Investigation of low-lying isovector octupole excitations in spherical nuclei

该项目的目的是进行中子诱发反应的实验，以确定最近提出的球核中低位置等向量八极激发的第一批候选者的光谱性质。由于历史原因，四极扇区中的模拟低位向量激发通常被称为混合对称态。对于被建议作为候选态的性质的确切赋值，必须以高精度确定连接候选态与第一激发的三态--等标量八极声子的跃迁的B(M1)值。因此，有必要用EXILL光谱仪测量这个跃迁的多极混合比、相对强度和辐射特性。此外，对于绝对值B(M1)，需要使用GAMS光谱仪在IIL时测量的寿命，因为它最适合飞秒范围内的预期寿命。在球核中建立这种新的低激发态集体模提供了几种理论模型，例如准粒子声子模型(QPM)或现代超越平均场模型，有机会将质子-中子剩余相互作用的矢量部分的耦合常数固定在有效核力的多极展开的八极部分。

Aim of the project is to conduct experiments with neutron-induced reactions to determine the spectroscopic properties of recently proposed first candidates for low-lying isovector octupole excitations in spherical nuclei. Due to historical reasons the analogue low-lying isovector excitations in the quadrupole sector are commonly referred to as mixed-symmetry states. For a firm assignment of the nature of the states proposed as candidates, it is mandatory to determine the B(M1) value of the transition connecting the candidate with the first excited 3- state, the isoscalar octupole phonon, with a high level of accuracy. Therefore, it is necessary to measure for this transition the multipole mixing ratio, the relative intensity and the radiation character which has been done already in the project with the EXILL spectrometer. In addition, for the absolute B(M1) value the lifetime is needed which will be measured with the GAMS spectrometer at ILL as it is suited best for the expected lifetime in the femtosecond range. Establishing this new kind of low-lying collective mode in spherical nuclei offers several theoretical models, such as, for example, the Quasiparticle Phonon Model (QPM) or modern beyond mean-field models, the opportunity to fix the coupling constant of theisovector part of proton-neutron residual interaction in the octupole part of the multipole expansion of the effective nuclear force.