Capture Gamma-Ray Spectroscopy and Related Topics

COMMITTEES

PREFACE

CONTENTS

We emphasize the point that every time that increased technical possibilities have become possible, on the level of accelerators, the use of exotic beams as well as on the data taking and analyses, new physics has never ceased to show up. I will illustrate this theme with a number of examples that have, over the last decades, expanded our horizon in observering and exploring the nuclear many-body system (such as superdeformation, ultra-high precision γ- spectroscopy, mass measurements, halo physics, exploration of superheavy elements,…) in various directions. I will also discuss recent developments in nuclear theory where various subfields like ab-initio methods, mean-field techniques, nuclear shell-model calculations, collective geometrical (shape) approaches and symmetry-guided and group-theoretical methods all tend to have more in common and can give rise to quite new fields of research.

A half century of neutron capture is reviewed. The development of this field and its personal impact on one research scientist are discussed.

Recent developments in the description of the structural evolution of nuclei invoke the concepts of phase transitions and phase coexistence. New, analytic, parameter-free symmetries have been developed that describe nuclei at the “critical points” of phase/shape transition regions. The entire N=90 region appears to be an example of the X(5) – spherical vibrator to axial rotor – phase transitional point. Consideration of both prolate and oblate shapes allows an extension and symmetrization of the symmetry triangle for nuclear structure.

We show that the second-order phase transition between spherical and deformed shapes of atomic nuclei is an isolated point following from the Landau theory of phase transitions. This point can only occur at the junction of two or more first-order phase transitions which explains why it is associated with one special type of structure and requires the recently proposed first-order phase transition between prolate and oblate nuclear shapes.

We discuss a microscopic framework for phenomenological boson-fermion models of nuclear structure based on the U(n/m) type of superalgebras. The generalized Dyson mapping of fermion collective superalgebras provides a basis to do so and to understand how collectivity selects the required preservation of boson plus fermion number as a good quantum number. We also consider the difference between dynamical and invariant supersymmetries based on possible supermultiplets of spectra of neighboring odd and even nuclei. We point out that different criteria exist for choosing the appropriate single particle transfer operators in the two cases and discuss a microscopically based method to construct these operators in the case of dynamical supersymmetry.

We outline a new iterative algorithm recently developed for generating a subset of eigenvalues and eigenvectors of large matrices and show how it can be efficiently adopted for nuclear shell model Hamiltonians.

Recent studies of Coulomb energy differences at high spin and excitation energy in the f_7/2 shell are discussed and an overview of the current status is given. New results are presented for the A=53 mirror pair and used to explore the values of the 2-proton Coulomb matrix elements necessary to successfully reproduce the empirical results from the shell model. An anomalous behaviour of the matrix elements as a function of spin emerges. A possible explanation for the anomaly is proposed.

Results are presented from experiments recently performed at the well-established bremsstrahlung facilities of the 4.3 MV Stuttgart DYNAMITRON accelerator. The surprisingly strong population of spin isomers in odd-mass nulclei near shell closures in photo-induced reactions have been studied by photoactivation and Nuclear Resonance Fluorescence (NRF) experiments on ^{113, 115}In (Z=49), ⁸⁷Sr (N=49), and in ^135,137Ba (N=79, 81). The generation of a population inversion of low-lying excited nuclear levels by feeding from higher-lying photo-excited states could be demonstrated for the first time by NRF experiments on ¹⁰³Rh. Such an inversion is the precondition for possible γ-ray laser schemes. The puzzling nucleosynthesis of nature's rarest isotope ¹⁸⁰Ta was studied by measuring the depopulation of the quasistable isomer in ¹⁸⁰Ta (J^π = 9⁻, t_1/2 ≥ 10¹⁵ yr) by resonant photoabsorption. The new results indicate a dramatic acceleration of the isomer decay to the short-lived J^π = 9⁺ ground state (t_1/2 = 8.1 h) under stellar s-process conditions. The astrophysical consequences and nuclear structure effects are discussed.

Isospin mixing of excited states with isospin quantum numbers T = 0 and T = 1 in odd-odd N=Z nuclei is analyzed. It is shown how the measured E2/M1 multipole mixing ratios can be used to estimate the degree of isospin breaking. The favorable conditions and the role of collective effects for the reliable estimations are discussed.

For nuclei far from stability the Relativistic Hartree-Bogoliubov model provides a unified and self-consistent description of mean-field and pairing correlations. The model is employed in the analysis of ground-state properties of A = 20 isobars and of light nuclei with 4 ≤ Z ≤ 10. The RHB maps of the proton drip-line for odd-Z nuclei in the regions 31 ≤ Z ≤ 49 and 73 ≤ Z ≤ 91, are compared with recent experimental data.

We adopt the quasiparticle-phonon model to investigate the microscopic structure of low-lying states recently discovered in nuclei around shell closure. The results are in overall agreement with experiments and consistent with the description provided by the interacting boson model and shell model.

Comprehensive investigations of off-yrast low-spin states of ⁹⁴Mo yielded evidence for one quadrupole phonon and two-phonon states with mixed proton-neutron symmetry. The mixed-symmetry assignments are based on the observation of strong M1 transitions and the measurement of their absolute M1 matrix elements at ≈1 μ_N. Corresponding structures were recently identified in the N = 52 neighboring even-even isotones, ⁹⁶Ru and ⁹²Zr, too. Progress on this topic is reported.

Vibrational excitations in a number of nearly spherical nuclei have been investigated with the (n, n′ γ) reaction. Studies of vibrations of the quadrupole, octupole, hexadecapole types are reviewed.

The nuclear phase/shape transition in the framework of the Interacting Boson Model is analyzed in detail and the predictions of this model are compared with the evolution of basic observables in the region Z=58-76, N=84-100.

B(E2) and level energy data have recently been claimed as evidence for a new muitiphonon interpretation of ¹⁵²Sm levels, which has been considered evidence for phase coexistence. However, closer examination shows these data are explained as well, or better, by traditional descriptions. The available data reveal there is not an adequate experimental foundation for the new interpretation, and the primary motivation for it appears to be from theoretical considerations. An important point is that B(E2) and level energy data are found to be consistent with several competing interpretations, and thus do not distinguish clearly which description is preferred. For future work (e.g., on nuclei far from stability) it is important to also obtain (and use) data of other types, to resolve the ambiguities.

Lifetimes of excited states in neutron-rich nuclei, produced in the spontaneous fission of ²⁵²Cf, were measured using the recoil distance method. The experiment was performed using the New Yale Plunger Device and the Gammasphere array. In this contribution we present results for ¹⁰⁰Zr, ^{104, 106}Mo and ¹⁴⁴Ba. The results for ¹⁰⁴Mo enable us to test if this nucleus is a further empirical realization of the critical point symmetry X(5) of the phase-/shape-transition between spherical and axially deformed nuclei. In ¹⁴⁴Ba we were for the first time able to directly measure the quadrupole and dipole moment of the negative parity octupole band.

It is shown that a self-consistent cranking approach with rotation around a symmetry axis is suitable to study a shape transition from axially–symmetric to triaxially deformed shapes. Such a shape transition is associated with bifurcation points of mean field solutions. The bifurcation points are related to critical rotational frequencies at which collective oscillations vanish in the rotating frame. Using this approach, we found the critical rotational frequency that corresponds to onset of the tilted rotation.

The self-consistent separable RPA (SRPA) method is proposed for arbitrary density- and current-dependent energy functionals. SRPA quickly converges to the exact RPA results and considerably simplifies the calculations. The method is specified for a Skyrme functional and numerically tested for the particular case of SkM* forces.

Based on new lifetime data from a (γ, γ′) experiment the second excited 2⁺ state in ⁹²Zr is identified as a mixed-symmetric 2⁺ state by its strong M1 decay to the state. Experimental data and shell model calculations support the mixed-symmetry character of the state, while the state is identified as a rather pure neutron excitation. The shell model calculations also show a very similar character of the lowest two 0⁺ states due to proton (2p_1/2)² and (1g_9/2)² configurations. Furthermore, two strong dipole excitations are observed in ⁹²Zr, which are candidates for two-phonon 1^π states.

The study of nuclei near the valley of stability profits from the wide variety of probes that can be employed. The nucleus ¹¹²Cd is an example of one that has been investigated throughly. State properties obtained from selective reactions, such as single-nucleon transfer, inelastic hadronic scattering, etc., complements those obtained from (typically) non-selective reactions employing γ-ray spectroscopy. By combining information from all available data, dominant configurations for all levels up to 2.5 MeV can be assigned. Additional experiments are suggested to further investigate the nature of levels in ¹¹²Cd.

A three-configuration mixing calculation is performed, for the first time, in the context of the interacting boson model with the aim to describe recently observed collective bands built on low-lying 0⁺ states in neutron-deficient lead isotopes. The configurations that are included correspond to the regular, spherical states as well as two-particle two-hole and four-particle four-hole excitations across the Z = 82 shell gap. The latter two configurations may be associated with oblate and prolate shapes, respectively.

Using the Gamma-Ray-Induced Dopplerbroadening (GRID) technique, lifetimes of states in the γ- and band in ¹⁶⁸Er were determined. The resulting absolute B(E2)-values are compared with theoretical models describing ¹⁶⁸Er within the framework of the Interacting Boson Model (IBM) and the Triaxial Projected Shell Model (TPSM). Special emphasis is put on a reinterpretation of the level scheme in terms of the new Partial Dynamical Symmetry (PDS) approach.

Systematics of quasiparticle and vibrational states in the chain of W isotopes (¹⁷⁹W, ¹⁸¹W, ¹⁸³W, ¹⁸⁵W, ¹⁸⁷W) is presented. Levels in ¹⁸³W, ¹⁸⁶W, and ¹⁸⁷W have been studied up to 2 MeV with the (d,p) or (d,t) and (n,γ) reactions. One-quasiparticle states below 2 MeV are interpreted in the framework of the quasiparticle-phonon model (QPM), rotational degrees of freedom using the particle-rotor model (PRM) including Coriolis interaction.

The nuclear resonance fluorescence method (NRF) is an ideal tool for the investigation of dipole excitations in nuclei because of the spin-selective excitation mechanism and the model-independent data analysis. This report focuses on the systematic study of E1-two-phonon excitations of the quadrupole-octupole-coupled type in nuclei around magic proton and/or neutron numbers. The experimental data from NRF experiments show enhanced E1 strengths, which are more than an order of magnitude higher than usual B(E1) values between low-lying states. The excitation energy of the 1⁻ states is very close to the sum energy of the one-phonon excitations, indicating a nearly harmonic coupling. The systematics of the ground-state transition strengths shows that the B(E1) values are highest for closed-shell nuclei and decrease going away from the magic numbers. This is an experimental evidence for the so-called Dipole Core Polarization (DCP) effect. The second observation that the decay branching to the first 2⁺ state is weak for magic nuclei and increases going away from the shell closure can also be explained by theoretical calculations.

Energies and B(E2) transition strengths of seven low-lying rotational bands with negative parity in ¹⁶³Dy are described using the pseudo SU(3) shell model. M1 excitation strengths to states in the scissors mode region (2–4 MeV) are also presented. Fragmentation patterns are found to be closely related with pseudo-spin excitations.

Single-nucleon knockout reactions in inverse kinematics at energies above 50 MeV/nucleon offer a powerful spectroscopic tool for identifying single-particle structure. Partial and differential cross sections to individual final levels can be obtained by measuring the momentum of the projectile residue with a high-resolution spectrograph in coincidence with gamma rays. The shape of the longitudinal momentum spectra identifies the orbital angular momentum l, while the absolute cross sections determine single-nucleon spectroscopic factors. Examples of results with this technique are given. The method has an extremely high sensitivity, and experiments have been carried out with incident beams of less than one atom/second. An analysis of precise data from high-energy experiments suggests that the method furnishes absolute occupancies and will allow the study of short-range correlations caused by the hard core in the nucleon-nucleon interaction. For the deeply bound p-shell neutron states of ¹⁶O and ¹²C the occupancies are reduced by factors R_s of 0.56(3) and 0.51(3), respectively, relative to the many-body shell model with effective interactions. Data for weakly bound neutron and proton states in radioactive nuclei give R_s values much closer to unity. This aspect of the nuclear knockout method will make it an interesting tool for understanding the fundamentals of the shell model.

We give a layout of the γ emission mechanism within the statistical description of the pre-equilibrium exciton model, show results compared to the data and estimate the predictive power and reliability of such calculations. Comparison of the two models, well-justified direct-semi-direct one and the pre-equilibrium exciton model, enables deeper insight into the γ emission mechanisms in nuclear reactions and also helps to specify conditions and circumstances suitable for the use of either of the two.

The main features of the Consistent Direct-semidirect (CDSD) model for radiative capture of nucleons in the energy region of giant multipole resonances are presented. The model successfully reproduces all of the experimental data available for light and heavy nuclei. The basic parameters of the model are constant over the entire mass and energy region. We believe that the model correctly represents the coupling of the simple modes of excitation to the complicated states, which are excited in the latter stages of the capture process. The model can therefore serve as a prototype for microscopic calculations, where the giant resonance is represented by an RPA approach. Even in its present form the model possesses strong predictive power. Some of the most convincing cases, indicating the deficiencies of previous approaches to the capture reactions, are presented.

Prompt gamma-ray and x-ray spectroscopy techniques are being employed to study fast-neutron-induced fission of actinides to determine independent (pre-beta-decay) yields for a wide range of product nuclides. Data are acquired using the GEANIE high-resolution gamma-ray spectrometer at the LANSCE/WNR unmoderated spallation neutron source providing neutrons with energies from below 1 MeV to over 400 MeV. Three different techniques (identification by characteristic gamma rays, by gamma-gamma coincidences, and by fission-gamma coincidences) are being used to gather complementary data sets from which detailed fission yields can be extracted. From these data, mass and charge distributions are determined over a wide incident-neutron energy range. The phenomena of interest include the transition from asymmetric to symmetric fission, the competition between neutron and gamma-ray emission, nuclear structure effects in fission and the angular momentum imparted to the fission products. Results for ²³⁸U and ²³⁵U are presented.

Experimental evidence for two types of collective excitations in nuclei generated by orbital motion is discussed, viz. a magnetic quadrupole twist mode observed in 180° electron scattering experiments and a toroidal electric dipole mode. The latter may be a source of low-energy pygmy dipole resonances observed in many nuclei. This is discussed in detail for the example of ²⁰⁸Pb based on the recent finding of a resonance at particle threshold in a high-resolution (γ, γ′) experiment.

The reaction processes going on at Coulomb barrier energies will be discussed on the light also of new experimental data on the systems ^9,10,11Be+²⁰⁹Bi (fusion) and ⁶Li+²⁰⁸Pb (breakup BU) and extended coupled channel calculations for the ⁶Li BU process.

From the Be beams data there is no significant evidence of subbarrier fusion cross section enhancement. The Li data indicate that the BU process has two components: the first with two fragments in the exit channel and the second with only one fragment. The present data point out that in this energy regime the interaction is mostly dominated by the BU process with moderate influence from the halo structure

The E1 radiative neutron strength functions of spherical and deformed nuclei are calculated within the framework of a generalized Landau Fermi liquid model and are compared with average 2-keV and 24-keV neutron capture measurements. The average total radiative widths of neutron resonances are estimated and compared with measurements. From studying of the damping widths of giant dipole- quadrupole- and octupole-resonances, three significant findings emerge: 1) quadratic energy dependence of the width is favored, 2) the ordinary two-body nuclear viscosity is determined as 0.022 ± 0.03 TP, and 3) the one-body dissipation mechanism is suppressed by a factor of 0.27.

To investigate the origin of the anomalous bump or shoulder observed in the γ-ray energy region from 1.5 to 3.5 MeV in the keV-neutron capture γ-ray spectra of lanthanide nuclides, the spectra of ¹⁴⁵Nd, ¹⁶¹Dy, and ¹⁶⁷Er were analyzed with the statistical model calculation from the viewpoint of the M1 scissors mode excitation, assuming the Brink hypothesis for the excitation. The values of B(M1) for the excitation were derived from the analysis, and the derived values were in good agreement with those derived from the (γ, γ′) reactions. From this agreement, the anomalous bump or shoulder in the keV-neutron capture γ-ray spectra of lanthanide nuclides may be ascribed to the M1 scissors mode excitation if we can assume the Brink hypothesis for the M1 scissors mode excitation.

A new method of partial capture cross section measurements in keV-neutron energy region was developed in Frank Laboratory of neutron Physics. The first results of the ⁵⁸Ni(n,γ0, 1), ⁵⁶Fe(n, γ0+1) and ⁴⁸Ti(n, γ1) cross sections and radiative strength functions of E1 and M1 multipolarity [F_γ(E1) and F_γ(M1)] are discussed in the report. The correlation of partial radiative widths with reduced neutron widths for s-wave resonances of ⁵⁸Ni and increase of F_γ(E1) due to the single particle component of the wave function were fixed. Experimental values of F_γ(E1) are compared with J. Kopecky and M.Uhl systematic.

We investigate the impact of nuclear structure on simulations of various nucleosynthesis processes. Examples are given from the rp-process (rapid proton capture) which is thought to occur in the atmosphere of x-ray bursting binary star systems. The rp-process proceeds by the hot CNO cycle, α-burning, and finally to masses above ⁵⁶Ni all the way to the Sn-Te region of nuclei. The rp-process is characterized by a number of even-even N=Z waiting point nuclei that determine the time-scale, the process path, the energy generation, and the resulting isotopic abundances. We investigate the impact of nuclear masses and nuclear structure near two of these rp-process waiting points at A=68 and 80.

The status of cross section measurements on unstable isotopes in the astrophysically relevant energy range between 0.3 to 300 keV is compared with the information requested from stellar evolution and explosive nucleosynthesis. Recently, this problem has attracted considerable experimental efforts as illustrated by some examples. New and promising perspectives for the near future are provided by the high fluxes available at spallation neutron sources which will allow for measurements on hitherto inaccessible cases. These data are very important for testing the current astrophysical scenarios of neutron capture nucleosynthesis.

Detailed measurements of (p, γ) reactions at energies below E_p = 200 keV are being used to obtain accurate information on astrophysical S-factors. This talk will describe three recent results obtained at TUNL. First, the ratio of the S-factors for the (p, γ0) reaction on ¹²C and ¹³C has been measured at E_p = 160 keV. The significance of this ratio will be discussed and a simple model will be used to interpret the result. Second, a report of our recent measurement of the slope of the S-factor in the ⁶Li(p, γ)⁷ Be reaction will be presented. The negative slope observed between E_p of 80 - to - 120 keV impacts the extrapolation of the S-factor, but remains unexplained. Finally, a recent study of the ¹⁰B(p, γ)¹¹C reaction using polarized protons between 100 and 160 keV revealed sizeable analyzing powers at 90°, indicating the presence of previously unobserved p-wave strength in the ground-state channel. The explanation and implications of this result will be presented.

Peripheral transfer reactions can be used to determine asymptotic normalization coefficients (ANCs). These coefficients, which provide the normalization of the tail of the overlap function for a proton bound to a nucleus, determine S factors for direct capture reactions at astrophysical energies. A variety of proton transfer reactions have been used to measure ANCs using both stable and radioactive beams. The technique of utilizing ANCs to predict astrophysical S factors has been tested for ⁹Be(p, γ)¹⁰B, ¹⁶O(p, γ)¹⁷F and recently ¹³C(p, γ)¹⁴N using ANCs from transfer reaction measurements. Recent applications include measurements of ANCs appropriate for determining stellar reaction rates for ⁷Be(p, γ)⁸B, ¹¹C(p, γ)¹²N and ¹⁴N(p, γ)¹⁵O.

The neutron capture reaction cross sections of light nuclei are quite important for constructing models of the primordial nucleosynthesis and stellar nucleosynthesis. In addition, they are very interesting from nuclear physics view points, since they provide crucial information of the reaction mechanism and nuclear structure. We report the measurement of the neutron capture reaction cross sections of D, ²⁰Ne and ²²Ne in the keV region using a discrete γ-ray detection method.

The present paper aims at understanding r-process nucleosynthesis by addressing the nuclear physics involved, the necessary environment conditions in the (stellar) production sites, and the observational constraints.We also summarize the remaining challenges and uncertainties which need to be overcome for a full understanding of the nature of the r-process.

The ²²Ne(α, n)²⁵Mg reaction is the neutron source during the s process in massive and intermediate mass stars as well as a secondary neutron source during the s process in low mass stars. Therefore, an accurate determination of this rate is important for a better understanding of the origin of nuclides heavier than iron as well as for improving s-process models. Also, because the s process produces seed nuclides for a later p process in massive stars, an accurate value for this rate is important for a better understanding of the p process. Because the lowest observed resonance in direct ²²Ne(α, n)²⁵Mg measurements is considerably above the most important energy range for s-process temperatures, the uncertainty in this rate is dominated by the poorly known properties of states in ²⁶Mg between this resonance and threshold. Neutron measurements can observe these states with much better sensitivity and determine their parameters much more accurately than direct ²²Ne(α, n)²⁵Mg measurements. I have analyzed previously reported ^natMg+n total and ²⁵Mg(n, γ) cross sections to obtain a much improved set of resonance parameters for states in ²⁶Mg in this region, and an improved estimate of the uncertainty in the ²²Ne(α, n)²⁵Mg reaction rate.

We compare r-process model predictions with recent astronomical observations from the solar system, ultra-metal-poor (UMP) halo stars and meteoritic r-process signatures, i.e. containing elemental as well as isotopic abundances. We deduce (1) astrophysical conditions (n_n-ranges for weak and main r-process components) under which such r-patterns can be obtained, and (2) criteria to determine Th/U chronometric ages.

Explosive nuclear burning in astrophysical environments produces unstable nuclei which again can be targets for subsequent reactions. In addition, it involves a large number of stable nuclides which are not fully explored by experiments, yet. Thus, it is necessary to be able to predict reaction cross sections and thermonuclear rates with the aid of theoretical models. Such predictions are also of interest for investigations at radioactive ion beam facilities. An extended library of theoretical cross sections and reaction rates is presented. The problem of α+nucleus potentials is addressed and new parametrizations presented. The problem of properly predicting cross sections at low level densities is illustrated by the ⁶²Ni(n, γ) reaction.

The Q_EC value of ⁸⁰Y has been measured by β-γ coincidence spectroscopy to be ≥8929(83) keV. Combing this result with the adopted mass excess of the daughter ⁸⁰Sr gives a mass excess for ⁸⁰Y of ≥ -61376(83) keV. Results are compared with other measurements, with Audi-Wapstra systematics, and with predictions of mass formulas. Implications for rp-process simulations are considered.

Using a combination of the RILIS (Resonance Ionization Laser Ion Source) and neutron-converter systems at the PS-Booster CERN-ISOLDE to achieve highest possible selectivity of Cd-beam production, we have performed first γ-spectroscopic measurements of the 162-ms isotope ¹³⁰Cd₈₂- This nuclide represents one of the most important classical, neutron-magic r-process “waiting-points”. The most challenging result compared to recent shell-model predictions is the position of the [πg_9/2⊗νg_7/2] 1+ state at 2.12 MeV. This surprisingly high excitation energy requires a rather high Q_β value of about 8.5 MeV in order to obtain a physically consistent picture of ¹³⁰Cd β-decay. Our new data are of particular importance for a better understanding of nuclear structure in the ¹³²Sn region. With the new experimental information, we have improved local shell-model predictions of so far unknown N≃82 nuclei and have tested their implications on the formation of the solar system r-abundance peak at A≃130.

A systematic investigation of (p, γ) cross sections of nuclei from Se to Sb is presented. In-beam cross section measurements were carried out at energies from 1.4 to 5 MeV by using either an array of 4 HPGe detectors of 100% relative efficiency shielded with BGO crystals, or a 4-π Nal summing detector. The astrophysical S factors obtained in the present work are compared with statistical model calculations performed using the code MOST.

The ¹⁸O(n, γ)¹⁹O reaction cross section, which is important for studies on the nucleosyntheses in the non-standard inhomogeneous big-bang model and the s process in cool giant stars, was measured by prompt γ-ray detection methods at thermal and keV energies. In the measurement at thermal energy, ten primary and secondary γ-ray transitions were identified and the total capture cross section was obtained as 156 ± 16 μb, assuming a new ¹⁹O neutron separation energy of 3.964 MeV. In the measurement at keV energies, three primary transitions were observed and the total capture cross sections were obtained as 8.5 ± 1.7 and 11.9 ± 1.8 μb at 42 and 65 keV, respectively.

We have used quantum Monte Carlo methods to calculate microscopically level densities in large model spaces within the framework of the interacting shell model. Calculated densities for A ~ 50 − 70 nuclei are in remarkably good agreement with experimental level densities and an improvement over empirical formulas. Nevertheless, current Monte Carlo calculations are limited to temperatures below ~ 1.5 − 2 MeV. We have recently introduced a framework that accounts for the full single-particle model space (bound states plus continuum) together with interaction effects in the truncated space. We find that the backshifted Bethe formula is valid in a larger temperature range (i.e., up to ~ 4 MeV). A signature of the pairing phase transition is clearly seen in the heat capacity of even-even nuclei.

Determination of accurate nuclear level densities is an important problem. In experiments which detect levels directly, some of the levels are not observed due to experimental limitations. The standard correction for missing levels uses the Porter-Thomas width distribution. A complementary method which utilizes spacing distributions was developed and applied to nuclear resonance data sets.

The effect of isospin symmetry breaking on level statistics is examined with the nuclear shell model. We find that greater symmetry-breaking is needed for these statistical measures than was necessary to reproduce isotopic mass differences. Effects of the broken symmetry on reduced transition probabilities are also examined.

Shell-model energy spectra of Ca, Sc and Ti isotopes are analyzed using standard statistics such as the nearest level spacing distribution and the Dyson-Mehta Δ₃. For all the Ca isotopes, in the ground state region the energy level fluctuations show strong deviations from GOE predictions. When one or two neutrons are replaced by protons, Sc is closer to GOE and Ti is even more chaotic. Thus we find a clear isospin dependence in the degree of nuclear chaoticity. It is also shown that the pairing force is largely responsible for the quasiregular motion in the ground state region of Ca isotopes.

Level density parameters of the back-shifted Fermi gas and of the constant temperature model were determined for 298 nuclei with a least squares fit to low energy levels and neutron resonances. The results are systematized with the valence correlation scheme.

A global formula of nuclear level densities, based on microscopic statistical calculations using the deformed Hartree-Fock-BCS predictions of the ground-state structure properties, is presented. The microscopic model includes a consistent treatment of the shell effects, pairing correlations, deformation effects and collective excitations. It predicts the experimental neutron resonance spacings with a degree of accuracy comparable to that of the phenomenological back-shifted Fermi gas-type formulae. The microscopic level densities are renormalized to the existing experimental data, namely the s-wave neutron resonance spacings and the cumulative number of low-lying levels. Level densities for more than 8000 nuclei are made available in a table format for practical applications.

There is no generally acceptable quantum chaos definition in physics yet, hence we believe that the application of existing approaches to various specific physical systems would help to solve this problem. In our approach to quantum chaos problem we employ the dynamical quantum chaos criterion κ_k = Γ_spr (k)/D₀, introduced by V.Bunakov, where Γ_spr (k) - fragmentation width of the unperturbed quantum state, D₀ - averaged distance between the levels of unperturbed system. This criterion is associated with physical system symmetries via the conservation laws for corresponding quantum numbers. We consider the application of Bunakov's criterion κ_k both to the traditional (Nilson single-particle) nuclear model as well as to the algebraic microscopic strictly restricted dynamics nuclear model (SRDM). In the case of SRDM dynamical criterion κ_k seems to be more sensitive indicator of quantum chaos, in comparision with the statistical one, associated with level spacing distributions.

Simple analytical models for E1 strength function calculations of the γ-decay are investigated. The MLO and GFL models¹⁻⁴ are recommended as the best models for E1 gamma-decay strength function calculations.

Radiative strength functions and level densities have been extracted from primary γ-ray spectra for ^{27, 28}Si, ^{56, 57}Fe, ^{96, 97}Mo, and several rare earth nuclei. An unexpectedly strong (~ 1 mb MeV) resonance at 3 MeV in the radiative strength function has been observed for well-deformed rare earth nuclei. The physical origin of this resonance and its connection to the scissors mode is discussed.

The shape of the spectrum associated with the quasicontinuous (QC) decay of superdeformed rotational bands in even- and odd-mass Pb isotopes is sensitive to the gap in level density at finite temperature and angular momentum at normal deformations. This gap in level density was deduced to be ~0.95 MeV at 6ħ for ¹⁹⁴Pb and ~0.4 MeV at 10ħ for ¹⁹²Pb, while the shape of the QC spectrum for ¹⁹⁵Pb is consistent with no gap in the level density at about 11ħ.

Neutron emission spectra from reactions induced by fast neutrons are of importance in basic physics and applications. Very few data are available in the literature over a wide range of incident neutron energies such as produced with a white neutron source. The FIGARO facility at the WNR/LANSCE neutron source has been established to measure such neutron emission over a range of incident neutron energies from 1 to over 100 MeV. Using the time-of-flight technique twice (once to determine the incident neutron energy, and again to determine the outgoing neutron energy), we are measuring neutron emission spectra for several reactions such as (n, n') and (n, f). Neutron emission from inelastic scattering gives information on the level density of excited states of the target nucleus. Our first measurements are on structural materials such as iron.

A new neutron time-of-flight facility has recently been constructed and become available at CERN. The high instantaneous neutron flux, high resolution and low background make this facility well suited for high quality neutron cross section measurements. The scientific programme of the nTOF collaboration include the measurements of neutron capture, fission and (n, xn) reaction cross sections for nuclear technology, nuclear astrophysics and fundamental nuclear physics. In this paper, the need for neutron induced reaction cross sections for ADS related applications will be mentioned. The nTOF facility will be described and the performed measurements will be illustrated with the measurement of ²³²Th.

The ultra high resolution crystal spectrometers GAMS4/5 are available at both sides of a tangential beam tube at the high flux reactor of the Institut Laue-Langevin. They permit to investigate gamma radiation by double Laue diffraction using perfect Si/Ge crystals. Recent significant improvements of the instrument characteristics and their impact on the scientific program will be discussed

A steady demand for precise nuclear structure information on very exotic nuclei is met by new means of production and identification of these nuclei, which in turn necessitate new spectroscopic techniques and instrumentation. This presentation is focused on the new applications of small arrays of BaF₂ detectors for studies of lifetimes of the excited states in exotic nuclei by the time-delayed multi-coincidence techniques. These arrays, which include from 4 to 8 BaF₂ detectors, were recently coupled with large Ge arrays, recoil separators, or the in-flight separators following the fragmentation reactions. The physics cases range from the neutron-rich to proton-rich (at N=Z) nuclei, and from medium heavy (A~60) to the heavy (A~235) isotopes. A selected physics case of ⁶⁹Ni is discussed in some detail.

Measurements of neutron capture on unstable nuclei are important for studies of s-process nucleosynthesis, nuclear waste transmutation, and other applications. A 160-element, 4π barium fluoride detector array, and associated neutron flight path, is being constructed to make capture measurements at the moderated neutron spallation source at LANSCE. Measurements can be made on as little as 1 mg of sample material over energies from near thermal to near 100 keV. The design of the DANCE array is described and neutron flux measurements from flight-path commissioning are shown. The array is expected to be complete by the end of 2002.

The Miniball array has been developed to meet the new challenges set by the use of post-accelerated exotic-beam facilities. High efficiency is needed as the beam intensities are very low, but high granularity is needed to perform Doppler corrections when working with inverse kinematics. Miniball is the first of a new generation of spectrometer which uses segmentation of the outer electrode of a Ge detector, combined with pulse-shape analysis, to increase this granularity by about two orders of magnitude. Digital electronics have been used to perform on-line pulse-shape analysis. Miniball has been operational at the REX-ISOLDE facility¹ at CERN since Easter.

We describe a method for determining cross sections of interest in neutron dosimetry. To illustrate the approach, which uses both experiment and theory, we describe our results for ²³⁹Pu(n, 2n)²³⁸Pu and ¹⁹³Ir(n, n′)^193mIr.

A new library comprising 30 thousand neutron capture gamma rays has been created by combining new measurements on natural elements from Budapest and literature data for all stable isotope targets. All energies and intensities are consistent in that they are based on the chlorine and nitrogen standards, respectively. Accurate neutron binding energies and thermal capture cross-sections could also be inferred for all cases where the level scheme is sufficiently complete. The new data can be used for nuclear structure investigations, reaction model calculations, and a number of applications, such as Prompt Gamma-ray Activation Analysis (PGAA).

Large-scale calculations of radiative neutron capture have been performed with the EMPIRE-II code for all stable nuclei, starting with ²⁷Al and using nine combinations of optical model parameters and level densities. Comparison with experimental data data shows that calculations with EMPIRE-specific level densities and optical model potential derived by Koning and Delaroche describe 70% of existing experimental results adequately. These approaches retain their validity up to about 200 MeV, providing for a consistent treatment of the radiative capture and more complex nuclear reactions.

Relative intensities were obtained for strong γ-rays from neutron capture on ³⁵Cl, ⁴⁸Ti and ^{50, 52, 53}Cr using the ¹⁴N(n, γ) capture γ-rays as efficiency standard. The new values are accurate to one percent or better, and can be used for detector efficiency calibrations up to 9.7 MeV energy. Using the same techniques, γ-ray intensities were also obtained for the high-energy emitter radionuclides ²²⁶Ra, ⁵⁶Co and ⁶⁶Ga applicable up to 4.8 MeV.

Nuclear reaction analysis with capture resonances has many applications in thin film technology. The present talk describes several applications of the ¹⁵N + ¹H 429 keV resonance (or its reverse reaction) for depth-profiling nitrogen (or hydrogen) after xenon ion beam irradiations of Ni₃N/Si bilayers and after laser irradiations of iron, titanium and silicon in the presence of nitrogen or hydrogen.

The the 4-meter Cauchois crystal diffraction γ-spectrometer with a very high energy resolution (PNPI, Gatchina) have been tested for in-pile control of fission nuclides in nuclear waste products transmutation. It is known that more than 90% of the waste activity after 10 years storage provides with two long-lived ⁹⁰Sr and ¹³⁷Cs nuclides. Their transmutation (burning up in a high neutron flux) produces the ⁹¹Sr and ¹³⁸Cs nuclides. Strong γ-transitions of ⁹¹Sr and ¹³⁸Cs have been measured with our crystal diffraction γ-spectrometer in a reactor core spectrum directly.

We have performed neutron irradiations at the High Flux Reactor of the Institut Laue-Langevin (Grenoble) on ²⁴³Am and ²⁴²Pu targets. Using γ and α spectroscopy of irradiated samples, we have measured Maxwell averaged neutron capture cross sections, respectively (74.8 ± 3.3) b and (22.7 ± 1.1) b, in a 98% thermal neutron spectrum at T ≃ 50° C.

The intense fluxes of epithermal neutrons, available at the ISIS facility, make possible Deep Inelastic Neutron Scattering (DINS) measurements in the 1-100 eV energy region. At present these are performed with the VESUVIO spectrometer, a Resonance Filter Spectrometer recently installed at ISIS. The physical quantities derived are the atom momentum distribution and the mean kinetic energy. The scattered neutrons are detected by ⁶Li-glass scintillation detectors, which however suffer of heavy efficiency loss at neutron energies above 30 eV. With the aim of extending the kinematic region (ħω) accessible in DINS measurements, the Resonance Detector Spectrometer (RDS) configuration has been recently revised, and encouraging results have been obtained both on eVS and VESUVIO spectrometers in DINS experiments up to final neutron energies of about 70 eV

The features of the upgraded Budapest neutron capture gamma-ray facilities are presented. The performance of the facilities is demonstrated by recent experiments on radioactive ⁹⁹Tc samples.

It has long been the goal of nuclear physics to understand and characterize the strong forces among baryons which make up the nuclear medium. This endeavor is best carried out in nuclei containing a limited number of interacting particles, in which the complications of nuclear structure can be stripped away. Hypernuclei are the best examples of such objects. High-resolution γ-ray spectroscopy, which was pioneered with neutron capture years ago, is now feasible with large germanium detector arrays(e.g. the “Hyperball”). The Hyperball is yielding important new information relevant to the YN interaction and its connection to QCD. This review summarizes the recent progress and the state of this field, with a assessment of near-term future capabilities.

The Doppler broadening of photons emitted during the n-p capture process in a nuclear reactor is considered in view of a new finding concerning the zero-point motion of the H-atom in the sample. The kinetic energy of the H-atoms of a Kapton sample (used in a recent determination of the deuteron binding energy¹) was measured directly at 296 K. It was found to correspond to an effective temperature of 991 K. This means that the H-atom contribution to the broadening of the n-p capture γ line is more important than that of the thermal neutrons. A simple expression for calculating the Doppler broadening of the emitted photons in terms of the kinetic energies of the neutron and the proton is deduced. The expression is applicable to any capturing nucleus and not just to the proton.

Time reversal invariance violation beyond the Standard model mechanism can be tested in free neutron beta decay. The triple correlation between neutron spin and the momenta of electron and antineutrino (D coefficient) tests T violating phases in non-standard couplings. We present a new preliminary limit for this correlation which was obtained by the Trine experiment: ^Dpreliminary = (−3.1 ± 6.2^stat ± 4.7^syst ± 4.7^syststat).10⁻⁴.

Hypernuclei are a convenient laboratory to study the baryon-baryon weak interaction and associated effective weak Hamiltonian. We will show how the nuclear structure aspects of the problem, often an unwelcomed detail of calculations attempting to understand basic two-body ΛN → NN interaction, can be used to pick out components of the effective weak Hamiltonian. It is well known that removing one nucleon from ⁹Be or ⁹B results in ⁸Be* with a subsequent αα-decay. Through this unique process it would be possible to identify final states of the residual nucleus. So, due to these specific properties of the core nuclei ⁹Be and ⁹B, it may be possible to measure the partial decay widths for the and hypernuclei.

The experiment under construction at LANSCE studies the weak interaction between neutrons and protons. The experiment will measure the directional dependence of the parity-violating γ-ray asymmetry, A_γ, in the polarized cold neutron capture by para-hydrogen. The goal is to measure A_γ with uncertainty of 0.5 × 10⁻⁸, 10% of its predicted value. A_γ primarily isolates the ΔI = 1 component of the hadronic weak interaction, and thus will determine the long-range weak pion-nucleon coupling constant . The experiment is carefully designed for the LANSCE pulsed spallation neutron source to achieve the proposed statistical precision and to control systematic errors. We discuss the experiment and its status.

A careful analysis of the available experimental data and theoretical calculations enabled us to reproduce the observed solar abundance of ¹⁸⁰Ta^m in the classical s-process. The observed intermediate state below 1.1 MeV that enables photon coupling between the isomer and the ground state has been taken into account.

Tissue samples (skin of mice, normal and tumor, skin of a woman, normal and tumor) were irradiated by protons from the Munich tandem accelerator. The samples were analysed using Raman spectroscopy at the Institute of Chemical Technology in Prague by measuring the intensity of signals sensitive to radiation damage. Effects depending on the delivered dose were found. Proton-irradiation effects are then compared to those of gamma-irradiation.

The main reasons limiting the bent crystal γ-spectrometer resolution are discussed. New solutions have been proposed and realized during modernization. Angular resolution as small as 0.34 arcsec has been achieved with this spectrometer in the γ-ray energy interval 95 ≤ Eγ ≤ 250 keV. Minimum line width of 0.19 arcsec has been achieved. The most general value conclusion, directly following from this work, is that the angle width of the mosaic block distribution in the quartz crystal (parameter describing the limiting angular resolution of crystal diffraction instruments on its basis) can take value ≤ 0.1 arcsec. The laser-reading device (designed in PNPI) on the basis of high-frequency holographic diffraction gratings is used for the first time. It yields an extremely small optical path (≤ 0.01 cm) and high angular sensitive (last digit in the reading corresponds to 0.002 arcsec).

Classification calculations are performed for nuclear ground states and low-lying excited levels, using unitary scheme basis in a case of ground SU₃ configuration. The quantum numbers, which are necessary for further investigations of these nuclear states within Strictly Restricted Dynamics Model (SRDM), have been obtained for all experimentally observed¹ nuclei (Z, A), corresponding to the ground SU₃ configuration. In 4 ≤ A ≤ 80 region these data have been evaluated using the complete classification results obtained by plethysm technique² for SU₃ ground configurations, but for A > 80 nuclei a method, proposed in ³ for the calculations of nuclear ground state classification characteristics has been employed.

Employing the ¹⁹³Ir(n, γ), (n, e⁻)¹⁹⁴Ir low-energy spectra above 500 keV, using recent ¹⁹⁶Pt(d, α)¹⁹⁴Ir data and taking into account all results known earlier, the ¹⁹⁴Ir level scheme has been developed to higher energies and higher spins. New levels have been established. Several more reliable Iπ values have been determined. Positive parity levels 270.918 keV 3⁺ and 519.518 keV 4⁺, probably, can be successfully interpreted only if a triaxial structure is assumed.

The level scheme of ¹⁶⁴Dy up to about 2.6 MeV energy is analyzed, using experimental data obtained in (n, γ), (n, e⁻) and (n, nγγ) reaction measurements at the high-flux reactor ILL Grenoble, as well as the (n, n'γ) reaction data, measured at the IRT reactor in Salaspils. The structure of low-lying levels of ¹⁶⁴Dy is interpreted in terms of two-quasiparticle states, interacting with collective excitations of axially-deformed core.

Several aspects related to the quality of the radiative capture measurements at the new n_TOF facility are reviewed following the initial measurements.

The behaviour of effective nucleon-nucleon (NN) interaction potential V_NN parameters in the framework of Strictly Restricted Dynamics Model (SRDM)¹ has been studied in the wide range of nuclear mass number A values. The easiest way to obtain such information is to employ a simplified version of this model (SSRDM)², when one can perform calculations without the use of collective U_A−1 invariant density matrices. Corresponding parameters of the effective quadrupole NN - interaction V_NN(r⁴) were evaluated for all experimentally observed α-cluster type nuclei in the mass number A region 4 ≤ A ≤ 100 by the fit of experimental ground state and excited states energies in the unitary scheme ground state SU₃ configuration bases. The Coulomb displacement energy calculations in the framework of SRDM are done for odd-even, even-odd mirror-type nuclei with mass number 41 ≤ A ≤ 91.

Project of the experiment on a search for a neutron electric dipole moment (EDM) using Laue diffraction in a noncentrosymmetric crystal is proposed. The method is based on the depolarization effect due to the interaction of the diffracted neutron with a strong (up to 10⁹ V/cm) interplanar electric field of a noncentrosymmetric crystal and on the possibility to increase considerably the time τ of neutron passage through the crystal (and so the time of the neutron interaction with the electric field), using Bragg angles close to π/2. The pilot setup was recently created and mounted at the WWR-M reactor in Gatchina to observe these phenomena. The first experimental results have shown that the sensitivity of the method can be better than 10⁻²⁵ e·cm per a day for really existing quartz crystal.

No abstract received.

To investigate the evolution of collectivity in a transition from nuclei with a collective behavior to nearly closed shell nuclei, the low-spin level scheme of ⁹²Zr has been investigated with the (n, n′γ) reaction. Special emphasis was placed on the investigation of collective excitations that are not fully symmetric with respect to the proton-neutron degree of freedom, the so-called mixed-symmetry (MS) states. From measured lifetimes, branching ratios, and E2/M1 mixing ratios, absolute transition strengths were determined permitting the interpretation of several low-spin states.

In order to investigate response of the high-resolution high-energy photon spectrometer (HHS) to high-energy gamma rays and obtain reliable response functions of the detector, the response to a monochromatic 10,763-keV gamma ray, produced through ²⁷Al(p,γ)²⁸Si reaction at Ep = 992 keV resonance, was measured using the HHS. The observed response of the HHS to the gamma ray shows supreme resolution and S/N ratio for the gamma ray.

No abstract received.

The well deformed (~ ∼ 0.3) nucleus ¹⁵⁹Gd was investigated by means of radiative neutron capture and single neutron transfer reactions. The (n, γ) reaction was studied with thermal and resonance neutrons. Secondary γ-rays were investigated with the GAMS2/3 bent-crystal spectrometers at the Institute Laue-Langevin Grenoble. Primary γ-rays were studied with 2 and 24 keV neutrons using the Average Resonance Capture technique and a pair spectrometer at Brookhaven. The single neutron stripping and pick up reactions (d,p) and were investigated at the Tandem van de Graaff accelerator of the University and Technical University Munich with unpolarized 18 MeV and polarized 22 MeV deuteron beams, respectively. All experiments resulted in the combined observation of about 300 levels up to 3 MeV. The decay scheme for this nucleus has been completed and expanded up to 1.2 MeV. Evidence of vibrational phonons coupled to single particle states is reported for the first time in this nucleus. The level density was derived and compared with the Fermi gas constant temperature model formula. A preliminary model calculation based on the quasi-particle-phonon model has been undertaken in an attempt to describe the level scheme and structure of this nucleus.

The capture to the fission cross-sections ratio (alpha value) was investigated by the measurements of gamma-rays time-of-flight spectra from the first to fifteenth multiplicities on the 500 m flight path of the IBR-30 reactor. One has used a 16-sections scintillation detector with NaI(Tl) crystals and two metal samples of 0.00137 and 0.00411 at/b thicknesses (the content: ²³⁵U :²³⁸ U = 90% : 10%.) The multiplicity spectra of coincidence and the alpha values α = σ_γ/σ_f were obtained for 212 resolved resonances in the neutron energy region E=19−1260 eV and for energy groups over the range 20-2000 eV. The experimental alpha values were compared with the similar values calculated on a basis of evaluated data from different libraries using the model of neutron cross sections description in multilevel R-matrix approximation.

As a continuation of a systematic study of reactions relevant to astrophysical p– process, the (p, γ) or (p,n) cross sections of three Se isotopes have been measured using an activation technique in the proton energy range between 1.3 and 3.6 MeV. The resulting cross sections of the ^{74, 76}Se(p,γ)^{75, 77}Br and ⁸²Se(p,n)⁸²Br reaction have been compared with the predictions of Hauser-Feshbach statistical model calculations using the NON-SMOKER and MOST codes. In all three cases the theory is in satisfactory agreement with the measurements.

The nuclear structure of ¹²⁷Te has been investigated with the ¹²⁶Te(n, γγ)¹²⁷Te reaction using thermal neutrons and with the reaction at E_d = 20MeV. More than 150 levels were identified already below 3.6 MeV excitation energy, in most cases including spin, parity and γ-decay. The γ-decay scheme after neutron capture is essentially complete containing about 100% of the population of the 11/2⁻ isomer and of the ground state. The thermal neutron capture cross section and isomer production of the 11/2- state at 88.3 keV were determined to be 0.38(4)b and 0.063(5)b, respectively. The experimental level scheme is compared with predictions of the Interacting Boson-Fermion Model (IBFM) and of the Quasiparticle Phonon Model (QPM).

Within the frameworks of plutonium recycling, of transmutation, of waste management and of the increase of fuel loading length, controlling and improving the accuracy on ²³⁸U conversion ratio becomes necessary. Indeed, ²³⁸U neutron capture generates transuranian elements as for instance the isotopes of plutonium, americium or curium. This study focuses on a new technique for measuring by gamma-spectrometry, the modified conversion ratio of ²³⁸U defined as the ratio of the capture rate of ²³⁸U to the total fission rate, in an irradiated fuel pin of a critical assembly.

In Accelerator Driven Systems the external neutron flux provided by the accelerator is produced by spallation reactions, which means that its spectrum extends to several hundreds of MeV. Thus (n, xn) reactions have more importance than in conventional reactors since they have a threshold increasing with x. On the other hand, the cross sections of these reactions are badly known. This is due only in part to the fact that few neutron beams with energies above 20 MeV were available up to now. Actually, no universal method exists so that only some systems could be measured, and data bases are often fed by model predictions, especially in the heavy nuclei. With a white beam like Gelina or nTOF, there is only one method which can be used, the in-beam γ-spectroscopy. This method was reported for the first time in 1994 by Los Alamos. We tested the feasibility of this method at the fixed beam facility of the cyclotron of Louvain-la Neuve (Belgium) and the Gelina white beam neutron source. The methodological tests performed so far towards these measurements are presented.

Quadrupole softness in the even-even N = Z nucleus ⁶⁴Ge is studied on the spherical shell model basis. We carry out the shell model calculation using the pairing plus quadrupole (QQ) plus octupole (OO) interaction with monopole corrections. The calculated results are in good agreement with the experimental data. It is shown that proton-neutron part Q_pQ_n of the QQ interaction is important for the γ-softness or triaxiality.

The new, model-independent method to estimate simultaneously the level densities excited in the (n,γ) reaction and the radiative strength functions of dipole primary transitions is developed. The method provides a sufficiently narrow intervals for the sum k(E1) + k(M1) of most probable radiative strength functions for primary dipole transitions and densities of levels populating by them.

The intensities of the two-step γ-cascades between the compound state and certain low-lying levels in a large group of nuclei from the mass region 28 ≤ A ≤ 200 have been analysed. This analysis gives the probable level density excited after thermal neutron capture and the sum of the radiative strength functions for E1 and M1 primary transitions in the excitation energy range almost up to B_n. Results of comparison of these data with the predictions of the early variant of the generalized model of the superfluid nucleus and a non-interacting Fermi-gas model lead to necessity of more correct accounting for superfluid properties of a nucleus in corresponding models.

Experimental investigation of the simple low-lying levels transformation into the Bohr's compound states process was performed in Dubna Riga and Řež in details for more than 50 nuclei from the mass region 28 ≤ A ≤ 200.

The study of the two-step γ-cascades proceeding between compound state and given low-lying levels allows one to obtain data on such nuclear parameters as, for example, level density and radiative strength functions. Such experiment in fissile compound nuclei ²³⁶U and ²⁴⁰Pu could be also realized at any existing neutron source.

Comparison between the experimental and calculated with the parameters [1] spectra demonstrates more or less improvement of their correspondence (up to practical coincidence) for the ¹¹⁴Cd, ¹⁵⁸Gd, ¹⁶⁶Ho, ¹⁶⁸Er, ¹⁸²Ta, ¹⁹⁶Pt, ¹⁹⁸Au, and ²⁰⁰Hg compound nuclei and some exceeding of the calculated intensity for ¹⁵⁰Sm, ¹⁵⁶Gd, ¹⁶⁰Tb, ¹⁶⁴Dy, ¹⁷⁶Lu, and ¹⁹²Ir at the γ-transition energy E_γ > 3 − 4 MeV.

The neutron capture cross sections of Dy isotopes (¹⁶¹Dy, ¹⁶²Dy, ¹⁶³Dy, and ¹⁶⁴Dy) have been measured in the neutron energy range from 10 to 90 keV using the 3-MV Pelletron accelerator of the Research Laboratory for Nuclear Reactors at the Tokyo Institute of Technology. Pulsed keV neutrons were produced from the ⁷Li(p, n)⁷Be reaction by bombarding the lithium target with the 1.5-ns bunched proton beam from the Pelletron accelerator. The incident neutron spectrum on a capture sample was measured by means of a TOF method with a ⁶Li-glass detector. Capture γ-rays were detected with a large anti-Compton NaI(Tl) spectrometer, employing a TOF method. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to derive capture yields. The capture cross sections were obtained by using the standard capture cross sections of ¹⁹⁷Au. The present results were compared with the previous measurements and the evaluated values of ENDF/B-VI.

Spectra of two-step γ cascades, following the thermal neutron capture in ¹⁶²Dy, have been measured. Conclusions regarding the role of the M1 scissors resonances built on the levels in the quasicontinuum of ¹⁶³Dy are made.

Prompt gamma-ray activation analysis (PGAA) is a non-destructive nuclear analytical technique used for identification and quantification of trace and other elements contained in an examined sample. PGAA consists in detection of prompt gamma rays emitted by a sample during irradiation by thermal or cold neutrons. These samples can be in solid, aqueous or gaseous form. In particular, PGAA is suitable for detection and measurement of boron concentration. The precision of measurement reaches ppm or even ppb levels depending on the sample matrix. Moreover, the risk of contamination by boron is extremely low compared to other analytical methods. Trace analysis of boron in liquid was performed and characterized at the PGA facility of the neutron spallation source SINQ (Paul Scherrer Institute, Switzerland). First, a calibration curve was determined with standard boron solutions. According to this curve, amount of boron in geological water samples was determined and compared with results of chemical method ICP-MS. Finally, application of boron solutions in medical treatment methods is discussed.

A new experimental program concerning the measurement of the neutron capture branching ratio of ²⁰⁹Bi as a function of neutron energy has been proposed recently. The preliminary results obtained at the high neutron flux reactor of ILL with a thermal neutron flux are presented in this paper. The neutron capture cross section and the corresponding branching ratio are measured with an on-line gamma-ray spectroscopy method. We find for the capture cross section 35±1.75 mb, what is in a good agreement with existing results. For the partial cross sections we get σ_210gs = 17.9±2 mb and σ_210m =17.1±2 mb giving a branching ratio of 51%±5%. This value is by 25% smaller than values from evaluated libraries.

The response of the RPI 16-segment NaI multiplicity detector system for the neutron radiative capture reactions ¹⁴⁹Sm(n, γ)¹⁵⁰Sm and ¹⁵⁰Sm(n, γ)¹⁵¹Sm was calculated. An algorithm which combined the Monte-Carlo based γ cascade code DICEBOX, based on the extreme statistical model, with the general MCNP(4C) monte carlo particle transport computer program was developed. Two processes, the individual events of the compound nucleus γ cascade de-excitation resulting from the neutron capture and the subsequent γ ray transport through the detector, were modeled. The cascade γ energy spectrum and γ multiplicity were derived from the DICEBOX results. These data were processed with MCNP(4C) to predict the detector γ multiplicity spectrum and efficiency. Comparison with measured data is presented.

A flash ADC with 100 MHz sampling frequency and 12 bit resolution is used to digitize the signals of several types of detectors. Mathematical algorithms are performed on the digitized signals to obtain energy, timing, and pulse shape information. The detectors involved in the study are HPGe (in-beam prompt gamma spectroscopy for (n,xn) reactions), NaI (possibility of high counting rates) and NE213 (neutron-gamma discrimination). Perspectives are explored for creation of pulse processing systems with performances significantly improved as compared to classical analogue chains.

Nuclear analytical methods are very important in materials research. We provide in Nuclear Physics Institute (NPI) several nuclear analytical methods, i.e. Rutherford Backscattering Spectrometry (RBS), Elastic Recoil Detection Analysis (ERDA), and Neutron Depth Profiling (NDP). The methods mentioned above were applied in the study of nonlinear optical materials, such as LiNbO₃, to explain the mechanism of ion exchange in the near-surface layers of the substrates and to determine the profile of laser active Er ions. Depth profiles of Li in LiNbO₃ were measured by NDP. We are able to evaluate the hydrogen depth profiles from ERDA measurement and Er depth profile from RBS measurement. We present the results of the complex study of LiNbO₃ treated by ion exchange and low temperature diffusion of laser active ions by NDP, RBS and ERDA.

Collective level structures of 232-U, 234-U, 238-U and 232-Th are described within soft rotator model. Even and odd parity collective band levels for excitations up to 2 MeV are identified. Direct excitation cross sections for quadrupole vibration as well as octupole vibration band levels were calculated within coupled channels approach. Simultaneous analysis of collective levels structure and differential scattering cross sections allows to define quadrupole/octupole nature of vibrational bands and relevant deformation parameters.

Neutron capture cross section data of 238-U(n,g) and 232-Th(n,g) are reproduced in a statistical model of compound nucleus reactions from a few keV up to 6 MeV. Calculated capture cross section in keV energy region is dependent on radiative strength function value S. Above 150 keV calculated cross section is defined by the energy dependence of radiative strength function and inelastic scattering/fission competition. Energy dependence of S is defined mainly by the level density of compound nucleus at excitation energies below three-quasi-particle excitation threshold. For incident neutron energies higher than 1 MeV, the (n,n'g) reaction competition to the “true” capture (n,gg) reaction cross section defines the capture cross section drop above 1 MeV.

The neutron capture cross sections of ¹²⁹I were measured in the energy region from 15 to 90 keV, using a 1.5-ns pulsed neutron beam and a Time-Of-Flight method. The capture γ rays were detected with a large anti-Compton NaI(Tl) spectrometer, and the capture yields were obtained by a pulse-height weighting technique. Using standard capture cross sections of ¹⁹⁷Au, the capture cross sections of ¹²⁹I were derived with the errors of about 7 %.

We have calculated potential-capture cross sections of ¹²C and ⁹Be for low-energy neutrons, using the valence-capture model in the reactance matrix representation. The calculations are in satisfactory agreement with observed non-resonance capture cross sections of those nuclei.

We provide evidence for transitions from a deformed equilibrium shape to a spherical equilibrium shape in the interacting boson model at finite temperature. The critical temperature strongly depends on the model used in estimating the effective boson number as a function of T.

We apply a point–symmetry based Quadrupole-Octupole Rotation Model (QORM) to study the collective motion of nuclei with simultaneous presence of octupole and quadrupole deformations. We demonstrate that it describes successfully the energy levels of alternating parity bands and reproduces their odd–even staggering structure. On this basis we are capable to determine quite accurately the regions of reflection asymmetry correlations in nuclear collective spectra.

We propose a new shell model method, combining the Lanczos diagonalization and extrapolation method. Linear and quasi-linear extrapolations can give an accurate shell model energy from a series of shell model calculations with various truncation spaces. Its feasibility is demonstrated by taking large-scale fp shell calculations.

A simple description for obtaining the parity distribution of nuclear levels in the pf + g_9/2 shell as a function of excitation energy was recently derived. We implement this in a global nuclear level density model. In the framework of the statistical model, cross sections and astrophysical reaction rates are calculated in the Fe region and compared to rates obtained with the common assumption of an equal distribution of parities. We find considerable differences, especially for reactions involving particles in the exit channel.

We have developed an exact isospin projection method in the framework of the shell model Monte Carlo approach that enables us to take into account the proper isospin dependence of the Hamiltonian. We apply the method to calculate level densities of N ~ Z nuclei, and demonstrate in the case of ⁵⁸Cu that the isospin projection can lead to significant corrections in N ~ Z nuclei.

The response function approach is proposed to include vibrational state in calculation of level density. The calculations show rather strong dependence of level density on the relaxation times of collective state damping.

The semiclassical method for description of the radiative strength function is used for asymmetric nuclei with N ≠ Z. The theory is based on the linearized Vlasov-Landau equations in two-component finite Fermi liquid. The dependence of the shape E1 strength on the coupling constant between proton and neutron subsystems was studied.