A. Kiss

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.

A large-area, position-sensitive neutron detector with neutron/γ-ray discrimination capabilities

Abstract To further study neutron-rich halo nuclei, we have constructed a neutron detector array. The array consists of two separate banks of detectors, each of area 2 × 2m 2 and containing 250l of liquid scintillator. Each bank is position-sensitive to better than 10 cm. For neutron time-of-flight measurements, the time resolution of the detector has been demonstrated to be about 1 ns. By using the scintillator NE-213, we are able to distinguish between neutron and γ-ray signals above 1 MeV electron equivalent energy. Although the detector array was constructed for a particular experiment it has also been used in a number of other experiments.

Observation of giant resonances in Mg24,25,26 and Al27 by inelastic α scattering

The E2 giant-resonance region in sd-shell nuclei was studied by inelastic scattering of 106-, 120-, 145-, and 172.5-MeV ..cap alpha.. particles. Strong excitation in this region was observed for all investigated nuclei (/sup 24/,/sup 25/,/sup 26/Mg and /sup 27/Al) at the higher incident energies. At lower incident energies, the tail of the giant resonance tends to be obscured by another broad distribution. In the E2 giant-resonance region, there is more fine structure in the Mg isotopes than in /sup 27/Al. (AIP)

Cross section for the astrophysical 14C(n, γ) 15C reaction via the inverse reaction

The 14C(n, γ)15C reaction is important in neutron-induced CNO cycles of stellar evolution phases beyond the main sequence. The chain of reactions in primordial nucleosynthesis in the neutron-rich environment of an inhomogeneous big bang also involves the 14C(n, γ)15C reaction. We have used a beam of 15C ions at E/A = 35 MeV to measure cross sections for 15C breakup on targets of C, Al, Zn, Sn, and Pb. The Coulomb part of the breakup cross section was determined as a function of decay energy. By the principle of detailed balance, the neutron capture cross section was then determined as a function of neutron energy. This excitation function rises with energy to 7 μbarns at 300 keV and falls to 2 μbarns at 1.2 MeV. In the stellar-burning region, our cross section exceeds a previous measurement by a factor of 3. Neither the shape nor the magnitude of the excitation function agrees with those of theoretical calculations based on the direct capture model. The reaction rate is calculated reliably up to ~T9 = 5. In the temperature range of inhomogeneous big bang models, our rate exceeds the rate of the 14C(p, γ)15N reaction for T9 < 1.2.

Observation of giant resonances in /sup 24/,/sup 25/,/sup 26/Mg and /sup 27/Al by inelastic. cap alpha. scattering. [106, 120, 145, 172. 5 MeV]

The E2 giant-resonance region in sd-shell nuclei was studied by inelastic scattering of 106-, 120-, 145-, and 172.5-MeV ..cap alpha.. particles. Strong excitation in this region was observed for all investigated nuclei (/sup 24/,/sup 25/,/sup 26/Mg and /sup 27/Al) at the higher incident energies. At lower incident energies, the tail of the giant resonance tends to be obscured by another broad distribution. In the E2 giant-resonance region, there is more fine structure in the Mg isotopes than in /sup 27/Al. (AIP)

Elastic and inelastic deuteron scattering on 12C in the energy range from 60 to 90 MeV

Abstract Elastic and inelastic scattering of deuterons on 12 C has been investigated in the incident energy range between 60 and 90 MeV. Angular distributions between 10° and 80° (lab) have been measured for states up to approximately 15 MeV; at still higher excitation energies some prominent groups of levels show up in the spectra, including a very broad structure peaked at 27 MeV. The elastic and inelastic scattering angular distributions were analyzed in the frame-work of standard optical model and coupled-channel analysis, respectively. The quadrupole deformation parameter was found to be β 2 = −0.48 ± 0.02, independent of incident energy. The angular distributions for the second 0 + and first 3 − state were analysed using a rotation-vibration coupling scheme.

Neutron cross-talk in a multi-detector system

Abstract Two 2 m × 2 m “walls” were built for experiments with two neutrons in the final state. Each wall consists of 25 rectangular cells filled with NE-213 liquid scintillator. The close-packed design of the array makes cross-talk an inevitable contributor to distortion of measurements with this system. For E n ≤ 25 MeV almost all the detection efficiency comes from n-p scattering, and the simple two-body kinematics can be used as the basis for identifying cross-talk events. A Monte-Carlo code was developed to simulate the detection process. We found that most cross-talk events could be distinguished from real two-neutron events. A test experiment for comparison with the code was performed with neutrons from the 7 Li(p, n) 7 Be reaction at E p = 30 MeV. With this reaction all two-detector coincidences are cross-talk events. Consistency between the experimental data and the simulation results was obtained.

Neutron yields from 155 MeV/nucleon carbon and helium stopping in aluminum

Neutron fluences have been measured from 155 MeV/nucleon 4He and 12C ions stopping in an Al target at laboratory angles between 10 and 160 deg. The resultant spectra were integrated over angle and energy above 10 MeV to produce total neutron yields. Comparison of the two systems shows that approximately two times as many neutrons are produced from 155 MeV/nucleon 4He stopping in Al and 155 MeV/nucleon 12C stopping in Al. Using an energy-dependent geometric cross-section formula to calculate the expected number of primary nuclear interactions shows that the 12C + Al system has, within uncertainties, the same number of neutrons per interaction (0.99 +/- 0.03) as does the 4He + Al system (1.02 +/- 0.04), despite the fact that 12C has three times as many neutrons as does 4He. Energy and angular distributions for both systems are also reported. No major differences can be seen between the two systems in those distributions, except for the overall magnitude. Where possible, the 4He + Al spectra are compared with previously measured spectra from 160 and 177.5 MeV/nucleon 4He interactions in a variety of stopping targets. The reported spectra are consistent with previously measured spectra. The data were acquired to provide data applicable to problems dealing with the determination of the radiation risk to humans engaged in long-term missions in space; however, the data are also of interest for issues related to the determination of the radiation environment in high-altitude flight, with shielding at high-energy heavy-ion accelerators and with doses delivered outside tumor sites treated with high-energy hadronic beams.

The (d, d) and (d, d′) scattering on 24Mg at energies between 60 and 90 MeV

Abstract Angular distributions for elastic and inelastic deuteron scattering on 24Mg have been measured from 10° to about 90° at 60.6, 77.3 and 90 MeV incident energies and from 10° to 70° between 60 and 80 MeV in steps of 2 MeV. The spectra were analyzed up to 12 MeV excitation energy. The elastic angular distributions were fitted by the standard optical model. The inelastic scattering data for the low-lying levels of 24Mg can be parameterized well in the framework of a coupled channels analysis assuming an axially deformed ground state. The quadrupole deformation parameter was found to be β2 = 0.40, independent of the incident energy. The analysis of experimental data is consistent with the existence of Kπ = 0− and 3− negative parity rotational bands in 24Mg.

Method for the study of neutron emission from light fragments in intermediate energy heavy-ion collisions

Abstract A procedure has been worked out for the study of neutron emission from the projectile-like fragments in intermediate energy heavy-ion collisions. We suggest presenting the collinear fragment-neutron coincidence data as relative velocity spectra. In 14 N + 165 Ho collisions at 35 MeV/u these spectra show that most of the neutrons are emitted via a few excited states of the light fragments. By comparison with Monte Carlo simulations which, in addition to the experimental details, take explicit account of kinematic effects, the excitation spectra for the major isotopes of Li, Be, B and C are explained.