Neutron emission

About: Neutron emission is a research topic. Over the lifetime, 3880 publications have been published within this topic receiving 49164 citations.

Fluctuations of Nuclear Reaction Widths

Abstract: The fluctuations of the neutron reduced widths from the resonance region of intermediate and heavy nuclei have been analyzed by a statistical procedure which is based on the method of maximum likelihood. It is found that a chi-squared distribution with one degree of freedom is quite consistent with the data while a chi-squared distribution with two degrees of freedom (an exponential distribution) is not. The former distribution corresponds to a Gaussian distribution for the reduced-width amplitude, and a plausibility argument is given for it which is based on the consideration of the matrix elements for neutron emission from the compound nucleus and of the central limit theorem of statistics. This argument also suggests that within the framework of the compound-nucleus theory all reduced-width amplitudes have Gaussian distributions, and that many of the distributions for the various channels may be independent. One consequence of the latter suggestion is that the total radiation width for a given spin state which is formed in neutron capture will be essentially constant, in agreement with some observations, because it is the sum of many partial radiation widths. The fluctuations of the provisional fission widths of ${\mathrm{U}}^{235}$ are best described by a chisquared distribution with about 2\textonehalf{} degrees of freedom, indicating that there are effectively only a few independently contributing fission channels.

Model for Nuclear Reactions with Neutrons

Abstract: A simple model is proposed for the description of the scattering and the compound nucleus formation by nucleons impinging upon complex nuclei. It is shown that, by making appropriate averages over resonances, an average problem can be defined which is referred to as the "gross-structure" problem. Solution of this problem permits the calculation of the average total cross section, the cross section for the formation of the compound nucleus, and the part of the elastic-scattering cross section which does not involve formation of the compound nucleus. Unambiguous definitions are given for the latter cross sections.The model describing these properties consists in replacing the nucleus by a one-body potential which acts upon the incident nucleon. This potential $V={V}_{0}+i{V}_{1}$ is complex; the real part represents the average potential in the nucleus; the imaginary part causes an absorption which describes the formation of the compound nucleus. As a first approximation a potential is used whose real part ${V}_{0}$ is a rectangular potential well and whose imaginary part is a constant fraction of the real part ${V}_{1}=\ensuremath{\zeta}{V}_{0}$.This model is used to reproduce the total cross sections for neutrons, the angular dependence of the elastic scattering, and the cross section for the formation of the compound nucleus. It is shown that the average properties of neutron resonances, in particular the ratio of the neutron width to the level spacing, are connected with the gross-structure problem and can be predicted by this model.The observed neutron total cross sections can be very well reproduced in the energy region between zero and 3 Mev with a well depth of 42 Mev, a factor $\ensuremath{\zeta}$ of 0.03, and a nuclear radius of $R=1.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{A}^{\frac{1}{3}}$ cm. The angular dependence of the scattering cross section at 1 Mev is fairly well reproduced by the same model. The theoretical and experimental values for the ratios of neutron width to level distance at low energies and the reaction cross sections at 1 Mev do not agree too well but they show a qualitative similarity.

The Nuclear Fission Process

Abstract: Introduction (C. Wagemans). Fission Barriers (H. Weigmann). Spontaneous Fission (C. Wagemans). Neutron Induced Fission Cross-Sections (H. Weigmann). Photon- and Electron-Induced Fission (E. Jacobs and U. Kneissl). Charged Particle Induced Fission (C. Wagemans). Theoretical Models of Fission Fragment Mass and Energy Distributions (J. Moreau and K. Heyde). Mass, Charge and Kinetic Energy of Fission Fragments (F. Goennenwein). Fission Fragment Angular Momentum (D. De Frenne). Fission Fragment Angular Distributions (H. -H. Knitter). Neutron and g-Emission in Fission (H. -H. Knitter, U. Brosa and C. Budtz-Jorgensen). Ternary Fission (C. Wagemans).