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

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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.