Depth-dose curves for 90Sr and natural and depleted uranium in Mylar

Strontium and uranium are common sources used to calibrate personal dosimeters and survey meters for beta radiation. Since the absorbed dose rate of low-penetrating beta radiation decreases significantly with increased depth in tissue or dosimeters, it is necessary to describe the dose rate accurately as a function of depth. Complete depth dose information from zero to 1000 mg/cm2 is currently unavailable for strontium and uranium. This paper describes the generation of accurate depth-dose curves in Mylar for 90Sr and slabs of natural and depleted uranium from 0 to 1000 mg/cm2, utilizing an extrapolation chamber. The absorbed dose rate produced by natural uranium was found to decrease from 235 mrad/hr at a depth of 7 mg/cm2 to 43 mrad/hr at 300 mg/cm2 to 4.8 mrad/hr at 1000 mg/cm2. The dose rate produced by depleted uranium was found to decrease from 205 mrad/hr at 7 mg/cm2 to 34 mrad/hr at 300 mg/cm2 to 2.3 mrad/hr at 1000 mg/cm2. A 1-mCi extended area 90Sr source (encapsulated in 50 mg/cm2 AG) was found to produce an absorbed dose rate of 16,997 mrad/hr at 57 mg/cm2, 5000 mrad/hr at 300 mg/cm2, and 27.1 mrad/hr at 1050 mg/cm2. Doses to deep organs have been determined historically at depths ranging from 300 to 1000 mg/cm2. The results of this study show that 90Sr and uranium produce significant absorbed dose rates at a depth of 300 mg/cm2. Thus, if doses for deep depths are determined at 300 mg/cm2 for mixed beta-gamma exposures, the dose will be overestimated due to the presence of the beta component. The depth-dose curves generated in this study can be used by dosimeter processors to normalize the response of their dosimeters to the shallow (7 mg/cm2) and deep (1000 mg/cm2) depths defined by the U.S. Nuclear Regulatory Commission.