Bochum 1998 – scientific programme

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HK: Hadronen und Kerne

HK 28: Spectroscopy: 70 ≤ A < 110

HK 28.7: Group Report

Tuesday, March 17, 1998, 15:45–16:15, D

Measurement of ps-lifetimes in the semi-magic nuclei ⁹⁵Rh and ⁹⁴Ru with the EUROBALL Cluster detectors — •Andrea Jungclaus¹, Dietrich Kast¹, Klaus-Peter Lieb¹, Christian Teich¹, Matthias Weiszflog¹, Thomas Härtlein², Christoph Ender², Frank Köck², Dirk Schwalm², Jens Reif³, Jürgen Eberth⁴, Heinz-Georg Thomas⁴, Rüdiger Peusquens⁴, Alfred Dewald⁴, and Hubert Grawe⁵ — ¹II. Physikalisches Institut, Universität Göttingen, Bunsenstr. 7-9, D-37073 Göttingen — ²Max-Planck-Institut für Kernphysik, D-69029 Heidelberg — ³Institut für Kern- und Hadronenphysik, FZ Rossendorf, D-01314 Dresden — ⁴Institut für Kernphysik, Universität zu Köln, Zülpicherstrasse, D-50937 Köln — ⁵GSI, Darmstadt

In the semi-magic N=50 nuclei ₄₄⁹⁴Ru and ₄₅⁹⁵Rh, the low-energy parts of the excitation schemes reflect the various valence proton configurations within the (p_1/2,g_9/2) space. In contrast, states with spins higher than 12⁺, 13⁻ in ⁹⁴Ru and 25/2⁺, 25/2⁻ in ⁹⁵Rh are generated by raising a neutron from the g_9/2 across the N=50 shell gap into the d_5/2 orbit [1]. Due to the large shell gap of about 2.5 MeV, these neutron particle-hole excitations decay via high-energy γ-rays. The main goal of our investigation was to measure the strengths of these ’single-particle’ transitions across the N=50 shell closure and to compare them to detailed shell model calculations within the (p_1/2, g_9/2, d_5/2) configuration space.
Because of their high efficiency for the detection of high-energy γ-ray, the EUROBALL Cluster detectors are perfectly suited for our purposes. We used six of these detectors, a 145 MeV ⁴⁰Ca beam from the MP accelerator at the MPI Heidelberg and a ⁵⁸Ni target foil to determine lifetimes in the range from 1-50 ps in the 3p and 4p reaction channels ⁹⁵Rh and ⁹⁴Ru via the RDDS method. The high statistics of these data allows us to address the advantages and disadvantages of various methods to analyse such coincidence RDDS data, namely the analysis of R(d) functions determined in coincidence with transitions above and below the transition of interest, and the DDCM method [2]. Only the richness of complementary information enabled us to reveal sources of systematic errors and to deduce reliable values for lifetimes and uncertainties.

[1] H. A. Roth et al., Phys. Rev. C50 (1994) 1330.

[2] G. Böhm et al., Nucl. Instr. Meth. A329 (1993) 248