Experiments with the Delayed Coincidence Method, including a Search for Short-lived Nuclear Isomers

Abstract

The delayed coincidence method has been studied in some detail and employed in a search for short-lived isomers in the product nuclei of β-emitters. The influence of counter time-lag fluctuations, which determine the short half-life limit in this method, is discussed in relation to the "differential" procedure for recording the delayed coincidences. It is shown that the time lags exhibited by a Geiger-Muller counter are distributed approximately according to a Gaussian law. A differential delayed coincidence recorder has been constructed in which the overall time-lags in the two input channels due to " rise-time " effects have been stabilized. At the coincidence stage of this recorder the quadratic mean deviation in the time-lag difference between the two channels due to " rise-time " effects has been kept as low as similar 5 × 10⁻⁹ sec. Details are given of the experimental procedure used in searching for short-lived isomers. The range of possible half-lives most thoroughly investigated extends from about 3 × 10⁻⁷ sec. to about 10⁻³ sec. for isomeric transitions giving conversion electron energies greater than about 100 kev. Possible improvements in the range of the experiments are discussed. The short-lived isomers of ¹⁸¹Ta and ¹⁸⁷Re have been confirmed. The half-life of the metastable state in ¹⁸⁷Re has been found to be (5.26±0.12)×10⁻⁷ sec. Negative results are reported for 19 isotopes. In particular the metastable states reported by other authors in ¹⁹⁸Hg, ¹⁴¹Pr, and ¹²⁴Te are not confirmed. The half-life of ThC' has been found to be (3.04±0.04)×10⁻⁷ sec.