Nuclear-level densities around Z = 50 from neutron evaporation spectra in ( p, n ) reactions
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CLEI Experiment
Nuclear-Level Densities around Z = 50 from Neutron Evaporation Spectra in (p, n) Reactions B. V. Zhuravlev, A. А. Lychagin, and N. N. Titarenko Institute for Physics and Power Engineering, pl. Bondarenko 1, Obninsk, Kaluga oblast, 249033 Russia Received January 17, 2005; in final form, June 21, 2005
Abstract—Neutron excitation functions, spectra, and angular distributions in the (p, n) reactions on the isotopes 116 Sn, 118 Sn, 122 Sn, and 124 Sn were measured in the proton-energy range 7–11 MeV. The measurements were performed by the time-of-flight method with the aid of a fast-neutron spectrometer at the EGP-15 pulsed tandem accelerator of the Institute of Physics and Power Engineering (Obninsk). A high resolution (about 0.6 ns/m) and a high stability of the time-of-flight spectrometer made it possible to identify reliably low-lying levels along with the continuous section of the neutron spectra. The data obtained in this way were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed with the aid of the precise Hauser–Feshbach formalism of statistical theory. The nuclear-level densities in the isotopes 116 Sb, 118 Sb, 122 Sb, and 124 Sb were determined, along with their energy dependences and model parameters. In the excitation-energy range 0–2 MeV, the energy dependence of the nuclear-level densities exhibits a structure that is associated with the shell inhomogeneities of the spectrum of single-particle states near filled shells. The isotopic dependence of the nuclear-level density is discovered and explained. It is also shown that the data obtained here for the nuclear-level density differ markedly from the predictions of model systematics of nuclear-level densities. PACS numbers : 21.10.Ma DOI: 10.1134/S106377880603001X
INTRODUCTION Investigation of nuclear-level densities is of great interest in nuclear physics, since they are of importance both in developing a consistent theoretical description of the properties of excited nuclei and in calculating cross sections for nuclear reactions within the statistical model. Such calculations include investigations into nuclear synthesis in nuclear astrophysics. The basic features of the nuclear-level density are well known, but existing data are frequently characterized by a scatter within a factor of up to 1.5; moreover, its isotopic dependence, which is of paramount importance for calculating nuclear synthesis in astrophysics, is very poorly understood because of the absence of experimental information. For many nuclei, experimental data on nuclear-level densities were obtained predominantly from an analysis of neutron resonances and low-lying levels. However, this information is available within rather narrow intervals of excitation energies, spins, and N − Z values, and its extrapolation may lead to significant errors both in the absolute value of the nuclear-level density and in its energy dependence, especially in the transition region between the region of well-
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