F. Camera

Constraints on the symmetry energy and neutron skins from pygmy resonances in Ni 68 and Sn 132

Correlations between the behavior of the nuclear symmetry energy, the neutron skins, and the percentage of energy-weighted sum rule (EWSR) exhausted by the Pygmy Dipole Resonance (PDR) in 68Ni and 132Sn have been investigated by using different Random Phase Approximation (RPA) models for the dipole response, based on a representative set of Skyrme effective forces plus meson-exchange effective Lagrangians. A comparison with the experimental data has allowed us to constrain the value of the derivative of the symmetry energy at saturation. The neutron skin radius is deduced under this constraint.

Angular distribution of photons from the delay of the GDR in hot and rotating light Yb nuclei from exclusive experiments

Abstract Angular distributions of photons associated with the damping of excited-state giant dipole resonances (GDR) in hot and rotating 161,162 Yb nuclei have been measured in exclusive experiments using the HECTOR array. In reactions with heavy ions ( 48 T) angular distributions are determined as a function of the angular momentum of the compound nuclei. In reactions with lighter ions ( 17,18 O) a difference method is applied to isolate GDR decays originating from specific excitation regions. The systematics of the measured angular distributions as a function of excitation energy and angular momentum are compared to theories taking into account fluctuations of the shape and orientation of the excited nuclei.

A novel technique for the characterization of a HPGe detector response based on pulse shape comparison

A novel technique for measuring a HPGe detector pulse shape as a function of the γ-ray interaction position inside the detector volume is presented. Exploiting a specific pulse shape comparison procedure, this technique allows to characterize the 3D position response of a HPGe segmented detector in a much shorter time as compared with the standard coincidence techniques. The method was first validated using a GEANT simulation of a 36-fold HPGe AGATA detector realized taking into account the effects of the electronic chain response and electrical noise on the calculated signal shape. This procedure was then applied to extract experimentally the position response of a non-segmented coaxial HPGe detector along the radial direction, using a 438 MBq 137Cs collimated γ-source. The results of this measurement show a dependence of the pulse shape as a function of γ-ray interaction radial coordinate consistent with that obtained with calculations. The signal acquisition rate reached using this characterization technique allows to realize a full scan of a large volume highly segmented HPGe detector in less than a week.

Structural Evolution in the Neutron-Rich Nuclei ^{106}Zr and ^{108}Zr

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

Pulse distributions and tracking in segmented detectors

A study of the performance of a cylindrical HPGe detector segmented in 25 segments is presented. It is based on simulations made with the computer code GEANT and focuses on the reconstruction of a g-ray path. The effects of the segmentation are initially discussed in terms of Doppler correction. The role of the pulse shape analysis and its effects on tracking algorithms are discussed as a function of the capability to reconstruct a g-ray path when multiple signals (direct and induced) are present in a single segment. It is found that it is critical to identify correctly at least two signals in a segment in order to have a reasonable efficiency and Compton suppression in the spectra and to make a good use of this type of detectors. r 2002 Elsevier Science B.V. All rights reserved.

Thermal fluctuation and collisional damping effects in the GDR observables in hot rotating 176W nuclei

Abstract The strength function and the angular distribution of the high energy γ-rays emitted by the giant dipole resonance (GDR) in hot rotating 176 W nuclei have been measured at temperature T ≈ 1.4 MeV and at 4 values of the angular momentum I in the interval I = 35–55, ħ. The measured GDR width is found not to change when the angular momentum is varied. An increase by a factor of ≈ 2 is measured for the a 2 ( E γ ) in the same I interval. The comparison of the data to model predictions shows that the collisional damping width is independent of temperature and rotational frequency and that thermal shape and orientation fluctuations are responsible for the measured widths and angular anisotropies.

The TMR network project "Development of γ-ray tracking detectors"

The next generation of 4 pi arrays for high-precision gamma -ray spectroscopy will involve gamma -ray tracking detectors. They consist of high-fold segmented Ge detectors and a front-end electronics, based on new digital signal processing techniques, which allows to extract energy, timing and spatial information for a gamma -ray by pulse shape analysis of the Ge detector signals. Utilizing the information on the positions of the interaction points and the energies released at each point the tracks of the gamma -rays in a Ge shell can be reconstructed in three dimensions.

Radiative fusion from very symmetric reactions: the giant dipole resonance in the 179Au nucleus

The spectra of high-energy γ rays emitted by the Giant Dipole Resonance (GDR) built on moderately excited states associated with the evaporation of 0, 1 and 2 nucleons were measured in the 90 Zr + 89 Y symmetric fusion reaction. The radiative fusion data suggest statistical emission from the compound nucleus. In addition, the analysis of the high-energy γ -ray spectra associated with the different evaporation channels at the present temperature of 0.7 MeV and spin range 15–20 ¯ h show a fairly narrow width of 5.0 ± 0.35 MeV. This value is smaller than what would be expected in a nucleus where shell effects do not play a role.  2003 Published by Elsevier Science B.V.

The GDR width in the excited 147 Eu compound nucleus at high angular momentum

Abstract High-energy γ -rays emitted in the decay of the hot compound nucleus 147 Eu have been measured in coincidence with low-energy γ -rays. The γ transitions from the different residual nuclei were detected by a multiplicity filter and by Ge detectors. The employed reaction was 37 Cl + 110 Pd at bombarding energies of 160, 165 and 170 MeV. The measured high-energy γ -ray spectra were analysed within the framework of the statistical model using the Cascade code. The GDR width in the angular momentum interval between 35 and 50 ℏ was found to increase weakly and to be rather well predicted by the thermal shape fluctuation model. Also the deformation parameter β as a function of the average angular momentum extracted from the data was found to be in general agreement with the model.

Large effects of orientation fluctuations in the angular distribution of the GDR photons

Abstract Measurements of the angular distribution of high energy γ rays emitted in the decay of compound nuclei 165–167 Er, 175,176 Hf, 110 Sn at E ∗ = 50–90 MeV and I ≤ 40 are presented. In all cases the measured anisotropies as a function of the gamma transition energy are small. The compound nuclei were formed with heavy ion fusion reactions and the high energy y rays were detected by the HECTOR array. A comparison between the data and model predictions shows that both shape and orientation fluctuations are very important and must be taken into account to reproduce the experimental results. The dominant role played by orientation fluctuations on the GDR lineshape and the effective induced nuclear deformations are discussed.