Experimental study of the isospin dependence of nuclear incompressibility

The equation of state (EoS) of nuclear matter not only governs the femto-scale quantum many-body system, namely nuclei, but also plays an important role in the structure of neutron stars, supernova phenomena and neutron-star mergers. In particular, the isospin dependence of EoS has attracted much interest from the viewpoint of the existence of heavy neutron stars. The asymmetric term of incompressibility, K_τ, can be a benchmark for various EoS models because it can be directly deduced from the energies of the isoscalar giant monopole resonance (ISGMR) measured along an isotopic chain.  The present value of K_τ is -550±100 MeV from the ISGMRs in tin isotopes [1]. In order to improve the K_τ value, the measurement on the isotopic chain is extended to unstable nuclei. The tin isotope, ¹³²Sn, has been chosen as a flagship for the measurements of unstable tin isotopes because of its large isospin asymmetry and doubly magic nature.

A measurement of ISGMR energy in ¹³²Sn was performed at RIBF using a gaseous active target CAT-S. The secondary beam including ¹³²Sn, ¹³³Sb, and ¹³⁴Te was produced through inflight fission of ²³⁸U at an incident energy of 345 MeV/u. Beam particles were identified on an event-by-event basis by measuring the time-of-flight and magnetic rigidity. The typical total beam intensity was 8×10⁵ and 3×10⁵ counts per second at the start point of optics and in front of the CAT-S, respectively. The kinetic energy of ¹³²Sn at the center of the CAT-S was about 100 MeV/u. The CAT-S was filled with 0.4-atm deuterium gas, and the trajectories of beam and recoil particles were simultaneously measured with the CAT-S; the energy spectrum was obtained by using the missing mass method. The bump around 15 MeV was observed that likely corresponds to the ISGMR. The deduced Kτ and the constraint for the EoS and outlook will be discussed.

[1]: T. Li et al., Phys. Rev. Let. 99 (2007) 162503