C. J. Lister

Lifetimes of levels inF20,21,Ne22, andN16

The recoil distance method has been used to measure the mean lifetime of the J/sup ..pi../ = (9/2)/sup +/ 1755-keV level of /sup 21/F, formed (predominantly) via the /sup 7/Li(/sup 18/O, ..cap alpha..)/sup 21/F fusion-evaporation reaction. The lifetimes of the 984- and 1309-keV levels of /sup 20/F and the 1275-keV level of /sup 22/Ne were obtained coincidentally. Similar measurements in the /sup 7/Li(/sup 11/B, pn)/sup 16/N reaction determine the lifetimes of the 298- and 397-keV levels of /sup 16/N. Data were acquired at theta/sub ..gamma../ = 0/sup 0/ using an escape suppression spectrometer, with /sup 18/O and /sup 11/B incident energies of 28.5 and 15.3 MeV, respectively. The experimental results are the following: /sup 16/N(298,3/sup -/), tau = 133 +- 4 ps; /sup 16/N(397,1/sup -/), tau = 6.60 +- 0.48 ps; /sup 20/F(984,1/sup -/), tau = 1.8 +- 0.3 ps; /sup 20/F(1309,2/sup -/), tau = 1.9 +- 0.3 ps; /sup 21/F(1755,(9/2)/sup +/), tau = 3.5 +- 0.5 ps; /sup 22/Ne(1275,2/sup +/), tau = 5.1 +- 0.2 ps. Shell-model calculations for /sup 16/N and /sup 20,21/F have been performed and are compared to experimental results.

Lifetimes of levels in /sup 20,21/F, /sup 22/Ne, and /sup 16/N

The recoil distance method has been used to measure the mean lifetime of the J/sup ..pi../ = (9/2)/sup +/ 1755-keV level of /sup 21/F, formed (predominantly) via the /sup 7/Li(/sup 18/O, ..cap alpha..)/sup 21/F fusion-evaporation reaction. The lifetimes of the 984- and 1309-keV levels of /sup 20/F and the 1275-keV level of /sup 22/Ne were obtained coincidentally. Similar measurements in the /sup 7/Li(/sup 11/B, pn)/sup 16/N reaction determine the lifetimes of the 298- and 397-keV levels of /sup 16/N. Data were acquired at theta/sub ..gamma../ = 0/sup 0/ using an escape suppression spectrometer, with /sup 18/O and /sup 11/B incident energies of 28.5 and 15.3 MeV, respectively. The experimental results are the following: /sup 16/N(298,3/sup -/), tau = 133 +- 4 ps; /sup 16/N(397,1/sup -/), tau = 6.60 +- 0.48 ps; /sup 20/F(984,1/sup -/), tau = 1.8 +- 0.3 ps; /sup 20/F(1309,2/sup -/), tau = 1.9 +- 0.3 ps; /sup 21/F(1755,(9/2)/sup +/), tau = 3.5 +- 0.5 ps; /sup 22/Ne(1275,2/sup +/), tau = 5.1 +- 0.2 ps. Shell-model calculations for /sup 16/N and /sup 20,21/F have been performed and are compared to experimental results.

New isotope Y-80, and the decays of Sr-79, Y-81, and Y-82

The helium-jet recoil transfer technique has been used to study the decays of /sup 79/Sr, /sup 80/Y, /sup 81/Y, and /sup 82/Y, produced by /sup 58/Ni(/sup 24/Mg,2pn), /sup 58/Ni(/sup 24/Mg,pn), /sup 58/Ni(/sup 25/Mg,pn), and /sup 60/Ni(/sup 24/Mg,pn) reactions, respectively. Decay properties of these four radioactivities have been deduced from the analysis of extensive singles and multiparameter spectroscopic measurements. The low lying level structure of /sup 80/Sr, as revealed by the decay of /sup 80/Y, can be reproduced through calculations of the interacting-boson-approximation model which place /sup 80/Sr at or near the O(6) limit. Positron end-point measurements (deduced from (..beta..,..gamma..) coincidence studies) have been used to determine decay energies for each of these nuclides. Nuclidic mass excesses inferred from these data show progressively larger deviations from the predictions of currently available mass models for the more neutron-deficient Y isotopes as the N = Z line is approached. It is suggested that this may reflect the influence of nuclear deformation of these isotopes on the mass surface in the vicinity of N = Z = 40.