Answer to Question #666 Submitted to "Ask the Experts"

• HPS Links to Other Web Sites of Interest
• Radiation and Health Physics Home Page, by University of Michigan Student Chapter of the Health Physics Society
• LBNL Isotopes Project - Nuclear Data Dissemination Home Page
• Idaho State University Department of Physics Program in Health Physics

If I can't find information quickly on known radiation-information sites, I'll often use a search engine such as Google and do a search on the subject; for example, "liquid scintillation counting" gave me several sites.

When performing LSC one needs to consider the following: the energy and radiation emitted from the sample, the efficiency of the specific counting solvent/solute "cocktail," counting vial size/type, the LS counting equipment used and conditions (e.g., background and counting temperature), potential quenching chemicals of materials in the sample matrix, etc. Also, when looking at LSC versus gamma counting, the complete decay scheme needs to be considered, for LSC will detect the beta and many low- (and some high-) energy photons and auger and conversion electrons. Similarly, with a sodium iodide (NaI) gamma counter, the size of the detector, type and thickness of the NaI encapsulation, sample size, geometry, etc., all factor into the efficiency of the system.

A very good reference to have for this particular problem is the Radioactive Decay Tables by D. Kocher (DOE/TIC-11026). The data tables for most radionuclides are presented, and energy and percent yield is easily found. It was available from the National Technical Information Center when published in 1981, and I believe the information is still up to date and accurate enough for most health physics work. For example, on the question of ¹²⁵I, the Radioactive Decay Tables notes it having an EC decay mode, but also having a number of low-energy x and gamma rays and conversion and auger electrons. Thus, these monoenergetic electrons will be detected as if they were low-energy betas, and the low-energy photons will have a fairly high probability of photoelectric capture by the counting fluid. Manufacturers of LSC equipment are also good sources of information. For example, an old LSC application note I have compares LSC to NaI gamma counting for several of the radionuclides given in this question. The percent counting efficiency data provided is as follows:

• Radionuclide: ¹²⁵I — LSC: 66 — NaI Gamma counting: 71
• Radionuclide: ⁵¹Cr — LSC: 80 — NaI Gamma counting: 6
• Radionuclide: ⁵⁹Fe — LSC: 89 — NaI Gamma counting: 51
• Radionuclide: ⁵⁷Co — LSC: 75 — NaI Gamma counting: 61

I would not consider the above data as absolute for any counting situation, but it may provide a fair relative comparison. Thus, it is recommended that National Institute of Standards and Technology traceable radionuclide standards, in a similar sample matrix and geometry, be used for LSC or NaI system calibrations. Nevertheless, if beta and photon energies are bracketed, one could infer an efficiency for another radionuclide through calculation by consideration of all the respective emitted radiations, yields, and energies.

David J. Allard, CHP