Systematics of fragment observables

Multifragmentation is observed in many reaction types: light-ion-induced reactions at large incident energies (in the GeV region), central heavy-ion collisions from 30 to 100MeV/u, and peripheral heavy-ion collisions between 30 and 1000MeV/u or above. When nucleus-nucleus collisions are considered, another entrance channel parameter is the corresponding mass asymmetry. The first question which is addressed in this contribution is: do we observe similar reactions in each case? Multifragmentation may be related to a phase transition of nuclear matter. Some others features indicate that dynamical features are dominant. It is a priori possible that the underlying mechanisms are different in proton- and nucleus-induced reactions, in central and in peripheral collisions, at limited and at large bombarding energies. In order to see to what extent they can reflect similar behaviour, it is useful to compare the results of various reactions. The observables can be the fragment multiplicity, the mass distributions or the kinematical properties. In this contribution, we are looking for such general features. We will limit the discussion to the observations themselves, rather than the interpretation, which is the subject of numerous entries in this volume. The experimental results indicate that multifragmentation exhibits at the same time universal and entrance-channel-dependent properties.     

Fluctuations of fragment observables

This contribution presents a review of our present theoretical as well as experimental knowledge of different fluctuation observables relevant to nuclear multifragmentation. The possible connection between the presence of a fluctuation peak and the occurrence of a phase transition or a critical phenomenon is critically analyzed. Many different phenomena can lead both to the creation and to the suppression of a fluctuation peak. In particular, the role of constraints due to conservation laws and to data sorting is shown to be essential. From the experimental point of view, a comparison of the available fragmentation data reveals that there is a good agreement between different data sets of basic fluctuation observables, if the fragmenting source is of comparable size. This compatibility suggests that the fragmentation process is largely independent of the reaction mechanism (central vs. peripheral collisions, symmetric vs. asymmetric systems, light ions vs. heavy-ion-induced reactions). Configurational energy fluctuations, that may give important information on the heat capacity of the fragmenting system at the freeze-out stage, are not fully compatible among different data sets and require further analysis to properly account for Coulomb effects and secondary decays. Some basic theoretical questions, concerning the interplay between the dynamics of the collision and the fragmentation process, and the cluster definition in dense and hot media, are still open and are addressed at the end of the paper. A comparison with realistic models and/or a quantitative analysis of the fluctuation properties will be needed to clarify in the next future the nature of the transition observed from compound nucleus evaporation to multi-fragment production.     

Fission fragment damage in zircon

Abstract

X-ray diffraction, electron microscope and optical studies have been : ade on zircons damaged by fission fragments derived from the action of neutrons on uranium impurities in the zircon lattice. A dosage of about 10¹⁴ fission events · cm^?3 was required to produce diffuse x-ray scattering, observable by conventional photographic methods, and a dosage of about 10¹⁶ fission events · cm^?3 appears to be required to render a zircon amorphous. An optical absorption band, generated apparently by γ-rays, was observed near 270 nm in undoped zircon. The valence state of the uranium impurities was unchanged by irradiation with 10¹⁴ fission events · cm^?3, 2 × 10¹⁸ nvt of fast neutrons, or ～10¹⁰ R of γ-rays. A fission event in zircon was deduced to produce ～10⁴ times more displacement damage than an α-recoil atom. Individual fission tracks were observed directly by electron microscopy after dosage of 10¹⁰-10¹³ fission events - cm^?3, the tracks being similar in appearance to those found by other workers in other materials. With increasing dosage, fission fragment uradiation appears to produce a progressive disordering of the lattice, as was previously deduced for the case of α-recoil irradiation, rather than the formation of new crystallographic phases.     

Dynamical models for fragment formation

The various dynamical models for fragment formation in nuclear collisions are discussed in order to bring out their relative advantages and shortcomings. After discussing the general requirements for dynamical models that aim to describe fragment formation, we consider the various mean-field models that incorporate fluctuations and then turn to models based on molecular dynamics.     

Scaling behavior of fragment shapes

We present an experimental and theoretical study of the shape of fragments generated by explosive and impact loading of closed shells. Based on high speed imaging, we have determined the fragmentation mechanism of shells. Experiments have shown that the fragments vary from completely isotropic to highly anisotropic elongated shapes, depending on the microscopic cracking mechanism of the shell. Anisotropic fragments proved to have a self-affine character described by a scaling exponent. The distribution of fragment shapes exhibits a power-law decay. The robustness of the scaling laws is illustrated by a stochastic hierarchical model of fragmentation. Our results provide a possible improvement of the representation of fragment shapes in models of space debris.     

Fission fragment γ-ray anisotropies

The anisotropy of fission fragment γ-rays in the reactions ²³⁹Pu(n_th, f) and ²³³U(n_th, f) have been measured. In this work, two γ energy bands were used to confirm the energy independence of the anisotropy. For the γ energy band of 0.49–0.65 MeV and the γ energy band of 0.83–0.92 MeV the anisotropies were found to be 0.18 ± 0.03, 0.13 ± 0.03 and 0.19 ± 0.03, 0.11 ± 0.03 for the ²³⁹Pu and ²³³U targets respectively. The data are corrected for the Doppler effect and the time of flight was large enough to eliminate the counting of neutrons. Comparisons with the theories of Nix and Swiatecki, and Strutinsky have been made and approximate correction for fragment rotation along fission axis made.     

放射性次级束流分离器的分离性能北大核心CSCD

放射性次级束流分离器(HFRS)是强流重离子加速器装置(HIAF)上开展放射性次级物理研究的重要装置。HFRS是飞行时间型(PF)碎片分离器,具有大磁刚度、大接受度、大孔径磁铁以及高动量分辨的特点。HFRS采用Bρ-ΔE-Bρ方法纯化弹核碎裂或裂变反应产生的放射性核素,是开展高精度储存环内实验及环外实验研究的重要工具。主要介绍HFRS分离纯化奇异核的能力,采用MOCADI程序模拟单降能器与双降能器下典型弹核碎裂反应和裂变反应中粒子的鉴别和纯化。模拟结果表明:HFRS具有很好的消色散和聚焦特性,对于弹核碎裂反应中轻核的分离采用单降能器系统即可得到很好的纯化效果;而弹核碎裂反应中重核的分离则需采用双降能器系统才可得到很好的纯化效果;对于裂变反应,由于裂变反应的能散较大,则在采用双降能器系统时也仅仅能得到一定的纯化效果。     

Projectile-like fragment production in heavy ion reactions

A model for peripheral heavy ion reactions is proposed in which the reaction mechanism is governed by the number of nucleon-nucléon collisions taking place during the interaction. A general random walk process leads to simple expressions for the mass yields. Angular distributions are obtained as the convolution of distributions due to deflection by the ion-ion potential and recoil effects due to the change in mass.     

Dynamical model of fission fragment angular distributions

A dynamical model of fission fragment angular distributions is developed. The experimental data on the angular anisotropy of fission fragments is analyzed for the ¹⁶O + ²⁰⁸Pb, ²³²Th, ²³⁸U, and ²⁴⁸Cm reactions at energies of the incident ¹⁶O ions ranging from 90 to 160 MeV. This analysis allows us to extract the relaxation time for the tilting mode. It was also demonstrated that the angular distributions are sensitive to the deformation dependence of the nuclear friction.

     

Flow angle from intermediate mass fragment measurements

Directed sideward flow of light charged particles and intermediate mass fragments was measured in different symmetric reactions at bombarding energies from 90 to 800 A MeV. The flow parameter is found to increase with the charge of the detected fragment up to Z = 3–4 and then turns into saturation for heavier fragments. Guided by simple simulations of an anisotropic expanding thermal source, we show that the value at saturation can provide a good estimate of the flow angle, Θ_flow, in the participant region. It is found that Θ_flow depends strongly on the impact parameter. The excitation function of Θ_folw reveals striking deviations from the ideal hydrodynamical scaling. The data exhibit a steep rise of Θ_flow to a maximum at around 250 – 400 A MeV, followed by a moderate decrease as the bombarding energy increases further.     

Beta-NMR detection of beta-emitting fragment43Ti

NMR on the beta emitter⁴³Ti has been observed by use of the -NMR technique. From the observed NMR spectrum, the magnetic moment of⁴³Ti was determined to be || =0.85±0.02 µ_N. The value is significantly quenched from the single particle value –1.91 µ_N, which shows a strong effect resulting from meson-exchange currents andconfiguration mixing.     

Systematics of complex fragment emission in niobium-induced reactions

Complex fragments of 3<Z⪅35 have been detected in the reverse-kinematics reactions of ⁹³Nb plus ⁹Be, ¹²C and ²⁷Al at bombarding energies of E/A=11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.     

Neutron emission and fragment yield in high-energy fission

The KRIS special library of spectra and emission probabilities in the decays of 1500 nuclei excited up to energies between 150 and 250 MeV was developed for correctly taking into account the decay of highly excited nuclei appearing as fission fragments. The emission of neutrons, protons, and photons was taken into account. Neutron emission fromprimary fragments was found to have a substantial effect on the formation of yields of postneutron nuclei. The library was tested by comparing the calculated and measured yields of products originating from the fission of nuclei that was induced by high-energy protons. The method for calculating these yields was tested on the basis of experimental data on the thermal-neutroninduced fission of ²³⁵U nuclei.     

A complete ridge-line potential for complex fragment emission

Cross sections were measured for fragments (4<><27) from=" the=" 5.0,6.2,6.9,8.0,10.2=" and=" 12.7=">⁶³Cu+¹²C reactions. Excitation functions were constructed for each Z value, and a nearly complete set of mass-asymmetric barriers has been obtained for⁷⁵Br. There is excellent agreement between the experimentally determined barriers and the finite-range model calculations, while there is strong disagreement with the liquid-drop model calculations.This work was supported by the Director, Office of the Energy Research, Office of High Energy and Nuclear Physics, Division of Nuclear Physics, of the U.S. Department of Energy under Contract No. DE-AC03SF00098.     

Fission fragment production from uranium carbide disc targets

A possible solution for a target system aimed at the production of exotic nuclei as a result of high-energy fissions in ²³⁸U compounds has been analyzed. The configuration proposed is constituted by a primary proton beam (40 MeV, 0.4 mA) directly impinging on a UC₂ multiple-disc target inserted within a cylindrical tungsten box. In order to extract the fission fragments, the tungsten box has to be kept at 2000 ^°C. This system has been conceived to obtain both a high number of fission fragments (about 2 ^. 10¹³ atoms/s) and a quite low power deposition in the target. The power release and the fragment distribution have been calculated by means of the Monte Carlo code MCNPX. The thermal analysis of the proposed configuration shows the capability of the thermal radiation to cool the discs with a reasonable margin below the material melting point. Moreover, the possibility of increasing such margin with simple modifications of the target design is shown by means of parametric analyses. The thermal analysis of the tungsten box, also cooled by radiation, points out the necessity to heat it and/or shield it thermally, in order to take it at the requested temperature. Preliminary calculations of the target-induced activity have also been performed.     

Single-particle spin effect on fission fragment angular momentum

Independent isomeric yield ratios (IYR) of ¹²⁸Sb, ¹³⁰Sb, ¹³²Sb, ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe, and ¹³⁸Cs have been determined in the fast neutron-induced fission of ²⁴³Am using the radiochemical and γ-ray spectrometric technique. From the IYR, fragment angular momenta (J _rms) have been deduced using the spin-dependent statistical model analysis. From the J _rms-values and experimental kinetic energy data deformation parameters (β) have been deduced using the pre-scission bending mode oscillation model and the statistical model. The J _rms- and β-values of fission fragments from the present and earlier work in the odd-Z fissioning systems ( ²³⁸Np ^* , ²⁴²Am ^* and ²⁴⁴Am ^* ) are compared with the literature data in the even-Z fissioning systems ( ^{230, 233}Th ^* , ^{233, 234, 236, 239}U ^* , ^{239, 240, 241, 242}Pu ^* , ²⁴⁴Cm(SF), ^{245, 246}Cm ^* , ²⁵⁰Cf ^* and ²⁵²Cf(SF)) to examine the role of single-particle (proton) spin effect. It was observed that i) in all the fissioning systems J _rms- and β-values of the fragments with spherical 82n shell and even-Z products are lower than the fragments away from the spherical neutron shell and odd-Z products, which indicate the effect of nuclear structure. ii) For both even-Z and odd-Z fission products J _rms-values increase with Z _F ²/A _F due to increase in Coulomb torque. iii) The J _rms- and β-values of even-Z fission products are comparable in all the fissioning systems. However, for odd-Z fission products they are slightly higher in the odd-Z fissioning systems compared to their adjacent even-Z fissioning systems. This is possible due to the contribution of the extra single-particle (proton) spin of the odd-Z fissioning systems to their odd-Z fragments. iv) The yield-weighted fragment angular momentum and elemental yields profile shows an anti-correlation in even-Z fissioning systems but not in the odd-Z fissioning systems.     

Fragment kinetic energies and modes of fragment formation

Kinetic energies of light fragments ( A相似文献   

Fission fragment angular momentum in ODD-Z fissioning systems

Independent isomeric yield ratios of ¹²⁸Sb, ¹³⁰Sb, ¹³², ¹³¹Te, ¹³³Te, ¹³²I, ¹³⁴I, ¹³⁶I, ¹³⁵Xe and ¹³⁸Cs have been determined in the fast neutron induced fission of ²³⁷Np and ²⁴¹Am using radiochemical and gamma spectrometric technique. From the independent isomeric yield ratios, fragment angular momenta (J_rms) have been deduced using spin-dependent statistical model analysis. Comparison of these data with the literature data for even-Z fissioning systems shows the following important features: (i) Angular momenta for fragments with spherical 82n shell and even-Z products are lower compared to the fragments with out the 82n shell and odd-Z products indicating the effect of nuclear structure. (ii) Angular momentum of even-Z products in all the fissioning systems are comparable where as for odd-Z products it is slightly higher in the odd-Z fissioning systems than in the adjacent even-Z fissioning systems. This indicates the role of single particle on fragment angular momentum in odd-Z fissioning systems. Received: 26 March 1999 / Revised version: 16 October 1999