Inhomogeneous isospin distribution in the reactions of28Si+112Snand

M. Veselský

¹

,

R. W. Ibbotson

²

,

Richard Laforest

³

et al.

Abstract

We have created quasiprojectiles of varying isospin via peripheral reactions of 28 Si + 112 Sn and 124 Sn at 30 and 50 MeV/nucleon. The quasiprojectiles have been reconstructed from completely isotopically identified fragments. The difference in N/Z of the reconstructed quasiprojectiles allows the investigation of the disassembly as a function of the isospin of the fragmenting system. The isobaric yield ratio 3 H/ 3 He depends strongly on N/Z ratio of quasiprojectiles. The dependences of mean fragment multiplicity and mean N/Z ratio of the fragments on N/Z ratio of the quasiprojectile are different for light charged particles and intermediate mass fragments. Observation of a different N/Z ratio of light charged particles and intermediate mass fragments is consistent with an inhomogeneous distribution of isospin in the fragmenting system. There has been significant interest in multifragmentation of excited nuclear matter for many years. While there has been some success in understanding the process of multifragmentation and describing this phenomenon in terms of a liquid gas phase transition [1], those efforts have often treated the nucleus as a single component nuclear liquid. In fact, the nucleus is a two component nuclear liquid. Early work by Lamb [2] laid the foundations for treating the nucleus as a two component system although the results were not connected to multifragmentation. Statistical calculations describing multifragment disassembly predicted that much of the neutron excess would be observed as free neutrons [3]. Thermodynamic calculations by Müller and Serot [4] lead us to the idea that, for the very neutron rich systems, there may exist a distribution of the excited nuclear matter into a neutron rich gas and a more symmetric liquid. It is predicted by both lattice gas and mean field calculations [5,6] that fragmentation of a system of asymmetric nuclear matter will express the characteristics of a two component system. This would result in a second order phase transition. Also within a dynamical code, constructed to study influence of charge asymmetry on spinoidal decomposition of nuclear matter at sub-saturation densities, differences in the fragmentation are seen as a function of isospin asymmetry [7].The difference of the mean N/Z ratio of the light charged particles with Z f ≤ 2 ( LCPs ) and of the intermediate mass fragments with Z f ≥ 3 ( IMFs ) may be a possible experimental signature of a separation into a gas ( resulting mostly in emitted LCPs ) and a liquid ( IMFs ). An enhanced production of neutron rich H and He isotopes from the neck region has been seen experimentally in mid-peripheral collisions [8,9], while a favored emission of more symmetric heavy clusters in the mid-rapidity region has also been shown [10]. Quite neutron deficient residues have also been seen in intermediate energy reactions [11,12]. Recent results from the reaction of 112,124 Sn + 112,124 Sn indicate that the relative abundance of free neutrons increases as the neutron content of the colliding system increas...

Effect of nuclear periphery on nucleon transfer in peripheral collisions

“…Peripheral nucleus-nucleus collisions can be described theoretically using the model of deep-inelastic transfer [8] in combination with an appropriate model of de-excitation. Very good description of experimental observables was obtained [2,5,6,9] using the DIT model of Tassan-Got [10] and the de-excitation codes SMM [11] and GEMINI [12]. SMM represents the statistical model of multifragmentation ( SMM ) and GEMINI invokes the model of sequential binary decay ( SBD ).…”

Isoscaling of heavy projectile residues andN/Zequilibration in peripheral heavy-ion collisions below the Fermi energy

“…We note that reactions between heavy ions around and below the Fermi energy [63] proceed via a deep-inelastic transfer mechanism involving substantial nucleon exchange [36,[79][80][81]. This mechanism is responsible for energy dissipation and the ensuing creation of highly excited primary fragments.…”

“…The DIT model has been recently applied in a variety of studies at and below the Fermi energy (e.g. [36,37,64,70,93,94]). In addition, the DIT code was employed to describe lower energy data [95].…”

“…Regarding the sharing of the total excitation energy E * between the quasiprojectile and the quasitarget, a reasonable assumption for peripheral collisions is equal division [36,83,84] at relatively low kinetic energy losses. This assumption was employed in our previous analysis of heavy-residue isoscaling data [64].…”

“…In these studies, isotopic or isobaric yield ratios of selected light or intermediatemass fragments were used to probe isospin equilibration in semi-peripheral to near-central collisions. Insight into isospin equilibration has also been provided in re-cent studies of reconstructed quasiprojectiles undergoing multifragmentation following a deep-inelastic collision [36,37].…”