J. A. Winger

Coulomb dissociation of 11Li.

Kinematically complete measurements for Coulomb dissociation of [sup 11]Li into [sup 9]Li+2[ital n] were made at 28 MeV/nucleon. The [ital n]-[ital n] correlation function suggests a large source size for the two-neutron emission. The electromagnetic excitation spectrum of [sup 11]Li has a peak, as anticipated in low-energy dipole resonance models, but a large post-breakup Coulomb acceleration of the [sup 9]Li fragment is observed, indicating a very short lifetime of the excited state and favoring direct breakup as the dissociation mechanism.

The A1200 projectile fragment separator

Abstract A beam analysis device, the A1200, has been constructed at the National Superconducting Cyclotron Laboratory (NSCL) for routine cyclotron beam analysis. This device can also be used to separate radioactive beams produced by projectile fragmentation. Since the A1200 begins the K1200 cyclotron beam lines, radioactive ions can be delivered to any experimental device. The details of the mechanical and optical designs are presented. In addition some of the planned experiments with the separated radioactive beams are discussed.

INTENSITY: a computer program for the estimation of secondary beam intensities from a projectile fragment separator

Abstract Radioactive ion beams are rapidly becoming an important new tool in nuclear science research. A number of facilities have been built or are under construction worldwide which will allow separation of relatively pure radioactive ion beams. A number of these facilities, including LISE at GANIL, RIPS at RIKEN, the FRS at GSI, and the A1200 at the NSCL, are based on the separation of projectile-like fragments using a doubly achromatic system. As part of the work leading up to experiments using the A1200, we have developed a computer code which allows fast, interactive estimation of secondary ion beam intensities and purities for use in the design and interpretation of experiments. The code has been designed to model and doubly achromatic device of design similar to the A1200 by using an idealized initial secondary beam from a production target. The various atomic effects involving the interaction of ion beams with targets and degraders, including the equilibrium charge states of beams leaving these materials, have been analyzed in detail and incorporated into the program. An analytical description of the beam transport system using first order beam optics has been developed. A comparison between the program and various other simulation methods as well as measured production rates have shown good agreement.

Single neutron emission following 11Li β-decay

Abstract The spectrum of neutrons emitted after the β-decay of 11 Li has been measured with a large time-of-flight array. The spectrum exhibits six peaks corresponding to neutron emission from known states in the 11 Be daughter nucleus. The measured branching ratios indicate significant feeding to states over a broad energy range. The energies and parities of the states are in very good agreement with the previous results of the 9 Be(t,p) reaction. Comparison of the present results with shell model calculations indicate the effect of the two neutron halo in 11 Li. The beta-delayed neutron spectrum of 8 He was determined in order to remove contributions from beam contamination.

The use of radioactive nuclear beams to study the equilibration of the N Z degree of freedom in intermediate-energy heavy-ion reactions

Abstract The effect of the neutron to proton ratio of the colliding system on the isotope ratio of the emitted fragments (1 ⩽ Z ⩽ 5) was studied for for a variety of constant mass beam-target combinations. To extend the N Z ratio as far as possible, one of the projectiles employed, 40Cl, was a radioactive secondary beam. The isotopic ratios depend on the N Z of the target and beam in a way which is not consistent with N Z > equilibration on the timescale of the emission of intermediate-mass fragments.

Halflife measurements of the rp-process nuclei 61Ga, 63Ge, and 65As

Abstract Using the A1200 beam analysis device at the National Superconducting Cyclotron Laboratory, we have measured the halflives of several nuclei along the rp-process path near the proton-drip line. Halflife results for 61 Ga, 63 Ge, and 65 As (0.15±0.03 s, 95 −25 +23 ms, and 0.19 −0.07 +0.11 s, respectively) and their implications for the rp-process are presented.

Initial operating experience with the A1200 fragment separator

Abstract A beam analysis device, the A1200, has been constructed at the National Superconducting Cyclotron Laboratory (NSCL) for routine cyclotron beam analysis, and separation of radioactive beams produced from projectile fragmentation. It has been in operation since October 1990. The A1200 begins the NSCL beamlines following the K1200 cyclotron, and hence can transport radioactive ions to any experimental device. Some details of the mechanical and optical designs are presented, along with a discussion of the initial operation.

Half-life measurements for Ga-61, Ge-63, and As-65 and their importance in the rp process

The [ital rp] process provides a mechanism whereby energy is generated and proton-rich nuclei are synthesized in various astrophysical environments. Understanding this process requires knowledge of the half-lives, binding energies, and reaction cross sections for the nuclei along its path, which for [ital A][approx gt]60 lies near the proton-drip line. Using the A1200 radioactive-beam facility at the National Superconducting Cyclotron Laboratory, we have performed an experiment to measure the [beta]-decay half-lives of several [ital T][sub [ital z]]=[minus]1/2 nuclei in this mass region by identification and implantation of projectile fragments followed by a rapid beam cutoff and observation of the [beta] decay. Using this technique, the half-lives of [sup 61]Ga, [sup 63]Ge, and [sup 65]As (0.15[plus minus]0.03 s, 95[sub [minus]20][sup +23] ms, and 0.19[sub [minus]0.07][sup +0.11] s, respectively) have been measured and are found to be consistent with expectations from [beta]-decay theory for decay between mirror states with [ital Q][sub [beta]][similar to]9 MeV. In addition, the proton-rich nucleus [sup 66]Se has been observed for the first time. The experimental method used for this measurement and the implications of these results for the [ital rp] process are discussed.

Study of the β-delayed neutron decay of 17C and 18C

Abstract The β-delayed neutron decays of 17C and 18C have been studied using a time-of-flight array with a high detection efficiency. The 17C and 18C ions were produced by fragmentation of an E A = 69 MeV 22Ne beam. Transitions to several neutron unbound states have been observed for the first time for both decays with total branching ratios of (10.8 ± 2.2)% and (21.4 ± 4.4)%. Half-lives of 193 ± 6 ms and 92 ± 2 ms were found for 17C and 18C, respectively. The results are compared with previous measurements of the β-decays and with shell-model calculations.

Search for double Gamow-Teller strength by heavy-ion double charge exchange

Abstract We have carried out a search for double Gamow-Teller excitations, employing the 24 Mg( 18 O, 18 Ne) 24 Ne reaction at 100 and 76 MeV/nucleon at NSCL-MSU and GANIL, respectively. The cross sections for low-lying excitations are typically a few nb/sr, providing evidence for a strong suppression of double Gamow-Teller excitations in heavy-ion double charge exchange, compared to pion-induced reactions. This result does not give good hope that heavy ions at intermediate energies could be used to probe double Gamow-Teller strength.