Statistical emission of large fragments: A general theoretical approach

A theory for the statistical emission of large fragments is developed. In analogy with the fission saddle point, a ridge line in the potential energy surface is defined which controls the decay width of the system into any two given fragments. The normal modes at the ridge are separated into three classes: decay modes, amplifying modes, and non-amplifying modes. Amplifying modes are those whose thermal fluctuations are amplified and lead to a broadening of the kinetic energy distribution. Analytical expressions for the kinetic energy distributions are developed for various combinations of amplifying and non-amplifying modes. The limit for large amplifications is a Gaussian kinetic energy distribution. The limit for no amplification is a Maxwellian-like distribution. Thus the formalism comprehends the fission decay on one hand and the neutron evaporation on the other. The angular distributions are evaluated in terms of the ridge line principal moments of inertia. A general analytical expression has been derived which predicts, correctly in both limits, the angular distributions of the evaporated neutrons and of the fission fragments.     

Recent experimental studies on 252Cf ternary fission

The rare ternary fission (TF) process was hitherto studied mainly by inclusive measurements of the energies and fractional yields of the light charged particles (LCPs) from fission, or by experiments on the angular and energy correlation between LCPs and fission fragments (FF). The present article briefly describes a series of recent correlation measurements on ²⁵²Cf(sf) TF that include either the registration of neutrons and γ rays with LCPs and FFs, or the coincident registration of two LCPs. The population of excited states in LCPs has been identified, as well as the formation of neutron-unstable nuclei as short-lived intermediated LCPs, the sequential decay of particle-unstable LCP species into charged particle pairs, and “quaternary” fission with the emission of two charged particles right at scission.     

Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.     

P-odd and P-even correlations for third particles in ternary fission

Within quantum-mechanical fission theory, P-odd and P-even correlations in angular distributions of products of the ternary fission of nuclei that is induced by polarized cold and thermal neutrons are investigated on the basis of a nonevaporative mechanism of third-particle emission and under the assumption that a two-humped fission barrier exists. It is shown that these correlations for third particles are induced by the analogous correlations for ternary-fission fragments, the latter being transferred to the third particle because of the kinematical conditions of third-particle emission that are associated with the charge and mass asymmetry of fragments. Optimum methods for observing the above correlations for third particles are discussed. The possibility of discovering the emission of prescission neutrons in the fission process against the background of evaporated neutrons by means of studying P-odd and P-even correlations is explored.     

Multidimensional dynamical-statistical model for describing the fission of excited nuclei

A multidimensional stochastic model for describing the decay of excited nuclei is presented. The model takes into account the dynamics of thermal fluctuations of collective variables, the dissipation of the kinetic energy of collective motion, and the emission of light particles from excited nuclei. The potential energy of a deformed nucleus is calculated within the liquid-drop model with a sharp surface and within the finiterange-interaction model. The friction parameters are calculated on the basis of the one-body-dissipation model. The inertia parameters are found in the Werner-Wheeler approximation. The drift components of forces are determined in terms of the entropy of an excited nucleus. The latter in turn is computed within the Fermi gas approximation with allowance for the deformation dependence of the density-level parameter. The fission probability, the mean multiplicity of neutrons emitted prior to scission (prescission neutrons), and the variances of the mass distributions of fission fragments at the most probable kinetic-energy value are calculated on the basis of the model developed here and are compared with experimental data. The dependences of these quantities on the model parameters are considered in detail.     

Unified Mechanism behind the Appearance of <Emphasis Type="Italic">T</Emphasis>-Odd TRI and ROT Asymmetries in Actinide Fission Induced by Cold Polarized Neutrons

Some shortcomings of the approaches that are used to describe T-odd ROT and TRI asymmetries in true ternary fission via reactions involving the emission of prescission alpha particles and which are based on employing the classical method of trajectory calculations are analyzed. These shortcomings are caused by the disregard of the interference between the fission widths of different sJ_s neutron resonance states formed in the first well of the deformation potential of fissile compound nuclei. It is shown that the method used in some studies to determine T-odd TRI-asymmetries for prescission alpha particles is at odds with basic concepts of the generalizedmodel of the nucleus and approaches to constructing collective (for example, bending) vibrations of a fissile compound nucleus. Quantum-mechanical fission theory is generalized via employing a unified mechanism of formation of T-odd TRI and ROT asymmetries for prescission alpha particles and evaporated photons (neutrons). The proposed mechanism takes correctly into account the effect of quantum rotation of a fissile compound nucleus on the angular distributions of fission fragments and alpha particles for true ternary fission, as well as on the angular distribution of prompt photons (neutrons) emitted by fragments originating from the delayed fission of the aforementioned nuclei.     

Correlations of neutral and charged particles in 40Ar- 58Ni reaction at 77 MeV/u

The measurement of the two-particle correlation function for different particle species allows to obtain information about the development of the particle emission process: the space-time properties of emitting sources and the emission time sequence of different particles. The single-particle characteristics and two-particle correlation functions for neutral and charged particles registered in forward direction are used to determine that the heavy fragments (deuterons and tritons) are emitted in the first stage of the reaction (pre-equilibrium source) while the majority of neutrons and protons originates from the long-lived quasi-projectile. The emission time sequence of protons, neutrons and deuterons has been obtained from the analysis of non-identical particle correlation functions.     

Effect of beta-electron capture to a bound state on delayed-neutron emission from fission fragments

The effect of ionization of the atomic electron shell on beta decay is investigated. The change in the beta-decay probability is due primarily to the appearance of the channel involving beta-electron capture to a bound state. It is shown that the effect may be significant at low beta-transition energies. The magnitude of the effect was studied in the case of the emission of one to three delayed neutrons following the beta decay of fission fragments. In the calculations, use was made of the beta-decay strength function with allowance for the population of isobaric resonances in daughter nuclei. The effect proved to be maximal for fragments where the numbers of protons and neutrons approach those in closed shells and subshells, thereby illustrating the role of the shell structure of the nucleus in fission.     

Quantum molecular dynamics approach to estimate spallation yield from<Emphasis Type="Italic">p</Emphasis> +<Superscript>208</Superscript>Pb reaction at 800 MeV

The spallation yield of neutrons and other mass fragments produced in 800 MeV proton induced reaction on²⁰⁸Pb have been calculated in the framework of quantum molecular dynamics (QMD) model. The energy spectra and angular distribution have been calculated. Also, multiplicity distributions of the emitted neutrons and kinetic energy carried away by them have been estimated and compared with the available experimental data. The agreement is satisfactory. A major contribution to the neutron emission comes from statistical decay of the fragments. For mass and charge distributions of spallation products the QMD process gives rise to target-like and projectile-like fragments only.     

Investigation of the maximum accessible kinetic energy of fragments in the neutron-induced fission of 238U nuclei

The masses, total kinetic energies (TKE), and emission angles of fragments originating from the fission of ²³⁸U nuclei that was induced by 5- and 6.5-MeV neutrons were measured by using digital methods for processing signals. A detailed analysis of the shape of digital signals made it possible to reduce substantially the contribution of fragments whose TKE values were distorted because of a superimposition of signals from recoil protons and from alpha particles produced in the spontaneous decay of uranium. The total statistics exceeded two million events for either neutron energy, and this permitted performing a detailed analysis of fission-fragment yields in the region of the highest attainable TKE values. An analysis of fragment yields made it possible to draw specific conclusions on the structure of the potential surface of fissile nuclei.     

裂变产物衰变链设计

根据核科学参考文献库,对半衰期、缓发中子发射几率等进行了更新评价,采用系统学研究和理论研究相结合的评价方法对裂变产物中不确定的基态进行了自旋指定,对部分存在问题的数据进行了必要修正;对衰变子核存在同质异能态的放射性核素,根据衰变纲图和内转换系数重新计算得到退激至子核同质异能态和基态的分支比数据。在此基础上,结合现有核结构和衰变数据库,研制了裂变产物衰变链设计专用衰变数据库,按照衰变规律,建立了裂变产物衰变路径和衰变信息的完整衰变链,本工作为裂变产物分析和裂变产额研究提供了一个便利参考。     

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.     

Clusters and covalently bound nuclear molecules

Nuclear clustering based on α particles and strongly bound substructures with N=Z has been studied for many decades. Of particular interest are excited states close to the decay thresholds into substructures, as described by the Ikeda diagram. This diagram can be extended to neutron-rich nuclei; in these cases strongly deformed isomeric states consisting of clusters and loosely bound neutrons will appear. A possible approach to describe these states is to use explicitly molecular concepts, with neutrons in covalent binding orbits. Examples for molecular structure in beryllium isotopes and in other neutron-rich light nuclei (carbon and neon) are discussed.     

Mass-energy distribution of fragments from the fission of excited nuclei within three-dimensional Langevin dynamics

Two-dimensional mass-energy distributions of fission fragments are calculated for the first time on the basis of three-dimensional stochastic Langevin equations. In these calculations, the emission of light prescission particles is taken into account within the statistical model. The results demonstrate that calculations within three-dimensional Langevin dynamics make it possible to describe most compre-hensively the properties of the mass-energy distribution of fission fragments and the mean multiplicity of prescission neutrons.     

Electromagnetic form factors of the nucleon

Dynamical and thermal characterizations of excited nuclear systems produced during the collisions between two heavy ions at intermediate incident energies are presented by means of a review of experimental and theoretical work performed in the last two decades. Intensity interferometry, applied to both charged particles (light particles and intermediate mass fragments) and to uncharged radiation (gamma rays and neutrons) has provided relevant information about the space-time properties of nuclear reactions. The volume, lifetime, density and relative chronology of particle emission from decaying nuclear sources have been extensively explored and have provided valuable information about the dynamics of heavy-ion collisions. Similar correlation techniques applied to coincidences between light particles and complex fragments are also presented as a tool to determine the internal excitation energy of excited primary fragments as it appears in secondary-decay phenomena.     

Neutron storage in a magnetic field

The motion of neutrons in magnetic traps is considered for various cases of neutron polarization. The results of implementing such traps in practice and special features of experiments studying magnetic neutron storage are discussed. The problem of neutron losses during injection via magnetic valves can be solved by conjoining a magnetic trap with a converter of cold neutrons into ultracold ones or with a source of ultracold neutrons. Prospects for expanding neutron-storage experiments by invoking a correlation analysis of neutron decay and by using the transport properties of charged particles in a nonuniform magnetic field are analyzed. In such an investigation, the recording of the storage time of neutrons proper can be supplemented with the detection of decay protons and electrons and with a parallel measurement of the asymmetries of proton and electron emission with respect to the magnetic field. A set of relative measurements permits improving the accuracy of an experimental determination of the neutron lifetime and combining this determination with the determination of correlation coefficients. On this basis, it is possible to find directly the ratio of the weak-interaction constants and the constants themselves. The application of the most advanced reactor and accelerator technologies to subcritical electric nuclear devices optimized for generating cold and ultracold neutrons, along with the use of solid deuterium and superfluid helium, creates preconditions for developing a neutron plant and for launching neutron studies at accelerators. Thus, the work that has been done as a development of V.V. Vladimirsky's proposals on magnetic neutron storage is analyzed, and the potential of a further use of ultracold neutrons and magnetic devices for deploying a full-scale precision experiment to study the beta decay of polarized neutrons is demonstrated.     

Angular correlations in ternary fission induced by polarized neutrons

Ternary fission induced by cold polarized neutrons was studied for the two isotopes ²³³U and ²³⁵U at the Institut Laue-Langevin in Grenoble, France. In particular two types of angular correlations between the spin of the incoming neutrons and the emission directions of both, the fission fragments (FF) and the ternary particles (TP), were investigated. For FF and TP detectors facing the target at right angles to the neutron beam, first, for longitudinally polarized neutrons a triple correlation between spin and the emission of outgoing particles was explored and, second, for transversally polarized neutrons parity violating asymmetries in the emission of FFs and TPs were analyzed. Nonzero expectation values for the triple correlation were oberserved in the present experiments for the first time.     

Threshold laws in delayed emission: an experimental approach

Delayed emission is a common decay process for very excited complex systems where the excitation energy is, to a large extent, statistically distributed over all accessible degrees of freedom. A rich variety of delayed decay processes is found in complex molecules and clusters such as thermionic emission, evaporation of heavy fragments or blackbody radiation. In this article, we present the general threshold laws that govern the kinetic energy spectrum of matter particles (electrons or fragments) ejected from spherically symmetric species in such processes, and we illustrate these laws by experimental results obtained in photoelectron and photoion spectroscopy. Deviations from simple laws for non spherical species are also evidenced.     

Decay processes of photoluminescence in a nanocrystalline SiC thin film

Nanocrystalline (nc)-SiC film has been deposited by helicon wave plasma enhanced chemical vapor deposition technique and intense blue-white light emission is obtained. Microstructure analyses show that the 3C–SiC particles are embed in amorphous SiC matrix, and the average size of the nc-SiC is 3.96 nm. The photon energy of the main photoluminescence (PL) band is higher than the band gap of bulk SiC, which indicates that the optical emission mainly occurs in quantum states of 3C–SiC nanocrystals. In addition, the band tail states of amorphous SiC also contribute to the optical emission. Three decay processes are obtained from time-resolved PL spectra by deconvolution treatment, and the decay components correspond to the quantum confinement effect (QCE), surface states of nc-SiC particles, and band tail of amorphous SiC, respectively. The fractional integrated PL intensity of QCE related decay process decreases dramatically in the lower PL photon energy, indicating that the QCE mainly contributes to the short wavelength optical emission.     

Heavy SUSY Higgs bosons at e + e − linear colliders

The production mechanisms and decay modes of the heavy neutral and charged Higgs bosons in the Minimal Supersymmetric Standard Model are investigated at future e ⁺ e ^? colliders in the TeV energy regime. We generate supersymmetric particle spectra by requiring the MSSM Higgs potential to produce correct radiative electroweak symmetry breaking, and we assume a common scalar mass m₀, gaugino mass m_1/2 and trilinear coupling A, as well as gauge and Yukawa coupling unification at the Grand Unification scale. Particular emphasis is put on the low tan β solution in this scenario where decays of the Higgs bosons to Standard Model particles compete with decays to supersymmetric charginos/neutralinos as well as sfermions. In the high tan β case, the supersymmetric spectrum is either too heavy or the supersymmetric decay modes are suppressed, since the Higgs bosons decay almost exclusively into b and τ pairs. The main production mechanisms for the heavy Higgs particles are the associated AH production and H ⁺H^? pair production with cross sections of the order of a few fb.