Pygmy resonance and low-energy enhancement in the γ\gamma-ray strength functions of Pd~isotopes

An unexpected enhancement in the $\gamma$-ray strength function, as compared to the low energy tail of the Giant Dipole Resonance (GDR), has been observed for Sc, Ti, V, Fe and Mo isotopes for $E_\gamma<4$ MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a Pygmy Dipole Resonance (PDR) centered at $E_\gamma\approx8$ MeV was however detected. The $\gamma$-ray strength functions measured for Cd isotopes exhibit both features over the range of isotopes, with the low-energy enhancement decreasing- and PDR strength increasing as a function of neutron number. This suggests a transitional region for the onset of low-energy enhancement, and also that the PDR strength depends on the number of neutrons. The $\gamma$-ray strength functions of $^{105-108}$Pd have been measured in order to further explore the proposed transitional region. Experimental data were obtained at the Oslo Cyclotron Laboratory by using the charged particle reactions ($^{3}$He, $^{3}$He$^{\prime}\gamma$) and ($^{3}$He, $\alpha$$\gamma$) on $^{106,108}$Pd target foils. Particle$-\gamma$ coincidence measurements provided information on initial excitation energies and the corresponding $\gamma$-ray spectra, which were used to extract the level densities and $\gamma$-ray strength functions according to the Oslo method. The $\gamma$-ray strength functions indicate a sudden increase in magnitude for $E_{\gamma}>4$ MeV, which is interpreted as a PDR centered at $E_{\gamma}\approx8$ MeV. An enhanced $\gamma$-ray strength at low energies is also observed for $^{105}$Pd, which is the lightest isotope measured in this work. Further, the results correspond and agree very well with the observations from the Cd isotopes, and support the suggested transitional region for the onset of low-energy enhancement with decreasing mass number. The neutron number dependency of the PDR strength is also evident

Eksperimentell nivåtetthet og gammastyrkefunksjon for 58 Ni.

Oppgaven tar for seg analyse av data fra et kjernefysisk forsøk utført ved Oslo syklotronlaboratorium våren 2005. En folie med naturlig nikkel ble bestrålet med helium-ioner. Naturlig nikkel består for det meste av isotopet med 58 kjernepartikler. Reaksjoner hvor heliumioner ble inelastisk spredt ble sett på. Energien til de spredte partiklene og samtidig gammastråling ble målt. Dette ble videre analysert ved hjelp av Oslometoden, slik at informasjon om kjernens statistiske egenskaper kunne bestemmes. Den eksperimentelle nivåtettheten og gammastyrkefunksjonen ble ekstrahert for 58 Ni. Nivåtettheten ble så brukt videre for å bestemme kjernens termodynamiske egenskaper

Design of a single magnet separator with mass resolving power mΔm≈20,000\frac{m}{\Delta m} \approx 20,000

ISOLDE at CERN is a leading radioactive ion beam facility. With its upgrade, the HIE-ISOLDE project, an increase in primary beam intensity and energy is envisaged and the aim is a significant increase in intensity of the exotic beams. The high resolution separator (HRS) after the upgrade is required to suppress contaminations almost completely when the masses differ to the beam of interest by Δm/m>1/20,000 . Here a 120° magnet with a bending radius of 1.25 m has been chosen. The magnetic rigidity is 0.625 Tm (B-field of 0.5 T) to allow for separation of molecules of up to a mass of 300 u. The magnet comprises a yoke in wedged H-type configuration for stability and precision and pole face conductors for focusing and compensation of aberrations. The concept was derived analytically, refined with the OPERA 2D software and tested with the ray-tracing module of OPERA 3D

Inclusive cross sections for one- and multi-nucleon removal from Sn, Sb, and Te projectiles beyond the N = 82 shell closure

Inclusive one- and multi-nucleon removal cross sections have been measured for several Sn, Sb and Te isotopes just beyond the neutron shell closure. The beams were produced in the projectile fission of a 238U beam at the Radioactive Isotope Beam Factory at RIKEN. The experimental cross sections are compared to predictions from the most recent version of the Liege intranuclear cascade model. Although the overall agreement is good, severe discrepancies are observed for the cases of one- and two-neutron removal from 134Sn and 135Sb projectiles and one-proton knockout from all measured isotones. These discrepancies, as well as the relevance of quasi-elastic reaction channels to the one-neutron removal cross sections, are discussed. In addition, the measured inclusive one-proton knockout cross section for the semi-magic 134Sn projectile is compared to eikonal direct reaction theory calculations to assess if the suppression factors to these calculated cross sections, deduced from data on reactions of lighter projectile nuclei, are also applicable to heavy nuclei

Is the generalized Brink-Axel hypothesis valid?

Experimental results of the 237Np(d, pγ) 238Np reaction are presented, which verifies the generalized Brink-Axel (gBA) hypothesis for γ transitions between states in the quasi-continuum. The gBA hypothesis holds not only for specific collective resonances, but for the full dipole strength below the neutron separation energy. We discuss the validity of the gBA hypothesis also for lighter systems like 92Zr where the concept of a unique γ-ray strength function (γSF) is problematic due to large Porter-Thomas fluctuations. Methods for studying the γSF and the fluctuations as function of excitation energy are presented

Validity of the Generalized Brink-Axel Hypothesis in Np 238

We analyze primary γ-ray spectra of the odd-odd 238Np nucleus extracted from 237Npðd; pγÞ238Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary γ spectra cover an excitationenergy region of 0 ≤ Ei ≤ 5.4 MeV, and allow us to perform a detailed study of the γ-ray strength as a function of excitation energy. Hence, we can test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the γ strength. In this work, using the available high-quality 238Np data, we show that the γ-ray strength function is to a very large extent independent of the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis is experimentally verified for γ transitions between states in the quasicontinuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined

Primary γ-ray intensities and γ-strength functions from discrete two-step γ-ray cascades in radiative proton-capture experiments

Background: Reaction rates of radiative capture reactions can play a crucial role in the nucleosynthesis of heavy nuclei in explosive stellar environments. These reaction rates depend strongly on γ-ray decay widths in the reaction products, which are, for nonresonant capture reactions at high excitation energies, derived from the γ-ray strength function and the nuclear level density. Recently, the ratio method was applied to primary γ rays observed from (d,p) reactions and nuclear resonance fluorescence measurements to extract the dipole strength in atomic nuclei and to test the generalized Brink-Axel hypothesis. Purpose: The purpose of this work is to apply the ratio method to primary γ-ray intensities of the 63,65Cu(p,γ) reactions to extract γ-ray strength information on the nuclei 64,66Zn. The impact of spin distribution, total γ-ray decay widths, level densities, and width fluctuations on the application of the ratio method will be discussed. Additionally, by comparing the relative γ-ray strength at different excitation energies, conclusions on the validity of the generalized Brink-Axel hypothesis can be made. Method: The radiative proton capture reaction measurements have been performed at the HORUS γ-ray spectrometer of the University of Cologne at one excitation energy for each reaction. Primary γ-ray intensities have been determined by normalizing secondary γ-ray transitions in two-step cascades using their absolute branching ratio. The ratio method was applied to the measured primary γ-ray intensities as well as to previous measurements by Erlandsson et al. at different excitation energies. Results: The relative strength function curve for 64Zn from our measurement shows no significant deviation from the previous measurement at a different excitation energy. The same is true for 66Zn where both measurements were at almost the same excitation energy. Absolute γ-strength function values have been obtained by normalizing the relative curves to quasiparticle random phase approximation calculations because of the absence of experimental data in the respective energy region. Conclusion: The generalized Brink-Axel hypothesis, i.e., the independence of the strength function on the excitation energy, seems to hold in the studied energy region and nuclei. The method to obtain primary γ-ray intensities from two-step cascade spectra was shown to be a valuable and sensitive tool although its uncertainties are connected to the knowledge of the low-energy level scheme of the investigated nucleus. The scaling in the ratio method should be taken with care, because the relative strength is not a simple sum of fE1 and fM1 but a somewhat complex linear combination dependent on the excitation energy of the nucleus

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