A. Grewe

The GSI projectile fragment separator (FRS): A Versatile magnetic system for relativistic heavy ions

The projectile fragment separator FRS designed for research and applied studies with relativistic heavy ions was installed at GSI as a part of the new high-energy SIS/ESR accelerator facility. This high-resolution forward spectrometer has been successfully used in first atomic and nuclear physics experiments using neon, argon, krypton, xenon, and gold beams in the energy range from 500 to 2000 MeV/u. For the first time relativistic xenon and gold fragments have been isotopically separated. In this contribution we describe first experiments characterizing the performance of this spectrometer.

Proton removal in peripheral nuclear collisions at relativistic energies

Abstract The projectile-fragment separator FRS, recently installed at the heavy-ion synchrotron SIS18 at GSI, was used to measure the formation cross sections and longitudinal-momentum distributions of fragments produced by removing one, two or three protons from 0.8 GeV · A 136 Xe and 1 GeV · A 197 Au projectiles impinging on targets of beryllium and aluminum, respectively. These data gave direct information on the high-energy nuclear collision with only little contribution from evaporation processes. The measured cross sections decrease by more than one order of magnitude per proton removed. They are well reproduced by the predictions of a statistical abrasion model and an intranuclear-cascade calculation. The momentum distributions are found to be much broader than predicted by empirical systematics. They are, however, in agreement with the momentum distribution of the abraded protons as calculated with the uniform-Fermi-gas model, if Fermi momenta as measured by quasi-elastic electron scattering are used.

The Scintillation detector equipment of the GSI projectile fragment separator

A complete identification in mass and nuclear charge for relativistic projectile fragments up to uranium is possible at the fragment separator (FRS) at GSI by using the scintillation detector equipment described in this paper. It was successfully used in experiments with primary beams ranging from oxygen to uranium with energies from 300 to 1000 A MeV. A mass-resolving power of A/ΔA = 407 (FWHM) was obtained for 0.95 A GeV 238U fragments.

Longitudinal momenta and production cross-sections of isotopes formed by fragmentation of a 500 A×MeV86Kr beam

Abstract At the projectile-fragment separator FRS at GSI, longitudinal momentum distributions and production cross-sections were investigated for reactions between a 500 A× MeV 86 Kr beam and beryllium, copper and tantalum targets. Fragments in a wide A Z range were studied. For fragments close to the projectile the measured longitudinal momentum distributions agree qualitatively with a semi-empirical parametrization. Deviations from this simple picture are found, however, for the momentum widths of light fragments and for the momentum transfer to those fragments that differ in their A Z ratio from the most probable value. The experimental data on production cross-sections are used for testing the predictions obtained from a semi-empirical parametrization, a geometrical abrasion model and an intranuclear-cascade model. The cross-section for the formation of the doubly magic nucleus 78Ni in 86Kr fragmentation is estimated to be 17 pb.

New neutron-rich isotopes in the scandium-to-nickel region, produced by fragmentation of a 500 MeV/u86Kr beam

We have measured production cross-sections of the new neutron-rich isotopes58Ti,61V,63Cr,66Mn,69Fe,71Co and neighbouring isotopes that have been identified as projectile fragments from reactions between a 500 MeV/u86Kr beam and a beryllium target. The isotope identification was performed with the zero-degree magnetic spectrometer FRS at GSI, using in addition time-of-flight and energy-loss measurements. The experimental production cross-sections for the new nuclides and neighbouring isotopes are compared with an empirical parametrization. The resulting prospects for reaching even more neutron-rich isotopes, such as the doubly-magic nuclide78Ni, are discussed.

β-delayed proton radioactivity of 44Cr, 47Mn, 48,49Fe and 50Co

Abstract The proton-rich nuclei 44 Cr, 47 Mn, 48,49 Fe and 50 Co have been produced by fragmentation of a 58 Ni beam at 650 MeV/u. The isotopic separation of these nuclei has been achieved with the GSI Projectile-Fragment Separator FRS. The isotopes have been identified in flight by ΔE -ToF- Bϱ measurements. After implantation in a stack of seven silicon detectors, the signals measured for implantation and radioactive decay were unambiguously correlated in time due to low counting rates. On the basis of the two proton peaks observed for 50 Co at (2034±30) keV and (2740±41) keV with a half-life of (44±4) ms, a partial decay scheme is proposed for this nucleus. A single proton peak at (959±33) keV was observed for 48 Fe with a half-life of (44±7) ms. This emission is attributed to the decay of the T = 2 isobaric analog state in 48 Mn. No deviation from the quadratic form of the isobaric multiplet mass equation is observed. Additional information on β-delayed proton branches of 49 Fe, 44 Cr and 47 Mn was also obtained.

First spatial isotopic separation of relativistic uranium projectile fragments

Abstract Spatial isotopic separation of relativistic uranium projectile fragments has been achieved for the first time. The fragments were produced in peripheral nuclear collisions and spatially separated in-flight with the fragment separator FRS at GSI. A two-fold magnetic-rigidity analysis was applied exploiting the atomic energy loss in specially shaped matter placed in the dispersive central focal plane. Systematic investigations with relativistic projectiles ranging from oxygen up to uranium demonstrate that the FRS is a universal and powerful facility for the production and in-flight separation of monoisotopic, exotic secondary beams of all elements up to Z = 92. This achievement has opened a new area in heavy-ion research and applications.

A self-calibrating ionisation chamber for the precise intensity calibration of high-energy heavy-ion beam monitors

The intensity of a 136Xe(600 A MeV) beam has been determined by simultaneously measuring the particle rate and the corresponding ionisation current with an ionisation chamber. The ionisation current of this self-calibrating device was compared at higher intensities with the current of a secondary-electron monitor and a calibration of the secondary-electron current was achieved with a precision of 2%. This method can be applied to all high-energy heavy-ion beams.

Momentum distributions of projectile fragments produced in the cold and hot fragmentation of relativistic136Xe and197Au projectiles

The longitudinal-momentum distributions of projectile fragments from 0.8 A GeV136Xe and 1 A GeV197Au projectiles impinging on targets of beryllium and aluminium, respectively, have been measured using the projectile-fragment separator FRS at GSI. Different momentum distributions have been found for two different classes of fragmentation processes: the abundant hot fragmentation with several nucleons evaporated from the prefragments, and the rare cold fragmentation with only protons removed from the projectile, but no nucleons evaporated. The data are compared to model calculations.

Few-neutron removal from238U at relativistic energies

As part of a comprehensive study of uranium fragmentation at relativistic energies at the GSI projectile fragment separator, FRS, inclusive neutron-removal cross sections have been measured for severalxn channels at projectile energies of 600 and 950A MeV using targets of Al, Cu and Pb. The variation of the experimental cross sections with target nuclear charge is used to disentangle nuclear and electromagnetic contributions. The electromagnetic cross sections agree surprisingly well with a simple harmonic oscillator calculation of giant dipole resonances based on measured photonuclear cross sections and do not require an extra enhancement of the two-phonon giant dipole excitation as concluded from similar measurements with197Au.