Modern African nuclear detector laboratory: Development of state-of-the-art in-house detector facility at the University of the Western Cape

The upcoming detector facility aims at developing new state-of-the-art particle detectors as well as providing hands-on training to postgraduate students using both analog and digital signal processing from nuclear radiation detectors. The project is two-fold and aims at developing: 1) ancillary detectors to be coupled with the new GAMKA array at iThemba LABS. Of particular interest to our group is the determination of nuclear shapes, which depend on the hyperfine splitting of magnetic substates; 2) PET scanners for cancer imaging using a cheaper technology. Performance of NaI(Tl) inorganic scintillator detectors has been evaluated using PIXIE-16 modules from XIA digital electronics. Gamma-ray energy spectra were acquired from 60Co and 137Cs radioactive sources to calculate the detector resolution as well as to optimize the digital parameters. The present study focuses on improving and optimizing the slow and fast filter parameters for NaI(Tl) detectors which can eventually be used in the list mode of data aquisition

Observation of the 0+ 2 and γ bands in 98Ru, and shape coexistence in the Ru isotopes

Excited states in 98Ru were investigated using γ-ray spectroscopy following the β-decay of 98Rh, and via the 100Ru(p,t) reaction. Combining the results from the two experiments, two states were revised to have spin-parity of 4+ and subsequently assigned to the 02+ and “γ” bands, respectively. The observed structures in 98Ru are suggested to be deformed and rotational, rather than spherical and vibrational, and fit well into the systematics of these excitations in the Ru isotopes. The 02+ excitation is suggested as a shape coexisting configuration. This observation eliminates some of the last remaining candidates for nearly harmonic vibrational nuclei in the Z≈50 region. Beyond-mean-field calculations are presented that support shape coexistence throughout the Ru isotopes with N=52–62, and suggest a smooth evolution of the shape

Recent experimental results in sub- and near-barrier heavy ion fusion reactions

Recent advances obtained in the field of near and sub-barrier heavy-ion fusion reactions are reviewed. Emphasis is given to the results obtained in the last decade, and focus will be mainly on the experimental work performed concerning the influence of transfer channels on fusion cross sections and the hindrance phenomenon far below the barrier. Indeed, early data of sub-barrier fusion taught us that cross sections may strongly depend on the low-energy collective modes of the colliding nuclei, and, possibly, on couplings to transfer channels. The coupled-channels (CC) model has been quite successful in the interpretation of the experimental evidences. Fusion barrier distributions often yield the fingerprint of the relevant coupled channels. Recent results obtained by using radioactive beams are reported. At deep sub-barrier energies, the slope of the excitation function in a semi-logarithmic plot keeps increasing in many cases and standard CC calculations over-predict the cross sections. This was named a hindrance phenomenon, and its physical origin is still a matter of debate. Recent theoretical developments suggest that this effect, at least partially, may be a consequence of the Pauli exclusion principle. The hindrance may have far-reaching consequences in astrophysics where fusion of light systems determines stellar evolution during the carbon and oxygen burning stages, and yields important information for exotic reactions that take place in the inner crust of accreting neutron stars.Comment: 40 pages, 63 figures, review paper accepted for EPJ

Capture cross sections for very heavy systems

In intermediate-mass systems, collective excitations of the target and projectile can greatly enhance the subbarrier capture cross section sigma(cap) by giving rise to a distribution of Coulomb barriers. For such systems, capture essentially leads directly to fusion [formation of a compound nucleus (CN)], which then decays through the emission of light particles (neutrons, protons, and alpha particles). Thus, the evaporation-residue (ER) cross section is essentially equal to sigma(cap). For heavier systems, the experimental situation is significantly more complicated owing to the presence of quasifission (QF) (rapid separation into two fragments before the CN is formed) and by fusion-fission (FF) of the CN itself. Thus, three cross sections need to be measured in order to evaluate sigma(cap). Although the ER essentially recoil along the beam direction, QF and FF fragments are scattered to all angles and require the measurement of angular distributions in order to obtain the excitation function and barrier distribution for capture. Two other approaches to this problem exist. If QF is not important, one can still measure just the ER cross section and try to reconstruct the corresponding sigma(cap) through use of an evaporation -model code that takes account of the FF degree of freedom. Some earlier results on sigma(cap) obtained in this way will be reanalyzed with detailed coupled-channels calculations, and the "extra-push" phenomenon discussed. One may also try to obtain sigma(cap) by exploiting unitarity, that is, by measuring instead the flux of particles corresponding to quasielastic (QE) scattering from the Coulomb barrier. Some new QE results obtained for the Kr-86 + Pb-208 system at iThemba LABS in South Africa will be presente

Stopping power of Nd, Pm and Sm ions in Cd determined with gamma-ray lineshape analysis

A semi-thick target has been used to measure the stopping power for recoils, produced in heavy ion induced reactions, by gamma-rayline shape analysis.The target thickness has been chosen such that the recoils are partially slowed down in the target and subsequently escape into vacuum. For transitions deexciting long-lived states the Doppler lineshape then depends only on the stopping power. Stopping-power parameters have been determined for Pm, Sm and Nd nuclei in Cd. The results for the electronic stopping power agree within uncertainties with the predictions of the Lindhard, Scharff and Schiøtt theory. The nuclear stopping power can be well described in the framework of the Ziegler, Biersack and Littmark parameterization using the software package Stopping and Range of Ions in Matter (SRIM)2006

Electric dipole moments in 230, 232U and implications for tetrahedral shapes

The nuclei 230U and 232U were populated in the compound nucleus reactions 232Th(α,6n) and 232Th(α,4n), respectively. Gamma rays from these nuclei were observed in coincidence with a recoil detector. Acomprehensive set of in-band E2 transitions were observed in the lowest lying negative-parity band of 232Uwhile one E2 transition was also observed for 230U. These allowed B(E1; I - → I + - 1)/B(E2; I - → I - - 2) ratios to be extracted and compared with systematics. The values are similar to those of their Th and Ra isotones. The possibility of a tetrahedral shape for the negative-parity U bands appears difficult to reconcile with the measured Q2 values for the isotone 226Ra. © 2010 The American Physical Society.Articl

Electric dipole moments in 230 , 232 U and implications for tetrahedral shapes

The nuclei 230U and 232U were populated in the compound nucleus reactions 232Th(α,6n) and 232Th(α,4n), respectively. Gamma rays from these nuclei were observed in coincidence with a recoil detector. Acomprehensive set of in-band E2 transitions were obs