C. Fransen

The γ-ray spectrometer HORUS and its applications for nuclear astrophysics

Abstract A dedicated setup for the in-beam measurement of absolute cross-sections of astrophysically relevant charged-particle induced reactions is presented. These, usually very low, cross-sections at energies of astrophysical interest are important to improve the modeling of the nucleosynthesis processes of heavy nuclei. Particular emphasis is put on the production of the p nuclei during the astrophysical γ process. The recently developed setup utilizes the high-efficiency γ-ray spectrometer HORUS, which is located at the 10xa0MV FN tandem ion accelerator of the Institute for Nuclear Physics in Cologne. The design of this setup will be presented and results of the recently measured 89 Y(p,γ) 90 Zr reaction will be discussed. The excellent agreement with existing data shows that the HORUS spectrometer is a powerful tool to determine total and partial cross-sections using the in-beam method with high-purity germanium detectors.

A method to correct differential nonlinearities in subranging analog-to-digital converters used for digital γ-ray spectroscopy

Abstract The influence on γ-ray spectra of differential nonlinearities (DNL) in subranging, pipelined analog-to-digital converts (ADCs) used for digital γ-ray spectroscopy was investigated. The influence of the DNL error on the γ-ray spectra, depending on the input count-rate and the dynamic range has been investigated systematically. It turned out that the DNL becomes more significant in γ-ray spectra with larger dynamic range of the spectroscopy system. An event-by-event offline correction algorithm was developed and tested extensively. This correction algorithm works especially well for high dynamic ranges.

First test results of the digital data acquisition at the HORUS spectrometer

The HORUS spectrometer at the 10MV Tandem accelerator at the Institute for Nuclear Physics in Cologne consists of 14 high-purity germanium γ-detectors. To extend the experimental opportunities, the new silicon-detector array SONIC was designed, housing eight ΔE-E sandwich silicon detectors for ion spectroscopy. In order to process all 30 detector signals, the analog data acquisition was replaced by a digital one using the commercially available DGF-4C modules from the company XIA. The new data acquisition system was tested in two experiments.

182Pt as a possible candidate for X(5) symmetry

Recently, a new island of X(5) nuclei has been suggested around A=180 exemplified by some Osmium isotopes. To investigate the limits of its region, a Recoil-distance Doppler shift lifetime measurement has been performed for 182Pt. For the data analysis, the Differential decay curve method has been applied in a newly developed version convenient for low recoil velocities and a non-negligible fraction of nuclei stopped already in the target. The level energies and the newly deduced transition quadrupole moments in the yrast band reveal the persistence of X(5) features in the investigated nucleus, but other spectroscopic data and IBM and GCM calculations indicate shape coexistence and a position of 182Pt close but not at the critical point of the shape-transition.

Different collectivity in the two signatures of the i13/2 stemming band in 167Yb

Six lifetimes have been determined in the 5/2+ [642] band from vi13/2 parentage in 167Yb by means of Recoil distance Doppler-shift (RDDS) measurements carried out at the Cologne FN tandem. The deduced transition strengths and the level scheme are reasonably described by Particle plus triaxial rotor model (PTRM) calculations except for the behavior of the quadrupole collectivity in the two signatures of the 5/2+[642] band. In that band, the quadrupole collectivity of the favored signature is appreciably larger than this of the unfavored signature. The effect increases with increasing the spin. Naturally, the rigid PTRM cannot explain these features, but the structure of its wave functions suggests a possible solution. It is associated with the enhanced contribution of low-Ω orbitals from vi13/2 parentage in the favored signature compared to the unfavored one. This could selectively increase the deformation of the favored signature band members and give rise to a dynamic shape coexistence taking place between the two signatures which needs quantitative explanation by future theoretical work.

Collectivity of Exotic Heavy Fe Isotopes

The properties of exotic neutron-rich nuclei between the proton shell closures Z = 20 and Z = 28 are of particular interest for the understanding of the shell structure for large neutron excess. Effects related to the energy gap between the neutron fp and 1g9/2 shells lead to a strong variation of collectivity for nuclei around N = 40. Whereas 68Ni was found to have doubly magic properties, this was not observed in neighbouring nuclei. Recent shell model calculations for the neutron rich iron isotopes clearly reveal the difficulty to describe nuclei in this mass region and resulted in large deviations of the predicted collectivity depending on the valence space. However, no experimental data on the transition strength existed for the very exotic nucleus 66Fe at N = 40. Here we present the newest results on absolute transition strengths of the lowest excited states in 62,64,66Fe measured model independently using the recoil distance Doppler-shift (RDDS) method. The experiments were performed at NSCL at Michigan State University with the Cologne/NSCL plunger device using Coulomb excitation in inverse kinematics at energies of 80 MeV/u. Our results yield a much higher collectivity for 64,66Fe than expected and allow tests of new calculations.

Neutron‐Rich 62,64,64Fe Show Enhanced Collectivity: The Washout of N = 40 in Terms of Experiment, Valence Proton Symmetry and Shell Model

Probing shell structure at a large neutron excess has been of particular interest in recent times. Neutron‐rich nuclei between the proton shell closures Zu2009=u200920 and Zu2009=u200928 offer an exotic testing ground for shell evolution. The development of the Nu2009=u200940 gap between neutron fp and lg9/2 shells gives rise to highly interesting variations of collectivity for nuclei in this region. While 68Ni shows doubly magic properties in level energies and transition strengths, this was not observed in neighbouring nuclei. Especially neutron‐rich Fe isotopes proved particularly resistant to calculational approaches using the canonical valence space (fpg) resulting in important deviations of the predicted collectivity. Only an inclusion of the d5/2‐orbital could solve the problem [1]. Hitherto no transition strengths for 66Fe have been reported. We determined B(E2,21+→01+) values from lifetimes measured with the recoil distance Doppler‐shift method using the Cologne plunger for radioactive beams at National Superconducting ...

Study of collectivity in 62Zn

An essential property of collectivity is the proton‐neutron degree of freedom, that leads to the so‐called states of mixed symmetry. Predictions for the electromagnetic decay of the lowest‐lying mixed‐symmetry state 2MS,1+ in vibrator‐like even‐A nuclei are a strong M1 decay to the fully‐symmetric 21+ state and a weak collective E2 decay to the ground state 01+. Excited states in 62Zn have been studied via the 61Ni(3He,2nγ) reaction using the HORUS‐cube spectrometer of the Institute for Nuclear Physics in Cologne. Two fragments of the mixed‐symmetry state 2MS,1+ could be identified by determining absolute M1 transition strengths to the 21+ state.