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Answer to Question #8534 Submitted to "Ask the Experts"Category: Doses and Dose Calculations — External dose calculations The following question was answered by an expert in the appropriate field: Q
Is it appropriate to use a code such as Varskin for the determination of skin dose from the handling of a sealed source? The source itself is encapsulated and therefore the skin dose would be from the photons not the betas. It seems as though Varskin 3 is equipped to handle such a problem, although I am not sure since I am getting very different results using other tools such as Microshield.
A
The photon dose model in Varskin 3 was not intended to be used as a general photon dose model because there are a number of limitations to it. Photon doses calculated by Varskin 3 (available at http://www-rsicc.ornl.gov/) use an expanded version of a simplified model developed by Lantz and Lambert (1990). Durham and Lantz (1991) verified the original photon dose model experimentally. The photon dose model assumes that the source is a point source. Therefore, the largest dimension of any source has been arbitrarily limited to a maximum of 1 mm. The photon dose is calculated (provided that the radionuclide emits photon radiation) for skin areas of up to 10 cm2 but not for larger areas. The method used to develop the photon dose model is described in Appendix A of the Varskin 3 user's manual.
It is important to note that the photon dose is calculated assuming that the source can be represented as a point source, regardless of the geometry package that is chosen by the user. If a three-dimensional source geometry is chosen, the photon source is assumed to be located at the bottom center of the source. An artifact of this is if the thickness or radius of the source is changed, the photon dose will remain the same for the same amount of activity. Please note that a programming error was recently discovered that caused an error in the calculation of the fraction of electronic equilibrium established. A bug report and notice was prepared by RSICC to correct this error. Please ensure that you are using Varskin 3 Version 3.1.0 when including photon dose in your calculations. M.C. Ryan, et al., from Oregon State University published an abstract describing a new model for photon dose to be incorporated into Varskin 4.Jim Durham Center for Nuclear Waste Regulatory Analyses
Answer posted on 15 September 2009. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
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