Nuclear fragmentation reactions in extended media studied with Geant4 toolkit

It is well-known from numerous experiments that nuclear multifragmentation is a dominating mechanism for production of intermediate mass fragments in nucleus-nucleus collisions at energies above . In this paper we investigate the validity and performance of the Fermi break-up model and the statistical multifragmentation model implemented as parts of the Geant4 toolkit. We study the impact of violent nuclear disintegration reactions on the depth-dose profiles and yields of secondary fragments for beams of light and medium-weight nuclei propagating in extended media. Implications for ion-beam cancer therapy and shielding from cosmic radiation are discussed.     

Investigation of isotope composition of nuclear fragments with angular momentum and Coulomb effects in peripheral ~(84)Kr+ ~(112,124)Sn collisons at 35 A MeV

New theoretical calculations are performed to investigate the Coulomb proximity and angular momentum effects on multifragmentation picture for84Kr+112,124 Sn collisions at an incident beam energy of 35 Me V/nucleon.Charge and isotopic distributions and the mean neutron-to-proton ratios of the fragments are reproduced within the microcanonical Markov chain calculations on the basis of Statistical Multifragmentation Model. It is shown that the Coulomb interactions and angular momentum effects are very important to reproduce isotopic composition of nuclear fragments in peripheral heavy-ion collisions at Fermi energies. Our results imply that it is possible to investigate in laboratories the modification of structure parameters of fragments, such as the symmetry energy coefficient, at subnuclear densities in dense environment of other species.     

Analysis of n + 6Li Reactions Using the Continuum-Discretized Coupled-Channels Method

The n + ⁶Li reactions are important not only from the basic interest but also from the application point of view. We study ⁶Li breakup reactions at incident neutron energies of 11.5, 14.1, and 18.0MeV applying a d + α cluster model to ⁶Li and using the continuum-discretized coupled-channels (CDCC) approach. At the present energies, our calculation well reproduces the experimental data. This successful result indicates the reliability of the CDCC approach based on the cluster model to the nuclear reaction evaluation of light nuclear reactions.     

Nuclear dynamics in multinucleon transfer reactions near Coulomb barrier energies

The mechanism of multinucleon transfer reactions has been investigated within the dinuclear system model, in which the sequential nucleon transfer is described by solving a set of microscopically derived master equations. The transfer dynamics in the reaction of~(136)Xe+~(208)Pb near Coulomb barrier energies is thoroughly analyzed. It is found that the total kinetic energies of primary fragments are dissipated from the relative motion energy of two touching nuclei and exhibit a symmetric distribution along the fragment mass. The angular distribution of the projectile-like fragments moves forward with increasing beam energy. However, the target-like fragments exhibit an opposite trend. The shell effect is pronounced due to the fragment yields in multinucleon transfer reactions.     

The excitation energy of fragments in nuclear fission

The multiplication factor for chain reactions in a nuclear reactor depends essentially on the number of secondary neutrons emitted during fission, and this is determined completely by the excitation energy of the fragments. Therefore, calculation of the excitation energy of the fragments as a function of the atomic number Z and the atomic weight A of the fissioning nucleus is of considerable interest in atomic energy.In the work it was shown that the excitation energy of the fragments may be found by solving a system of equations for the deformation parameters of the fragments and the distance between them. The basic excitation energies of the fragments were found for some nuclei. An investigation was made of the dependence of the excitation energy on Z and A of the fissioning nucleus.In conclusion, the author would like to thank V. M. Galitskii and V. M. Strutinskii for an interesting discussion and I. G. Krutikova for carrying out the numerical calculations.     

Interpolation of nuclear reaction energy distributions

This paper discusses several methods of interpolation of data for tables of energy distributions for particles emitted by nuclear reactions. The methods are also applicable to angular distributions. Our primary interest is in interpolation with respect to the energy of the incident particle. We first examine four interpolation methods currently used in the nuclear data community: direct interpolation, unit-base interpolation, interpolation by averages on equiprobable bins, and interpolation by corresponding energies. Because each of these methods has shortcomings, a modified version of interpolation by corresponding energies is proposed here. We call this new method interpolation by cumulative points.     

Antiproton-nucleus reactions at intermediate energies

Antiproton-induced reactions on nuclei at the beam energies from hundreds Me V up to several Ge V provide an excellent opportunity to study interactions between the antiproton and secondary particles(mesons, baryons and antibaryons) with nucleons. The antiproton projectile is unique in the sense that most of the annihilation particles are relatively slow in the target nucleus frame. Hence, the prehadronic effects do not much influence their interactions with the nucleons of the nuclear residue. Moreover, the particles with momenta less than about 1 Ge V/c are sensitive to nuclear mean field potentials. This paper discusses the microscopic transport calculations of the antiproton-nucleus reactions and is focused on three related problems:(i) antiproton potential determination,(ii) possible formation of strongly bound antiproton-nucleus systems, and(iii) strangeness production.     

Model-based cross section calculations on production of ~(43,34)Sc,~(45)Ti,~(51)Cr,~(54)Mn,and ~(55)Fe radioisotopes

A cross section database on excitation functions of reactions produced by charged particles is essential for many areas of nuclear research. Particularly, accurate knowledge on nuclear cross sections for the cyclotron production of radioisotopes is very important for nuclear medicine. In the present paper, the cross section calculations for the production of~(43),~(34)Sc,~(45)Ti,~(51)Cr,~(54)Mn, and~(55) Fe radioisotopes were carried out by the use of ALICE/ASH code using the Fermi gas model, Kataria Ramamurthy Fermi gas model, and superfluid nuclear model for nuclear level density. Thereby, these model calculations were compared with the available measured data.     

Theoretical estimates of cross sections for neutron–nucleus collisions

We construct an analytical model derived from nuclear reaction theory and having a simple functional form to demonstrate the quantitative agreement with the measured cross sections for neutron induced reactions. The neutron–nucleus total, reaction and scattering cross sections, for energies ranging from 5 to 700 MeV and for several nuclei spanning a wide mass range are estimated. Systematics of neutron scattering cross sections on various materials for neutron energies up to several hundred MeV are important for ADSS applications. The reaction cross sections of neutrons are useful for determining the neutron induced fission yields in actinides and pre-actinides. The present model based on nuclear reaction theory provides good estimates of the total cross section for neutron induced reaction.     

Nuclear reactions and screened-coulomb fusion rates

We discuss properties of nuclear reactions (branching ratios, reaction constants, etc.) between hydrogen isotopes from the standpoint of recent R-matrix calculation.. Using R-matrix theory to include nuclear effects at small distances, we then develop a general expression for fusion rates in complex systems that, in principle, allows a more complex interplay of the short-ranged (nuclear) and long-ranged (screened Coulomb) forces than does the familiar separable form. Calculations using the latest nuclear R-matrix information in this more correct formulation appear to agree with the separable approximation for screened Coulomb potentials of the Hulthén form, however. They indicate that fusion rates of the order of 10⁻²⁴s⁻¹ require unreasonably large electron densities if they are to result from screening in the lattice, and are more likely to arise from non-equilibrium processes producing particles with relative energies greater than 100 eV.     

基于协变密度泛函理论的258Fm诱发裂变动力学研究

采用基于协变密度泛函理论的含时生成坐标方法研究了258Fm低能诱发裂变动力学性质,重点探讨了裂变位能曲面、裂变碎片总动能分布和碎片质量分布等。研究表明,258Fm位能曲面中存在显著的对称裂变谷,因而其低能裂变碎片总动能分布与质量分布均呈单峰结构,且随剪裂线判据Qn（脖子处粒子数）从4减至1,碎片总动能分布变窄,碎片质量分布的峰值从988%增至1028%。此外,随初态激发能从83 MeV增至173 MeV,碎片质量分布峰值从988%降至855%。     

Theoretical calculations and evaluations of n+23Na reaction

The data for neutron-induced reactions are indispensable in a lot of applications of nuclear science and technologies. All reaction cross sections, angular distributions, energy spectra, and double-differential cross sections of neutron, proton, deuteron, triton, and alpha-particle emissions are consistently calculated and analyzed for n+~(23)Na reactions at incident neutron energies below200 Me V, based on nuclear theoretical models. The calculated results are compared with the experimental data and the evaluated data in the ENDF/B-VII, JENDL-4.0,and JEFF-3.2 libraries. In most cases, the calculated results describe the corresponding experimental data well. At the resonance energy region, evaluated experimental data are adopted to fit to the resonance structures.     

Symmetry energy extraction from primary fragments in intermediate heavy-ion collisions

An improved method is proposed for the extraction of the symmetry energy coefficient relative to the temperature, asym/T, for the heavy-ion reactions near the Fermi energy region, based on the modified Fisher Model(MFM). This method is applied to the primary fragments of the Anti-symmetrized Molecular Dynamics(AMD)simulations for the reactions of40 Ca + 40 Ca at 35 Me V/nucleon. The density and the temperature at the fragment formation stage are extracted using a self-consistent method.     

Proton-Induced Nuclear Reactions in Silicon

Measurements of the energy deposited in silicon surface-barrier detectors as a result of proton-induced nuclear reactions were carried out at the Harvard Cyclotron for protons with incident energies ranging from 50 to 158 MeV and detectors with thicknesses of 2.5, 4.2, 24.1, 100, and 200 ?m. The number of events in which a given threshold amount of energy is deposited in a 4.2 ?m detector varied with incident proton energy in a manner similar to previous measurements of the proton-induced soft-error cross section. The number of events in which at least a threshold amount of energy was deposited in the detector fell off in a near exponential manner with increasing threshold energy. The data were found to be in reasonable agreement with a computer simulation model developed in our laboratory. The model is used to illustrate how the mass spectra of the residual nuclear fragments shifts towards lower masses with increasing recoil energy. Lighter recoils have longer ranges and a greater chance of leaving a microscopically thin sensitive volume element before coming to their end of range.     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.     

Comparison of oxidation model predictions with gasification data of IG-110, IG-430 and NBG-25 nuclear graphite

A phenomenological oxidation kinetics model of graphite is presented and its results are compared with the reported experimental gasification data for nuclear graphite of IG-110, IG-430 and NBG-25. The model uses four elementary chemical kinetics reactions, employs Gaussian-like distributions of the specific activation energies for adsorption of oxygen and desorption of CO gas, and accounts for the changes in the effective surface areas of free active sites and stable oxide complexes with weight loss. The distributions of the specific activation energies for adsorption and desorption, the values of the pre-exponential rate coefficients for the four elementary chemical reactions and the surface area of free active sites are obtained from the reported measurements using a multi-parameter optimization algorithm. At high temperatures, when gasification is diffusion limited, the model calculates the diffusion velocity of oxygen in the boundary layer using a semi-empirical correlation developed for air flows at Reynolds numbers ranging from 0.001 to 100. The model also accounts for the changes in the external surface area, the oxygen pressure in the bulk gas mixture and the effective diffusion coefficient in the boundary layer with weight loss. The model results of the total gasification rate and weight loss with time in the experiments agree well with the reported measurements for the three types of nuclear graphite investigated, at temperatures from 723 to 1226 K and weight loss fractions up to ～0.86.     

The asymmetry of nuclear fission

One of the essential characteristics of nuclear fission and of processes occuring in nuclear reactors is the distribution of fission fragments by mass. According to the drop model of the nucleus, a symmetrical mass distribution of the fragments is most probable, but this, however, is contrary to experiment.To find the mass distribution of the fragments, in this work we calculated the energy of the fissionable nucleus before rapture of the neck, taking into account shell effects, for several nuclei. It was shown that the energy minimum corresponds to asymmetric fission.     

Principle validation of nuclear fuel material isotopic composition measurement method based on photofission reactions

We have developed a new nondestructive assay technique based on the photonuclear reaction, which is aimed at measuring the isotopic composition of nuclear fuel materials without relying on their self-generated neutron information. This methodology enables measurement of the number of neutrons produced by the photofission reactions at different specific photon energies, and only information on the relative counts and not the absolute values or energies, which are considerably affected by backgrounds, are required for the enrichment induction according to a mathematical process. The present methodology estimated the ²³⁵U enrichment value with 8% accuracy when the cross section uncertainty is 5%.     

用转移反应研究6He奇特核体系的光学势

由于受到放射性束强度弱、品质差的限制,奇特核体系的光学势性质一直是亟待解决的国际难题。本工作利用稳定束的转移反应作为探针,深入研究了反应出射道奇特核体系的光学势性质。利用中国原子能科学研究院的HI-13串列加速器在近库仑位垒能区高精度测量了⁷Li+⁶³Cu、²⁰⁸Pb的弹性散射以及单质子转移反应角分布,并利用扭曲波玻恩近似（DWBA）和耦合反应道（CRC）方法对实验数据进行了拟合,抽取了出射道⁶He+⁶⁴Zn、²⁰⁹Bi晕核体系的光学势参数。所得参数可重现文献中已有的6He体系的弹性散射角分布,验证了这种方法的可靠性。对所得势参数的能量相依性的分析表明,在重体系⁶He+²⁰⁹Bi中,基于因果律的色散关系并不适用,其潜在的物理原因还需进一步深入研究。     

High-energy nuclear physics and nuclear astrophysics at the Radium Institute

Research into high-energy nuclear physics and nuclear astrophysics at the Radium Institute is briefly reviewed. The history of cosmic-ray research is outlined, from the early years of the Institute, as well as the history of research on high-energy physics. The basic work on nuclear astrophysics, cosmochronology, and astrochemistry is described. Research at the Institute on direct nuclear reactions, nuclear fragmentation and multifragmentation, deeply inelastic nuclear processes, nuclear fission at high exciting energies, nuclide formation at the limit of stability, multiple particle formation in relativistic internuclear reactions, and other topics is considered. Scientific and methodological accomplishments are noted. V. G. Khlopin Radium Institute. Translated from Atomnaya énergiya, Vol. 86, No. 6, pp. 419–426, June, 1999.