Answer to Question #8534 Submitted to "Ask the Experts"

Category: Radiation Workers

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 on the Radiation Safety Information Computational Center website) 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.

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

 

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