both beta and gamma contribution, is generally assumed to be in the range of 200 to 230 mrad/h--depending on depletion or enrichment. The value is mostly dependent on the beta contribution and will therefore vary depending on additional layers of absorber. The following is an excellent reference for published data on natural and depleted uranium (no, I am not the author of the paper): Preferred Reference:

Coleman, R.L.; Hudson, C. G.; Plato, P. A. Depth-dose curves for ⁹⁰SR and natural and depleted uranium in mylar. Health Phys. 44(4): 395-402; 1983.

Other References:

Handbook of Safety Procedures for Processing Depleted Uranium, Army Material Command Handbook, No. AMCHDBK-385-1.1-89, Department of the Army, Washington, D.C. The relevant charts from the handbook are shown in Question #5 of the 1997 ABHP Exam (the view at this Web site is in a not-completely-clear format).
Radiological Health Handbook, Bureau of Radiological Health, Jan. 1970, Page 204. Rules of thumb for dose rates at the surface of uranium materials (no supporting data).

Robert L. Coleman Oak Ridge National Laboratory

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