Production of medium-mass neutron-rich nuclei from fragmentation of Sn

Medium-mass neutron-rich nuclei are important for nuclear structure investigations (e.g. shell evolution with neutron excess) and nuclear astrophysics (e.g. r-process in nucleosynthesis). However, these investigations are limited because of the difficulties in producing medium-mass nuclei with a large neutron excess. The traditional way for producing neutron rich nuclei was fission of actinides [1]. A new idea is to use a two step reaction scheme. Medium-mass neutron-rich isotopes are produced with high intensities as fission fragments. Then, they are used as projectiles in a second step to produce even more neutronrich nuclei by cold fragmentation. In fact, this two step reaction might be a tool for producing beams of extremely neutron rich isotopes of refractory elements and short lived nuclei in ISOL facilities [2]. In a recent work [3] the feasibility of the two step reaction scheme was investigated by calculating the production cross sections of residual nuclei in this kind of reactions. Two different model calculations were used, EPAX [4], the semiempirical parameterisation for fragmentation cross sections, and the cold fragmentation code COFRA, which is described in ref. [5]. It has been shown that the predictions of both model calculations applied for fragmentation of neutron-rich projectiles differ considerably. In order to obtain a clear answer for these discrepancies, an experiment was performed at GSI Fragment Separator (FRS) in November 2006. A 950 MeV/u U beam with an intensity of 10 projectiles per spill impinged onto a a 650 mg/cm Pb target at the entrance of the FRS to produce fission fragments. The forward emitted fission products were isotopically identified by the first section of the FRS by using and improved time-of-flight system and TPC chambers for position and angle measurements. The tracking correction to the time-of-flight allowed for the first time to isotopically separate at the FRS A≈130 fragments with a limited flight path (20 m). These fully identified fission residues impinged onto a secondary beryllium target (2.6 g/cm) located at the intermediate focal plane. The fragmentation residues were also isotopically identified in the second section of the spectrometer using a similar technique but with a longer flight path (36 m). The production cross sections of these fragmentation residues were obtained normalizing the measured yields to the secondary beam current (fission fragments) and the beryllium target thickness. Figure 1 shows the measured cross sections of residual nuclei produced in the fragmentation of Sn with a beryllium target. As expected, this two step reaction scheme leads to the production of very neutron-rich medium-mass fragments. Indeed, we have observed for the first time Mass number 115 120 125 130 ( m b )