P. A. Wilmarth

Structure in β-delayed proton spectra of N=81 precursors

Abstract Intense peaks, superposed on a structureless spectrum, were observed in 151Yb β-delayed proton decay. It is shown that the peak and statistical protons follow the decay of the s 1 2 groundand h 11 2 isomeric states in 151Yb, respectively. It is concluded that the peaks, also seen in 147Dy and 149Er decays, are due to a low density of 1 2 and 3 2 levels at ≈ 4.5 MeV in the N = 82 daughters.

Alpha-decaying low-spin levels in155Lu and157Lu

Using an on-line mass separator, the α-particle decays of155Lu and157Lu were investigated. A new α-emitting level in157Lu was identified [T1/2=5.7(5) s,Eβ=4924(20) keV]. In addition, a half-life of 140(20) ms was measured for the recently discovered second, low-lying, α-decaying level [Eβ=5579 (5)keV] in155Lu.

Spectroscopy of proton-rich nuclei in the rare earth region

Abstract The isotope separator facility OASIS, on-line at the Lawrence Berkeley Laboratory Super-HILAC, was used to investigate proton-rich rare earth nuclei. Single-particle states near the 82-neutron shell were delineated, numerous new isotopes, isomers and β-delayed proton emitters were discovered and the α-decay properties of some nuclides with N ⩾ 84 were re-examined. In this contribution the experimental program is summarized briefly, the excitation energies of the s 1 2 and h 11 2 proton states in this mass region are discussed, and results on the β-delayed-proton spectra of 145Dy and 147Er are presented.

Nuclear Decay Studies Far from Stability

The decays of rare-earth nuclei in the region near N = 82 and Z = 64 and extending toward the proton drip line have been studied with the OASIS mass-separation facility on-line at the Lawrence Berkeley Laboratory SuperHILAC. Spectra of x rays, γ rays, and β-delayed protons and a particles have been measured in both singles and coincidence modes. Decay schemes have been derived, in many instances, with absolute branching intensities for electron capture, positron emission, and β-delayed particle decay. In favorable cases, Q(EC) values have been determined from the measured electron-capture and positron-decay branchings. Nuclear structure near the closed shells has been qualitatively explained by a simple weak-coupling model, and the disappearance of Z = 64 shell effects near N = 78 has been explored. The systematics of Gamow-Teller spin-flip transitions have been investigated and will be compared with shell-model predictions.

Decay studies of neutron-deficient nuclei near the Z=64 subshell: Dy141, Tb141, Gdg+m141, and Eum141

The {beta}/{gamma} and delayed proton decays of mass separated {ital A}=141 isotopes with {ital Z}=63--66 were investigated with an on-line mass separator. Branching ratios have been redetermined for the total {beta} decay of the 2.7(3) s, (11/2){sup {minus}} isomer {sup 141}Eu{sup {ital m}} at 96.5 keV (13{sub {minus}2}{sup +4}%), and for the direct {beta} feeding of the 1.58 keV (3/2){sup +} level of {sup 141}Sm in the decay of 40-s {sup 141}Eu{sup {ital g}} (73(5)%). Detailed decay schemes have been constructed for the decays of the (11/2){sup {minus}} isomer {sup 141}Gd{sup {ital m}} (11(2)% isomeric transition) and the (1/2){sup +} ground state, {sup 141}Gd{sup {ital g}}. Half-lives of 24.5(5) s and 14(4) s, respectively, have been determined for these isomers. A delayed proton branch of 0.03(1)% was identified in the decay of {sup 141}Gd{sup {ital g}}. A detailed decay scheme was also constructed for 3.5(2)-s {sup 141}Tb and a {ital J}{sup {pi}}=(5/2{sup {minus}} ) was assigned to its ground state, at variance with the systematics of heavier, odd-mass Tb isotopes. All {gamma}-ray assignments to the various isotopes are based on {ital x}{gamma} and {gamma}{gamma} coincidences and on half-life measurements. A 0.9(2)-s half-life for {sup 141}Dy was determined from its delayedmorexa0» proton decay. All spin assignments proposed in this paper are discussed in terms of Nilsson level diagrams and nuclear shapes.«xa0less

Direct and β-delayed particle decays near closed shells

Abstract We have been involved in the investigation of nuclear structure properties of nuclei close to the proton drip line. In these studies we have used on-line isotope separators at Oak Ridge (UNISOR) and Berkeley (OASIS), the ORNL velocity filter, and a fast helium-gas-jet transport system at the LBL 88-in. cyclotron. We describe briefly three areas of our research that have dealt with particle decays: (1) α-decay in medium-weight nuclei, (2) α-decay rates for even-even isotopes, and (3) s-delayed-proton decay of N = 81 precursors.

Identification of153Lu β decay

Using an on-line mass separator the decay of153Lu was studied in64Zn bombardments of92Mo. The T1/2=0.9 ±0.2 s activity was assigned to the β decay of the (11/2−) isomer.

Identification of the neutron-rich isotope174Er

The heaviest known erbium isotope, 3.3(2) min.174Er, has been identified using on- line isotope separation and by ß and γ- ray spectroscopy. Twelve γ rays were placed in a preliminary decay scheme. The mass of the new isotope was established by the separator and the Z was deduced from Tm K x ray coincident ß and γ rays. In addition, the heaviest known isotope of holmium171Ho was observed with a half-life of 49(5) s.

Beta decay of Lu154 and Yb154

Using mass-separated sources, the ..beta..-decay properties of /sup 154/Lu and /sup 154/Yb were investigated. Limits of /sup 154/Lu decay to the first 8/sup +/ and 6/sup +/ levels in /sup 154/Yb suggest a 7/sup +/ spin for the odd-odd parent; also delayed proton emission and an indication of delayed ..cap alpha..-particle emission were observed to follow /sup 154/Lu ..beta.. decay. The ..beta..-decay branch of the ..cap alpha..-emitting nucleus /sup 154/Yb was identified for the first time by the observation of one intense 133.2-keV ..gamma.. ray. This transition deexcites a 1/sup +/ 133.2-keV level in /sup 154/Tm which is fed by an allowed 0/sup +/..-->..1/sup +/..beta.. transition with a logft value of 3.6 +- 0.3.

Beta decay of Lu 154 and Yb 154

Using mass-separated sources, the ..beta..-decay properties of /sup 154/Lu and /sup 154/Yb were investigated. Limits of /sup 154/Lu decay to the first 8/sup +/ and 6/sup +/ levels in /sup 154/Yb suggest a 7/sup +/ spin for the odd-odd parent; also delayed proton emission and an indication of delayed ..cap alpha..-particle emission were observed to follow /sup 154/Lu ..beta.. decay. The ..beta..-decay branch of the ..cap alpha..-emitting nucleus /sup 154/Yb was identified for the first time by the observation of one intense 133.2-keV ..gamma.. ray. This transition deexcites a 1/sup +/ 133.2-keV level in /sup 154/Tm which is fed by an allowed 0/sup +/..-->..1/sup +/..beta.. transition with a logft value of 3.6 +- 0.3.