Answer to Question #438 Submitted to "Ask the Experts"

Q

Having ⁵⁸Co as a large contributor to our isotopic mix, RM14/HP210 probe efficiency is not adequate, and I am curious as to specific meter we should use to detect contamination levels.

A

Cobalt-58 undergoes transformation by positron emission and electron capture. The principle radiation emissions and their intensities include a 475 keV E-max positron (about 15 percent), 511 keV annihilation radiation (about 30 percent), and an 811 keV gamma ray (99+ percent). Other radiations include low- energy auger electrons, low-energy x rays of 7 keV or less, and several additional gamma rays of low abundance. The measurement system you describe is a ratemeter-type monitor with a pancake-type GM tube with a window thickness on the order of 1.4 to 2.0 mg/sq. cm supplied by one of the more well-known suppliers of health physics instrumentation. This detector will respond to the positron, the gamma radiation, and the x rays, but apparently you desire a greater sensitivity. An NaI gamma scintillation probe coupled to the same or a similar monitor would have a higher efficiency for gamma radiation and should give you greater sensitivity. If you are concerned about specifically detecting ⁵⁸Co in the presence of other radionuclides, you might consider using the scintillation probe with a monitor containing an analyzer with one or more channels—of course, the specificity will depend upon the complexity of the energy spectrum produced by the radionuclide mix. I suggest you contact the vendor of your instrument or one of the other suppliers of health physics instrumentation for a recommendation. If you tell us what industry or setting you are working in, we also may be able to refer your question to someone else in a similar setting.

C. E. Roessler, PhD, CHP