中子辐射俘获反应机制的质量能量相关性研究

对A?<10?0核质量区内的核素,研究中子辐射俘获反应中不同反应机制对辐射俘获截面的贡献随靶核质量数和中子入射能量变化的规律.所考虑的反应机制有复合核统计过程和复合核弹性散射道中的辐射俘获及形状弹性散射道中的直接–半直接辐射俘获两种非统计过程.在中子入射能量0.1—20MeV区间,给出了²⁷Al,⁴⁰Ca,⁶³Cu和⁹³Nb的理论计算结果及与实验数据的比较,并对呈现的变化规律进行了分析讨论     

Analysis of E1 radiative neutron capture based on the optical model

The optical model formula of Lane and Mughabghab for E1 radiative neutron capture has been applied to resonance capture in ⁵⁶Fe and ⁹⁰Zr. It is shown that in general the optical model predicts the contributions to the radiative widths which are correlated with the neutron widths, except for a depletion factor representing the proportion of E1 single-particle strength remaining near the neutron threshold. The degree of correlation between measured and calculated radiative widths is related to the ratio of the variances of these widths. This enables an estimate of the factor . It is shown that the theoretical value for the ratio of the radiative to neutron widths is to a good approximation independent of the imaginary part of the optical potential. The calculated radiative width is thus well defined. The results are compared with those from the valence model and from experiment. The direct background contribution is found to be negligible compared to the average compound cross section. The contribution of d-waves to the radiative capture cross section is discussed.     

Stuty on the Process of the Fast Neutron Radiative Capture for 40Ca and 208Pb

The pre-equilibrium correction for the nuclear reactions induced by fast neutron is considered as follows: the nuclear reaction processes with the exciton number equal to or larger than five can still be described by the statistical theory of the nuclear reaction. The particle emission processes, in which less than five excitons are involved, are calculated by means of the exciton model and the γ emission of one exciton state is calculated by the direct capture mechanism. For the three-exciton state, only the semidirect capture mechanism, which plays the main role in the researched energy region, is taken into account. The interference effect between the direct and semidirect capture is also considered. The radiative capture cross sections for ⁴⁰Ca and ²⁰⁸Pb in the neutron incident energy region from 3 MeV to 20 MeV are calculated and a better coincidence with the experimental values is obtained. At the same time, the contribution to the (n, γ) reaction cross sections of the γ emissions before and after statistical equilibriums as well as the characteristics of the direct capture, semidirect capture and their interferece terms are discussed.     

An interference effect in the channel radiative neutron capture process

This study shows that the small thermal neutron radiative capture cross sections in ¹²C and neighbouring nuclides are the result of destructive interference between the potential scattering wave and the resonance scattering wave near the nuclear surface, resulting in a drastic cancellation in the radial integral. The behaviour of the scattering wave function is examined, and the general condition for the occurrence of such cancellation is discussed. The expression for the channel radiative capture cross section which has been derived has the same structure as the Lane-Lynn formula but is expressed in terms of different parameters. In addition, this investigation shows that if the optical model well depth is adjusted so that the binding energy of the $\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ orbit in ¹²C is kept at the experimental value, then the calculated results for the potential and channel radiative neutron capture cross sections are insensitive to the value of the nuclear radius.     

Cross sections for the photodisintegration of a deuteron and neutron capture by a proton calculated for the case of nonlocal potential

The processes of deuteron photodisintegration by a gamma-quantum and the radiative capture of a neutron by a proton with the emission of a gamma-quantum are considered. Interaction between nucleons is described by a nonlocal potential of the Yamaguchi type but allowing for repulsion due to the nucleon cores. In contrast to other potentials, the Schroedinger equation is solved exactly for the proposed potential. The potential is more exactly defined in comparison with the previously obtained values for the parameters; information on this potential is important in solving certain fundamental problems in nuclear theory. The effective cross sections for photodisintegration of a deuteron and for radiative capture are computed. Calculations show that the proposed potential makes it possible to describe the photodisintegration processes quite accurately for intermediate (up to 20 MeV) energies. The cross section computed for radiative capture is in somewhat better agreement with experiment than is the same cross section determined for other potentials.     

Gamma decay of nuclear molecular states

Signatures of deformation and clustering in nuclei can be found in the γ-decay properties of their excited states. Such signatures are still missing for the so-called nuclear ‘molecular’ states which correspond to the resonant states observed in light heavy-ion reactions. Among the new proposed experiments to search for γ transitions between highly deformed molecular states, the radiative capture studies look quite promising.     

Direct Radiative Capture to Continuum States

A formula for the calculation of the cross sections,of nuclear transition to continuum states induced by direct radiative capture of intermediate-energy proton is deduced.The transition amplitude includes two terms corresponding to potential-scattering to potential-scattering transitions and potential-scattering to resonance-scatting transitions,respectively.The model is compard with available experimental data of the ¹¹B(p,γ₁₉)¹²C reaction,and the results show that the direct capture mechanism is capable to account for the main features of the measured data within the reasonable parameter scope,and,in the present case,the contributions from two terms are of the same order of magnitude.The physical significance of the results is also discussed.     

Towards a better understanding of O nuclear structure: Muon capture and radiative pion capture reactions

Muon capture and radiative pion capture reactions on ¹⁶O have been analysed with the aim to extract more information on the importance of higher components of the A = 16 nuclear ground and excited states. Good agreement with the experimental data for both reactions may be achieved if at least 2p2h components of the nuclear wave functions are taken into account. The positive parity excited states in ¹⁶N are shown to contribute 43 % to the radiative π⁻ capture but only ≈ 10 % to the muon capture rate.     

Optical model calculation for neutron capture in the mass region A=40–70

Calculations of thes, p andd-wave neutron strength functions based on the spherical optical model have been performed for the mass region A=40–70. The aim was to investigate the radiative neutron capture observed in this region leading to final states with spin J=5^?/2 and J=7^?/2. Results of the calculation indicate thatd-wave neutron capture followed by E1 radiative decay is the most likely explanation of the process.     

Radiative electron capture accompanying resonant nuclear scattering

A quantum mechanical theory for the radiative capture (REC) of a target electron by a heavy, swift projectile is formulated, allowing for resonant nuclear scattering through the use of distorted waves. Calculations are performed for the systems O¹⁶, Ne²⁰→He within the exact strong potential Born theory and the impulse approximation. Similar structures as in the case of Coulomb capture are found in the transition probability.     

Attenuation of solar radiation by a water mist from the ultraviolet to the infrared range

A model is developed for the hemispherical transmittance of direct and scattered solar radiation from a cloudless atmosphere by a mist layer of water droplets in order to investigate the potential of water misting systems to serve as a protection from solar irradiation with particular emphasis on harmful UV radiation. The proposed model is based on published spectral experimental data for solar irradiation, Mie theory for interaction of the radiation with single spherical droplets, and radiative transfer theory. Known limiting solutions are employed to simplify the Mie calculations. The modified two-flux approximation is used to account for both direct and diffuse irradiation in lieu of a numerical solution for the full radiative transfer equation in anisotropically scattering media. The role of the governing parameters of a disperse water curtain of water droplets, water content, and droplet size for sample conditions is studied in some detail, particularly in the near-ultraviolet part of the spectrum where radiation can result in human tissue damage.     

Direct and sequential radiative three-body reaction rates at low temperatures

We investigate the low-temperature reaction rates for radiative capture processes of three particles. We compare direct and sequential capture mechanisms and rates using realistic phenomenological parametrizations of the corresponding photodissociation cross-sections. Energy conservation prohibits sequential capture for energies smaller than that of the intermediate two-body structure. A finite width or a finite temperature allows this capture mechanism. We study generic effects of positions and widths of two- and three-body resonances for very low temperatures. We focus on nuclear reactions relevant for astrophysics, and we illustrate with realistic estimates for the α-α-α and α-α-n radiative capture processes. The direct capture mechanism leads to reaction rates which for temperatures smaller than 0.1 GK can be several orders of magnitude larger than those of the NACRE compilation.     

Interaction and fusion of deformed nuclei

It is shown that the height of the barrier and its position, as well the depth of the capture well, are highly sensitive to the relative orientation of colliding strongly deformed nuclei. It is found that the fusion/capture cross sections and the nucleus-nucleus potential for heavy nuclear systems depend greatly on the magnitude and sign of the quadrupole deformation of nuclear surfaces. In order to describe correctly the cross section for the capture of heavy strongly deformed nuclei, it is necessary to perform averaging over all three angles that describe their relative orientation. Allowance for a hexadecapole deformation leads to a significant increase in the capture cross section for very heavy nucleus-nucleus systems.     

Nuclear deformations in the pairing-plus-quadrupole model (III). Static nuclear shapes in the rare-earth region

The potential energy of deformation (β, γ), is calculated with the pairing-plus-quadrupole model for nuclei with N=82–126, Z=50–82. There is a sudden onset of deformation in the N=86–90 region, and the static nuclear shape, the lowest minimum of the potential function, changes from spherical to prolate. The disappearance of deformation in the Z=74–80 region is more gradual, and the static shape changes from prolate to asymmetric to oblate to spherical. The energy of zero-point motion is calculated, and it is concluded that all the stable deformed shapes of the region are prolate. Proton and neutron energy gaps, intrinsic quadrupole moments, moments of inertia and gyromagnetic ratios of the doubly even nuclei of the rare-earth region are calculated and compared with the experimental data.     

On the valence model for radiative capture

We give several parametrizations for the elastic scattering and radiative capture cross sections for low neutron bombarding energy and discuss the relationship between the corresponding resonance parameters. We then perform an extensive investigation of the valence radiative capture model of Lane and Lynn. This model is formulated here in the frame of the shell-model approach. We exhibit the similarities and differences between our results and those derived from the R-matrix approach by Lane and Lynn on the one hand and from the optical-model approach by Lane and Mughabghab on the other hand. Particular attention is paid to the choice of the average potential well in the shell-model approach, in relation to the proper way to identify theoretical quantities and phenomenological parameters. We show that practically equivalent results can be obtained from a complex average potential well and from a suitably chosen real potential well, respectively. The following topics are investigated formally and numerically: dependence of the various theoretical expressions on the choice of the (real or complex) average potential well; relative importance of external and internal capture; dependence of photon widths and background cross section on mass number (for thermal energy and for E = 100 keV); dependence of the resonance parameters and background cross sections on energy, for A = 60; comparison between experimental data and theoretical values for radiative capture on ⁵⁶Fe and ⁶⁰Ni. We discuss the conditions of validity of the valence capture model. In particular, we investigate the role of the giant dipole resonance and of the closed channels. We argue that the success of the valence capture model is intimately related to the importance of external capture. The contribution of the low-lying excited target states is investigated formally and numerically; it increases with mass number and tends to diminish the correlation between neutron and photon widths, which is implied by the valence capture model.     

Toroidal,compression, and vortical dipole strengths in 144-154Sm: Skyrme-RPA exploration of the deformation effect

All transfer reactions and radiative capture nuclear-astrophysical reactions at low energies measured so far are analysed using a reaction theory that contains overlap functions between the wave functions of the target and residual nuclei. These overlaps are assumed to have an asymptotic form determined by the separation energy of the transferred (or radiative captured) cluster and such an assumption is incorporated into all reaction codes. We point out that although this asymptotic form is dominant for the majority of the transfer reactions and the nuclear-astrophysical radiative capture reactions, for some cases the overlap function has anomalous asymptotic behavior. This behavior originates from virtual decays of the complex nucleus into intermediate channels and, mathematically, is generated by contributions from the singularities of the triangle Feynman diagram and the generalised triangle diagram containing a loop. In the present work, these contributions are investigated in detail and expressions are derived for the strengths of the anomalous terms taking spin variables and the Coulomb effects into account. We present specific examples of nuclear vertices with anomalous asymptotics and discuss their application for peripheral nuclear processes.     

On the shell model and its application to the deformation energy of heavy nuclei

The technical realisation of the shell model with arbitrary fields is presented in detail, with special emphasis of the unusual and large deformations of the nuclear shape as they may occur in the fission process. We discuss how realistic parametrisations of the nuclear shape and the potential well can be developed and how the parameters of the average fields can be determined. We restrict ourselves to wells with a Woods-Saxon distribution in the radial coordinate. By means of Strutinsky's shell correction approach, the single particle energies deserve to calculate the potential part of a collective Hamiltonian. Its behaviour with varying deformation is discussed both qualitatively and quantitatively. Considered are the deformation types of elongation (1), necking (2), reflection (3) and axial (4) asymmetry of the nuclear shape. Evidence is given for geometrical symmetries which can be correlated to normal modes in finite nuclei. The transition from spherical to deformed nuclei is presented in detail for the radium isotopes, revealing the importance of hexadecapole deformations. Finally, we give an extensive and systematic presentation of the energies and of the deformations at the various stationary points of the deformation energy for the nuclei in the actinide region and for the hypothetical superheavy nuclei.