We investigate the energy and multiplicity distributions of gamma-rays following neutron capture in Mo-94,Mo-95. By comparing simulated gamma cascades following radiative capture with experimental spectra we try to learn something about the photon strength function at energies below the separation energy. The point of this comparison is to try and confirm recent experiments that found an anomalous low energy enhancement in the photon strength function for molybdenum isotopes.

We have performed a study of the statistical mechanics of correlated spectra first introduced by Dyson and Mehta some 40 years ago. We have derived a modified thermodynamical statistics (a number and its variance) for linear spectra. This approach was used to analyze the statistical properties of the eigenvalues of random matrices of Gaussian ensembles and of the experimental eigenfrequencies of acoustic resonances of an aluminum plate cut in the shape of a chaotic billiard. The results obtained suggest that this statistics provides a robust tool for the investigation of spectral properties.

Level densities for Mo93-98 have been extracted using the ( He-3, alpha gamma) and ( He-3, He-3'gamma) reactions. From the level densities thermodynamical quantities such as temperature and heat capacity can be deduced. Data have been analyzed by utilizing both the microcanonical and the canonical ensemble. Structures in the microcanonical temperature are consistent with the breaking of nucleon Cooper pairs. The S shape of the heat capacity curves found within the canonical ensemble is interpreted as consistent with a pairing phase transition with a critical temperature for the quenching of pairing correlations at T-c similar to 0.7-1.0MeV.

The neutron spectrum from the (55)mn(d,n)Fe-56 reaction has been measured at E-d = 7 MeV. The level density of Fe-56 obtained from neutron evaporation spectrum has been compared to the level density obtained from Oslo-type Fe-57(He-3, alpha gamma)16 Fe experiment [1]. The good agreement supports the recent results [1, 2] including the low-energy enhancement in the gamma-strength function for iron isotopes. The new level density function allowed us to investigate an excitation energy dependence of this enhancement, which is shown to increase with increasing excitation energy.

Radiative strength functions in (152)'Eu-154 nuclei for gamma-ray energies below 6 MeV have been investigated. Neutron capture for incident neutron energies < leV up to 100 keV has been measured for Eu-151,Eu-153 targets. Properties Of 7 decay of neutron resonances in Eu-152,Eu-154 nuclei are examined. The results of measurements are compared to outcome of simulation of gamma cascades based on various models for the radiative strength function. Comparison between experimental data and simulation suggests existence of the low-energy resonance in these two nuclei.

Cross-section measurements were made of prompt gamma-ray production as a function of incident neutron energy on a Ti-48 sample. Partial gamma-ray cross sections for transitions in Ti45-48, Sc44-48, Ca42-45, K41-44, and Ar41-42 have been determined. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the LANSCE/WNR facility. The prompt-reaction gamma rays were detected with the large-scale Compton-suppressed germanium array for neutron-induced excitations (GEANIE). Neutron energies were determined by the time-of-flight technique. The gamma-ray excitation functions were converted to partial gamma-ray cross sections taking into account the dead-time correction, target thickness, detector efficiency, and neutron flux (monitored with an in-line fission chamber). The data will be presented for neutron energies between 1 to 250 MeV. These results are compared with model calculations that include compound nuclear and pre-equilibrium emission.

We have analyzed constraints on parity-odd time-reversal noninvariant interactions derived from measurements of the energy dependence of parity-violating polarized neutron capture on unpolarized targets. As previous authors found, a perturbation in energy dependence due to a parity (P)-odd time (T)-odd interaction is present. However, the perturbation competes with T-even terms which can obscure the T-odd signature. We estimate the magnitudes of these competing terms and suggest strategies for a practicable experiment.

A direct measurement of nn-scattering by colliding free neutrons has never been performed. Indirect measurements continue to provide inconsistent results, leaving the issue of charge symmetry in the nuclear force unresolved. At present the Russian pulsed reactor YAGUAR is the best neutron source for such a measurement. A neutron moderator is installed in the central through channel and the scattered neutrons are detected at a distance of 12 m from the reactor. An instantaneous value of 1.1 × 1018/cm2s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as ISTC project No. 2286.

Shell-model calculations in the $s\ensuremath{-}d$ shell have been utilized to examine how the statistical behavior of eigenvalues and reduced transition probabilities are affected by broken isospin symmetry. Calculations have been performed for the nuclides $^{22}\mathrm{Na},^{26}\mathrm{Al}$, and $^{34}\mathrm{Cl}$ and have been compared to existing experimental data for $^{26}\mathrm{Al}$ and $^{30}\mathrm{P}$. The eigenvalue statistics depend on the magnitude of the Coulomb matrix element, and this is reflected in a sensitivity to the choice of single-particle energies. The distributions of reduced transition probabilities are not universal but depend upon the particular transition mode chosen; this behavior is qualitatively similar to experimental results.

Radiative strength functions (RSFs) in 93-98Mo have been extracted using the (3He,alpha gamma) and (3He,3He' gamma) reactions. The RSFs are U-shaped as function of gamma energy with a minimum at around E_gamma=3 MeV. The minimum values increase with neutron number due to the increase in the low-energy tail of the giant electric dipole resonance with nuclear deformation. The unexpected strong increase in strength below E_gamma=3 MeV, here called soft pole, is established for all 93-98Mo isotopes. The soft pole is present at all initial excitation energies in the 5-8 MeV region.

A high resolution measurement of the cross sections of the {sup 44}Ca(p,p{sub 0}) and {sup 44}Ca(p,p{sub 1}) reactions was performed over the energy range E{sub p}=2.50-3.53 MeV. A total of 809 resonances were observed. The purity and completeness of the {sup 45}Sc data were tested with a variety of statistical analyses. These analyses suggest that the 1/2{sup +} and 1/2{sup -} resonance sequences are the most pure and complete ever measured. The resulting level densities are {rho}(1/2){sup +}=127{sub -8}{sup +7} MeV{sup -1} and {rho}(1/2{sup -})=132{sub -9}{sup +8} MeV{sup -1}; there is no evidence of parity dependence of the level density of J=1/2 states at E{sub x}=10 MeV in {sup 45}Sc.

Although crucial for resolving the issue of charge symmetry in the nuclear force, direct measurement of nn-scattering by colliding free neutrons has never been performed. At present the Russian pulsed reactor YAGUAR is the best neutron source for performing such a measurement. It has a through channel where the neutron moderator is installed. The neutrons are counted by a neutron detector located 12 m from the reactor. In preliminary experiments an instantaneous value of 1.1 × 10(18)/cm(2)s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as International Science & Technology Center (ISTC) project No. 2286.

An accurate value of the nuclear resonance spacing is crucial for determination of level densities. Level densities are key input for the calculation of nuclear reaction rates and cross sections. This paper discusses various effects that can adversely impact the average level spacing, with special emphasis on the issue of quantum number assignment. The most striking property of spacings of resonances with the same quantum number is level repulsion. We investigate how a simple test based on level repulsion can be used in the identification of spin misassignment and provide new experimental verification of the proposed test. Proton resonances obtained in the Ca-44+p reaction are used as an example. In addition, s-wave neutron resonances in the U-238+n reaction are considered.

The radiative strength function (RSF) is key for understanding nuclear reaction rates in areas ranging from astrophysics to radiochemical diagnostics. The RSF also provides a tool for understanding the characteristics of gamma-ray cascades. Unresolved transitions in nuclear deexcitation processes are best described by statistical properties such as the radiative strength function. The sequential extraction method developed at the Oslo Cyclotron Laboratory provides data for radiative strength functions for gamma-ray energies from 1MeV to the neutron binding energy. The data are averaged over about 100keV energy bin and normalized to the average total radiative width of neutron resonances. The radiative strength functions in all nuclei studied show a characteristic increase with increasing gamma-ray energy. However, the detailed structures in the radiative strength function for various nuclei show different behavior in various mass regions. In rare-earth nuclei, a resonance structure near 3MeV is observed. For several lighter nuclei (A<100), a large enhancement for low transition energies is observed. This unexpected phenomenon is not explained by existing theoretical models. Additional investigations using independent measurements are underway. The experiments and analysis methods will be described briefly. Experimental data for Fe, Sn, and Yb will be shown to illustrate the variety of behavior observed for the radiative strength function.

The level density and the γ‐strength function have been extracted experimentally. From the level densities thermodynamical quantities such as temperature and heat capacity can be found. Structures in the micro‐canonical temperature are interpreted as the onset of new degrees of freedom by the breaking of Cooper pairs. The S‐shape in the heat capacity curves, found within the canonical ensemble, indicates the pairing‐phase transition, and a critical temperature for the quenching of pair correlations is found. The pygmy resonance at 3.3(1) MeV in 172Yb has now been established with a strength of B(M1)=6.5(15)μN2 and M1 multipolarity, the so‐called scissors mode. In addition, a strong unexpected enhancement of the radiative strength function (RSF) has been found at low γ‐ray energy in medium light Fe nuclei and also in the heavier Mo nuclei.

The effect of isospin symmetry breaking on level statistics has been examined with the nuclear shell model. The eigenvalues and electromagnetic transitions were calculated with the program OXBASH for the nuclide 26 Al for conserved isospin and for broken isospin. The long-range correlations of the eigenvalues, as measured by Δ 3 , show good agreement between the experimental results and the calculations. However, there are discrepancies between data and calculations for the short-range correlations of the eigenvalues.

We present a statistical analysis of proton resonances in the compound nucleus 93Tc in terms of random matrix theory (RMT). The fluctuation properties of energy levels and reduced widths from data measured by Bilpuch et al. [Phys. Rev. C 9 (1974) 1589] are studied. We conclude that one T>=9/2 isobaric analog state does not affect the spectral correlations of a sequence of 124 T<=7/2 states, and that the observed deviations from RMT are due to unobserved levels. For the reduced widths, however, certain deviations from Porter–Thomas statistics are attributed to the effect of isospin mixing.

Level densities and radiative strength functions in Yb-171 and Yb-170 nuclei have been measured using the Yb-171(He-3,He-3'gamma)Yb-171 and Yb-171(He-3,alphagamma)Yb-171 reactions. New data on Yb-171 are compared to previous measurements for Yb-171 from the Yb-172(He-3,alphagamma)Yb-171 reaction. The systematics of level densities and radiative strength functions in Yb-170,Yb-171,Yb-172 are established. The entropy excess in Yb-171 relative to the even-even nuclei Yb-170,Yb-172 due to the impaired neutron quasiparticle is found to be approximately 2k(B). Results for the radiative strength function from the two reactions lead to consistent parameters characterizing the "pygmy" resonances. Pygmy resonances in the Yb-170,Yb-172 populated by the (He-3,alpha) reaction appear to be split into two components for both of which a complete set of resonance parameters is obtained.

Symmetries and statistical properties in nuclei are closely related. The most striking example is the extremely large enhancement of parity violation in neutron resonances. Statistical distributions can provide information about the underlying character of nuclear properties. Level statistics and electromagnetic transition distributions have been used successfully to provide unique tests of predictions of random matrix theory.