Decay Detector for the Study of Giant Monopole Resonance in Unstable Nuclei
Giant Resonances (GR) are the broad resonances that occur at excitation energies between 10 and 30 MeV. They correspond to the collective motion of nucleons within the nucleus. The GR modes can be classified according to their multipolarity L, spin S and isospin T quantum numbers. In the microscopic description, the GR modes can be understood as the collective particle-hole excitations characterized by certain values of the angular momentum and parity (J?), orbital momentum, spin, and isospin.
The Giant Monopole Resonance (GMR) is interesting because its excitation energy is directly related to the incompressibility of the nucleus KA. KA can be used to derive the incompressibility of nuclear matter KNM, but this extrapolation from the data for real nuclei is not straightforward due to contributions from surface, Coulomb and asymmetry effects. Thus, improvements to the extrapolated KNM can be made by measuring the GMR for increasing (N-Z)/A. The incompressibility of nuclear matter is of importance in the nuclear equation of state (EOS) which describes a number of phenomena: collective excitations of nuclei, supernova explosions and radii of neutron stars.
In order to study the Isoscalar Giant Monopole Resonance in unstable nuclei, a ?E-?E-E decay detector composed of plastic scintillator arrays has been built and tested. The measurement of the ISGMR in unstable nuclei will be done using inverse kinematics, with a 40 MeV per nucleon beam of the unstable nucleus incident on a 6Li target. Xinfeng Chen studied the viability of this approach, taking data for elastic scattering and inelastic scattering to low-lying states and giant resonances of 240 MeV 6Li ions on 24Mg, 28Si, and 116Sn.
Nuclei excited to the GMR region are particle unstable, and will decay by p, ? or n decay shortly after excitation. To reconstruct the event it is necessary to measure the energy and angle of the decay particle and of the residual heavy ion. In many lighter nuclei a few nucleons off stability, and in light proton rich nuclei, the neutron threshold is above the region of interest.