Beta Decay Half-Lives of Neutron Rich Isotopes in the Germanium-Bromine Region

Understanding the origin of the heaviest elements in the universe is one of thegreat outstanding problems in physics. More than half of these elements are thoughtto have been created by a process called the rapid neutron capture, or R process.This process involves many nuclei that are very far from the line of stability. Becausethese nuclei have such short beta decay half-lives, they are very difficult to produceand study. The vast majority of r-process nuclei have not been observed at all.This work details the measurement of several r-process nuclei that have neverbeen produced before. These nuclei lie in the r-process path near the Z = 40,N = 60 region. This region of the chart of the nuclides is also interesting because arapid change in the shape of nuclei is seen. With the addition or removal of just afew nucleons, nuclei go from a spherical shape to a strongly deformed shape.For both r-process abundance calculations and nuclear deformation calculations,the beta decay half life and beta-delayed neutron emission ratio Pn values are importantparameters. This experiment determined these values for the r-process nucleiin the Germanium-Bromine region. The experimental work was carried out at theNational Superconducting Cyclotron Laboratory at Michigan State University.Twenty-two halflives were determined in our measurments. Three nuclei, 90Se,88As, and 89As, were measured for the first time. In addition, we have confirmedpreviously measured half-lives for Y, Sr, Rb, Kr, Br, Se, and As isotopes. The newmeasurements were used to calculate the Pn values for 92Ì¢è '90Br and 96,94Rb.An r-process simulation was performed with the results from this measurement.Because of the possible change in shape of the Se nuclei in particular, the uncertaintyof r-process production abundances in the A = 90 Ì¢è ' 130 region are as large as afactor of two. This work determined the shape of the Se isotopes in this region tobe deformed, which removed this large uncertainty in the r-process calculations.