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20 March 2010

Answer to Question #5483 Submitted to "Ask the Experts"

Category: Doses and Dose Calculations — Basic dose information, dose quantities, units

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

Our laboratory is proposing to use ³²Si as a tracer for aquatic environmental levels. I realize this isotope decays to ³²P, but I do not know the limits of ³²Si use before dosimetry is required when practicing ALARA (as low as reasonably achievable). Can you help, please?

Silicon-32 will be in secular equilibrium with ³²P (the activity of the ³²P and ³²Si will be equal). The maximum beta energy for ³²Si is 213 keV with an average beta energy of ~65 keV. Beta particles must have a maximum beta energy greater than 250 keV to be detected by commercially available dosimeters. Therefore, the beta dose from the ³²Si will not be detected by a commercially available dosimeter. Also, the beta dose from the ³²Si is insignificant when compared to the beta dose from the ³²P progeny.

The ³²P progeny have a maximum beta energy of 1.71 MeV with an average beta energy of 698 keV. A conservative calculation can be used to estimate the dose from the ³²P. I have calculated beta dose constants based on conservative assumptions. The beta dose constants for ³²P are:

Hands: 660 mrem/mCi-hr
Skin: 19.9 mrem/mCi-hr
Lens of Eye: 1.35 mrem/mCi-hr
Whole Body: 0.00218 mrem/mCi-hr

To estimate the dose for a year, multiply the ³²Si/³²P activity per experiment, the number of experiments per year, and the handling time by the appropriate beta dose constant plus (number of experiments per year X five minutes per aliquot X stock vial activity X beta dose constant for hands). The second component is to account for beta dose from aliquoting the stock solution. The hand dose will be the limiting factor. Therefore, an extremity dosimeter is required when the hand dose is equal to or greater than 5,000 mrem. A whole-body dosimeter will need to be issued when the skin dose is equal to or exceeds 5,000 mrem or the whole-body dose is equal to or exceeds 500 mrem.

Depending on the activity of ³²Si/³²P used, you may want to consider using L-blocks constructed of Plexiglas^TM.

David Burkett, CHP

Answer posted on 8 May 2006. 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.