Description Of Nuclear Reactions Research Articles

Relativistic Effects in Light Nuclei and Nuclear Reactions

A brief review of the results obtained in the description of nuclear reactions in systems containing up to four nucleons in the approach based on the Faddeev equations is given. The methods for consideration of relativistic effects and their influence on observables are discussed.

Exchange Effects in the Radiative Capture Reactions3H(α, γ)7Li and3He(α, γ)7Be

The mirror 3 H(α, γ) 7 Li and 3 He(α, γ) 7 Be reactions have been considered using the algebraic versions of the resonating group model and of the orthogonality conditions model. Exchange effects in interaction of the colliding nuclei and influence of the corresponding exchange terms in matrix elements of the interaction potential on calculated astrophysical S-factors for the reactions have been studied. At the present time one of the most important aims of nuclear reaction theory is to build so called ab initio approaches to description of nuclear reactions, which use realistic NN-potentials and exactly account for all exchange effects caused by the antisymmetry of a wave function of nuclear system. Such approaches are in continuous progress but, nevertheless, turn out to be very complicated procedures, which are, in fact, unrealizable for sufficiently heavy colliding nuclei. Though, there are alternative approaches suitable for heavier systems. They are based on the resonating group model (RGM) [1] or its algebraic version (AVRGM) [2] with effective NN-potentials and, as above mentioned approaches, rigorously allow for the exchange effects. However, complexities rapidly grow even in such approaches at increasing system mass. Thus, a development of reliable approximations within the existing microscopic approaches is rather topical problem of nuclear reaction theory. Such approximations will allow to simplify the calculations significantly and even to make ones realizable for sufficiently complicated systems.

Microscopic nature of the radiative strength function: Structures, coupling with phonons

The microscopic nature of the radiative strength function, which is the most important characteristic necessary for the description of nuclear reactions involving gamma-ray photons both in astrophysics and in the theory of nuclear reactors, has been discussed. It has been shown that, in contrast to phenomenological approaches based on various modifications of the Lorentzian dependence for this function, the microscopic approach gives structures that are due to the effects both within the standard random phase approximation and of coupling with low-lying collective excitations (phonons), i.e., beyond the standard random phase approximation. Microscopic calculations of the strength function for several Sn and Ni isotopes have been performed within the self-consistent version of the extended theory of finite Fermi systems, where both of these effects are taken into account and the SLy4 Skyrme forces are used to calculate the mean field, effective interaction between nucleons, and characteristics of phonons. Microscopic radiative E1 strength functions have been used in the modern EMPIRE 3.1 code for the calculation of the cross sections for the radiative capture of neutrons and average radiative widths of neutron resonances. Reasonable agreement with the existing experimental data has been obtained with allowance for coupling with phonons. The integral characteristics of the pygmy dipole resonance in the unstable 68Ni nucleus have been explained. The existence of this resonance has been predicted in the unstable 72Ni nucleus.

Bayesian Evaluation Including Covariance Matrices of Neutron-induced Reaction Cross Sections of 181Ta

A new evaluation of neutron-induced reactions on 181Ta using a consistent procedure based on Bayesian statistics is presented. Starting point of the evaluation is the description of nuclear reactions via nuclear models implemented in TALYS 1.4. A retrieval of experimental data was performed and covariance matrices of the experiments were generated from an extensive study of the corresponding literature. All reaction channels required for a transport file up to 200 MeV have been considered and the covariance matrices of cross section uncertainties for the most important channels are determined. The evaluation has been performed in one step including all available experimental data. A comparison of the evaluated cross sections and spectra with experimental data and available evaluations is performed. In general the evaluated cross section reflect our best knowledge and give a fair description of the observables. However, there are few deviations from expectation which clearly indicate the impact of the prior and the need to account for model defects. Using the results of the evaluation a complete ENDF-file similarly to those of the TENDL library is generated.

QGSM development for spallation reactions modeling

The growing interest in spallation neutron sources, accelerator-driven systems, R&D of rare isotope beams, and development of external beam radiation therapy necessitated the improvement of nuclear reaction models for both stand-alone codes for the analysis of nuclear reactions and event generators within the Monte Carlo transport systems for calculations of interactions of high-energy particles with matter in a wide range of energy and in arbitrary 3D geometry of multicomponent targets. The exclusive approach to the description of nuclear reactions is the most effective for detailed calculation of inelastic interactions with atomic nuclei. It provides the correct description of particle production, single- and double-differential spectra, recoil, and fission product yields. This approach has been realized in the Quark Gluon String Model (QGSM) for nuclear reactions induced by photons, hadrons, and high energy heavy ions. In this article, improved versions of the QGSM model and a corresponding code have been developed tested and bench marked against experimental data for neutron production in spallation reactions on thin and thick targets in the energy range from a few MeV to several GeV/nucleon.

Decoherence and localization in the Fermi accelerator

The one-body dissipation mechanism has been successfully employed in the description of nuclear reactions at intermediate energies. However, detailed simulations show that quantum localization would strongly suppress this dissipation mechanism. The fact that particle emission accompanies the reaction process, has not been taken into account. We show, through Fermi accelerator simulations, that the changes in the system's parameters brought by this effect alone are enough to destroy the quantum localization.

Relativistic transport theory for hadronic matter

Abstract We derive coupled equations of motion for the density matrices for nucleons, Δ resonances, and π mesons, as well as for the pion-baryon interaction vertex function for the description of nuclear reactions at intermediate energies. We start from an effective hadronic Lagrangian density with minimal coupling between baryons and mesons. By truncating at the level of three-body correlations and using the G -matrix method to solve the equations of motion for the two-body correlation functions, a closed equation of motion for the one-body density matrices is obtained. A subsequent Wigner transformation then leads to a tractable set of relativistic transport equations for interacting nucleons, deltas, and pions.

Theory of Nuclear Reactions

The book presents an extended version of the lecture course on the theory of nuclear reactions that has been given by the author for some years in Kiev State University. An account is given of the nonrelativistic nuclear reaction theory. The R - matrix description of nuclear reactions is considered and the dispersion method is formulated. Mechanisms of nuclear reactions and their relationship are studied in detail. Attention is paid to nuclear reactions involving the compound nuclear formation and to direct nuclear processes. The optical model, the diffraction approach and high - energy diffraction nuclear processes involving composite particles are discussed. It also deals with some problems treated only in special journal papers.

Dispersion K-matrix approach to nuclear reactions and its application to Nα scattering

A description of nuclear reactions is suggested based on the assumption that the main contribution to the reaction K-matrix elements, regarded as analytic functions of the variable z = cos (being the scattering angle), comes from the singularities of these functions which are nearest to the physical region. Analytic properties of K-matrix elements with respect to z are discussed. As an illustration, elastic Nα scattering is examined and vertex constants values are found for the vertices α⇄t + p and α⇄ τ + n.

Multistep contributions in 88Sr(h, t) 88Y

Using a coupled-reaction-channels (CRC) description of nuclear reactions, we have studied the reaction 88Sr(h, t) 88Y to 4 −, 5 −, and 2 +, 3 + doublet states. The data, taken at 23 MeV, show angular distributions with anomalous shapes, which cannot be explained by simple distorted-wave Born approximation calculations; our calculations, including (h, α) − (α, t) and (h, d) − (d, t) intermediate processes, but neglecting the direct (h, t) mechanism, give a good description of the shapes and relative magnitudes of the angular distributions. Effects of configuration mixing are also studied.

A study of intermediate nuclear resonances by nonlocal separable potentials

It is well known that if one considers the possible excitation of the internal degrees of freedom of the target in the description of a nucleon-nucleus scattering process, some typical effects of the coupling between channels, like threshold effects and compound resonances, are found. Different kinds of interactions may be employed in this context provided that they possess tensorial properties able to couple the elastic and inelastic channels together. In this context we examine here the use of nonlocal separable potentials, whose remarkable increasing importance in the two- and three-body description of nuclear reactions is well known. The model is outlined in its general form, and then the potential, compound and quasi-compound resonances are analysed under some restrictive conditions. A numerical example is given as a particular application.

Description of nuclear reactions in the harmonic oscillator shell-model function scheme

The general reaction theory formalism given by Heiss and this author — based on the Kohn variational principle — is applied to the case that the two reaction partners in every channel are described in the harmonic oscillator shell model. One of them should be large so that translational invariance may be neglected; the other one consists of a maximum number of four nucleons so that its wave function is given by the translationally invariant shell model without difficulty. The Hamiltonian used in this theory is composed from the elementary nucleonnucleon interactions. It is shown how some matrix elements of this Hamiltonian can be computed; these matrix elements are the basic quantities in the reaction equation.

Gamma Ray Energy Spectrum from Iron Bombarded by 14-Mev Neutrons

The energy spectrum of gamma rays from iron when bombarded by 14-Mev neutrons is calculated on the basis of the compound nucle~s formalism. The cross sections for various competing processes are found to be considerably dependent upon the magnitude of temperature. We have assumed three values of a, the quantity closely related to temperature. Although they are based upon the experimen­ tal evidences of different sources, none of them is found to be able to account for the experimental energy spectrum. There seems to remain a difficulty in the statistical theory pointed out by Cohen. § I. Introduction In the previous paper1>, cited as I, we have investigated the yield of gamma rays excited by fast neutrons, aiming at the study of the mass number dependence of nuclear temperature. We were able to show that the statistical theory could account for the over­ all behaviour of the gamma ray yields for various nuclei, provided that the empirical values of temperatures as well as compound nucleus formation cross sections were assumed. Although this result may be thought to support the statistical description of nuclear reactions, the material2' we employed is not always suitable to our purpose by several reasons. Firstly, the energy of incident neutrons was not monochromatic, but had a broad width around 10 Mev. Secondly, the number of gamma qu~ta was not directly counted, but was observed with a counter whose efficiency is assumed to be proportional to the energy of gamma-rays. Thirdly, we were only concerned with the over-all A-dependence, but ignored the detailed properties of individual nuclei, except for three cases where the detailed pro­ perties would be exhibited. These drawbatks are forced by the shortage of material availa­ ble in those days, while our analysis seemed enough to prove the essential correctness of the statistical theory. Recent experiments with a monochromatic beam have facilitated one to make a closer examination of the statistical theory by observing the energy spectrum of emitted neutrons. 3>4> The nuclear temperature thus obtained shows the A-dependence similiar to that obtained by Blatt and Weisskopf/' as employed in I, but its absolute magnitude is