Answer to Question #7168 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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


I want to measure the radioactivity of a sample containing 111In using a well counter. Based on the decay scheme, I understand that two photons are emitted almost simultaneously (171 and 245 keV). Can a multichannel analyzer (MCA) separate the two signals or does it detect only one peak at (171 + 245 = 416 keV)?


The two photons are sufficiently separated in energy that you should readily be able to resolve them using most any conventional detector such as a 3" x 3" NaI(Tl) detector or a germanium detector with a multichannel analyzer. A typical NaI(Tl) well detector will probably have an energy resolution of around 20 percent at the energies of interest, which means that the full peak widths at half the maximum peak heights will be in the range of 30 to 50 keV; so, if you use a NaI(Tl) detector, the tail of the 171 keV photopeak may overlap somewhat with the leading edge of the 245 keV peak, but it should still be easy to distinguish the two. Naturally, a typical germanium detector has much better resolution and would produce complete separation between the two photopeaks. The use of a well counter encourages the detection of the sum peak that you refer to, but its detection efficiency will be considerably less than that of either of the individual peaks. You should expect to be successful in your measurement. Good luck.

George Chabot, PhD, CHP

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