Answer to Question #4428 Submitted to "Ask the Experts"

How is activity releated to the number of gamma rays emitted per second? I understand that activity is the number of disintegrations per second but how can I find out how many gamma rays of what energy are given off per disintegration?

In general, there is no governing relationship that allows one to determine the gamma emission rate from radionuclide activity unless one also knows the specific characteristics of the nuclear excited states that are produced in the decay process and that are the source(s) of the gamma rays that are produced. Some radionuclides emit no gamma rays during the decay process, while others yield many gamma rays of many energies.

There are available a variety of sources that do give detailed information as to the gamma-ray energies and frequencies produced as specific radionuclides decay. Nuclear Data Sheets, published monthly by Elsevier Publishing, contain an abundance of nuclear structure data, including gamma-ray information. Individuals and groups have compiled selected portions of these and other data into more compact forms that are easier to use for many routine requirements. One example is The 8th edition of the Table of Isotopes, book and CD-ROM, by Richard B. Firestone, Virginia S. Shirley, Coral M. Baglin, S.Y. Frank Chu, and Jean Zipkin, published by John Wiley & Sons, Inc., 1996; this two-volume compilation also has updates produced in later years.  An older and simplified, but still commonly used, source for routine decay data is Radioactive Decay Data Tables—a handbook of decay data for application to radiation dosimetry and radiological assessments, by David C. Kocher, Oak Ridge National Laboratory (1981), Tech. Inf. Center, US Department of Energy.

Two very useful sources that are available online and that may be helpful, especially if you are interested in gamma spectrometry, are the Heath catalogs. These compilations include specific gamma-ray information for many radionuclides along with measured gamma-ray spectra for both sodium iodide and germanium (or sometimes, silicon) detectors. These are available at the Idaho National Laboratory Web site on their Gamma-ray Spectrometry Center - Online Catalogs page. As an example, if you select the NaI(Tl) Gamma Spectrum Catalogue (23.7 Mb) on this page, the sodium iodide catalog will load in Adobe Acrobat Reader. The first 53 pages give much explanatory information and discussion; the catalog data begin after page 54. If you point to and click on a listed radionuclide of interest, the sodium iodide derived gamma spectrum will be displayed. The specific gamma-ray energies and their intensities are also shown (they will either be in a table insert on the page with the spectrum, or on the page following the spectrum). The absolute intensity is given as a percentage and labeled I; if you divide this number by 100 you will obtain the number of gamma rays of that energy emitted per disintegration.

There is a variety of other related information available on the Internet that may be of interest if you have time and interest in pursuing it. I hope the information we have provided here is sufficient to get you started.

George Chabot, PhD, CHP