Answer to Question #5559 Submitted to "Ask the Experts"

How is the half-life of a very, very short-lived radionuclide determined?

Half-lives on the order of seconds or somewhat less can be measured by rather conventional methods. Instead of using conventional scalers, however, multichannel scaling techniques, available with most modern multichannel analyzers, can be used. In these approaches the analyzer operates as if it were a large number of individual scalers, each channel representing one scaler with the analog-to-digital converter being bypassed, and counts accumulate sequentially in each scaler. The dwell time for accumulating each count can be preset to allow sufficient counts and acceptable decay. With reasonable source activities and an appropriate detector, the decay of quite short-lived sources can be followed.

For much shorter half-lives, on the order of 10^-9 to 10^-3 seconds, other more specialized approaches are necessary. These frequently involve specialized coincidence counting techniques, probably most often involving gamma rays emitted from specific short-lived nuclear species, in which the emission of a specific gamma ray acts as a trigger to begin counting, and two or more detectors operate to record specific-energy gamma rays emitted in the decay of the species of interest.

A highly specialized system, known as the gammasphere, has been constructed as a joint project among a number of nuclear groups, and various measurements have been made at the ATLAS accelerator facility at Argonne National Laboratory. The gammasphere consists of more than 100 detectors, 98 of them being large-volume coaxial Compton-suppressed germanium detectors and three being low-energy planar detectors. The system has been used in a delayed coincidence counting process to investigate lifetimes of various species produced when selected target nuclei are bombarded with heavy particles from a high-energy accelerator. The accelerator beam is directed at the target for a short interval and then terminated. Gamma counting begins when the first gamma ray is detected by the detector array; gamma-gamma separation times are set at a desired level and gamma rays are detected within a specified interval that extends from the gamma-gamma separation time to the end-time of the detector array. Here is one example of such an application in which the authors have used the gammasphere at ATLAS to investigate characteristics of a specific nuclear isomer of lutetium. (The authors are not specifically investigating half-lives, but the techniques used are basically the same.)

Other approaches have also been used for measuring short lifetimes. You can find some specific experiments available from ORTEC on its Website. One method uses a time-to amplitude converter method to investigate excited nuclear state lifetimes and a second shows an application of the delayed coincidence approach. There have also been some methods used in which specific short-lived nuclear species have been subjected to mass spectrometry as an isolation technique and decay assessed with scintillation detectors.

George Chabot, PhD, CHP