The little hagedorn that could

The Hagedorn exponential mass spectrum with slope 1/TH was erroneously interpreted as fixing an upper limiting temperature TH that the system can achieve. To the contrary, such spectrum indicates a 1st order phase transition at a fixed temperature TH. A much lower energy example is the log linear level nuclear density below the neutron binding energy that prevails throughout the nuclear chart. We show that, for non-magic nuclei, such linearity implies a 1st order phase transition from the pairing superfluid to an ideal gas of quasi particles

Experimentally constrained (n, γ) reaction rates relevant to r- and i-process nucleosynthesis

The element distribution that we observe in the Universe today tells a fascinating story of nucleosynthesis events that have taken place throughout the 13.7-billion-years-long history starting with the Big Bang. It has been known for a long time that radiative neutron-capture reactions play a major role in synthesizing elements heavier than iron. However, many questions remain when it comes to our understanding of neutron-capture processes in extreme stellar environments. In particular, the intermediate and rapid neutron-capture processes are very challenging to describe, as there exist little or no data on the much-needed neutron-capture rates. In this work, we discuss possibilities to obtain indirect, experimental constrains on these rates by means of the Oslo and the β-Oslo methods

Experimental first order pairing phase transition in atomic nuclei

The natural log of experimental nuclear level densities at low energy is linear with energy. This can be interpreted in terms of a nearly 1st order phase transition from a superfluid to an ideal gas of quasi particles. The transition temperature coincides with the BCS critical temperature and yields gap parameters in good agreement with the values extracted from even- odd mass differences from rotational states. This converging evidence supports the relevance of the BCS theory to atomic nuclei

Extreme nonstatistical effects in γ decay of 95Mo neutron resonances

We demonstrate that high-quality total radiation width (Γγ) data are a virtually untapped resource for testing and improving nuclear models. To this end, we obtained unprecedentedly large sets of Γγ values for all six s- and p-wave Jπ values for 95Mo neutron resonances. We show that Γγ distributions simulated in the framework of the nuclear statistical model are in sharp disagreement with the data. Simulations modified to include doorway effects resulted in much better agreement. These results call into question the reliability of the nuclear statistical model. © 2013 American Physical Societ

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

Independent normalization for gamma-ray strength functions: The shape method

The shape method, a novel approach to obtain the functional form of the γ-ray strength function (γSF), is introduced. In connection with the Oslo method the slope of the nuclear level density (NLD) and γSF can be obtained simultaneously even in the absence of neutron resonance spacing data. The foundation of the shape method lies in the primary γ-ray transitions which preserve information on the functional form of the γSF. The shape method has been applied to 56Fe, 92Zr, and 164Dy, which are representative cases for the variety of situations encountered in typical NLD and γSF studies. The comparisons of results from the shape method to those from the Oslo method demonstrate that the functional form of the γSF is retained regardless of nuclear structure details or Jπ values of the states fed by the primary transitions

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

Level densities of 74,76Ge from compound nuclear reactions

The level densities of 74 , 76 Ge nuclei are studied with 68 , 70 Zn ( 7 Li , Xp ) reactions. Proton evaporation spectra are measured at backward angles in a wide energy region, from about 2 to 25 MeV. The analysis of spectra allows for the testing of level density models used in modern reaction codes for practical cross-section calculations. Our results show that at excitation energies above the discrete level region, all level density models tested in this work overestimate the level densities that are needed to reproduce proton spectra from these reactions. The Gilbert and Cameron model, which includes the constant-temperature energy dependence of the level density, shows the best agreement with experiment, however, its parameters need to be adjusted to reflect the observed reduction of the level density at higher excitation energies