D. Pansegrau

Conversion electron spectroscopy in the superdeformed minimum of 240Pu

Abstract Conversion electrons from electromagnetic transitions preceding the fission of the 3.7 ns 240f Pu shape isomer have been measured. The electron spectrum shows a broad structure of individually resolved lines with energies between 620 and 650 keV, which are interpreted as the (Jβ→Jg) E0 transitions from the decay of the lowest β-vibrational band with a phonon energy of 770 keV feeding to the ground state band of 240f Pu . In a combined analysis of e− and high resolution γ-ray spectroscopy data previously established octupole bands could also be studied in more detail.

γ Spectroscopy in the superdeformed minimum of 240Pu

Abstract The γ decay of excited states built on the superdeformed shape isomer 240 f Pu was measured for the first time with an array of EUROBALL CLUSTER detectors in coincidence with delayed fission decays using the 238 U( α ,2n) reaction. The strongest γ transitions can be assigned to the out-of-band decays of three excited superdeformed rotational sequences: A K π =2 − and K π =1 − (2 − ) band with respective bandhead energies of 806.2 keV and 836.0 (846.8) keV decaying to the ground state band of 240 f Pu, and a K π =2(1) band at 1360.9 (1344) keV populating mainly the K π =2 − (806.2 keV) band.

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

Abstract:As a tool for studying the structure of nuclei far off stability the technique of γ-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the Doppler broadening of γ lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 106 particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the γ background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross-sections of neutron-rich light nuclei on 2H and 9Be targets at REX-ISOLDE energies of 2.2 MeV . A are confirmed.

Investigations of 159-163Dy using Incomplete Fusion Reactions

The isotopes 159–163Dy have been investigated using the incomplete fusion reactions 7Li → 158,160Gd at beam energies of 8 MeV/u. The γ-rays were detected in the GASP spectrometer in coincidence with the fast charged particles detected in the ISIS silicon ball. The level schemes of all five isotopes could be amply extended. The most important result was the observation of both the ground state and Stockholm bands in 162Dy up to high spin covering the band crossing region. The crossing in this nucleus is shifted to a higher frequency in comparison to the lighter Dy isotopes. The interaction strength between the two bands could be determined with high precision and with this result, a full oscillation of the interaction strength from one node to the next within an isotopic chain could be observed for the first time.

Detailed spectroscopy in the superdeformed second minimum of 240Pu

Spectroscopic studies in the superdeformed shape isomer of 240Pu using γ-spectroscopy, conversion electron spectroscopy and transmission resonance spectroscopy have been performed. In a high-resolution and high-efficiency γ-spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K=2− and 1− could be identified together with evidence for a weakly populated 0− octupole band. Surprisingly, no low-lying collective quadrupole excitations could be observed. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest s-vibrational K=0+ band. For all rotational bands the variation of the moment of inertia with spin could be studied. A predominant population of negative parity states in the second well could be observed, which can be explained by the selective population and depopulation of the second minimum. Complementary transmission resonance measurements have been performed, yielding new information on the fine structure of high-lying (s-)vibrational multi-phonon states. A new method could be established to determine the excitation energy of the fission isomer ground state from measured level densities.

New insights in the first backbending region of heavy Dy isotopes

SummaryThe7Li-induced incomplete fusion reaction158/160Gd(7Li,p(d,t)xn) (beam energy 56 MeV) has been used to investigate in detail the first backbending region of the heavy stable Dy nuclei160/162Dy. In162Dy, the measured low interaction strength between the ground band and the Stockholm band of |Vgs| ≈ 17 keV confirms the predicted existence of a node in the interaction strength for nuclei with neutron number betweenN = 96 andN = 98, which has not been accessed earlier. The crossing frequency between the ground band and the Stockholm band in162Dy was measured to be hwc = 320 keV, which is about 40 keV higher than the expected value of hwc = 280 keV known from other nuclei in this mass region. In160Dy, an yrare band could be established for the first time in a nucleus with a very large interaction strength. From fits to level energies as well as from branching ratios, an interaction strength of |Vgs| ≈ 220 keV was extracted.

Rotational bands in neutron-rich 160-162Ho

We have studied the high spin states in 160–162Ho in order to investigate the properties of the rotational bands and their dependence on the single particle orbits involved. The reaction 158,160Gd(7Li,xn) at 56 MeV were used to produce the Ho isotopes of interest. In all three Ho isotopes the known rotational bands have been significantly extended. New band‐crossings have been observed for the first time in this work.

Investigation of the backbending region of heavy Dy isotopes via incomplete fusion

The interpretation of the first backbending point in deformed rare earth nuclei as a crossing of the ground state band with a rotational band built on an aligned i132 neutron pair is rather well established. However, in terms of such a two-band mixing model one cannot understand the exclusive population of yrast states in and above the backbending point observed in Coulomb excitation experiments of neutron-rich, stable rare earth nuclei. To investigate this problem in more detail for the heavy Dy nuclei 160, 162Dy, an Incomplete Fusion experiment has been performed at the MPI fur Kernphysik in Heidelberg in order to investigate its suitability for studying these n-rich nuclei. A cross section of σ ≈ 150 mbarn for producing the nuclei with a spin distribution covering the first backbending region could be established.