Abstract
In radioactive ion beam experiments, beams containing isomers can be of interest in probing nuclear structure and informing astrophysical reaction rates. While the production of mixed in-flight ground state and isomer beams using nucleon transfer can be generally understood through distorted wave Born approximation methodology, low-spin isomer production via fast fragmentation is relatively unstudied. To attain a practical understanding of low-spin isomer production using fast fragmentation beams, a test case of $^{38}\mathrm{K}/^{38m}\mathrm{K}$ was studied at the National Superconducting Cyclotron Laboratory's ReAccelerated Beam facility. Starting from lise++ predictions, the fragmentation momentum distribution was sampled to determine isomer production. In addition, the effects of the gas stopper gradient and charge breeding times were examined. In the case of $^{38}\mathrm{K}$, isomer production peaks at $\ensuremath{\sim}57%$. This maximum is observed just off the lise++ predicted optimum magnetic rigidity, with only small losses in beam intensity within a few percent of this optimum rigidity setting. Control of the isomer fraction was also achieved through the modification of charge breeding times. Fast fragmentation appears to be a feasible method for production of low-spin isomeric beams, but additional study is necessary to better describe the mechanism involved.
Highlights
As techniques for producing radioactive ion beams continue to improve, additional opportunities arise
An isomer may be of interest for probing otherwise inaccessible nuclear structure information [1,2,3,4]; in other cases, such as 26Al and 38K, astrophysical reactions on the isomer can play an important role in the reaction network [5]
In addition to the samples taken along this momentum distribution, a measurement was taken with the fragmentation target at a higher reaction angle with respect to the beam or separator; to accomplish this, the primary beam tune was adjusted to impinge on the fragmentation target at the largest incident angle allowed by the diameter of the upstream beam pipe, sampling a different angular range with respect to the reaction
Summary
As techniques for producing radioactive ion beams continue to improve, additional opportunities arise. Among these is the possibility to study nuclear reactions on strongly populated isomeric states within the beam. In fragmentation reactions on fast beams, low-spin isomer production is not well reproduced by models, though higher spins in higher mass regions are more accurately predicted 38K was studied as an example case for the production of low-spin isomer beams using fast fragmentation. While the proton capture on the ground state has been studied [16,17], capture on the isomer may play a significant nucleosynthesis role, as the decay branch from 38Ca to the 38K isomer is 76.5% [13]
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