Table of contents

Neutron capture gamma ray spectroscopy has been a very powerful method for the study of nuclear structure. In particular the nucleus ¹⁶⁸Er has been studied very precisely and almost complete information has been provided. The author takes ¹⁶⁸Er to investigate how far the interacting boson model can go in explaining the structure of ¹⁶⁸Er. The necessity of using hexadecapole (g) bosons is pointed out. The author discusses briefly the results of the spdf-boson model obtained by Otsuka and Sugita (1988). Their results are very promising for explaining the structure of actinide nuclei.

The concept of a dynamical symmetry is re-examined. Starting from elementary considerations, it is shown that dynamical symmetries also occur in transitional Hamiltonians of the interacting boson model. Examples are given for the IBM-1 and IBM-2. Next, the breaking of dynamical symmetries is discussed. One specific case, namely the breaking of F spin, is of importance in the study of magnetic dipole properties, and is analyzed in detail with reference to observed E2/M1 mixing ratios in deformed rare-earth nuclei.

An overview of the status of development of some microscopic nuclear models is presented. Special attention is paid to the calculations starting from the effective nucleon-nucleon force, to the angular momentum projection method before variation, to the multiphonon method and to the selfconsistent coordinate method. The success and the limitations of the three last mentioned models are illustrated in the example of ¹⁶⁸Er.

The results of calculations in the QPNM and the IBM are compared in order to elucidate the structure of excited states of deformed nuclei. It is shown that there are essential differences in describing some states in the QPNM and sdg IBM. For a common description of collective states with positive and negative parity, one should develop a version of the IBM with spdfg bosons. A better understanding of excited states of deformed nuclei necessitates experimental investigation of the structure of K_n^pi =0₂^-, 0₃^+or-, 0₄^+or-, 1₂^+or-, 1₃^+or-, 2₂^+or-, 2₃^+or-, 2₄^+or-, 3₂^+or-, 3₃^+or-, 4₁⁺ and 4₂⁺ states. Critical comments on the description of giant resonances in the IBM are given. Possible experimental verification of the formation of a giant resonance by one boson is discussed.

A general microscopic approach of low-energy coupling of individual and collective degrees of freedom is presented. The ingredients of a Bohr-Mottelson unified model description are determined consistently from the Skyrme SIII effective interaction, through the adiabatic limit of the time-dependent Hartree-Fock-Bogoliubov approximation. Three specific aspects are developed: (i) the effect of pairing correlations on adiabatic mass parameters and collective dynamics; (ii) a consistent coupling of collective and individual degrees of freedom to describe odd nuclei and (iii) a study of spectroscopic data in odd-odd nuclei as a test of effective nucleon-nucleon interactions.

A microscopic fermion model of nuclear collective motion is presented. The model has a SU₃ rotational limit, a SO₆ gamma -soft limit and three vibrational limits (SU₂, SO₇ and SO₅*SU₂), with fully microscopic connections between these dynamical symmetries and the underlying shell structure. It is shown that both the phenomenological interacting boson model (IBM) and the geometrical model (GM) can be recovered from this model under certain approximations.

The Glauber approximation for medium energy scattering of hadronic projectiles from nuclei is combined with the interacting boson model to produce a transition matrix for elastic and inelastic scattering in algebraic form which includes coupling to all intermediate states. Examples of application of this formalism to proton scattering from ¹⁵⁴Gd are presented.

The author reviews several aspects, and empirical consequences, of the p-n interaction as it relates to the structure, phase transitions and collectivity of medium and heavy nuclei. The N_pN_n scheme is reviewed as background material while the emphasis centers on a discussion of a related quantity, the P factor, and on the relationship of the integrated strength of the p-n interaction to the empirically observed 'saturation' of B(E2) values in the mid-shell regions of deformed nuclei.

The level schemes of the Ag isotopes are discussed in the light of investigations of the odd-odd isotopes ¹⁰⁸Ag and ¹¹⁰Ag. Information about the valence proton (hole) properties is deduced from the odd-A Ag isotopes, whereas the Pd isotopes are used to study the characteristics of the core and the valence neutrons. The predictions of various models are discussed.

Subnanosecond lifetimes have been measured in deformed and transitional even-even nuclei. Strong evidence is reported for spin-related octupole vibrational admixtures in 2QP band members in ¹⁶⁸Er. The 0⁺ level in ¹⁷²Yb is a candidate for a mixed symmetry state. Forbidden transitions in the 0(6) nuclei ¹⁹⁶Pt and ¹²⁴Te are considered.

Multiple neutron-capture measurements on ¹⁹³Ir have populated numerous low-spin positive-parity states in ¹⁹⁵Ir. These states form a complete set of 1/2, 3/2, 5/2 states with sigma =13/2, 11/2 and tau =1/2, 3/2, 5/2 expected in a U(6/4) contains/implies Spin(6) boson-fermion symmetry.

Progress made in proton-capture gamma -ray studies is discussed under four headings. (a) Precision; the recent evaluation of the fundamental constants has direct implications for capture gamma -ray work. (b) Completeness; modern detection techniques applied to (p, gamma ) reactions lead to a completeness of information which is essential in the comparison of experiment and theory, and thus crucial for the progress of nuclear physics. (c) Spin-assignments; the large amount of precise information may be used to assign spins in non-traditional ways. (d) Applications; new techniques, e.g. for the production of variable-energy gamma -rays, enlarge the scope of the capture gamma -ray field. These lines of progress are all four made possible by instrumental developments.

M1 transitions between low lying collective levels in deformed nuclei are described within the IBM-2 framework. This is done by a special choice of the Hamiltonian and transition operators which allow a simultaneous fit of energies, E2 and M1 transitions. Finally, the results are interpreted using the F-spin concept.

The ¹⁹⁷Au(d,p)¹⁹⁸Au reaction was measured with the high resolution Q3D-spectrograph at the Munich tandem accelerator at Theta _lab=35 degrees , using 20 MeV deuterons. An average resolution of 3 keV was achieved up to 1600 keV excitation energy. The level scheme of ¹⁹⁸Au could thus be extended and compared with new model predictions. The ¹⁶⁷Er(d,p)¹⁶⁸Er reaction was measured at Theta _lab=30 degrees with an average resolution of 3.5 keV using 15 MeV and 22 MeV deuterons. States between 0 and 2600 keV were populated and new levels were observed.

Neutron capture gamma -ray spectroscopy now employs the most sophisticated techniques to study nuclear structure, yet efforts are continuing to improve still further sensitivity and resolving power of the spectrometers used. The combination of precise measurements of primary gamma -transitions with the precision obtained to date with curved and flat crystal and conversion electron spectrometers has considerably extended the possibilities in the study of nuclear structure. Techniques are also available for the investigation of multiple neutron capture leading to nuclei which are up to three neutrons beyond stability. The nearly perfect resolution obtained with the most modern double flat crystal spectrometers allow the measurement of lifetimes of excited nuclear states in the range <10^-12 s via the observation of the Doppler-broadening of gamma -rays due to the recoil of the nucleus following emission of a primary gamma -ray.

The (n, gamma )-facilities at the HFR of the ILL Grenoble have been used intensively for several years by a large group of experimentalists. The bent crystal spectrometers GAMS1 and GAMS2/3 serve to investigate secondary gamma -ray transitions after thermal neutron capture. The double flat crystal spectrometer GAMS4 is used for high accuracy, absolute wavelength measurements and for direct measurements of Doppler broadened capture lines. Furthermore a pair spectrometer using a three crystal arrangement, and conventional HPGe-detectors, serve to investigate the high energy part of the gamma -ray spectrum after thermal neutron capture. The HFR was shut down for general renovation and improvement from October 1984 until September 1985. During this time the tangential beam tube for the bent crystal spectrometers had to be replaced. This shut down was used to improve the in-pile target facility and the instruments and the authors report on these developments and give some results.

A substantially improved double flat crystal gamma ray spectrometer (GAMS4) has been put into operation as a joint NBS/ILL facility at the HFR, Grenoble. This facility has the capability of determining gamma ray wavelengths, in absolute units, with uncertainties on the order of 0.1 ppm in the energy range from 0.1 to approximately=5 MeV. This new facility incorporates a number of major improvements over the earlier double flat crystal spectrometer used to determine the n-p 2.2 MeV line with an error of 1 ppm (2 eV). The operation of the instrument and the nature of the improvements is discussed. Experimental results related to the direct determination of the binding energy of several light nuclei (for example D, ¹³C, ¹⁵N) are given. The implications of this work for the atomic mass scale and for the determination of fundamental constants is reviewed.

Statistical and non-statistical aspects of nuclear reactions and levels are discussed and related to the onset of chaos in nuclei.

Level densities and primary gamma -ray transition strengths in odd-odd nuclei with A=20-80, obtained from thermal neutron capture reactions, have been analyzed with statistical models. The authors extend these calculations to the flux pattern of primary and secondary transitions, e.g. to the shapes of the spectra and the time development of the cascades.

Nuclear level schemes and neutron resonance spacings yield information on level densities and level spacing distributions. A total of 75 nuclear level schemes with 1761 levels and known spins and parities was investigated. The A-dependence of level density parameters is discussed. The spacing distributions of levels near the ground state indicate transitional character between regular and chaotic properties while chaos dominates near the neutron binding energy.

Elastic photon scattering experiments are discussed which have been used to investigate the average distribution of E1 and M1 strengths below particle threshold at energy regions of high nuclear level density. An experiment with quasimonochromatic tagged photons has confirmed the smooth energy dependence of the average radiative strength in ²³⁸U below threshold. The inferred total photoabsorption cross sections are in agreement with the extrapolated tail of the giant dipole resonance. At energies below 5.3 MeV, however, local concentrations of E1 strength have been observed which probably can be attributed to strong isolated resonances seen in previous NRF experiments using neutron capture gamma rays. Large amounts of strongly fragmented M1 strength have been found in ⁹⁰Zr, ¹⁴⁰Ce and ²⁰⁶Pb using linearly polarized tagged photons. The total M1 transition strength is in agreement with recent microscopic calculations and can account for the giant M1 resonance in these nuclei.

The authors report the progress made in understanding the concept of nucleon effective charge and its application in interpreting nuclear E1 and E2 transitions. Effective charge values often display greatly varying characteristics in different situations, e.g. small effective charge for E1 transitions between bound states, -Ze/A for E1 valence capture. In the E2 case, similar values of effective charge can be used both for transitions between bound states and for direct E2 neutron capture. The role of effective charge in different radiative transition mechanisms is discussed and a consistent explanation of its behaviour is presented.

Capture gamma-ray spectra of ¹⁶O, ²⁸Si and ³²S have been measured to investigate the mechanism of neutron capture in s-wave and p-wave resonances with large reduced neutron width. Observed gamma-ray transitions from the 434-keV p₃/₂-wave resonance of ¹⁶O exhibit typical features of valence neutron capture in light nuclei. For the 565-keV and 806-keV p₃/₂-wave resonances of ²⁸Si, a particle-vibrator coupling model indicates that in these resonance states core excitation is essential to explain the observed partial radiative widths. There is also some possibility of core excitation in the 203-keV p₁/₂-wave resonance capture in ³²S. Moreover, a strong correlation between partial radiative widths and spectroscopic factors of the final states was found for gamma-ray transitions from the 188-keV s-wave resonance of ²⁸Si and the 103-keV s-wave resonance of ³²S. A discussion follows on the particle-core coupling scheme in these resonance capture transitions.

The authors discuss the direct capture theory pertaining to primary electric dipole (E1) transitions following slow neutron capture. For light nuclides that they have studied (including ⁹Be, ¹²C, ¹³C, ²⁴Mg, ²⁵Mg, ²⁶Mg, ³²S, ³³S, ³⁴S, ⁴⁰Ca, and ⁴⁴Ca), estimates of direct capture cross sections using optical model potentials with physically realistic parameters are in reasonable agreement with the data. Minor disagreements that exist are consistent with extrapolations to light nuclides of generally accepted formulations of compound nucleus capture. They also discuss the channel capture approximation which is, in general, a good representation of these cross sections in heavier nuclei, particularly if the scattering lengths are not different from the corresponding potential radii. They also draw attention to cases where the use of this formula leads to inaccurate predictions.

A reanalysis of the ⁹Be(n, gamma )¹⁰Be data yields a revised value of 1.25+or-0.15 MeV for the spin-spin potential, V_{I sigma} . Other evidence for such a term is derived from ¹⁰B(n, gamma )¹¹B where V_{I sigma} =1.6+or-0.6 MeV. The Lane isovector symmetry potential is determined from ⁴⁸Ca(n, gamma )⁴⁹Ca where W_D=11.3-67.8((N-Z)/A) MeV.

Selected topics of photon-induced reactions are reviewed with emphasis on recent developments.

The threshold photo-neutron technique explores the process of resonant absorption of gamma-rays and the decay of the compound state by neutron emission. The authors examine, on the basis of currently obtainable experimental conditions, the class of nuclei and the characteristics which can be investigated to advantage by this technique. With reference to the Bologna facility, some improvements are discussed concerning the measurement of angular distributions, the efficiency and time properties of neutron detectors and the development of computer codes for data analysis.

A study of the statistical decay of the giant dipole resonance (GDR) built on excited nuclear states provides information on nuclear properties at finite temperature and moderate spin. The results of a systematic investigation of GDR properties at high excitation covering a wide mass range, A approximately 40-170 are presented. The observed resonance strengths and mean resonance energies vary smoothly as a function of mass and are in agreement with the properties of the ground-state GDR. The widths and resonance shapes of the GDR in highly excited nuclei reflect the deformation of the nucleus at finite temperature and spin.

Angular asymmetries in the ( gamma ,n) reaction in natural calcium, cadmium, and lead have been measured in the energy region of the isovector quadruple resonance (20 to 40 MeV). The angular asymmetries increase from low values, about 0.2, to high values, about 0.6, where this resonance is expected. This increasing asymmetry is interpreted as resulting from the interference between electric dipole and quadruple amplitudes. The locations of the resonance are well determined. The widths are found to be considerably larger than those of the giant dipole resonance.

New probes are being used to study old and new giant resonances. In particular, charge exchange reactions such as (n,p), ( pi ^+or-, gamma ), and ( pi ^+or-, pi ⁰) are providing new insight into the nature of giant resonances. A brief discussion is given of some of these developments. The problem of isospin mixing in ⁴He is also discussed.

The chemical elements in nature are the products of stellar burning processes. If the underlying nuclear physics is sufficiently known, it may be possible to decipher the respective nucleosynthesis mechanisms, e.g. the production of the heavy elements by slow neutron capture (s-process). The observed abundance patterns can be strongly affected by temperature because beta decay rates and capture cross sections may become temperature dependent via significant population of excited nuclear states in the hot stellar photon bath; in turn, this allows derivation of estimates for the temperature. The status of such analyses is presented with particular emphasis on the example of ⁷⁹Se. The results of the (empirical) classical model constitute important constraints for stellar models of helium burning zones in red giant stars. The long-lived radioisotope ¹⁷⁶Lu represents a potential cosmic clock for the age of the s-process elements. For some time it is suspected that the decay of ¹⁷⁶Lu was temporarily enhanced and that this clock is therefore misadjusted. In spite of several attempts, this enhancement is not yet understood quantitatively.

The authors have calculated the production and destruction of the 3.7-h, 123-keV isomeric state of ¹⁷⁶Lu, by photons as well as by neutrons. They used absolute photon strength functions and discrete nuclear level sets for ^175-177Lu. Their results for the photon-induced production of ^176mLu are remarkably different from the experimental measurements in the current literature. They calculate much larger cross sections for this reaction, and a lower threshold than had previously been assumed. These two effects combine to indicate an enormous correction, a factor of 10⁷, needs be applied to shorten the current estimate of the half-life against photoexcitation of ^176mLu as a function of temperature.

Charged-particle-induced capture reactions play a crucial role in the understanding of primordial and stellar nucleosynthesis as well as of the evolution of various astrophysical scenarios. The experimental investigation of such reactions, in principle to be carried out over a wide range of energies and to as low an energy as is technically feasible (subcoulomb energies), often requires the use of novel experimental approaches.

The measured beta -decay properties of the 'waiting-point' nuclei ¹³⁰Cd and ⁸⁰Zn, together with improved shell model predictions of T_1/2 and P_n-values for their experimentally unknown N approximately=82 and 50 neighbours, can be directly related to the observed r-abundances in the A approximately=130 and 80 abundance peaks. Based on this result, new constraints are given on the stellar conditions under which the r-process has operated.

Astrophysical r-process calculations of transbismuth elements are of interest because certain actinide pairs can be treated as chronometers in determining the duration of nucleosynthesis. For one such calculation where a particularly long galactic age was derived, 21+2-4 Gyr, the author presents evidence that the effect of beta-delayed fission appears to be seriously overestimated in uranium decay chains with A=252 to 257. With this conclusion, it follows that this estimate of the galactic age must be considered more uncertain than if the calculated rates of beta-delayed fission were found to be acceptable. The nuclear level structure of ²³⁸Np has been investigated using the ²³⁷Np(n, gamma )²³⁸Np reaction and the alpha decay of ^242mAm as experimental probes. Having established a level scheme for ²³⁸Np that includes 47 excited levels and 93 secondary transitions, the author finds a high degree of correspondence between the experimental band structure and that of a semi-empirical model developed to predict excitations in odd-odd deformed nuclei.

In the light of the description of the nucleon-nucleon potential in terms of meson-exchange, the author discusses the most interesting contributions to the nuclear wave function, paying particular attention to 2 pi exchange associated with isobar components in the wave function. Direct evidence for mesonic degrees of freedom in nuclei is revealed by electromagnetic interactions, giving rise to the so-called meson exchange currents. They detail the important aspects of these currents at low and medium energies by studying neutron radiative capture by protons and deuterons on the one hand, and the photodisintegration of the deuteron on the other hand.

Three beams of the high flux reactor HFR in Petten are being used for nuclear physics research primarily aiming at a test of the validity of models describing mesons in nuclei. Mesonic exchange currents (MEC) manifest themselves in the cross section as well as in the channel spin interference of radiative capture of neutrons in H, D and ³He. Experiments with polarized neutrons and polarized target nuclei clearly indicate the existence of MEC. In the low cross section reaction ³He(n, gamma ), which partly proceeds by double-photon emission, regular internucleon contributions are much more suppressed than MEC.

The history of studies of parity nonconserving (PNC) effects in nuclear interactions was initiated nearly 25 years ago by the first successful observation of two such effects in electromagnetic nuclear decays, namely, gamma -radiation asymmetry in the capture of polarized neutrons by nuclei and gamma -ray circular polarization in the decay of polarized nuclei. In recent years a number of other effects have been observed including: (a) parity-forbidden alpha -decay of ¹⁶O; (b) asymmetry in nuclear fission fragment ejection with respect to spin direction of captured neutron; (c) spin rotation of neutrons passing through matter; (d) dependence of total thermal and resonant neutron interaction cross sections on neutron helicity; (e) gamma -ray asymmetry and circular polarization in the integral gamma -ray spectrum of the (n, gamma ) reaction on nuclei; (f) asymmetry of proton ejection in the (n,p) reaction on polarized neutrons.

Experiments on the particle properties of the neutron and on P and T violation in neutron capture reactions are reviewed. The related activities at the ILL High Flux Reactor, Grenoble are summarized.

The extraordinary neutron intensities available from the new spallation pulsed neutron sources open up exciting opportunities for basic and applied research in neutron nuclear physics. Prospective experiments are reviewed with particular attention to those with a strong connection to capture gamma-ray spectroscopy.

A new white neutron source, which can produce intense neutron fluxes with energies from less than 1 MeV to greater than 400 MeV, is being constructed at the Los Alamos National Laboratory. The neutrons are produced in a spallation reaction using the 800 MeV pulsed proton beam from the Los Alamos Meson Physics Facility (LAMPF) accelerator. This source is designed to allow several experiments to be run simultaneously. A gamma-ray detection system has been instrumented at 15 degrees with respect to the incident proton beam with a flight path of 18 m. The apparatus consists of five 7.6 cm long*7.6 cm diameter BGO scintillators which span an angular range from 40 degrees to 140 degrees in the reaction plane. Excitation functions and angular distributions of the 4.44 and 15.1 MeV gamma rays produced following the ¹²C(n,' gamma ) reaction have been measured in a preliminary run. Several gamma-ray experiments are planned for the coming run cycle including extending the data on the ⁴⁰Ca(n, gamma ) reaction in the region of the isovector giant quadrupole resonance.