Answer to Question #1225 Submitted to "Ask the Experts"

Q

When using a Ludlum model 14C with a pancake probe checking for ^99mTc contamination, does the plastic beta shield need to be taken off or can it be left on? Does ⁹⁹Mo breakthrough contribute significantly in checking for ^99mTc contamination in a hospital setting?

A

The decay scheme for ^99mTc notes that over 10% of the decays have conversion electrons over 100 keV, with ~ 89% of the gamma photons at 140 keV. The plastic beta shield will attenuate these conversion electrons and, to a very minor extent, the low-energy gammas. I'd remove the beta shield cap from the probe and use the open mica window for contamination surveys. You could prove this by simulating some contamination. Try to keep a fixed geometry and the count rate about 10,000 counts per minute (for statistics and dead time concerns). Measure a spot with the shield and without. My gut feeling is you'll see a slight increase without the attenuation of the beta shield. There should not be much ⁹⁹Mo breakthrough, but you should know that from daily dose calibrator measurements. The Nuclear Regulatory Commission sets a limit on breakthrough in 10CFR35.204: "Permissible molybdenum-99 concentration. (a) A licensee may not administer to humans a radiopharmaceutical containing more than 0.15 microcurie of molybdenum-99 per millicurie of technetium-99m." Only 0.00015 decays should be from ⁹⁹Mo per decay of ^99mTc. And although ⁹⁹Mo is a beta emitter, even at maximum allowed concentration, it will not contribute much to your contamination survey results.

David J. Allard, CHP