Measurements of isomer ratios and half-life of isomeric state in ¹²Be

Isomeric states can have significantly different structures from the ground state, so isomeric states are interesting and important for structure. In the case of ¹²Be, it has an isomeric state 0⁺₂ located about 2.3 MeV above the ground state 0⁺₁, and the half-life of this state is 230 ns[1]. This isomeric state has two decay modes, E2 decays through an intermediate 2⁺₁ state emitting gamma rays, or an E0 decay directly to the ground state emitting an electron-positron pair by internal pair production. Previous studies utilizing gamma ray measurements on the isomeric state of ¹²Be suggested the expanding nuclear radius compared to that of the ground state. This means ¹²Be may have a two-neutron halo in the isomeric state. We aim to measure nuclear radius directly from the reaction cross section measurements of the ground and isomeric states. However, it is impossible to produce a complete isomeric beam, we need information on the isomer ratio of the ¹²Be beam. In this study, at the HIMAC synchrotron facility, secondary beams of 220 MeV/u ¹²Be were produced from an ¹⁸O primary beam and we measured its isomer ratios. Additionally, we determined the half-life of the isomeric state with updated precision. We will present the details and results of the measurements and future plans.

References

[1]S. Shimoura et al., Phys. Lett. B 654 (2007) 87