The cross sections and velocity distributions of projectile-like fragments from the reaction of 25 MeV/nucleon 86 Kr + 64 Ni have been measured using the MARS recoil separator at Texas A&M, with special emphasis on the neutron rich isotopes. Proton-removal and neutron pick-up isotopes have been observed with large cross sections. A model of deep-inelastic transfer (DIT) for the primary interaction stage and the statistical evaporation code GEMINI for the deexcitation stage have been used to describe the properties of the product distributions. The results have also been compared with the EPAX parametrization of high-energy fragmentation yields. The experimental data show an enhancement in the production of neutron-rich isotopes close to the projectile, relative to the predictions of DIT/GEMINI and the expectations of EPAX. We attribute this enhancement mainly to the effect of the extended neutron distribution (neutron "skin") of the 64 Ni target in peripheral interactions of 86 Kr with 64 Ni. The large cross sections of such reactions near the Fermi energy, involving peripheral nucleon exchange, suggest that, not only the N/Z of the projectile and the target, but also the N/Z distribution at the nuclear surface may properly be exploited in the production of neutron-rich rare isotopes. This synthesis approach may offer a fruitful pathway to extremely neutron-rich nuclei, towards the neutron-drip line.

The ground and first excited states in 15 F were studied in resonant elastic scattering using the thick ͑CH 4 ͒ gas target method in inverse kinematics with a separated 14 O beam. An analysis of the excitation functions of the elastic scattering was carried out with the potential model. The quantum numbers 1 / 2 + (ground state) and 5/2 + (first excited state) were assigned to the lowest two states in 15 F. Also, the widths and the proton decay energies of the unbound levels were obtained. The analysis of the data indicates that a large diffuseness is needed in the Woods-Saxon potential in order to describe single-particle features in drip-line nuclei. Over the past decade it has become clear that drip-line nuclei demonstrate a number of phenomena which are not found in nuclei close to the line of stability. One such feature is the change in magic numbers, which are generated by a conventional Woods-Saxon potential with parameters fitted for stable nuclei [1][2][3]. One example is the intruder singleparticle 2s 1/2 state, which appears to be the ground state in 11 Be [4] and 11 N [5] instead of the 1p 1/2 state. As another example, it has been predicted [1-3] that the diffuseness of nuclear densities for intermediate mass nuclei increases dramatically when approaching the neutron drip line. These phenomena indicate that the parameters of the shell model potential can be unusual for nuclei at the borders of nuclear stability. Figure 1 shows how, for a light nucleus, the energies of the shell model levels in a Woods-Saxon potential depend on the ratio of the diffuseness to radius parameters. It can be seen in Fig. 1 that the 1p 1/2 and 2s 1/2 levels approach each other as the ratio increases. One way to explain the phenomena observed near the drip line is that the singleparticle potential changes its shape giving way to new shell structure. This effect can be tested by an analysis of the nucleon widths of the single-particle states in drip-line nuclei, which are mainly dependent upon the geometrical parameters of the well.15 F is a good system to check the considerations above. The lowest states in 15 F are unstable to proton decay and should have dominantly single-particle structure. The theoretical predictions for the single-particle spectroscopic factors are 0. O on hydrogen with the thick target inverse kinematics method [5,10,11]. An approach similar to Ref.[9] was used in the present experiment. However, there are important differences in the details of the two measurements. In the present work, a gas target, CH 4 , was used instead of a solid CH 2 target. The gas target results in a drastic decrease of the background (see below). Also, only the excitation function at 180°͑c.m.͒ in arbitrary units was measured in Ref. [9], while in the experiment reported here measurements were made at several angles. As a result, we have a better determination of the positions and the widths of the levels which allows us to make conclusions about the parameters of the interaction potential between 14 O and protons. The expe...

A large enhancement in the production of neutron-rich projectile residues is observed in the reactions of a 25 MeV/nucleon 86 Kr beam with the neutron rich 124 Sn and 64 Ni targets relative to the predictions of the EPAX parametrization of high-energy fragmentation, as well as relative to the reaction with the less neutron-rich 112 Sn target. The data demonstrate the significant effect of the target neutron-to-proton ratio (N/Z) in peripheral collisions at Fermi energies. A hybrid model based on a deep-inelastic transfer code (DIT) followed by a statistical de-excitation code accounts for part of the observed large cross sections. The DIT simulation indicates that the production of neutron-rich nuclides in these reactions is associated with peripheral nucleon exchange in which the neutron skins of the neutron-rich 124 Sn and 64 Ni target nuclei may play an important role. From a practical viewpoint, such reactions between massive neutron-rich nuclei offer a novel synthetic avenue to access extremely neutron-rich rare isotopes towards the neutron-drip line.PACS numbers: 25.70.Hi,25.70.Lm Exploration of the nuclear landscape towards the neutron-drip line [1] is currently of great interest in order to elucidate the evolution of nuclear structure with increasing neutron-to-proton ratio (N/Z) [2,3] and understand important nucleosynthesis pathways [4], most notably the r-process [5]. Reactions induced by neutronrich nuclei provide invaluable information on the isospin dependence of the nuclear equation of state [6,7]. Extremely neutron-rich nuclei offer the unprecedented opportunity to extrapolate our knowledge to the properties of bulk isospin-rich matter, such as neutron stars [8,9]. The efficient production of very neutron-rich nuclides is a key issue in current and future rare isotope beam facilities around the world [10,11,12] and, in parallel, the search for new synthetic approaches is of exceptional importance.Neutron-rich nuclides have traditionally been produced in spallation reactions, fission, and projectile fragmentation [13]. In high-energy fragmentation reactions, the production of the most neutron-rich isotopes is based on a "clean-cut" removal of protons from the projectile. The world's data on fragmentation cross sections are well represented by the empirical parametrization EPAX [14]. EPAX is currently the common basis for predictions to plan rare beam experiments and facilities. In addition to the widely used projectile fragmentation approach, neutron-rich nuclides can be produced in multinucleon transfer reactions [15] and deep-inelastic reactions near the Coulomb barrier (e.g. [16,17,18]). In such reactions, the target N/Z significantly affects the production cross sections, but the low velocities of the fragments and the ensuing wide angular and ionic charge state distributions render practical applications rather limited. The Fermi energy regime (20-40 MeV/nucleon) [19] offers the unique opportunity to combine the advantages of both low-and high-energy reactions. At this energy, the synergy...

No abstract

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86 Kr projectiles with 124 Sn, 112 Sn and 64 Ni, 58 Ni targets is studied. Isotopically resolved yield distributions of projectile fragments in the range Zϭ10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7°-5.4°. For these deep inelastic collisions, the velocities of the residues, monotonically decreasing with Z down to ZӍ26-28, are employed to characterize the excitation energy. The ratios R 21 (N,Z) of the yields of a given fragment (N,Z) from each pair of systems are found to exhibit isotopic scaling ͑isoscaling͒, namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are ␣ϭ0.43 and ␤ϭϪ0.50 for the KrϩSn system and ␣ϭ0.27 and ␤ϭϪ0.34 for the KrϩNi system. For the KrϩSn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Zр26 and Nр34. For heavier fragments from KrϩSn, the parameters vary monotonically, ␣ decreasing with Z and ␤ increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Interestingly, such high-resolution mass spectrometric data can provide important information on the role of isospin and isospin equilibration in peripheral and midperipheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.