Answer to Question #2135 Submitted to "Ask the Experts"

Q

Is there any published information on radioactive decay which states whether or not there is a measurable time between the emission of multiple radiations from a single decay of a radioactive atom (for example, the ⁶⁰Co beta particle and its two gamma rays or the ¹¹⁰Ag beta particle and its multiple gamma rays, etc.)? I am talking about the time between the different gamma ray emissions as well as between the beta particle emission and gamma ray emissions.

A

Intuitively there must be a finite time period between an alpha or beta decay, and subsequent gamma-ray emission, or cascade of gamma rays as the decay product nucleus de-excites. In most practical applications in medicine or industry, such a time period is irrelevant. One notable exception is the rare-but-possible metastable state of a radionuclide, such as ⁹⁹Mo beta decay to ^99mTc, with an isomeric transition to ⁹⁹Tc (ground state) through a gamma-ray emission. ^99mTc has a relatively long half-life of about six hours. Similarly, there are the ¹³⁷Cs beta decay to ^137mBa, and ²³⁴Th beta decay to ^234mPa. All these metastable state radionuclides have measurable half-lives.

I'm sure if you searched the published nuclear physics literature you could find a great deal of theoretical and experimental information on any particular radionuclide decay you may be interested in (that is, ⁶⁰Co or ¹¹⁰Ag). You might want to borrow the two-volume text Alpha-, Beta-, Gamma-Ray Spectroscopy edited by Kai Siebbahn (published by North-Holland Publishing Company, Amsterdam). These two volumes are circa 1965, but might be a good starting point. However, as Friedlander et al. have noted on page 94 in the 1981 3rd edition of their text Nuclear and Radiochemistry, published by John Wiley and Sons, New York, "Life-time of Excited States - The overwhelming majority of gamma transitions take place on a time scale too short for direct measurement, that is, in less than 1×10^-12 seconds . . ."


David J. Allard, CHP