Evaporation barriers for4He indicate very extended forms for many emitting nuclei

We have analyzed a large set of mean energies and angular anisotropies for evaporative⁴He emission to obtain barriers to evaporation,B. These exit channel barriers are often substantially smaller than the corresponding empirical s-wave fusion barriersE ₀. The differences (E ₀-B) are interpreted as indicators of the extent of distortion of the emitters. These distortions have in turn been characterized by the deformation parameter for a spheroid α₂₀. ForZ=80 the dependence ofB or {α}₂₀ on spin is somewhat suggestive of the superdeformed shapes predicted by the liquid drop model. ForZ>70 significant distortions are indicated for emitters of both large and small spin.     

Particle evaporation from hot nuclei withA ≦85 mainly prior to scission

We report on measurements of fusion-fission fragments and evaporation residues from the reaction⁵⁸Ni +²⁷Al at 835 MeV incident energy leading to compound nucleiA ≦ 85 with high excitation energies. In the case of fission the mass dependence of the velocity widths of separated fragments indicate that the evaporation of light particles predominantly occurs from the composite nucleus prior to scission. The fraction evaporated from the fully separated fragments after scission can be estimated to approximately 20%.     

Alpha clustering in 4n nuclei from 4He to 40Ca

The α clustering in nuclei from ⁴He to ⁴⁰Ca has been presented on a systematic footing which depicts the similarities from nucleus to nucleus. Here, the isomorphic shell model has been employed, which is a hybrid between the conventional shell model and liquid drop model in conjunction with the nucleon finite size and which, in addition, uses no adjustable parameters. In the framework of the model an α-like particle is defined as four close-by nucleons (two neutrons and two protons) in relative angular momentum zero. Thus, up to ⁴⁰Ca nine such α-like particles and two deuterons are formed whose average positions are well specified in the model. Hence, each time an α-like particle is formed (following the aforementioned definition), this could have an average position only at one of the above nine available positions for such particles. Any 4n nucleus arranges its n α-like particles in the same way and any such arrangement corresponds to the ground state or to an excited state of this nucleus and serves as the band head of a rotational band. For ²⁰Ne nine such bands have been found, while for ¹²C and ²⁸Si two and five bands, respectively. The linear α-chain for ¹²C and persisting α-planar structures for heavier nuclei appear in a natural way in the framework of the model and are supported by many observables. The real novelty of this presentation is the fact that the axis of rotation and the number of rotating nucleons inside the same rotational band may change in such a way that the relevant moment of inertia increases monotonically in steps forming for each step a new branch of the band. Thus, several such bands have the same band head, a fact which closely resembles the phenomenon of superdeformation. This phenomenon here is the result of existence of several axes of symmetry and of several axes of rotation which, by changing the axis of rotation, permit the moment of inertia to increase up to the solid body limit.     

Analytical calculation of fusion barriers and cross-sections for spin-saturated colliding nuclei

We present an analytical formula for calculating the fusion barriers and fusion cross-sections. This is based on Skyrme energy density formalism and the Proximity force theorem. Comparisons with experimental data are also presented.     

Fusion barriers,empirical and theoretical: Evidence for dynamic deformation in subbarrier fusion

Semiempirical fusion barriers have been obtained by analysis of 87 excitation functions for complete fusion. These barriers are used as a test for several theoretical potentials. For most of the reactions at energies below the operational s-wave fusion barrier, the cross sections are greater than expected from considerations of simple barrier penetration alone. We ascribe this to dynamic effects, such as neck formation which may be expected as precursors of fusion. The effects of static deformation do not appear to be sufficient to explain the discrepancies.     

Expansion and evaporation of hot nuclei: Comparison between semi-classical and quantal mean-field approaches

We present a general discussion of the mean field dynamics of finite nuclei prepared under extreme conditions of temperature and pressure We compare the prediction of semi-classical approximation with complete quantum simulations. Many features of the dynamics are carefully studied such as the collective expansion, the evaporation process, the different time-scale, etc. This study points out many quantitative differences between quantum and semi-classical approaches. Part of the differences are related to numerical features inherent in semi-classical simulations but most of them are a direct consequence of the non-treatment of nuclei as quantal objects. In particular, we show that because of a too strong damping in semi-classical approaches the expansion of hot nuclei is quenched and the speed of the collective motion reduced.     

Neutron emission in interactions of 1H, 2H, 4He, and 12C nuclei with lead nuclei at 1–2 GeV per nucleon

The double differential cross sections for neutron production that were measured by the time-of-flight method for interactions of 2-GeV p and d, 4-GeV ⁴He, and 24-GeV ¹²C with Pb nuclei are discussed. In the phenomenological model of four moving sources, the neutron energy distribution shape at emission angles above 30° is well reproduced with the temperature parameters for all sources that are almost independent of the type and the energy of incident nuclei. Using the developed model, we estimate the mean neutron multiplicity and the energy removed by neutron emission.     

Comparison of Brueckner theory with “exact” results for H3 and He4 nuclei

The consistency of the second-order Brueckner-Baranger approximation to the ground-state energyE of H³ and He⁴ nuclei is confirmed by the agreement ofE with the recent Faddeev and Faddeev-Yakubovsky equation results for Reid local potential, by the dependence ofE on different phase-equivalent transforms (= off-shell changes) of the Reid potential which agrees with Faddeev equation and nuclear matter calculations, and by the Coester line well reproduced for a set of realistic potentials in He⁴. The influence of approximations in solving Bethe-Goldstone equation is also discussed.     

Deuteron size effects on its elastic scattering from1H and4He

The effect of change in deuteron size on its elastic scattering from protons and alphas is investigated by varying the Hulthen parameters of the deuteron wave function in the scattering process. The cross sections forp-d scattering, calculated in the Born approximation, are found to increase substantially at backward angles even when the deuteron size is reduced by a small amount, whereas the shape of the angular distribution does not change significantly. For theα-d elastic scattering, interaction potential is obtained by folding the deuteron wave function and the optical potential for nucleon-scattering. The cross sections calculated atE _d = 13·7 MeV, shows that the first minimum around Θ_cm = 60° is deepend as the deuteron size is reduced, while at 52 MeV bombarding energy, the size effects are not very distinct. These observations are useful in the interpretation of deuteron cluster knockout reactions.     

Microscopic study of the low-energy3He(3He, 2p)4He and3H(3H, 2n)4He fusion cross sections

We have calculated the³He(³He, 2p)⁴He and³H(³H, 2n)⁴He reaction cross sections at low energies within the microscopic multichannel resonating group method. For both reactions, we find good agreement with experiment. For the³H(³H, 2n)⁴He reaction, our calculated energy dependence reproduces that of each individual low-energy experimental data set, except for a normalization constant. Using this fact, we derive at a low-energy³H(³H, 2n)⁴He rate by taking the averaged mean of these fits.This work has been supported in part by the National Science Foundation, Grants PHY86-04197 and PHY88-17296.     

超重核合成时的驱动势与热熔合反应截面

在双核模型框架下,双核系统生成超重复合核的机理是由双核中的弹核的核子全部转移到靶核产生的,而核子转移是由双核系统驱动势确定的.对有的反应道,核子转移与中质比变化路径之间有比较复杂的关系.原则上动力学方程与驱动势都应该是中子和质子的二维显函数.为处理方便,采用与中质比相关的核子转移路径的选择来取驱动势,得到了接近实验值的超重核合成蒸发剩余截面. 关键词： 超重核 熔合反应 驱动势 激发函数     

Threshold enhancement in η photoproduction from 2H and 4He

The photoproduction of η-mesons from ²H and ⁴He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The η-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross-sections, angular and momentum distributions of the η-mesons have been determined for both reactions. The total cross-sections in the threshold region show a large enhancement over the predictions of a participant-spectator model, indicating significant final-state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-η final-state interaction effects on different levels of sophistication. Received: 23 November 2001 / Accepted: 11 February 2002     

Relativistic corrections to electron scattering by 2H, 3He,and 4He

Relativistic corrections to the scattering of electrons by a nonrelativistic nucleus are calculated, using the method of Close and Copley, and Krajcik and Foldy. Particular attention is paid to the deuteron, ³He and ⁴He. It is found that these corrections are substantially the same as those calculated by F. Gross for the deuteron. The corrections to ³He have the effect of moving the diffraction minimum, as calculated in a nonrelativistic model, outward by as much as 0.32 fm^?2. Smaller corrections are found for ⁴He.     

Noneikonal approach to the scattering of hadrons from nuclei. II. Proton and pion scattering from D, 3He and 4He (kL ≳ 1 GeV/c)

A previously formulated noneikonal theory for hadron-nucleus scattering valid for all angles is applied to p and π scattering on the lightest targets (k_L ? 1 GeV/c). When the required input is known, calculated differential cross sections agree well with experiments out to medium angles, far beyond the region of previous analyses. In the case of pA scattering at k_L = 1.2 GeV/c at which momentum some parameters in the elementary amplitudes F_xN are badly known, limits on their values have been obtained by demanding simultaneous agreement between theory and data for several targets. Generally speaking the agreement evidences the existence of some major corrections to the eikonal amplitude, namely, propagator off-shell and nucleon recoil effects. Inclusion of the latter primarily affects the single scattering amplitude, which may dominate the large-q² cross section if F_xN (like F_pp) itself rises with q². The calculation of pD and p⁴He cross sections requires knowledge of body form factors S_A often unknown for large q². Consistency requirements for angular distributions at several k_L lead to a conjectured large-q² behavior of S_A(q). In the case of the D the conjecture has been largely confirmed by a recent measurement of the electric form factor. The agreement leaves little room for $\text{NN}{}^1$ components in the D as heretofore derived from pD scattering. We speculate that in general, reasonable estimates for nuclear form factors out to very large q² may be obtained from hadron-nucleus scattering data with existing facilities. Since higher q² values can be reached in hadron-nucleus than in eA scattering, the former may well provide the first tests for predicted scaling of nuclear form factors.     

Dispersion relations and low-energy elastic scattering of charged particles from nuclei: (II). Application to N-4He scattering

A “Coulomb-modified” dispersion relation is applied to low-energy p-⁴He forward elastic scattering. To check the validity of the modification, the results are compared with those deduced from n-⁴He elastic scattering. Completely analogous information is obtained for the two processes. The exchange of three bound nucleons is found to contribute strongly to N-⁴He forward scattering. The corresponding ⁴He-³H-p and ⁴He-³He-n coupling constants are evaluated as R_p = 3.8±0.3 and R_n = 3.0±0.3, respectively. These constants are related to the strength of the asymptotic wave function of nucleons in ⁴He and thus to the tail of the nucleon distribution. A comparison of R_p with the empirical proton distribution in ⁴He as deduced from e-⁴He elastic scattering shows excellent agreement. From R_p and R_n the effective ranges of the singlet ³H-p and ³He-n interactions in the ground state of ⁴He are determined to be equal, in accordance with charge symmetry, and to have a value of r_eff = 1.072±0.006 fm.     

Large-space cluster model calculations for the 3He(3He,2p)4He and 3H(3H,2n)4He reactions

The ³He(³He,2p)⁴He and ³H(³H,2n)su4He reactions are studied in a microscopic cluster model. We search for resonances in the ³He+³He and ⁴He + p + p channels using methods that treat the two- and three-body resonance asymptotics correctly. Our results show that the existence of a low-energy resonance or virtual state, which could influence the ⁷Be and ⁸B solar neutrino fluxes, is rather unlikely. Our calculated ³He(³He,2p)⁴He and ³H(³H,2n)⁴He cross sections are in a good general agreement with the experimental data.     

Accretion model for a hot source formation in 72MeV proton induced 3He and 4He emission

Energy spectra and angular distributions of the (p, ^3,4He) reaction on ⁵⁸Ni, ⁹⁰Zr, ²⁰⁹Bi and ²³²Th are analyzed in the framework of an extended, moving-hot-source model. The mass-temperature relation of the hot source is described as the result of an accretion process. The cooling curve and the total emission cross sections are calculated and presented as a function of the emission time.     

Excitation functions for complete fusion and transfer reactions in the interaction of 6He with 197Au nuclei

The excitation functions for the fusion reactions with subsequent neutron evaporation, ¹⁹⁷Au(⁶He, xn)^{203 ? xn}Tl (x = 2?7), and the neutron transfer reactions with production of the ¹⁹⁶Au and ¹⁹⁸Au isotopes have been measured. Unusually high cross section of the ¹⁹⁸Au isotope production at energies below the Coulomb barrier has been observed. Possible mechanisms of production and decay of the transfer reaction products are considered.