Half-life measurement of isomeric state in ²⁵Si

The rapid proton capture process (rp process) passing through proton-rich nuclei plays an important role in the nucleosynthesis as well as the rapid neutron capture process. However, a possible path of the rp process ²⁴Al(p,γ)²⁵Si(p,γ)²⁶P(p,γ)²⁷S has not been well studied yet due to lack of experimental data for their nuclear structure. Since the temperature in the candidate of the site of rp process is around 10⁹ K, low-lying excited states can affect the rp process taking into account the Maxwell-Boltzmann distribution. The first excited state at 40 keV in ²⁵Si, which is a candidate for the isomer, has not been researched yet in detail. To measure the half-life of the first excited state in ²⁵Si, the γ-ray spectroscopy have been performed at QST-HIMAC.

A secondary beam including ²⁵Si was produced by the projectile fragmentation of a 300-MeV/nucleon primary beam of ²⁸Si on a 20-mm-thick CH2 target. The secondary beam was separated and identified by using the SB2 beamline and was implanted in an active stopper consisting of three plastic scintillators. The γ rays were measured with three kinds of detectors surrounding the stopper, four LaBr₃(Ce) detectors, a HPGe detector, and a NaI(Tl) detector. By observing the delayed γ rays with these detectors, the half-life of 40-keV isomeric state in ²⁵Si has successfully been determined for the first time. We will discuss the deduced transition probability of the isomeric state in comparison with the isomeric state in the mirror nucleus of ²⁵Na and theoretical predictions.