The Oslo Method in Inverse Kinematics

The first ever investigation of the application of the Oslo Method in inverse kinematics experiments has been performed by analysis of an experiment where a 86Kr beam at 300 MeV was used on deuterated polyethylene/polystyrene targets at iThemba LABS to induce d(86Kr,p)87Kr reactions. The experiment was done at the AFRODITE array featuring two large volume (3.5x8") LaBr3(Ce) detectors, eight CLOVER detectors and two particle telescopes each consisting of two segmented silicon detectors. Problematic effects only seen in inverse kinematics such as Doppler shift, large angular dependencies, etc. have been solved. A new software code for analyzing inverse kinematics experiments with the Oslo Method was developed. The gamma-ray strength function and the level density of 87Kr was extracted and a large enhancement of the gamma-ray strength function at low energy was found. The experimental level density found was consistent with a constant temperature model for the level density

The Oslo Method in Inverse Kinematics

The statistical properties of nuclei are important to our understanding of the nuclear structure and are input to astrophysical models for the formation of elements heavier than iron. Currently there are little data on these properties for unstable nuclei, which makes evaluation of models for these properties difficult. In addition, the lack of data strongly affects the accuracy of astrophysical models describing the production of elements heavier than iron. This doctorial work explores how one can measure statistical properties of nuclei that cannot be reached using the traditional experiments of light ions impinging on a solid target. Specifically, the thesis presents the statistical properties of 87Kr and 67Ni from experiments where heavy ions impinge on deuteron enriched plastic targets. These are the first-ever measurements of statistical properties using this technique. The thesis also includes the development of state-of-the-art detections systems for nuclear experiments. The result of the thesis paves the way for measuring statistical properties in previously inaccessible nuclei, which will help constrain models of the nucleus and improve the accuracy of astrophysical models for production of heavy elements

The gamma-ray energy response of the Oslo Scintillator Array OSCAR

The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo Cyclotron Laboratory (OCL). It consists of 30 large volume (⌀ 3.5 × 8 inches) LaBr3(Ce) detectors that are used for -ray spectroscopy. The response functions for incident rays up to 20 MeV are simulated with Geant4. In addition, the resolution, and the total and full-energy peak efficiencies are extracted. The results are in very good agreement with measurements from calibration sources and experimentally obtained mono-energetic in-beam -ray spectra

γ-ray strength function for barium isotopes

Photoneutron cross sections were measured for 137 Ba and 138 Ba at energies below two-neutron threshold using quasimonochromatic γ -ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the γ -ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasiparticle random phase approximation using the Gogny D1M interaction. Supplementing the experimentally constrained γ -ray strength function with the zero-limit E1 and M1 contributions, which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the s-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82

Photoneutron cross section measurements on

Photoneutron reactions on 208Pb in the Giant Dipole Resonance (GDR) energy range have been investigated at the γ-ray beam line of the NewSUBARU synchrotron radiation facility in Japan. Making use of quasi-monochromatic laser Compton scattering (LCS) γ-ray beams and of a novel flat-efficiency neutron detection system along with associated neutron-multiplicity sorting method, total and partial (γ,xn) photoneutron cross sections with x = 1 to 4 have been measured for 208Pb in a broad energy range covering the neutron threshold up to 38 MeV

Photoneutron cross sections for Ni isotopes: Toward understanding (n, γ ) cross sections relevant to weak s-process nucleosynthesis

Photoneutron cross sections were measured for 58Ni, 60Ni, 61Ni, and 64Ni at energies between the one-neutron and two-neutron thresholds using quasimonochromatic γ -ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. These photoneutron data are used to extract the γ -ray strength function above the neutron threshold, complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron-capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including 63Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained γ -ray strength functions from the Hartree-Fock-Bogolyubov plus quasiparticle–random-phase approximation based on the Gogny D1M interaction for both E1 and M1 components and supplemented with the M1 upbend

Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes 142,144−151Nd which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of ≈150 at an excitation energy of 6 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement

Nuclear level densities and γ-ray strength functions of 180,181Ta and neutron capture cross sections

The γ -ray strength functions and nuclear level densities in the quasi-continuum of 180,181Ta are extracted from particle-γ coincidence events with the Oslo Method, below the Sn . The data were used as input in the TALYS reaction code for calculations of the astrophysical Maxwellian-averaged (n, γ ) cross-sections to investigate nucleosynthesis of nature’s rarest stable isotope 180Ta

Resonances in odd-odd 182Ta

Enhanced γ -decay on the tail of the giant electric dipole resonance, such as the scissors or pygmy resonances, can have significant impact on (n,γ ) reaction rates. These rates are important input for modeling processes that take place in astrophysical environments and nuclear reactors. Recent results from the University of Oslo indicate the existence of a significant enhancement in the photon strength function for nuclei in the actinide region due to the scissors resonance. Further, the M1 strength distribution of the scissors resonances in rare earth nuclei has been studied extensively over the years. To investigate the evolution and persistence of the scissor resonance in other mass regions, an experiment was performed utilizing the NaI(Tl) γ -ray detector array (CACTUS) and silicon particle telescopes (SiRi) at the University of Oslo Cyclotron laboratory. Particle-γ coincidences from the 181Ta(d,p)182Ta and 181Ta(d,d’)181Ta reactions were used to measure the nuclear level density and photon strength function of the well-deformed 181Ta and 182Ta systems, to investigate the existence of resonances below the neutron separation energy

Novel Techniques for Constraining Neutron-capture Rates relevant to Heavy-element Nucleosynthesis

In this contribution we discuss new experimental approaches to indirectly provide information on neutron-capture rates relevant to the r-process. In particular, we focus on applications of the Oslo method to extract fundamental nuclear properties for reaction-rate calculations: the nuclear level density and the γ strength function. Two methods are discussed in detail, the Oslo method in inverse kinematics and the beta-Oslo method. These methods present a first step towards constraining neutron-capture rates of importance to the r-process