A study of compressibility in nuclei far from the stability line

我们用逆运动学方法研究了丰质子或丰中子不稳定核的（α，α '）反应，为研究E0巨共振和核压缩性提供了一个里程碑.构建了一个液态氦靶系统，其中激发态伽马射线路径中的材料被最小化，温度的稳定性通过2周的实验得以保持。靶的厚度可在100-250 mg/cm^2的范围内选择。利用RIKEN加速器碎片分离器RIPS的60 MeV次级束流，测量了（a）^<14>O+α和（B）^<12>Be+α散射。该探测器系统由一个用于带电粒子探测的塑料衰减器和一个68个用于伽马射线探测的NaI（TI）衰减器阵列组成。(a)确定了^ O分子到0^+、1^-、2^+和3^-激发态的跃迁<14>，并利用转移角动量表征了微分截面的角分布。我们还测量了各种碎片的产生截面，发现碳同位素的产生比半经验规则的预测大得多。(b)我们发现了一个能量为2.24 MeV的同分异构0^+态<12>，它的自旋是由γ-γ角关联模式决定的，它的位置正好在^ Be的第一激发态上方。还测量了2+和1-态的α非弹性散射，其中角分布的特征在于转移的角动量。我们新发现（α，t）和（α，^3He）核子转移反应具有显著的反应截面，这为研究不稳定核中的单粒子态提供了一种新的光谱工具.

We have studied (α, α') reactions for proton- or neutron-rich unstable nuclei by using inverse kinematics in order to get a milestone for investigating E0 giant resonances and nuclear compressibility. A Liquid He target system was constructed where materials in a path of gamma rays from excited states were minimized and stability of the temperature were maintained through a 2-weeks experiment. The thickness of the target are selectable in a range of 100-250 mg/cm^2. By using 60-MeV secondary beams from the fragment separator RIPS in RIKEN accelerator facility, (a) ^<14>O+α and (b) ^<12>Be+α scatterings were measured. The detector system consisted of a hodoscope of plastic scintillators for charged particle detection and an array of 68 NaI(TI) scintillators for gamma rays. (a) The transition to 0^+, 1^-, 2^+ and 3^- excited states in ^<14>O were identified and the angular distributions of the differential cross sections were characterized by the transferred angular momenta. We also measured production cross sections of various fragments and found that production of carbon isotopes are much larger than a prediction of a semi-empirical rule. (b) We found an isomeric 0^+ state at 2.24 MeV just above the first excited state of the ^<12>Be, where its spin was determined by a gamma-gamma angular correlation pattern. Alpha inelastic scattering to the 2+ and 1- states were also measured, where the angular distributions are characterized by the transferred angular momenta. We newly found that the (α, t) and (α, ^3He) nucleon transfer reactions have significant cross sections, which gives a new spectroscopic tool for single particle states in unstable nuclei.