V. A. Khitrov

Excitation study of high-lying states of differently shaped heavy nuclei by the method of two-step cascades

Abstract Recently obtained primary transition intensities from two-step gamma-ray cascade investigations of 14 heavy nuclei: 137,138,139 Ba, 146 Nd, 150 Sm, 156,158 Gd, 160 Tb, 164 Dy, 168 Er, 174 Yb, 181 Hf, 196 Pt and 198 Au are analysed. Experiments based on thermal neutron capture were undertaken mainly at the Frank Laboratory of Neutron Physics. Comparing these measured intensities and statistical model predictions, some properties of compound-state depopulation were derived. For the spherical nuclei of 137,138,139 Ba, 146 Nd, 196 Pt and 198 Au very intense cascades with high-energy primary transitions were observed. Rather different is the behaviour of the cascade intensity in the strongly deformed nuclei of 156,158 Gd, 160 Tb, 164 Dy, 168 Er and in the transitional nucleus of 150 Sm - where the intensity of primary γ-rays E 1 ∼ 2–3 MeV exceeds the calculated value.

Levels of 137Ba studied with neutron-induced reactions

Abstract The nucleus 137 Ba has been studied via primary and secondary γ-rays as well as γγ-coincidences following thermal-neutron capture and inelastic neutron scattering. A level scheme was established up to 4.7 MeV consisting of 37 levels including 91 transitions. The neutron binding energy was determined to be 6905.59(8) keV. The thermal-neutron-capture cross section and isomer production of the 11 2 − state at 661.6 keV were determined to be 0.65 −0.04 +0.08 and 0.0098(4) b respectively. The experimental data on excitation energies and electromagnetic properties are compared with predictions of the SU(5) limit of the interacting boson-fermion model. A satisfactorily good agreement between experiment and theory can be stated.

Properties of the 158Gd compound state gamma-decay cascades

Photon spectra from thermal neutron capture in natural gadolinium were measured, using the technique of summed amplitudes of coinciding pulses, to study the compound state decay in the 158Gd nucleus and to reveal its main peculiarities. The two-step cascades leading to the first three levels of the rotational band Kpi =0+ based on the ground state are reported, and differences from previously published data are noted. Decay modes for many levels up to the excitation energy of approximately=4 MeV are established for the first time. Experimental cascade intensity distributions as a function of the primary transition energy are compared with calculations using different model predictions for level density. Conclusions about the gamma -decay characteristics of complex nuclei, which form the beginning of the region of the 4s-maximum neutron resonance strength function, are drawn.

Direct experimental estimate of parameters that determine the cascade gamma decay of compound states of heavy nuclei

A method is proposed for simultaneously determining the interval of the most probable values of the density of levels excited in the radiative capture of slow neutrons and the sum of radiative E1 and M1 strength functions in the excitation-energy interval extending nearly up to the neutron binding energy. Experimental data on the intensities of two-step photon cascades between the compound state and a given low-lying level of the nucleus being studied are analyzed together with the total radiative widths of neutron resonances. Such an analysis can be performed for nuclei having an arbitrary level density, including deformed ones. The resulting data demonstrate that there are significant deviations from the predictions of commonly accepted level-density models—for example, the Fermi gas model—and specify the range of nuclei and the regions of their excitation energies where a further experimental investigation can furnish new important information about the properties of nuclear matter.

Possibility of implementing a nondirect experiment aimed at determining the cross section for nucleon interaction with an excited nucleus

With the aid of experimental data on the level densities in the even-odd tungsten isotopes 183,185,187W as determined in the respective (n, 2ρ) reactions, the ratio of the cross section for the interaction of an excited nucleus with a neutron to its counterpart calculated on the basis of the optical model of the nucleus was obtained from the spectra of evaporated neutrons in the reaction 181Ta(p, n)181W. A significant local increase in this ratio is qualitatively interpreted as that which is due to the possible increase in the penetrability of the nuclear surface for nucleon reaction products because of the effect of phonon-type excitations. Sources of possible nonstandard systematic errors in experimentally determining the level density in an arbitrary nucleus at energies below the nucleon binding energy Bn are analyzed. The extraction of information from the distribution of the intensities of cascades belonging to the nucleon and photon type is simulated. The resulting information may be highly reliable if use is made of the entire body of experimental data on the nucleus under analysis.

Semiphenomenological approximation of the sums of experimental radiative strength functions for dipole gamma transitions of energy E γ below the neutron binding energy B n for mass numbers in the range 40 ≤ A ≤ 200

The sums of radiative strength functions for primary dipole gamma transitions, k(E1) + k(M1), are approximated to a high precision by a superposition of two functional dependences in the energy range 0.5 < E1 < Bn − 0.5 MeV for the 40K, 60Co, 71,74Ge, 80Br, 114Cd, 118Sn, 124,125Te, 128I, 137,138,139Ba, 140La, 150Sm, 156,158Gd, 160Tb, 163,164,165Dy, 166Ho, 168Er, 170Tm, 174Yb, 176,177Lu, 181Hf, 182Ta, 183,184,185,187W, 188,190,191,193Os, 192Ir, 196Pt, 198Au, and 200Hg nuclei. It is shown that, in any nuclei, radiative strength functions are a dynamical quantity and that the values of k(E1) + k(M1) for specific energies of gamma transitions and specific nuclei are determined by the structure of decaying and excited levels, at least up to the neutron binding energy Bn.

Nuclear structure of 156Gd studied with (n, γ), (n, e−), (d, p), (d, t) reactions and lifetime measurements

Abstract The nucleus 156 Gd was studied with (n, γ) and (n, e − ) reactions at the Institut Laue-Langevin (ILL) in Grenoble. The (d, t) and (d, p) reactions were investigated at the Munich Tandem Accelerator. Primary gamma rays following average resonance capture were observed at the Institut for Nuclear Research in Kiev. Summed coincidences measurements were carried out at the Reactor IBR-30 in Dubna. An extensive level scheme was constructed up to 2.35 MeV including 413 transitions and 18 band assignments. Lifetimes of 16 selected levels were measured with the Gamma-Ray Induced Doppler broadening method at the ILL. Low-lying bands of positive and negative parity were interpreted in the framework of the spdf-IBA. The agreement was found to be quite good.

Thermal-neutron-capture studies on 135Ba

Abstract The nucleus 135 Ba has been studied via primary, secondary γ-rays, γγ coincidences following thermal-neutron capture. A level scheme was established up to 3.7 MeV consisting of 41 levels including 114 γ-transitions. The neutron binding energy was determined to be 6971.78(17) keV. The total thermal cross section and isomer production of the 11 2 − state were determined to be σ(tot) = 1.4(4) and σ( 135 Ba) = 0.11(2) barn. The level energies, E2 and M1 transitions and electromagnetic moments were compared with model predictions of the interacting boson-fermion model (IBFM).

Precise model approximation of JINR (Dubna) data on nuclear level densities in the region 40 ≤ A ≤ 200 below Bn

The experimental nuclear level density below the neutron binding energy Bn in the mass region 40 ≤ A ≤ 200 is approximated to a high precision on the basis of the Strutinsky model combined with the assumption that the coefficient of collective enhancement of the level density for a given number of excited quasiparticles decreases exponentially with increasing excitation energy. This combination of model concepts makes it possible to reproduce faithfully not only the general trend revealed by the method developed at the Joint Institute for Nuclear Research (JINR, Dubna) in the change in the level density with increasing excitation energy in any nuclei but also the fine structure of this level density. Realistic experimental information about the change in the relationship between the densities of quasiparticle- and vibrational-type states was obtained for the first time for any nuclei virtually up to the neutron binding energy Bn.

Study of the187W states excited in the (n, γ) reaction

A singlesγ-ray spectrum and a spectrum of summed amplitudes of coinciding pulses (SACP) were measured in the186W(n, γ)187W reaction experiment on the thermal neutron beam. It was for the first time that the data on187Wγ-transitions were obtained in the excitation energy region from 1500–2500 keV. The neutron binding energy was determined to beBn=5467.25 (4) keV (statistical error only). In result the level scheme of187W was developed in the excitation energy interval 1<Ef< 3.4 MeV, which contained 105 levels with about 70 of them being identified for the first time. The experimental values for summed intensities of two-step cascades were established to exceed those predicted by the modern statistical theory (by 36±6%). This is explained by a considerable contribution of few-quasiparticle components to wave functions of compound and intermediate states.