While the existence of isomeric states (states with exceptionally long half-lives, from pico seconds to years) has been known since the 40's, their method of population is still an area of intense research. There are three types of isomers: spin-isomers, k-isomers, and shape isomers. Of primary importance to researchers at TUNL are spin isomers. Spin isomers occur when a low-lying excited state has a large spin difference with the ground state.
One particular question is the role that neutron and/or proton structure have on isomer population. This is an on going study at the HIGS facility by using a (g,n) on the N=82 isotones 138Ba, 140Ce, and 142Nd reaction to populate the h11/2 isomeric state in the N=81 isotones. The isomeric states are classified as h11/2 while that is the predominate shell contribution to the neutron wave function. These three isotones were chosen while having a simple neutron configuration (one neutron short of a closed shell), thus any variation in the population of the isomeric state should be from differences due to proton configuration.
Previous studies on isomers in the region of the N=81 isotones, isomer population has primarily been studied using an activation technique. At HIGS we have used in-beam spectroscopy techniques to observe the direct population of the isomeric state. Following the emission of the neutron in the reaction (g,n), the residual nucleus is left in an excited state, and undergoes a cascade of gamma-rays eventually ending at the ground state or the isomer state. We utilize 4 60% HPGe detectors at a polar angle of 90 degs, and at azimuthal angles of 0, 90, 180, and 270 degs. This arrangement is called the "Gamma Quartet".