Answer to Question #7257 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

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

I want to make shielding calculations for a ⁹⁹Mo/^99mTc production laboratory. I need some information like TVL (tenth value layers) (in cm Pb and cm concrete) and the specific gamma ray constant (in μSv×m²/MBq×h) of ⁹⁹Mo.

There is no easy answer to your question. ⁹⁹Mo produces 30 gammas, in energies from 40.5 keV to 1.05 MeV, plus K x-rays of 18-21 keV. The most abundant gammas are 140 keV at 4.52 percent, 181 keV at 5.99 percent, 366 keV at 1.2 percent, 740 keV at 12.1 percent, and 778 keV at 4.26 percent (National Nuclear Data Center, Brookhaven National Laboratories). (I am neglecting the gammas from ^99mTc.) Thus, as shielding is added, the radiation gets "harder" and the TVL gets larger. With heavy shielding, the 740 keV and the 778 keV photons become the ones of concern. The TVL for 750 keV photons is 2.03 cm in lead or 13.4 cm in concrete. This does not include buildup. Calculating shielding for these multienergetic sources requires more than a simple evaluation by TVL.

The reference I found for the gamma ray constant for ⁹⁹Mo is 1.8 R×cm²/mCi×hr (Radiological Health Handbook 1970). Of course, that value is for an unshielded source. Please consider contracting with a health physicist in your area to provide support in this project.

David Close, CHP

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