Meson-exchange contributions to the nuclear charge operator

The role of the meson-exchange current correction to the nuclear charge operator is studied in electron scattering processes involving the excitation of medium and heavy nuclei to energies up to the quasi-elastic peak. The effect of these contributions in the quasi-free electron scattering process is a reduction of at most a 3% in the longitudinal response at the energy of the peak, a value which is below the experimental error and must not be taken into account in calculations in this energy region. On the other hand, the excitation of low-lying nuclear levels of neutronic character shows, with respect to the protonic ones, a considerable effect due to the inclusion of the two-body term in the charge operator. More realistic calculations, such as those performed in the random-phase approximation framework, give rise to a mixing of one particle-one hole configurations of both kinds which reduce these effects. However, it has been found that the excitation of some of these levels is sizably affected by the meson-exchange contribution. More precise experimental data concerning some of these states, such as e.g. the high-spin states in ²⁰⁸Pb, could throw some light in the problem of a more feasible determination of these effects and, as a consequence, could provide an alternative procedure to determine the charge neutron form factor.     

Excitation of collective states in spherical nuclei in high-energy proton interactions

The excitation of collective states in spherical nuclei is investigated in interactions with ≈ 1 GeV protons. Collective states of dipole and quadrupole type in a large energy range, including the isoscalar and isovector giant resonances, are considered. The cross sections for proton-nucleus interactions are found within the single-inelastic-scattering approximation of Glauber theory with the use of semi-microscopic nuclear wave functions. The largest probability of excitation is proved to take place for states of isoscalar type. Isovector states are strongly suppressed. It is shown that 1 GeV proton inelastic scattering can be used for the excitation of giant resonances of isoscalar type. The probability of their excitation is of the same order as that for low-lying collective states.     

Inelastic scattering of heavy ions at intermediate energies with excitations of nuclear collective states

Excitation of low-lying nuclear collective states upon scattering of heavy ions with energies of several tens of MeV/nucleon has been studied. The interaction potential leading to excitation is chosen in the form of a derivative of the microscopic (or semimicroscopic) nucleus-nucleus double-folding optical potential. Elastic and inelastic scattering cross sections have been calculated within the high-energy approximation; the inelastic scattering amplitude was obtained in the first order in the deformation parameter. The cross sections are compared with the experimental data on scattering of ¹⁷O from a series of nuclei with excitation of the 2⁺ level.     

Giant magnetic dipole analog excitations by the allowed muon capture in 1p-shell nuclei

The distributions of the allowed muon capture strengths in 1p-shell nuclei are shown to exhibit a tendency to concentrate in a few low-lying daughter states. These strong excitations are analogs of the giant magnetic dipole transitions, predicted by Kurath, and observed in inelastic electron scattering. Similar excitations are expected in 1S radiative pion capture and low-energy charged pion photoproduction processes.     

Electron impact excitation of tetrasulfur S4 molecule

Electron impact excitation of tetrasulfur S₄ molecule is investigated in this work using R-matrix method. The twelve low-lying electronic states are used in close-coupling expansion to represent the target states, and the integral cross sections for elastic scattering and excitation into the six lowest electronic states are calculated.     

Electromagnetic transition properties and isospin excitation in the cross-conjugate nuclei 44Ti and 52Fe

The interacting boson model with isospin (IBM-3) is applied to study the band structure and electromagnetic transition properties of the low-lying states in the cross-conjugate nuclei 44Ti and S2Fe. The isospin excitation states with T=0, 1 and 2 are identified and compared with available data. The E2 and M1 matrix elements for the low-lying states have been investigated. According to this study, the 2+3 state is the lowest mixed symmetry state in the cross-conjugate nuclei 44Ti and 52Fe. The excitation energy of the second 0+2 and 2+2 states with T=0 in the nucleus 52Fe are identified. The agreement between the model calculations and data is reasonably good.     

A study of the nuclear response function

We examine the excitation properties of spherical nuclei in the Random Phase Approximation using the Green's function method. The calculations are done with interactions of the Skyrme type for nuclei from ¹⁶O to ²⁰⁸Pb. Different Skyrme interactions can have the same predictions for ground state Hartree-Fock properties, but give quite different predictions for the dynamic response. Our calculations favor a mild velocity-dependence in the interaction, such as given by Skyrme I. The level of agreement with empirical properties is as follows: energies of low-lying states, ≈25%; positions of giant resonances, ≈10%; transition rates of low states, factor of 2 typical. Inelastic scattering of electrons is reasonably accounted for by the model, but nucleon inelastic scattering has difficulties with the noncollective strength.     

Electron form factors of deformable nuclei

Using the smallness of the deformation parameter of the nucleus, we obtain simple explicit expressions for the form factors of electroexcitation of the low-lying rotation-vibration states of light, deformable, even-even nuclei. The expressions satisfactorily describe the experimental data on the excitation of collective nuclear states by the inelastic scattering of fast electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 62–66, September, 1987.     

多任务神经网络对原子核低激发谱的研究

原子核低激发谱对深入理解原子核结构具有重要作用。采用多任务反向传播(Back Propagation,BP)的神经网络方法系统研究了原子核2 ₁⁺和4 ₁⁺的激发能量。除了质子数和中子数外,通过在网络输入层增加一个有关原子核集体性的物理量,BP神经网络在0.1 MeV到数MeV的能量范围内很好地拟合了原子核的低激发能。相比五维集体哈密顿量(Five-Dimensional Collective Hamiltonian,5DCH)方法,BP神经网络更好地再现了原子核低激发能的同位素趋势,以及由壳效应导致的幻数原子核低激发能的突然增大,并且将2 ₁⁺和4₁⁺激发能的预言精度分别提高了约80%和75%,该预言精度与单任务神经网络基本一致,但是改进了对轻核区与缺中子核区低激发谱的学习能力,这说明多任务神经网络可以实现多种激发能量的统一精确计算。     

Excitation and decay of low-lying nuclear states in a dense plasma produced by a subpicosecond laser pulse

The excitation of low-lying nuclear levels in a hot, dense plasma, produced by a subpicosecond pulse with intensity exceeding 10¹⁶ W/cm², is investigated theoretically and experimentally. The basic channels of electronic (inelastic scattering and inverse internal electron convergence) and photon (photoexcitation) excitations of such states as well as the influence of the broadening of a nuclear level on the excitation efficiency and the presence of hot electronic component are examined. The experimental data from measurements of the decay kinetics of the low-lying nuclear level 6.238 keV of the stable isotope ¹⁸¹Ta, which were obtained on two experimental laser systems, are presented.     

Gamow-teller strength function and the missing strength

The Gamow-Teller strength function including all 2p2h states with excitation energy up to 4 ?ω is calculated for the ⁴⁸Ca(p, n)⁴⁸ Sn reaction. A detailed discussion on the treatment of the self-energy term in connection with the non-energy-weighted sum rule is presented. The quenching of the Gamow-Teller strength in the low-lying region may chiefly be explained by the tensor correlation, and partly by some vertex corrections due to the Δ-hole and other exchange currents.     

Predicting narrow states in the spectrum of a nucleus beyond the proton drip line

Properties of particle-unstable nuclei lying beyond the proton drip line can be ascertained by considering the (usually known) properties of its mirror neutron-rich system. We have used a multichannel algebraic scattering theory to map the known properties of the neutron-14C system to those of the proton-14O one from which we deduce that the particle-unstable 15F will have a spectrum of two low-lying broad resonances of positive parity and, at higher excitation, three narrow negative-parity ones. A key feature is to use coupling to Pauli-hindered states in the target.     

Excitation of discrete levels of <Superscript>63</Superscript>Cu and <Superscript>65</Superscript>Cu nuclei in (<Emphasis Type="Italic">e,e′</Emphasis>) reactions

Inelastic electron scattering on ⁶³Cu and ⁶⁵Cu nuclei at excitation energies of up to 5 MeV was studied. Information about the reduced probability and multipolarity of transitions was obtained for 11 low-lying levels and groups of levels of the ⁶³Cu nucleus and for 17 such levels and groups of levels of the ⁶⁵Cu nucleus. It was shown that the experimental form factors for inelastic scattering accompanied by the excitation of discrete levels and group of levels of these nuclei with a nonzero spin can be described in terms of a single multipolarity, even though the law of angular-momentum conservation admits the existence of several more multipolarities yielding the same parity of an excited state.     

Interaction of 11 MeV neutrons with Y

Differential cross section for scattering of 11 MeV neutrons by ⁸⁹Y were measured using the Ohio University beam swinger time-of-flight facility. Measurements were taken in the angular range between 15° and 145°. Empirical optical model parameters have been obtained from the measured elastic scattering data. Deformation parameters were obtained for low-lying excited states using these optical-model parameters in a DWBA collective formalism. A comparison with deformation parameters in neighboring even-even nuclei ⁸⁸Sr and ⁹⁰Zr is presented. The weak-coupling model is used to describe neutron scattering to low-lying states in ⁸⁹Y.     

Analyses of electron and proton scattering to low-excitation isoscalar states in 20Ne, 24Mg and 28Si

The excitation of low-lying isoscalar 2⁺ and 4⁺ states in ²⁰Ne, ²⁴Mg and ²⁸Si by electron and proton scattering is studied. Large basis models of nuclear structure are used to determine both the electromagnetic and hadronic transition densities. The analyses of the longitudinal form factors obtained from electron scattering show that little or no effective charges are required with these nuclear structure models. Proton inelastic scattering to these states then is analysed to test effective forces based upon the Paris and Hamada-Johnston interactions. At intermediate energies (155 MeV) density-dependent t-matrices from both potentials were used with fits to data giving a clear preference for that based upon the Paris interaction. For lower energies only the Hamada-Johnston t-matrix is available and comparison of analyses of 24 and 49 MeV data made using this (complex) t-matrix with those in which the (real) Paris G-matrix is used as the effective force show a clear preference for the t-matrix. This is particularly the case with analyses of polarization data and suggests that the use of the G-matrix as an effective force in nuclear reaction calculations is inadequate even at low energies.     

对力对希土原子核性质的影响

本文利用粒子数守恒方法分析对力对希土区变形偶偶核的各种性质的影响,其中包括低内部激发态(E≤2MeV)的激发机构、有关的β衰变和奇偶质量差等。为进行这些分析,本文先确定希土变形核内的单粒子能级。分析表明:适当改变参数μ和к以后的Nilsson能极,在考虑对力影响后,能较好地解释奇A核的低内部激发谱的自旋和宇称,并能近似地解释其能级间隔。参数μ,к,和η是由奇A核的实验能谱来确定的。在希土转动区中,к～0.067—0.072,η～4—4.6.     

Photoneutron cross sections for unstable neutron-rich oxygen isotopes.

The dipole response of stable and unstable neutron-rich oxygen nuclei of masses A = 17 to A = 22 has been investigated experimentally utilizing electromagnetic excitation in heavy-ion collisions at beam energies about 600 MeV/nucleon. A kinematically complete measurement of the neutron decay channel in inelastic scattering of the secondary beam projectiles from a Pb target was performed. Differential electromagnetic excitation cross sections d sigma/dE were derived up to 30 MeV excitation energy. In contrast to stable nuclei, the deduced dipole strength distribution appears to be strongly fragmented and systematically exhibits a considerable fraction of low-lying strength.     

Nilsson mean-field plus the extended pairing model description of rare earth nuclei

The Nilsson mean-field plus the extended pairing model for well-deformed nuclei is applied to some representative rare earth examples. The binding energies, some low-lying pair-excited states and even-odd mass differences of Er, Yb and Hf isotopes are calculated systematically within the proton frozen-pair excitation limit. A comparison with experimental data for these nuclei shows that the results of the extended pairing model are better than those for the standard pairing model with the BCS approximation and the nearest-orbit pairing model.     

Lasers as a bridge between atomic and nuclear physics

This paper reviews the application of visible and ultraviolet laser radiation to several topics in low-energy nuclear physics. We consider laser-induced nuclear anti-Stokes transitions, laser-assisted and laser-induced internal conversion, and the electron bridge and inverse electron bridge mechanisms as tools for deexcitation and excitation of low-lying nuclear isomeric states. A study of the anomalously low-lying nuclear isomeric states (in the case of the ²²⁹Th nucleus) is presented in detail.     

Large-angle scattering of heavy ions: (II). General structure of inelastic cross section

Closed-form expressions are derived for the differential cross section of inelastic heavy-ion scattering at large angles. The inelastic transition amplitude given by the distorted-waves theory for excitation of low-lying collective states is evaluated by an extension of analytic methods developed in the preceding paper for elastic large-angle scattering. The very close relation between the inelastic and elastic cross sections is displayed. In particular it is shown that the angular distributions have a universal form, and that the backward-angle inelastic excitation function has, aside from a slowly varying overall energy dependence, an oscillatory gross structure of diffractive origin which is nearly identical to that of the elastic excitation function, irrespective of the physical mechanism involved.