S. Skoda

Development of segmented Ge detectors for future γ-ray arrays

The EUROBALL Cluster detector is composed of seven encapsulated Ge detectors in a common cryostat with a total volume of 2000 ccm of HP Gemanium. The development and the performance of the Cluster detector is summarized. Up to six Cluster detectors were used in pre-EUROBALL experiments at the S-DALINAC Darmstadt, at the tandem-postaccelerator facility of the MPI-K Heidelberg and at the UNILAC at GSI. Examples of these experiments - the excitation of dipole modes with (γ, γ′)-reactions and the first coincidence spectroscopy of the N=Z-nucleus 68Se with a CLUSTER Cube - are discussed. The development of segmented encapsulated Ge detectors for a MINIBALL at the radioactive beam facility REX-ISOLDE has been launched. The status of the project is presented.

Fine structure of the E1 response in 140Ce below the particle threshold

Abstract The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

Delayed g9/2 Alignment in the N=Z Nucleus 72Kr

The structure of Kr-72 has been investigated at GASP through the Ca-40(Ca-40,2 alpha) reaction at a beam energy of 160 MeV using the 4 pi ISIS Si-ball for reaction channel selection. The level scheme has been extended up to an excitation energy of approximate to 8.5 MeV. The spins and parities of the observed levels are assigned tentatively. The observed band shows the predicted change from oblate to prolate shape. The four quasi-particle g(9/2) alignment is found to be significantly delayed in rotational frequency with respect to the heavier Kr isotopes. Such a delay contradicts the predictions of standard mean-field calculations and may reflect either additional correlations in the T = 0 pairing channel or coupling to vibrational degrees of freedom or both

High-spin states and shape evolution in 70,72Se

High-spin states in 70,72Se have been extended up to spin 14+ and 28+, respectively. The yrast bands in both nuclei exhibit a very regular rotational pattern up to the highest spins observed, with no indication of band crossings above hw approximately=0.5 MeV. According to deformed shell modeling calculations the regular yrast structures can be associated with aligned configurations involving g9/2 quasiparticles.

Two Compton polarimeter constructions for modern standard γ-spectroscopy

Abstract Two types of Compton polarimeters for γ-linear polarization measurements are presented. The first one consists of standard Ge-detectors and unites an easy handling and high grade of universality with high polarization sensitivity in the γ-ray energy range 180–2500 keV. In this device, a low efficiency Ge-crystal scatterer is used with a special diameter-to-length ratio which optimizes the probability for Compton scattering. In addition, the background in the summed energy spectra is significantly reduced by requiring coincident γ-ray energies to obey the Compton scattering formula. The second device fits the need of accessory units which can be used with the new generation of multi-detector arrays. It is based on a segmented planar Ge-crystal surrounded by an anti-Compton shield and possesses a high coincidence efficiency in the 50–1000 keV range. For both spectrometers we performed calibration measurements to obtain the sensitivity Q and the coincidence efficiency e c . From these values we deduced the “figure of merit” F = Q 2 e c which gives a kind of quality description for polarimeters.

Complete scissors mode strength in heavy deformed odd-mass nuclei: a case study of 165Ho and 169Tm

Abstract The low-energy dipole strength distributions in 165Ho and 169Tm have been studied in nuclear resonance fluorescence experiments at the S-DALINAC utilizing a Euroball Cluster detector. In the energy interval between 2.5 and 4.0 MeV, where the scissors mode is expected, 35 ground state transitions could be observed in 165Ho and 53 in 169Tm. Assuming M1 character for all these transitions corresponds to ΣB(M1) ↑= 1.54(23), μN2 and ΣB(M1) ↑= 2.15(63), μN2 for 165Ho and 169Tm, respectively. A statistical analysis is applied to the measured spectra which is capable of reconstructing the complete strength, including contributions of transitions below the observation limit, with the ratio between M1 and E1 excitations and their total strengths taken as an average over the even-mass neighbours. This results in summed M1 transition strengths of ΣB(M1) ↑= 3.54−0.95+0.75, μN2 (165Ho) and ΣB(M1) ↑= 3.36−0.57+1.00μN2 (169Tm). The large variations of the summed dipole strength and the fragmentation in other deformed odd-mass nuclei can be reproduced by the same statistical ansatz. The total low-lying M1 strength in heavy deformed odd-mass nuclei is in agreement with the findings in the neighbouring even-mass nuclides as well as with sum-rule predictions.

Photon scattering off 52Cr: two-phonon E1 strength at the N = 28 shell closure?

Abstract Results of a nat Cr(γ,γ′) experiment at the S-DALINAC are reported for energies up to 7 MeV utilizing a Euroball Cluster detector. The excitation of a J = 1 state at 5544 keV is observed which is believed to belong to 52 Cr because of the strength of the signal and the large natural abundance. By a variety of empirical arguments a quadrupole-octupole-coupled two-phonon 1 − character is suggested for this transition. Quasiparticle random-phase approximation (QRPA) calculations are able to reproduce the energy and B (E1) transition strength remarkably well. The underlying microscopic structure is suggested to be more complex than a pure two-phonon picture. Furthermore, the calculations indicate a pure neutron ( 2p 3 2 −1 2p 1 2 ) magnetic dipole structure for an excitation experimentally seen at 5098 keV which would provide a direct measure of ground-state correlations in 52 Cr.

Side-feeding times and level lifetimes measured in the reaction 54Fe(32S, 3p)83Y

Abstract We have performed recoil-distance and Doppler-shift-attenuation lifetime measurements in the reaction 54Fe(32S, 3p)83Y at 103–107 MeV beam energy. By analyzing the Doppler-broadened line shapes of the 964 keV 21 2 + → 17 2 + yrast transition in coincidence with feeding and depopulating transitions, the average side-feeding time and lifetime of this state have been deduced as τF = 0.6(2) ps and τ = 0.62(10) ps, respectively. A similar analysis of the 811 keV 17 2 + → 13 2 + transition gave values of τF = 0.65(25) ps and τ = 1.5(2) ps, respectively for the 17 2 + yrast state. Feeding times of the higher transitions in the positive-parity yrast band have been estimated from their Dopplerbroadened lineshapes measured in coincidence with depopulating transitions only. The feeding times are compared with the results of Monte Carlo calculations of the time development of the continuum γ-ray flux, and a damping width of 2–3 MeV has been deduced. The measured level lifetimes support the regular band structures with the deformation β2 = 0.33 previously inferred from the transition energies.

Identification of excited levels in 69Se and evidence for oblate deformed g92 bands in 69, 71Se

Abstract γ-rays from the decay of excited states in the neutron deficient nucleus 69Se could be identified from the reaction 32S(40Ca, 2pn)69Se. In 71Se the unfavoured g 9 2 band was established using the reaction 16O(58Ni, 2pn) 71Se. γ-spectroscopic investigation revealed an oblate structure of the g 9 2 bands in both nuclei.

Structure and evolution of electric dipole strength in 204,206,208Pb below the neutron emission threshold

Abstract The electric dipole strength distributions below threshold in stable even-mass Pb isotopes have been extracted from photon scattering experiments at the superconducting Darmstadt electron linear accelerator S-DALINAC. Between 4 and 6.5 MeV excitation energy a resonance-like clustering of strength is observed. A strong evolution of a fragmentation of the dipole strength with opening the neutron shell is found. The fine structure of the strength is compared to microscopic quasiparticle-phonon model calculations that quantitatively reproduce the data quite well and explain the fragmentation of the dipole strength. Models suggesting the oscillation of excess neutrons with respect to an N ≃ Z core as a possible origin of low-lying E1 strength are critically examined.