Delayed-neutrons from arsenic isotopes 84As, 85As and 86As

Abstract

Short-lived arsenic isotopes were produced by thermal-neutron fission of 235U and isolated within 2·5 sec by volatilization of arsenic hydride. The existence of a strong delayed-neutron precursor of 2·05±0·05 sec half-life among the arsenic isotopes is confirmed and its assignment to 85As ascertained by milking of the daughter product 33 sec 85Se and 3·1 min 84Se, which is the final nucleus in the neutron decay branch of 85As. A delayed-neutron yield of 7·8 ± 1·2 neutrons/10 4 fissions was obtained for 85As. A new isotope, 0·9 ± 0·2 sec 86As, was detected by delayed-neutron counting and by following the decay of its most prominent γ-ray. The mass assignment was verified by milking of 54 sec 86Br. The delayed-neutron yield of 86As amounts to 0·57 (+0·25/−0·15) neutrons/10 4 fissions. Neutron emission was also detected in the decay of the 5·5 sec 84As, with a yield of 0·02 ± 0·01 neutrons/10 4 fissions. Using the measured delayed-neutron yields and measured or estimated fission yields, the following neutron emission probabilities were obtained: P n 84 = 0·13 ± 0·06%, P n 85 = 23 ± 3% and P n 85 = 3·8 (+1·7/−1·0)%. From the growth of 87Br in arsenic samples a half-life of about 0·3 sec and a fractional cumulative yield of 1·8 ± 0·9% were deduced for 87As.