Two-step cascades of 117Sn(n, 2γ) reaction and problems of describing main nuclear properties below Bn

Main properties of the excited states of ¹¹⁸Sn manifesting themselves in cascade γ decay of its compound state are studied. As in heavier nuclei studied earlier, qualitative interpretation of the whole set of observed properties of the nucleus appears to be impossible without accounting for coexistence and interaction of quasiparticle and collective nuclear excitations and their considerable influence on the main parameters of the process under study. From Yadernaya Fizika, Vol. 67, No. 10, 2004, pp. 1845–1849. Original English Text Copyright ? 2004 by Khitrov, Panteleev, Sukhovoj, Honzátko, Tomandl. This article was submitted by the authors in English.     

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.     

Potential,problems, and prospects of experimental investigations of superfluidity of heated nuclei

The status of modern experiments aimed at determining level densities and radiative strength functions in complex nuclei from spectra of products of various reactions via recording a cascade or a single photon (nucleon) is analyzed. It is shown that the most precise data for these parameters and, accordingly, the best possibilities for studying the superfluidity of heated nuclei can be obtained only from spectra of two-step reactions, the detection of nucleons or light nuclei at the first step being mandatory.     

Intensities of two-quanta cascades at different excitation energies of compound nuclei146Nd,174Yb and183W

Intensities of two-quanta cascades are obtained (measured) for 2–3 final low-lying levels of the following nuclei¹⁴⁶Nd,¹⁷⁴Yb and¹⁸³W. These measured intensities are compared with the intensities calculated in the frame of various models at primary transition energies ranging from about 0.5 MeV to the neutron binding energy. Some excitation energy intervals are revealed, experimentally obtained intensities of cascade are inconsistent with model calculations.     

Experimental estimates on radiative strength function of low-energy γ-quanta following even-odd heavy nuclei decay

The applicability of the method of summed amplitudes of coinciding pulses to determine radiative strength functions of heavy nuclei by measuring two-step cascade-ray spectra is discussed. Experiments based on thermal neutron capture in¹³⁶Ba and¹⁸⁰Hf were carried out to test this method. The values obtained for the radiative strength functions of a wide range of primary transitions were compared with the predictions of two modifications of the giant dipole resonance model. Some insight into the characteristics of the-decay of compound-states is provided by analyzing the information obtained on the two-step-decay of a number of nuclei in the mass region 137A187.Authors' thanks are due to Mrs. T.F. Drozdova and Dr. M.A. Ali for their help in the preparation of the English version of this paper.     

Estimations of level densities and radiative strength functions from experimental data and probable change in the properties of nuclei at E exc ⋍ 3–4 MeV

Investigations of two-step γ cascades following thermal-neutron capture by heavy nuclei reveal that the density of excited levels is significantly smaller than that which is predicted by an exponential extrapolation like that of the back-shifted Fermi gas model. Data on two-step cascades allow one to determine in a model-independent way the most probable energy dependences of the level density and radiative strength functions, virtually over the whole excitation-energy interval below the neutron binding energy B _n . Data for more than 30 heavy nuclei cannot be understood without assuming a considerable change in the nuclear properties at E _exc ? 3–4 MeV.     

Precise model approximation of JINR (Dubna) data on nuclear level densities in the region 40 ≤ <Emphasis Type="Italic">A</Emphasis> ≤ 200 below <Emphasis Type="Italic">B</Emphasis><Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

The experimental nuclear level density below the neutron binding energy B _n 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 B _n .     

Potential of present-day experiments for determining the parameters of the gamma decay of compound states of heavy nuclei in a model-independent way

Experimental data on two-step cascades initiated by thermal-neutron capture in ^184,186W and ^190,192Os nuclei are analyzed from the point of view of prospects for improving the reliability of a model-independent determination of the density of levels in a given interval of J ^π and the radiative strength functions for E1 and M1 transitions exciting these levels in the region E _exc≤B _n .

     

Partial level density of n-quasiparticle excitations, radiative strength functions and new experimental information on the dynamics of nuclear structure change in the B n range

The most reliable at present values of the level density in the fixed spin window and the sums of radiative strength functions of cascade gamma transitions are obtained from analysis of intensities of two-step cascades excited upon thermal neutron capture for approximately 40 nuclei in the mass range 40 ≤ A ≤ 200. The maximal reliability of these data is provided by the experimental conditions—minimum possible propagation error coefficients and practically unique solution of the problem of determination of gamma decay parameters from measured spectra. The experimental data are approximated by the sum of partial level densities corresponding to excitation of n quasiparticles. Steplike structures in the level density at excitation energies smaller than 3–4 MeV are described with good accuracy as the superposition of two-quasiparticle (three-quasiparticle in odd A nuclei) and vibrational excitations with the coefficient of collective density enhancement K _coll ≈ 10?20. They correspond to excitation-energy-correlated maximum enhancement of the radiative strength functions of primary gamma transitions. The level density at larger excitation energies is well reproduced if the breakup of at least two more Cooper pairs of nucleons is taken into account. The increase in the number of excited quasiparticles in the nucleus corresponds to unconditional reduction of the radiative strength functions of primary gamma transitions of the compound state decay. However, the maximum possible value of partial widths of primary transitions increases regularly with decreasing energy. Some ambiguity in the results of approximation and divergence from existing theoretical ideas of the energy dependence of nucleon correlation functions in an excited nucleus point to the possibility of direct extraction from experiment of fundamentally new information on the structure of excited nuclear levels in the range of the neutron binding energy. These are, first of all, the parameters of dependence of nucleon correlation functions on the excitation energy of the nucleus.