Gamma-ray Strength Functions Research Articles

Nuclear level density andγ-ray strength function of43Sc

The nuclear level density and the gamma-ray strength function have been determined for 43Sc in the energy range up to 2 MeV below the neutron separation energy using the Oslo method with the 46Ti(p,alpha)43Sc reaction. A comparison to 45Sc shows that the level density of 43Sc is smaller by an approximately constant factor of two. This behaviour is well reproduced in a microscopical/combinatorial model calculation. The gamma-ray strength function is showing an increase at low gamma-ray energies, a feature which has been observed in several nuclei but which still awaits theoretical explanation.

Exclusive giant dipole resonance measurement on the Jacobi transition in the19F+27Al system

Evaporation-residue-gated high-energy gamma-ray spectra were measured in the ${}^{19}$F+${}^{27}$Al reaction at two beam energies of $\ensuremath{\sim}$75 and 125 MeV. The compound nuclear angular momenta populated at the higher energy should extend beyond the critical value for the Jacobi shape transition, while those at the lower energy should be mostly restricted below the critical value. A sharp low-energy (at $\ensuremath{\sim}$10 MeV) component has been observed in the gamma-ray strength function superimposed on a broad distribution. At the higher beam energy, the component becomes more prominent at higher angular momentum, whereas an opposite behavior is witnessed at the lower beam energy. The implication of these observations in connection with the occurrence of the Jacobi shape transition is discussed.

Level Densities in the actinide region and indirect n,y cross section measurements using the surrogate method

Results from a program of measurements of level densities and gamma ray strength functions in the actinide region are presented. Experiments at the Oslo cyclotron involving the Cactus/Siri detectors and 232 Th(d,x) and 232 Th(3 He,x) reactions were carried out to help answer the question of which level density model is the most appropriate for actinide nuclei, since it will have an impact on cross section calculations important for reactor physics simulations. A new technique for extracting level densities and gamma ray strength functions from particle-gamma coincidence data is proposed and results from the development of this technique are presented.In addition, simultaneous measurements of compound nuclear gamma decay probabilities have been performed for the key thorium cycle nuclei 233 Th, 231 Th and 232 Pa up to around 1MeV above the neutron binding energy and have enabled extraction of indirect neutron induced capture cross sections for the 232 Th, 231 Pa and 230 Th nuclei using the surrogate reaction method. Since the neutron capture cross section for 232 Th is already well known from direct measurements a comparison provides a stringent test of the applicability of the surrogate technique in the actinide region.

Impact of a low-energy enhancement in theγ-ray strength function on the neutron-capture cross section

A low-energy enhancement of the gamma-ray strength function in several light and medium-mass nuclei has been observed recently in 3He-induced reactions. The effect of this enhancement on (n,gamma) cross-sections is investigated for stable and unstable neutron-rich Fe, Mo and Cd isotopes. Our results indicate that the radiative neutron capture cross sections may increase considerably due to the low-energy enhancement when approaching the neutron drip line. This could have non-negligible consequences on r-process nucleosynthesis calculations.

RIPL – Reference Input Parameter Library for Calculation of Nuclear Reactions and Nuclear Data Evaluations

We describe the physics and data included in the Reference Input Parameter Library, which is devoted to input parameters needed in calculations of nuclear reactions and nuclear data evaluations. Advanced modelling codes require substantial numerical input, therefore the International Atomic Energy Agency (IAEA) has worked extensively since 1993 on a library of validated nuclear-model input parameters, referred to as the Reference Input Parameter Library (RIPL). A final RIPL coordinated research project (RIPL-3) was brought to a successful conclusion in December 2008, after 15 years of challenging work carried out through three consecutive IAEA projects. The RIPL-3 library was released in January 2009, and is available on the Web through (http://www-nds.iaea.org/RIPL-3/). This work and the resulting database are extremely important to theoreticians involved in the development and use of nuclear reaction modelling (ALICE, EMPIRE, GNASH, UNF, TALYS) both for theoretical research and nuclear data evaluations. The numerical data and computer codes included in RIPL-3 are arranged in seven segments: MASSES contains ground-state properties of nuclei for about 9000 nuclei, including three theoretical predictions of masses and the evaluated experimental masses of Audi et al. (2003). DISCRETE LEVELS contains 117 datasets (one for each element) with all known level schemes, electromagnetic andmore » {gamma}-ray decay probabilities available from ENSDF in October 2007. NEUTRON RESONANCES contains average resonance parameters prepared on the basis of the evaluations performed by Ignatyuk and Mughabghab. OPTICAL MODEL contains 495 sets of phenomenological optical model parameters defined in a wide energy range. When there are insufficient experimental data, the evaluator has to resort to either global parameterizations or microscopic approaches. Radial density distributions to be used as input for microscopic calculations are stored in the MASSES segment. LEVEL DENSITIES contains phenomenological parameterizations based on the modified Fermi gas and superfluid models and microscopic calculations which are based on a realistic microscopic single-particle level scheme. Partial level densities formulae are also recommended. All tabulated total level densities are consistent with both the recommended average neutron resonance parameters and discrete levels. GAMMA contains parameters that quantify giant resonances, experimental gamma-ray strength functions and methods for calculating gamma emission in statistical model codes. The experimental GDR parameters are represented by Lorentzian fits to the photo-absorption cross sections for 102 nuclides ranging from {sup 51}V to {sup 239}Pu. FISSION includes global prescriptions for fission barriers and nuclear level densities at fission saddle points based on microscopic HFB calculations constrained by experimental fission cross sections.« less

Evaluation of Neutron Capture Gamma-ray Spectra for 89Y, 93Nb, 127I, 133Cs, 141Pr, 197Au, natTl, and 209Bi

The neutron capture gamma-ray spectra for 8 nuclides, 89Y, 93Nb, 127I, 133Cs, 141Pr, 197Au, natTl, and 209Bi, were calculated by using the Hauser-Feshbach statistical model, and their results were compared with the available experimental data. Two dominant ingredients to perform the statistical calculation were the level densities described by the Gilbert-Cameron approach with an improved systematics, and the gamma-ray transmission coefficients described by gamma-ray strength functions. Although various gamma-ray strength functions with a Lorentzian formula have been developed by using the photonuclear data or a microscopic analysis, they have failed to reproduce the occasional anomalous bumps observed near or below a neutron binding. In this work, we could reproduce the bumps well by adding a Lorentzian with an energy-temperature dependent width into a giant electric dipole resonance with an enhanced generalized Lorentzian. In addition, we introduced a correction function so as to compensate for the shortcomings due to missing levels or level-cuts.

Testing and Improvements of Gamma-Ray Strength Functions for Nuclear Model Calculations

The dependence of the gamma-decay and photoabsorption dipole strength functions on the excitation energy, gamma-ray energy and mass number is studied within framework of the closed-form model with the microcanonical ensemble for initial states. The approach covers relatively wide gamma-ray energy interval, ranging from zero to the values above the GDR energy and gives a rather accurate tool for simultaneous description of the gamma-decay and photoabsorption strength functions in the medium and heavy nuclei.

γ-ray strength function and pygmy resonance in rare earth nuclei

The gamma-ray strength function for gamma energies in the 1-7 MeV region has been measured for 161,162-Dy and 171,172-Yb using the (3-He,alpha gamma) reaction. Various models are tested against the observed gamma-ray strength functions. The best description is based on the Kadmenskii, Markushev and Furman E1 model with constant temperature and the Lorentzian M1 model. A gamma-ray bump observed at E_gamma=3 MeV is interpreted as the so-called pygmy resonance, which has also been observed previously in (n,gamma) experiments. The parameters for this resonance have been determined and compared to the available systematics.

Thermal and electromagnetic properties of166Erand167Er

The primary gamma-ray spectra of 166-Er and 167-Er are deduced from the (3-He,alpha gamma) and (3-He,3-He' gamma) reaction, respectively, enabling a simultaneous extraction of the level density and the gamma-ray strength function. Entropy, temperature and heat capacity are deduced from the level density within the micro-canonical and the canonical ensemble, displaying signals of a phase-like transition from the pair-correlated ground state to an uncorrelated state at Tc=0.5 MeV. The gamma-ray strength function displays a bump around E-gamma=3 MeV, interpreted as the pygmy resonance.

Gamma-ray strength functions at finite temperature.

Two approaches to modeling of average partial widths of {gamma} transitions connecting excited nuclear levels are discussed. The first one is based on calculations of thermal photoabsorption cross sections, the second on calculations of thermal-radiation rates. Limitations of the finite-temperature approach are studied.

Radiative strength in the compound nucleus 157Gd.

The distribution of E1 and M1 strength in $^{157}\mathrm{Gd}$ compound nucleus has been studied. An investigation of capture of 2 and 24 keV neutrons in a $^{156}\mathrm{Gd}$ target resulted in resonance-averaged intensities of primary gamma rays between 3.7 and 6.4 MeV. From these intensities the gamma-ray strength functions have been derived for E1 and M1 radiation. We compare several formulations of strength functions to these resonance capture and/or photoabsorption data. Further we use these prescriptions in calculations of the total average radiation width, radiative capture cross sections, and gamma-ray spectra, and compare them to available experimental information. By analyzing these results strong evidence was found for an E1 strength function which is based on a generalized Lorentzian, enhanced compared to spherical nuclei, with an energy-dependent spreading width and a nonzero limit as the energy tends to zero. For M1 radiation the giant resonance spin-flip mode is favored.

Test of gamma-ray strength functions in nuclear reaction model calculations.

The impact of models for E1 and M1 gamma-ray strength functions on the results of nuclear model calculations of total average radiation widths, radiative capture cross sections, and gamma-ray spectra has been studied. We considered strength functions that reproduce photoabsorption and/or average resonance data but significantly differ from each other at low gamma-ray energies. As the calculated quantities critically depend on the strength functions in this energy region, model calculations can be used to test the low-energy behavior of strength functions. By analyzing the $^{197}\mathrm{Au}$, $^{143}\mathrm{Nd}$, $^{105}\mathrm{Pd}$, and $^{93}\mathrm{Nd}$ neutron capture reactions we found strong evidence for a model of the E1 strength function, which is characterized by the following properties: (i) an energy dependent spreading width of the underlying Lorentzian for the photoabsorption cross section and (ii) a nonzero, temperature dependent, limit as the transition energy tends to zero. This model is founded in theoretical work by Zaretskij, Sirotkin, and Kadmenskij and represents a partial breakdown of Brink's hypothesis.

Calculation of Capture Cross Sections and Gamma-Ray Spectra Following the Interaction of Neutrons with181Ta and197Au

This article describes the calculation of capture cross sections and gamma-ray spectra for the neutron-induced reactions with /sup 181/Ta and /sup 197/Au using the level density parameters derived from a new low-lying level fitting method in which the constant temperature partial level density formula and the spin-selected level cumulation were used. The gamma-ray profile functions, of the Brink-Axel type with an anomalous bump, were employed for both nuclei. The capture cross sections, spectra, and gamma-ray strength functions obtained in this calculation agree well with the experimental ones.

Observation of the M1 giant resonance by resonance averaging in 106Pd

An investigation of capture of 2 keV and 24 keV neutrons in a 105Pd target resulted in resonance-averaged intensities of primary gamma rays with energies between 5.2 and 9.5 MeV. From these intensities the gamma ray strength functions have been evaluated for E1, M1 and E2 radiation and compared with predictions of the giant resonance theory. The inclusion of an energy dependent spreading width for the E1 giant resonance is necessary. The energy distribution of M1 reduced strength is consistent with an interpretation of a broad resonance around 8.8 MeV. E2 data agrees satisfactorily with the giant resonance extrapolation.

KeV-Neutron Capture in Cesium-133, Gold-197 and Tantalum-181

The keV-neutron capture in 133Cs, 197Au and 181Ta was studied by measuring the cross section and the emitted gamma-ray spectrum. The neutron was produced with the KUR linear accelerator and a photoneutron target, and the samples were placed at 11.7 m from the neutron source. The gamma-rays following the keV-neutron capture were detected with a pair of C6D6 scintillators. The relative capture cross section between 3.2 and 270 keV was normalized to the absolute cross section at 24 keV obtained with an iron-filtered neutron beam. The results of 133Cs and 197Au are in good agreement with the recently measured and evaluated capture cross sections. The capture gamma-ray spectrum was obtained by unfolding the observed spectrum with the response functions of the detector. The gamma-ray strength function, which represents the average gamma-ray transition probability, was derived with the spectrum fitting method. The level density distributions used in the calculation of spectrum fitting were investigated and it is found that the constant temperature level density is preferable. The gamma-ray strength functions for 198Au and 134Cs show the 5.5-MeV bump and the deficiency of strength compared with the tail of giant dipole resonance, and that for 182Ta does not give the bump but shows the similarity to the tail of giant resonance. The results were compared with other data.

KeV-neutron capture in cesium-133, gold-197 and tantalum-181.

The keV-neutron capture in 133Cs, 197Au and 181Ta was studied by measuring the cross section and the emitted gamma-ray spectrum. The neutron was produced with the KUR linear accelerator and a photoneutron target, and the samples were placed at 11.7 m from the neutron source. The gamma-rays following the keV-neutron capture were detected with a pair of C6D6 scintillators. The relative capture cross section between 3.2 and 270 keV was normalized to the absolute cross section at 24 keV obtained with an iron-filtered neutron beam. The results of 133Cs and 197Au are in good agreement with the recently measured and evaluated capture cross sections. The capture gamma-ray spectrum was obtained by unfolding the observed spectrum with the response functions of the detector. The gamma-ray strength function, which represents the average gamma-ray transition probability, was derived with the spectrum fitting method. The level density distributions used in the calculation of spectrum fitting were investigated and it is found that the constant temperature level density is preferable. The gamma-ray strength functions for 198Au and 134Cs show the 5.5-MeV bump and the deficiency of strength compared with the tail of giant dipole resonance, and that for 182Ta does not give the bump but shows the similarity to the tail of giant resonance. The results were compared with other data.

Proposal on gamma-ray strength function for calculation of gamma-ray production cross sections.

Click to increase image sizeClick to decrease image sizeKEYWORDS: gamma-ray strength functiongamma-ray production cross sectionprofile functionnuclei near closed shellgold 197sum-ruleMeV range 01-10

Gamma-ray strength functions for A = 70–90 nuclei

The radiative proton capture reaction has been used to study the energy dependence of the El γ-ray strength function for A = 70–90 nuclei in the region between 6 and 10 MeV. Using a Ge(Li) detector, γ-ray spectra of primary radiative transitions have been measured in order to obtain γ-ray intensities averaged over many neighbouring resonances. The γ-ray strength functions were deduced for decay to the levels with well-known J π values. The values of the γ-ray strength function, in the energy region investigated, are well described by a Lorentzian curve extrapolated from the giant resonance region.

Investigation of65Cu by means of the average resonance proton capture method

The64Ni(p, γ)65Cu reaction has been studied in the proton energy rangeEp=2.05–2.55 MeV. The gamma-ray spectra were recorded with a three-crystal pair spectrometer at proton energy differences of 19 keV covering the proton energy range. An average gamma-ray spectrum was formed by adding all the individual spectra after proper adjustment as a result of the alterations in proton energy. The intensities of the gamma rays to final states with knownJπ-values were tested against theoretical calculations based on the Hauser-Feshbach theory. The gamma-ray strength function for energies lower than 9 MeV has been extracted from the experiment.

Resonance neutron capture in 198,199,201Hg

Abstract Partial radiation widths of γ-rays of energies E γ > 4.1 MeV following capture of neutrons with E n 199 Hg, 200 Hg and 202 Hg. Gamma-ray tables and decay schemes are given. Gamma-ray strength functions and level density distributions are derived. Departures from statistical behaviour are observed in 199 Hg. It is found that the energy dependence of the level density below 3 MeV differs in functional form in the three isotopes.