Th. W. Elze

Invariant-mass spectroscopy of 10Li and 11Li

Break-up of secondary Li-11 ion beams (280 MeV/nucleon) on C and Pb targets into Li-9 and neutrons is studied experimentally. Cross sections and neutron multiplicity distributions are obtained, characterizing different reaction mechanisms. Invariant-mass spectroscopy for Li-11 and Li-10 is performed. The E1 strength distribution, deduced from electromagnetic excitation of Li-11 up to an excitation energy of 4 MeV comprises similar to 8% of the Thomas-Reiche-Kuhn energy-weighted sumrule strength. Two low-lying resonance-like structures are observed for Li-10 at decay energies of 0.21(5) and 0.62(10) MeV, the former one carrying 26(10)% of the strength and likely to be associated with an s-wave neutron decay. A strong di-neutron correlation in Li-11 can be discarded. Calculations in a quasi-particle RPA approach are compared with the experimental results for Li-10 and Li-11

A Large area detector for high-energy neutrons

Abstract We present design studies, results of test measurements, and Monte Carlo simulations which served as a basis for the realization of a large area neutron detector (LAND). It has a front area of 2m×2m and a depth of 1 m, and features a multilayer structure of passive converter and active scintillator material. The detector is subdivided in independently operating paddles which allow time-of-flight and position measurement. An energy resolution of ΔT n / T n =5.3% for a flight path of 15 m and an overall detection efficiency of ϵ ≈ 1 is anticipated for neutrons with T n ≈ 1 GeV. The operation of LAND at the SIS facility of GSI is described.

Structure of high-spin states in 232Th, 234U and 236U

Abstract Multiple Coulomb excitation of 232 Th, 234 U and 236 U by 5.3 MeV u 208 Pb ions has been studied using γ-ray spectroscopy. Excitation of ground-band levels is observed up to spin I π = 26 + (tentatively 28 + ) in 232 Th and 234 U and I π = 28 + (tentatively 30 + )in 236 U. High-spin levels of the K π = 0 − octupole-vibrational bands are also observed in these nuclei. The measured transition energies between ground-band levels suggest that at I≈ 28 h several units of angular momentum are carried by single particles aligned with the rotation axis.This result can be understood in terms of a super band built on aligned two-quasiparticle configurations which crosses the ground-state rotational band at a rotational frequency of h ω ≳ 0.25 MeV (I ≳ 28 h ) . The E2 transition-matrix elements deduced from the experimental γ-yields agree within their errors with the rigid-rotor predictions up to the highest spins observed.The experimental results are discussed using the concept of rotation alignment and are compared with predictions of the rotation-vibration model and the interacting-boson model.

Collective states of Gd isotopes from (p, t) reactions

Abstract The reactions 160,158,156,154,152Gd(p, t) have been studied with 18 MeV protons. The angular distributions of the emergent tritons allow an unambiguous identification of l = 0 transitions and, in some cases, a classification of l = 2 transitions in terms of excitations of β- or γ-vibrations. Excited collective 0+ states are populated with approximately 15 % of the ground state strength. In the 154Gd(p, t)152Gd reaction which connects a deformed with a spherical nucleus, a third 0+ state is observed at 1053 keV. The present results and previous data from (p, t) and (t, p) reactions on Sm isotopes support the interpretation of the 1053 keV state being a deformed 0+ excited state in the spherical 152Gd nucleus.

8He-6He: a comparative study of nuclear fragmentation reactions

Dissociation of 227 MeV/u He-8 in a carbon target has been studied in kinematically complete experiments. The data include the relative energy spectrum, angular distributions in the neutron knock-out channel (He-6 + n) as well as diffractive dissociation and inelastic scattering into the (He-6 + 2n) channel. The data are compared with corresponding results from the well-known halo nucleus He-6. In both cases it is found that neutron knock-out is the: dominating reaction channel. The relative energy spectrum (He-6 + n) shows a structure, which is interpreted as being due to the I-pi = 3/2(-) resonance in the He-7 ground state with about equal contribution from its I-pi = 1/2(-) spin-orbit partner. The He-7 resonance shows a spin alignment similar to that observed in He-5, but with a smaller anisotropy indicating that the structure of the He-8 ground state is more complicated than that of He-6. The data in the (He-6 + 2n) channel were used to identify resonances in the excitation energy spectrum of He-8. If the spectrum is interpreted as two overlapping resonances, the spin-parity assignment for these is found to be 2(+) and 1(-), respectively.

Analysis of simulated and measured pulse shapes of closed-ended HPGe detectors☆

Abstract A program for simulating pulse shapes of HPGe detectors has been developed taking into account the closed-ended geometry as well as multiple interactions of γ-rays. Good agreement between simulated and experimentally determined pulse shapes is achieved. From a detailed investigation of risetime distributions a method to determine the radial coordinate of the first point of interaction has been substantiated. For a large Ge detector with 70 mm diameter a radial resolution of 4 to 8 mm has been obtained. This method, combined with segmentation of the detector, reduces the Doppler broadening of γ-lines significantly.

Searching for the 5H resonance in the t+n+n system

19 pages, 7 figures, 2 tables, 2 appendices.-- PACS nrs.: 27.10.+h; 25.60.Gc.-- Printed version published Jul 28, 2003.

Invariant mass spectrum and alpha-n correlation function studied in the fragmentation of He-6 on a carbon target

Momentum distributions and invariant mass spectra from the breakup of He-6 ions with an energy of 240 MeV/u interacting with a carbon target have been studied. The data were used to extract information about the reaction mechanism which is influenced by the structure of He-6. It is found that the dominant reaction mechanism is a two-step process: knock out of one neutron followed by the decay of the He-5 resonance. The shape of the (alpha+n) two-body invariant mass spectrum is interpreted as mainly reflecting the 5He ground state which is a J(pi) = 3/2(-) resonance. However, no evidence for correlations between cu particles and neutrons is observed in the momentum widths of the distributions. It is demonstrated that a combined analysis of the two-body invariant mass spectrum and an appropriate correlation function may be used to determine the properties of the intermediate resonance

Longitudinal and transverse momentum distributions of 9Li fragments from break-up of 11Li

Transverse and longitudinal momentum distributions of Li-9 fragments from Li-11 break-up reactions in C, Al and Pb targets have been measured at 280 MeV/u. The two-neutron removal cross-section was measured to be sigma(-2n), = 0.26 +/- 0.02 b for the carbon target, sigma(-2n) = 0.47 +/- 0.08 b for the aluminum target and sigma(-2n), = 1.9 +/- 0.4 b for the lead target. No significant difference is observed between the narrow widths (FWHM approximate to 47 MeV/c) of the transverse and longitudinal momentum distributions of the Li-9 fragments. The physical implications of this are discussed.

Hexadecapole moments obtained by semiclassical and quantum mechanical analyses of Coulomb excitation

Electric quadrupole and hexadecapole matrix elements of rare-earth nuclei were determined from Coulomb excitation of the 2+ and 4+ rotational states in the ground-state band. The E4-matrix elements obtained second-order perturbation expansion of the quantum mechanical theory and by a coupled channel code agree within 20%.