Th. Happ

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.

Positron production in heavy ion collisions: Current status of the problem

Narrow peaks have been observed at GSI Darmstadt in the energy spectra of positrons and sum-energy spectra of positron-electron pairs, produced in collisions of very heavy ions. To date, there is no satisfactory explanation of the origin of these lines although many differing models have been proposed. In this contribution, the authors describe the features of a new experiment aimed at the study of the line phenomenon and present the results of their first experiments. The specific goals of their experiment are to clarify the experimental situation regarding the lines through high-resolution, high-statistics data and, by direct measurement of the vector momenta of the peak pairs, to determine their kinematics.

A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

Abstract A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.

Inelastic scattering and nucleon transfer in the system232Th+206Pb near the Coulomb barrier

Nucleon transfer accompanied by Coulomb excitation was studied in the system206Pb+232Th atELab=6.4 MeV/u. Particle-particle-gamma coincidence techniques were used to identify excited states of reaction products populated through inelastic scattering and nucleon transfer reactions. The mean excitation energy was measured by means of aγ-ray energy and multiplicity filter consisting of 6 NaI detectors. Large cross sections for one-neutron and two-neutron pick-up from232Th are observed. The impact-parameter dependence of the neutron transfer is analyzed in terms of semiclassical barrier penetration models. Using realistic neutron potentials with a diffuse surface, the experimental data are in accordance with the assumption of a “cold” transfer to states near the yrast line.

Coulomb excitation and transfer reactions

Abstract Using the γ-spectrometer GASP and the PYRAMID array of position sensitive particle counters enabled improved nuclear structure studies employing direct reactions. Coulomb excitation of 232 Th by 58 Ni at a beam energy of 265 MeV was used to investigate multi-phonon surface vibrations. For 232 Th a new band has been found which is interpreted as harmonic two-phonon octupole vibrational band with K π =0 + . The system 110 Pd + 52 Cr was chosen to study the mechanism of multinucleon transfer at energies in the vicinity of the Coulomb barrier. It was possible to identify 10 different transfer channels by their characteristic γ-decay. The total excitation energy versus γ-multiplicity correlations for the most prominent even-even transfer channels show a ‘cold’ component which is a strong indication for correlated pair transfer. For the 2n and 2p channel this fraction is 30(4)%, respectively 16(8)%.