Production of Neutron-Rich Nuclides by Fragmentation of 212-MeV/amuCa48

Abstract

Yields of neutron-rich projectile fragments have been measured at 0° for the reaction of 212-MeV/amu 48 Ca ions on an 890-mg-cnT 2 beryllium target. Fourteen nuclides have been observed for the first time. The systematics of production cross sections are discussed.The limits of stability for nuclei have been established up to sodium and beryllium for protonrich and neutron-rich nuclides, respectively. Recently, the techniques of relativistic heavy-ion fragmentation, 1 deeply inelastic scattering, 2 and spallation induced by high-energy protons 3 have been used to produce neutron-rich nuclei near the limit of particle stability. In this Letter, we present the first experimental evidence for the particle stability of fourteen nuclides, 22 N, 26 F, 33,34 Mg> 36,37 A1> 38,39^ 41,42 p> 43,44^ and 44,45 Cl, produced in the fragmentation of 212-MeV/amu 48 Ca. In addition, the recent observation 2 of 37 Si, 40 P, and 41 ' 42 s is confirmed. Predictions for the masses of neutron-rich light nuclei have been made based on several methods, including iterative techniques such as the modified GarveyKelson relations, 4,5 the liquid-droplet model, 6,7 and large-basis shell-model calculations. 8 The energy levels of such nuclei have also been predicted using the same shell-model calculations. 8 ' 9 From the present experiment it appears that the production cross sections for very neutron-rich light nuclei may be quite sensitive to their detailed nuclear structure.The experimental arrangement used for the present work was similar to that described in Ref. 1. The fragments, which emerge from the reaction at nearly the beam velocity, were detected in a zero-degree magnetic spectrometer with an acceptance of 0.94 msr. A detector telescope consisting of twelve Si(Li) detectors, two position-sensitive Si(Li) detectors (PSD), and a veto scintillator was placed in the focal plane of the spectrometer. Each of the twelve Si(Li) detectors was 5 mm thick and 5 cm in diameter while the PSD's were 500 /xm thick and 6 cm in diameter. The PSD's were arranged to measure horizontal and vertical position with a resolution of ~ 1 mm. The beam current of 48 Ca ions from the Bevalac was ~10 7 particles/sec and was monitored directly with plastic scintillators, an ion chamber, and a scintillator telescope that monitored particles scattered from the target. The target consisted of 890 mg cm" 2 of beryllium and the beam lost approximately 35 MeV/amu passing through it.Combining the spectrometry with the energyloss measurements in the Si(Li) detectors made it possible to measure M and Z unambiguously as described in Ref. 1. The mass resolution obtained was 0.3 amu. The mass-and atomic-number scales were calibrated by use of the direct 48 Ca beam and also beams of 20 Ne and ^Ar of high energy that were progressively degraded to provide a continuous spectrum of 20 Ne and 40 Ar ions stopping in each detector. Since the detector thicknesses were precisely known, it was then possible to use a range-energy table to make an accurate channel-to-energy calibratio...

Projectile fragmentation reactions and production of nuclei near the neutron drip line

“…The concept of geometrical separation in an overlap or participant zone does not specify its proton-to-neutron ratio. A few different methods to calculate the charge distribution of the prefragments are so far proposed, a correlation model [49], hypergeometrical model [50], and GDR model [51]. After all, the charge distribution of prefragments from these methods depends on the projectile nuclei but has no target dependence.…”

Structure ofN22and theN=14subshell

“…This is done in a theoretical analysis of the experimental momentum distributions of fragments after one-neutron knockout reactions from a chain of nitrogen isotopes measured at GSI [4,5], ranging from A = 18 to 22. The role of the s-d shell and the crossing of the N = 14 neutron number are discussed with special emphasis on the possible halo structure of 22 The first observation of 22 N, with isospin projection of 4, was done by Westfall et al [6] in the pioneering nuclear fragmentation experiments with a 212 MeV/nucleon 48 Ca beam. Its half life, the beta-delayed neutron-emission probabilities P n [7], and its mass [8] were measured at the Grand Accélérateur National d'Ions Lourds (GANIL).…”

Production of unstable nuclei far from the stability line: its scope and limitations

“…Isotope production with the projectile fragmentation process was clearly demonstrated in 1979 by Westfall et al [9] in 48Ca on 9Be collisions. It was shown that a variety of nuclei could be produced using high energy heavy ion collisions.…”