W. von Witsch

Search for highly excited states in light nuclei with three-body reactions

Abstract Various three-body break-up reactions resulting from the bombardment of 7 Li with 120 MeV 3 He have been measured simultaneously in a kinematically complete experiment. Missing-mass spectra have been deduced for 4 H, 4 He, 4 Li, 5 He, 5 Li, 6 He, 6 Li and 7 Li. The ground states of 4 H and 5 He are well described as n-t and n-α P-wave final-state interactions, respectively. Evidence is found for relatively narrow states at very high excitation energies in several nuclei. The results are compared with those of other authors and with various theoretical predictions.

High-pressure gas target for the production of intense fast-neutron beams

Abstract A high-pressure, liquid-nitrogen cooled gas target is described which is suitable as an intense high-energy neutron source using the 2 H(d, n) reaction. Details of the design and construction are given and the performance of the target with respect to gas pressure, beam intensity, and gas density is discussed and illustrated. Measured neutron spectra and intensities agree with predictions.

Final-state interactions and quasi-free scattering in the 2H(α, pα)n reaction

Abstract The 2H(α, pα)n reaction has been studied in a kinematically complete experiment at several angles and energies suitable for observing proton-α quasi-free scattering (QFS) and proton-neutron final-state interaction (FSI). The data have been analyzed in terms of various quasi two-body models, taking into account the strong contributions from p-α and n-α interactions in the ground states of 5Li and 5He which are present in most of the spectra. A modified impulse approximation was found to describe reasonably well the spectra dominated by p-α QFS and FSI. A characteristic increase of yield has been observed in the region of low relative p-n energy which can only be explained by the admission of at least 1% of isospin forbidden proton-neutron FSI in the spin-singlet state.

A thin-film scintillator detector for light ions in low-energy nuclear physics

Abstract A thin-film plastic scintillator detector has been developed for use in time-of-flight measurements of fast light ions in low-energy nuclear physics experiments. The design of the detector is described, and its response to protons, deuterons and alpha particles is discussed with respect to light output, time resolution, and detection efficiency.

The influence of convection on high-pressure gas target densities

Abstract It is demonstrated that free convection is the dominant mechanism of heat transfer when a liquid-nitrogen-cooled high-pressure gas target is bombarded with intense ion beams.

Neutron-neutron effective range from a comparison of n-n and n-p quasi-free scattering at 24 MeV

Abstract Neutron-neutron and neutron-proton quasi-free scattering have been measured at E n = 24 MeV the d + n reaction to deduce the n-n effective range from a comparison of relative cross section, reducing considerably experimental as well as theoretical uncertainties. A Monte Carlo analysis with exact three-body calculations yields r nn = 2.65 ± 0.18 fm.

Isospin purity of the 5.37 MeV state in 6Li

Abstract The 6Li(p, pα)d, 6Li(p, pα)np and 6Li(p, pp)5He (g.s.) reactions were measured at 11 and 12 MeV bombarding energy, θ1 = 135° and θ2 = 30°. In the coincidence spectra, final state interactions (FSI) corresponding to the Jπ = 2+ doublet in 6Li were clearly seen and their widths and energy positions determined. In addition, the p-α spectrum gives evidence for a level in 6Li at 5.7 MeV excitation with a width of approximately 1 MeV. No indication for decay of the 6Li(5.37 MeV, 2+, T = 1) state in the d-α channel was found. By comparing the corresponding yields in the isospin allowed and forbidden channels, an upper limit of α2 ≦ 8 × 10−3 was deduced for a possible T = 0 admixture in the T = 1 state at 5.37 MeV.

Efficiency and directional effects in the detection of low-energy recoil nuclei in a NaI(Tl) single crystal

The pulse shape for low-energy 127I recoils in a NaI(Tl) single crystal was investigated as a function of the crystal orientation, using elastic neutron scattering. For comparison, the pulse shape was also studied for Compton-scattered electrons at small pulse heights. In addition, relative and absolute yields were measured for the elastic scattering of neutrons from 23Na and 127I. Large deficits were observed in the count rates, indicating detection efficiencies for the recoiling nuclei which are much smaller than 100%.

Neutron-neutron scattering length from the 2H(n, np)n reaction at bombarding energies up to 27 MeV

Abstract The n-n interaction has been investigated in a kinematically complete experiment with high statistical accuracy via the 2H(n, np)n reaction at bombarding energies up to 27 MeV, extending previous measurements to higher energies and to a different geometry. The analysis with exact three-body calculations yields ann = −16.9 ± 0.6 fm.

Measurement of the imaginary part of the I=1 N̄N S‐wave scattering length

The survival time spectrum of slow antineutrons produced in a LH2 target has been measured. From these data the imaginary part of the I=1 spin averaged S‐wave antineutron proton scattering length has been deduced to be Im a1= −0.83±0.07 fm. The result lies within the range of values calculated from current potential models.