Answer to Question #4979 Submitted to "Ask the Experts"

While searching the Internet for depleted uranium, I found these statements in the Christian Science Monitor for 15 May 2003: " . . . a Geiger counter carried by a visiting reporter starts singing when it nears a DU bullet fragment no bigger than a pencil eraser. It registers nearly 1,000 times normal background radiation levels on the digital readout. . . . a 3-foot-long DU dart from a 120 mm tank shell, was found producing radiation at more than 1,300 times background levels. . . . [where] the Monitor found the 'hot' DU tank round. This burned dart pushed the radiation meter to the far edge of the 'red zone' limit. A similar DU tank round recovered in Saudi Arabia in 1991, that was found by a US Army radiological team to be emitting 260 to 270 millirads of radiation per hour . . ." Are these reports anywhere near the right ballpark for DU? Can you tell me what the dose rate would be for beta/gamma radiation at the surface of a large thick slab of DU? My interest arose while trying to refute statements found in publications of the ban-DU lobby.

The media reports are actually quite reasonable and certainly in the appropriate ballpark, as you say, for depleted uranium (DU). However, they need to be considered in a somewhat broader and more rigorous scientific context.
Normal ambient background levels from cosmic rays and terrestrial radiation are typically on the order of 10 to 20 microrads per hour, which would produce about 100 to 200 counts per minute in a typical Geiger-Mueller survey meter. Thus, a measurement of 1,000 times this level from a piece of depleted uranium, with the survey meter probe held next to the DU so as to give the highest reading as would likely be the case, is not at all surprising. In fact, this very crude measurement does not take into account certain technical factors such as resolving time losses in counts registered or geometry considerations which might actually result in somewhat higher values.

Measurements of the surface dose rates from a slab of uranium have been reported by a number of investigators in the scientific literature, and recently calculated by Fetter and von Hippel (Global Security 8:125-161, 1999) for a slab of DU. The calculated value for DU was 250 mrad/h which was in good agreement with the measured value of 233 mrad/h of beta radiation plus an additional 23 mrad/h from from photons (i.e., x and gamma radiation) for a total of about 255 mrad/h for a slab of natural uranium, made through a 7 mg per square centimeter filter to mimic the cornified (i.e., dead) layer of the skin (Health Phys. 29:143-152, 1975). The beta dose rate from a uranium slab is quite constant for the first few millimeters away from the slab and then drops off rapidly. At 10 cm (approximately 4") from the slab, the dose rate is only about 11.5 mrad/h, about 80 percent of which is from beta radiation.
 
Ronald L. Kathren, CHP