Fast neutron emission from partial fusion heavy ion reactions

The Boltzmann master equation theory which succesfully reproduces the spectra of fast nucleons emitted in fusion and quasi-fusion heavy ion reactions is used to predict the spectra of fast nucleons in coincidence with a projectile-like fragment in the interaction of 35 MeV/ nucleon¹⁴N with¹⁶⁵Ho. In particular it is shown that the spectra of neutrons in coincidence with a high energy projectile-like fragment emitted at a very forward angle with energy corresponding to the beam velocity may be reproduced satisfactorily assuming an elastic breakup of the projectile followed by fusion of one of the fragments with the target nucleus.     

Origin of projectile-like fragments from40Ar +68Zn collisions between 15 and 35 MeV/nucleon: Direct transfer of nucleons and projectile fragmentation

The projectile-like fragments emitted in the⁴⁰Ar +⁶⁸Zn reaction performed at 14.6, 19.6, 27.6 and 35 MeV/nucleon are studied. Their energy spectra and angular distributions have been measured. Velocities, widths of the linear momentum distributions and cross sections have been deduced. The results are discussedi) in terms of transfer of a few nucleons and analysed with a diffractional model. They are consistent, for stripping reactions, with a direct transfer of nucleons and a target excitation yielding multiparticlemultihole configurations,ii) In terms of projectile fragmentation-like process with a modified abrasion model taking into account the energy separation of the participant nucleons. The projectile fragmentation-like process appears above 20 MeV/nucleon and strongly competes with transfer of nucleons at 35 MeV/nucleon. The evolution of the mechanisms with incident energy is discussed on the basis of the reduced widths of the linear momentum distributions and on those of velocities and cross sections.     

MULTIPLICITY DISTRIBUTIONS OF PRODUCED PARTICLES IN α-α COLLISIONS AT HIGH ENERGIES

We analyse the multiplicity distributions of negative particles produced in α-α collisions at S_NN=31.2GeV by using a Glauber-type multiple collision model in which the projectile nucleons degrade in energy as they make collisions with target nucleons.The energy loss suffered by the projectile nucleons in the passage through the target nucleus is explicitly considered in the calculation.The agreement with experiments is good.     

Nucleus excitation and deexcitation following at rest

The evolution of a nucleus after at rest is studied. The process is divided into two successive phases: a rapid phase, described by an intranuclear cascade model and a slow phase, akin to an evaporation process. The distribution of the residue masses after the second phase is calculated and compared to the recent radiochemical measurements for ⁹⁸Mo. The evolution of the number of pions, and the distribution of the number of fast ejected nucleons and of participants is studied. The latter two are fitted by negative binomials. The meaning of this observation is discussed. The mass dependence of several quantities, like the maximum excitation energy and the average mass loss is also investigated. Finally, the possibility of a multifragmentation is discussed.     

用高能核子非弹性散射研究核力有效势

本文用扭曲波玻恩近似法及多体高能近似法,处理了原子核对高能核子的非弹性散射现象。在具有可靠的靶核激发态波函数的情况下,可利用这些理论处理方法研究核内两核子间的有效势,本文具体就碳核对185MeV入射核子的非弹性散射进行了计算。在计算中利用了粒子-空穴模型核波函数。在采用了具有各种交换性质并包含自旋轨道耦合项的有效势后,用一组合理的位阱参数,由多体高能近似法计算的理论值可与几个微分截面及极化实验曲线同时符合。     

Phenomenological analysis of fission induced by high-energy protons

High-energy proton induced fission is studied in the framework of a two-step model. In the first step, the projectile penetrates the target nucleus, knocks out few nucleons and leaves the residual nucleus with a spectrum of excitation energies depending upon the number of projectile-nucleon collisions. This stage is described in terms of a simplified version of Glauber's multiple-scattering theory. The second stage in which the residual nucleus fissions, is treated by assuming phenomenological expressions for the dependence of the fission probability on excitation energy which take into account the onset of fragmentation at a certain “crack” energy. Comparison with experimental data suggests that high energy fission of heavy nuclei proceeds in a way similar to low-energy fission. Light nuclei, however, require a more violent fission mechanism.     

High Energy Nucleus–Nucleus Collisions in View of the Multi-Peripheral Model

The hypothesis of the ulti-peripheral odel is extended to the hadron –nucleus interactions and then generalized to the nucleus –nucleus case. The processing of the odel depends on input parameters that are extracted fro the features of the experiments in this .eld.The number of encountered nucleons from both target and projectile are estimated according to the eikonal scattering approach.The screening effect due to the interaction of the projectile nucleons in successive manner with the target nucleus is considered.The rapidity distributions of fast particles are reproduced for the successive collisions in p –S and 32 S –32 S interactions at 200 A GeV.A global fair agreement is found in comparison with data of the experi ent CERN-NA-035.     

Multifragmentation and incomplete fusion in central heavy ion collisions within a schematic phenomenological model

The projectile-energy thresholds for nuclear multifragmentation in the collisions of ¹²C, ²⁰Ne and ⁴⁰Ar with ¹⁹⁷Au have been calculated using a schematic model applicable to central collisions of intermediate energy heavy ions. The properties of the composite nucleus which remains following the incomplete fusion of the projectile and target nuclei are calculated using the master equation approach of Harp and Miller to determine the number of preequilibrium nucleons emitted and the energy and linear momentum removal by these nucleons. This nucleus is followed through an isentropic expansion phase and the probability distribution of clusters is determined using a percolation approach based on a 3-dimensional cubic lattice. The input parameters for the percolation calculation are derived from the results of the preequilibrium calculation. While the calculated thresholds in projectile energy per nucleon decrease rapidly as the projectile mass increases, the excitation energy at which multifragmentation is first predicted to occur is approximately the same for each system, i.e., 4.5 to 5 MeV/u. This result is consistent with the apparent disappearance of residual heavy products at such bombarding energies.     

Decay Analysis of Ge Isotopes Formed in Reactions Induced by Tightly and Loosely Bound Projectiles

The dynamical cluster-decay model (DCM) is employed to investigate the decay of ^68,70Ge^* compound nuclei formed respectively via tightly (⁴He) and loosely (⁶He) bound projectiles, using ⁶⁴Zn target. The study is carried out over a wide energy range (E_c.m.~5 MeV to 16 MeV) by including the quadrupole deformations (β2i) and optimum orientations (θ_i^opt) of the decaying fragments. The fusion cross-sections, obtained by adding various evaporation channels show nice agreement with the experimental data for ⁴He+⁶⁴Zn reaction. The contribution from competing compound inelastic scattering channel is also analyzed particularly for ⁶⁸Ge^* nucleus at above barrier energies. On the other hand, the decrement in the fusion cross-sections of ⁷⁰Ge^* nuclear system is addressed by presuming that ⁶⁵Zn ER is formed via two different modes:(i) the αn evaporation of ⁷⁰Ge^* nucleus, and (ii) 1n-evaporation of ⁶⁶Zn^* nuclear system, formed via breakup and 2n-transfer channels due to halo structure of the ⁶He projectile. Besides this, the suppression in 2np evaporation cross-sections suggests the contribution of another breakup and transfer process of ⁶He i.e. ⁴He+⁶⁴Zn. The contribution of breakup+transfer channels for ⁶He+⁶⁴Zn reaction is duly addressed by applying relevant energy corrections due to the breakup of " ⁶He" projectile into 2n and ⁴He. In addition to this, the barrier lowering, angular momentum and energy dependence effects are also explored in view of the dynamics of chosen reactions.     

Low-energy antiproton annihilation in nuclei

The annihilation of 100 MeV antiprotons in atomic nuclei is studied in the frame of an intranuclear cascade model. It is found that the antiproton is annihilated about 1 fm inside the nucleus on the average. The energy released by the annihilation is carried by an average of 5 pions, which cascade throughout the nucleus. About 5% (10%) of the primordial pions are absorbed by the ⁴⁰Ca (¹⁰⁸Ag) target. The pions transfer around 5̃50 MeV (7̃00 MeV) to the nucleons and eject about one-fifth of the nucleons. The pion and proton cross sections are calculated. In particular, the relative transparency of the nucleus to high-energy pions (and not to pions in the Δ-resonance region) gives rise to a peculiar pion emission pattern. The time evolution of the baryon density and of the spectrum of the participant nucleons is investigated. It turns out that the cascade does not generate high energy density.     

Scattering of deuterons by nuclei with breakup effects

The deuteron direct-breakup reaction is explicitly incorporated into the d + nucleus formulation of model K which is a simplified version of the microscopic resonating-group method. By employing the same nucleon-nucleon potential as that used in the corresponding nucleon + nucleus system and by adopting an imaginary potential with a single adjustable parameter to crudely represent other reactions not explicitly included in the formulation, it is shown that a good agreement between calculated and experimental differential cross sections can be obtained for deuteron scattering by a wide variety of target nuclei, ranging from ²⁸Si to ²⁰⁸Pb. In addition, the results also indicate that the deuteron breakup reaction is a significant, but not dominant, reaction in the d + nucleus system. To make model K into a truly microscopic model without any phenomenological imaginary potential, one must further explicitly include the (d, p) and (d, n) one-nucleon-transfer or stripping reactions into the model-K formulation.     

Intra-Nuclear Cascade (INC) code for simulation of meson photoproduction in nuclei

η meson photoproduction in the ¹⁴N nucleus was studied using simulations done with INC (Intra-Nuclear Cascade) code. The probability for mesons and nucleons to escape the nucleus on different steps of the reaction was evaluated. These results predict the possibility of studying elastic and inelastic η meson interactions with intra-nuclear nucleons using the recoil proton as a tag for the case of slow η meson and fast recoil proton.     

Gross properties of heavy-ion transfer reactions: (II). Comparison with experiment

A semiclassical theory of heavy-ion transfer reactions developed in our previous paper is applied to the analysis of the energy spectra, angular distributions and polarizations of outgoing particles. We treat reactions involving transfers of one, two and seven nucleons, and one and two α-particles. A one- or two-step process is assumed. A straight-line trajectory of relative motion is used in most cases. But better agreement for the polarizations can be obtained if we assume that the incident ion rolls around the target nucleus for a short time interval.     

Inelastic interactions of fast nucleons with nuclei and fission of nuclei

The cross section for the fission of actinide nuclei that is induced by fast neutrons is considered as a fraction of the cross section for inelastic nucleon interaction with nuclei. In turn, inelastic nucleon interaction with a nucleus is treated as scattering on intranuclear nucleons. It is shown that this interaction model describes satisfactorily the cross section for the inelastic interaction of 60- to 2200-MeV nucleons for a broad set of nuclei and that the energy dependence of the cross section for the fission of actinide nuclei that is induced by 400- to 1000–MeV protons replicates the energy dependence of the cross section for inelastic interactions with respective nuclei. From the model used, it follows that the cross sections for proton-nucleus interactions exceed cross sections for respective neutron-nucleus interactions in the energy range extending up to 550 MeV; at higher energies neutron cross sections are larger than proton cross sections.     

Pauli exchange effects in the elastic scattering of O + O

The real optical potential for ¹⁶O+¹⁶O system is calculated within a generalized version of the double-folding model with the Pauli knock-on exchange effects between the projectile nucleons and the target nucleons taken into account from first principle. The elastic scattering data at E_lab=350 MeV supplemented by the new measurement at larger angles seem to be the first case in heavy-ion scattering where one can test the reliability of different theoretical heavy-ion optical potentials. Predictions are made for the elastic scattering of ¹⁶O+¹⁶O at laboratory energies of 240–480 MeV to illustrare the energy dependence of the rainbow structure which has been clearly observed in experiment at 350 MeV.     

Possibility of ~5He emission in neutron induced reactions

There are some statistical model codes[1—4] as the evaluation tool that have been long and widely used to set up neutron data library below 20 MeV, which is the most important energy region in the application of nuclear engineering. The emitted particles considered in these codes mentioned above are neutrons, protons, as well as the compos-ite particles, such as deuterons, tritons, 3He and alpha particles. These emitted particles and nuclei can be treated as the nucleon or stable clusters. H…     

A Simulation of Emitted Nucleons:An Extended BUU Model

On the basis of the old BUU method,the neutrons and protons of the projectile and target nucleus was distinguished,meanwhile,the droplet model which included the surface diffusion parameter,neutron skin and deformation parameter etc.was considered during simulating original nucleon's distribution.Condersing the fact σ_np≈3σ_nn≈3σ_pp in intermediate energy domain,it was incorapated into the collision term of BUU.Before studying 44MeV/A ⁸⁶Kr+¹⁹⁷Au reaction by the improved BUU model,the stability of the model has been checked,then the condition of nucleons emission was defined,finally the impact parameter dependence of the abraded neutrons and protons from the projectile was obtained,so was the correlation of TLF and PLF.