Seminar on Fission

ORGANIZATIONS

ORGANISING AND SCIENTIFIC ADVISORY COMMITTEES

PREFACE

CONTENTS

Fission is an important process which is responsible not only for the yields of transuranium isotopes, but may have a strong influence on the formation of the majority of heavy nuclei due to fission recycling in the r-process. Calculations of beta-delayed and neutron-induced fission rates were performed taking into account different fission barriers and mass formulae were done. It is shown that an increase of fission barriers results in naturally changes of fission rates, but that the absolute values of fission rates are nevertheless sufficiently high and lead to the termination of the r-process. Furthermore it is discussed, that the probability of triple fission could be high for A > 260 and have an affect on the formation on the heaviest nuclei.

We calculate fission barriers with the Skyrme-Hartree-Fock-Bogoliubov plus particle number projection (SHFB+PLN) method, using a Skyrme force fitted to essentially all the nuclear mass data with the same method. The reflection asymmetry is introduced to study the lowering of the outer barrier of three selected nuclei. We discuss the feasability of performing large-scale SHFB+PLN fission-barrier calculations, i.e. for about 2000 nuclei of astrophysical interest.

Fission potential-energy surfaces are calculated from a microscopic Hartree-Fock-Bogoliubov description of the deformed nucleus, fully constrained in the three deformation coordinates c, h, α. The dynamical fission paths along the multidimensional deformation space are determined by applying the classical least action principle. The resulting dynamical fission barriers and spontaneous fission half-life are compared with static calculations and experimental values for ²⁴⁰Pu. Furthermore, microscopic calculations of fission barriers and nuclear level densities are used to obtain neutron-induced fission cross sections. The results for the actinides are compared with existing experimental data in the low energy region ≈100 keV relevant to the r-process nucleosynthesis.

The astrophysical r-process is responsible for the synthesis of half the nuclear species beyond iron, including the heaviest elements in nature like Th, U and Pu. These long-lived elements are applied as cosmochronometers in determining the age of the Universe. Their relative abundances are strongly dependent on the processes terminating the r-process: spontaneous, neutron-induced and β-delayed fission.

A time dependent Generator Coordinate Method based on self-consistent intrinsic nuclear wave-function has been developed in order to derive fragment mass distribution in a completely microscopic way. Calculations are performed in the two-dimensional space generated by the quadrupole and octupole collective variables. The basis set of wave functions is calculated using the Hartree-Fock-Bogoliubov method and the Gogny force. First calculations with selected initial states indicate that the main features of experimental mass distributions in ²³⁸U are reproduced. It appears that the peak-to-valley ratio significantly depends on the parity of the initial state and that dynamical effects are responsible for the spreading of the peak of the distributions.

The statistical model for fission cross-section evaluation is extended by the concept of multi-modality of the fission process. The three most dominant fission modes, the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric superlong (SL) mode are taken into account. Deconvoluted fission cross-sections for S1, S2 and SL modes for ^235,238U(n, f) and ²³⁷Np(n, f), based on experimental branching ratios, were calculated for the first time in the incident neutron energy range from 0.01 to 5.5 MeV. Good agreement was found with the experimental fission cross-section data. Branching ratios were deduced for the different modes over the whole incident neutron energy region. Fission fragment mass yield distributions have been calculated at incident neutron energies, where no experimental data exist.

We have precisely measured mass yields and kinetic-energy distributions of fission fragments from the thermal-neutron-induced fission of ²³⁵U and ²³⁹Pu, in the symmetry region, with the Lohengrin mass separator at the Institut Laue-Langevin (Grenoble). The dip in the kinetic energy between asymmetric and symmetric fission regions amounts to 28 MeV for ²³⁵U and 23 MeV for ²³⁹Pu, which agrees with other experimental findings. A pronounced asymmetry in the yields was found for masses from A=119 to A=125. For the ²³⁵U case, this asymmetry is at variance with the data given by libraries. An attempt has been made to explain this asymmetry in the mass distribution as resulting from neutron evaporation. The presence of a five-mass structure found in the mass yields (²³⁵U nucleus) tentatively indicates the presence of a proton odd-even effect in the isotopic yields.

Prompt gamma-ray and x-ray spectroscopy techniques are being employed to study fission induced on ²³⁸U targets by neutrons of energy from 1 MeV to 150 MeV. Data are acquired using the GEANIE high resolution gamma-ray spectrometer at the LANSCE/WNR unmoderated spallation neutron source. Information on the predecay yields (excitation functions and isotopic distributions) were extracted from gamma and gamma-gamma coincidence data for a wide range of nucleides. Charge yields were extracted by measuring prompt x-rays from a thin, fission-sensitive target using photovoltaic cells specially designed. The experimental results are presented and compared to model calculations.

The number of prompt neutrons emitted in the fission event has been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of ²⁵²Cf and ²⁴⁸Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration. Approximately 3·10⁶ fission events coincident with prompt neutron emission have been accumulated for each isotope. Two-dimensional neutron multiplicity distributions corrected for efficiency, background and pile-up have been reconstructed for each value of the fission fragment mass and total kinetic energy (TKE). Based on these unfolded multiplicity distributions fragment mass and TKE distributions for specific numbers of emitted neutrons from each of the complementary fragments have been obtained. These distributions exhibit pronounced structures reflecting the fine structures in the potential-energy surface. Structures showing a periodicity of two-masses at the edges of the mass-TKE distributions corresponding to the odd-even effect of the neutron pairing in the fissioning nucleus have been observed for the first time at large values of the fragment total excitation energy.

Radioactive ion beam intensities have been measured at ISOL (isotope separation on-line) facilities from many different targets, but only rarely these intensities are converted into production cross-sections. Here we discuss the method and possible problems in this conversion at the examples of Kr and Xe produced by 1.4 GeV-proton-induced fission of ²³⁸U at ISOLDE and Rb and Cs produced by ≈10 MeV-neutron-induced fission of ²³⁸U at PARRNe.

The Random Excitation Model (REX-M) in nuclear fission is formulated with the level density formula from the Fermi-gas model. It is assumed that excitation of fission fragments is totally determined by a temperature calculated from the reaction Q-value. From this assumption fragment excitation, moments of kinetic energy distributions, and neutron evaporation are calculated. It is shown that the measured distributions and the neutron evaporation characteristics are in good agreement with the model calculations. Finally we extend the REX-model to describe aspects of ternary fission.

We present the latest experimental evidence for a new exotic decay mode – Collinear Cluster Tripartition. Among 2 million binary fission events from ²⁵²Cf detected with FOBOS spectrometer we have found over 30 events with clear mass deficit of about 100 atomic mass units. 20 of these events form a distinct and well-separated rectangle in the mass-mass plot gated by velocities and momenta. The corners of the rectangle correspond to the masses of magic nuclei: ⁶⁸Ni and ⁸⁴Se/⁸⁶Kr. We thus interpret these events as originating from the three-fold decay of an elongated Cf nucleus dominated by the 3 clusters: ⁶⁸Ni, ⁸⁴Se and ⁸⁶Kr. Due to the collinear configuration at scission, most energy is carried out by the 2 external clusters leaving the central fragment nearly at rest.

Highlights from more than 60 years of research on neutron emission in fission reactions are surveyed. The emphasis is put on low energy fission. First, global average multiplicities, multiplicity distributions and neutron spectra are discussed. Next, angular distributions of neutrons relative to the fission axis are shown to point to the existence of neutrons emitted right at scission. Then the dependence of neutron emission on the masses and kinetic energies of fragments are considered, including correlations between neutrons emitted from complementary fragments. Finally heavy ion induced fission of compound nuclei with masses up to the region of superheavy nuclei is reviewed. Throughout the role neutron research has played in contributing to our understanding of fission is stressed.

In the past, the rare ternary fission (TF) process was mainly studied either 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). These previous studies, although having revealed valuable insight into many aspects of the TF process, are inherently limited to the emission of stable or β-radioactive LCP species. The present article describes a number of recent more elaborate correlation measurements that include either the registration of neutrons and γ-rays with LCPs and FFs, or the coincident registration of two LCPs. These experimental approaches have permitted to identify the population of excited states in LCPs, the formation of neutron-unstable nuclei as short-lived intermediated LCPs, as well as the sequential decay of particle-unstable LCP species into charged particle pairs. “Quaternary” fission with an apparently independent emission of two charged particles has also been observed. These experimental studies, performed on either ²⁵²Cf(sf) or ^233,235U(n_th,f), and the applied technologies are briefly summarized, and particular results are presented and discussed.

The shape of the energy distribution of the particles emitted in ternary fission has been studied since the discovery of the phenomenon for a large variety of fissioning systems. The general tendency of the observations is that most particles have a Gaussian-shaped energy distribution, except the α-particles, for which mostly an important non-Gaussian tailing on the low-energy side is reported. The origin of this tailing is generally ascribed to the decay of ternary ⁵He particles in an α-particle and a neutron. Since the experiments reported in the literature are rarely optimised for measuring the low-energy part of the α-spectrum, we realised good experimental conditions for studying the ²³⁵U(n_th,f) ternary α energy distribution at the High Flux Reactor of the ILL in Grenoble. Thanks to a very intense and clean neutron beam, a small, very thin sample of highly enriched U could be used, with an activity of only 1.6 Bq. So the measurements could be done without absorber in between the sample and the ΔE-E detector. With the resulting low detection limit of 6 MeV, a clearly asymmetric energy distribution was obtained, in agreement with most data in the literature.

Recently, the influence of the excitation energy of the fissioning nucleus on the ternary triton emission probability (noted t/B) has been investigated for ²⁴⁸Cm. This was done by comparing the (t/B)-data obtained from the ²⁴⁷Cm(n_th,f) reaction (where the excitation energy of the fissioning nucleus corresponds to the neutron binding energy) and from ²⁴⁸Cm(sf) decay (where the excitation energy is zero). This study has revealed a slight increase of t/B with the excitation energy indicating that tritons do not behave in the same way as ternary alpha particles. The aim of this paper is to study this effect on the ²⁴⁶Cm compound nucleus and on various Cf-isotopes.

For that purpose, the energy distributions and yields of the ternary triton and alpha particles emitted from ²⁴⁶Cm(sf) have been measured, using a double telescope consisting of a twin ionization chamber coupled to two surface barrier detectors. The corresponding ²⁴⁵Cm(n_th,f) data are available from a previous experiment. For the investigation of the Cf-isotopes, a ²⁵¹Cf-sample also containing ²⁴⁹Cf, ²⁵⁰Cf and ²⁵²Cf has been used. The measurement was performed at the high flux reactor of the Institute Laue Langevin in Grenoble (France), using a vacuum chamber with a single ΔE-E telescope, consisting of two suited surface barrier detectors. This measurement was performed respectively with the neutron beam closed and opened in order to investigate contributions from (sf)-decays as well as (n_th,f)-reactions.

For each fissioning nucleus studied, a comparison of the spontaneous and thermal neutron induced fission data confirms the previously observed behaviour for the ²⁴⁸Cm compound nucleus, namely: a decrease of the ternary alpha emission probability and an increase of the triton emission probability with the excitation energy of the compound nucleus.

The paper presents a review of the experimental works dedicated to the study of the multimodal fission phenomenon. These works were performed in a wide collaboration namely Flerov Laboratory of Nuclear Reaction (JINR, Russia), Institute of Nuclear Physics (National Nuclear Center, Kazakhstan), Institut de Recherches Subatomiques (France), Universite Libre de Bruxelles (Belgium), Laboratorio Nazionale del Sud (INFN, Italy), Laboratorio Nazionali di Legnaro (INFN, Italy) and The Cyclotron Institute (Texas A&M University, USA). Mass and energy distributions (MED) of the fission fragments from the fission of compound-nuclei ^216,218,220Ra, ^220,224,226Th, ²⁷⁰Sg and ²⁷⁴Hs were measured and analyzed with relation to the fission modes presence. Neutron and γ-quanta emissions accompanied the fission process of ²²⁶Th compound-nucleus were measured and analyzed the same way.

We analyze the effect of entrance channels having very different mass asymmetries on the quasifission process in competition with the fusion, and on the fissility of the compound nucleus in competition with the evaporation residue production. According to the dinuclear system (DNS) concept, in reactions with massive nuclei, we estimate the capture and fusion cross sections. Using the calculated partial fusion cross section and the advanced statistical model, we also estimate the evaporation residue cross sections for the superheavy nucleus formation.

The competition between fusion-fission and quasi-fission in the reactions ⁴⁸Ca +²⁰⁸Pb and ⁴⁸Ca +²⁴⁴Pu (E* = 40 MeV) is investigated with the CORSET and DEMON detectors. The development of a new analysis method, THOMATE, enables us to obtain the pre-scission neutron multiplicity distributions (PSNMD) that allows to disentangle the contributions of fusion-fission and quasi-fission for the first time for a superheavy system at such a low excitation energy. The ratio of the fusion cross-section over the capture cross-section is found to be of the order of 10%.

Spectroscopic beta-gamma investigations of neutron-rich nuclei close to the r-process path have been planned. These nuclei will be produced by photofission and their mass and charge numbers will be determined by time-of-flight (TOF) measurements. A novel time-of-flight spectrometer has been designed for this purpose. The coincidence spectra of the double-TOF measurements with the designed spectrometer have been simulated using a Monte Carlo method, which results in a mass and charge separation for the fission fragments (FF). The test of the spectrometer is under way by using a spontaneous fission source.

Since 1998, SCK•CEN in partnership with IBA s.a., is designing a multipurpose ADS for R&D applications –MYRRHA - and is conducting an associated R&D support programme. MYRRHA is an Accelerator Driven System (ADS) under development at Mol in Belgium and aiming to serve as a basis for the European experimental ADS to provide protons and neutrons for various R&D applications. It consists of a proton accelerator delivering a 350 MeV*5 mA proton beam to a liquid Pb-Bi spallation target that in turn couples to a Pb-Bi cooled, subcritical fast core.

In a first stage, the project focuses mainly on demonstration of the ADS concept, safety research on sub-critical systems and nuclear waste transmutation studies. In a later stage, the device will also be dedicated to research on structural materials, nuclear fuel, liquid metal technology and associated aspects and on sub-critical reactor physics. Subsequently, it will be used for research on applications such as radioisotope production. The MYRRHA system is expected to become a major research infrastructure for the European partners involved in the P&T and ADS Demo development.

The production cross sections and the kinematical properties of fission fragment residues have been studied in the reaction ²³⁸U (1 A.GeV) + p. Isotopic distributions were measured for all elements from O (Z = 8) to Gd (Z= 64). The distribution of fission velocities and of production cross sections as function of Z of the fragments, provide relevant informations on the intermediate fissioning nuclei.

In this work, the particle evaporation “clock” method has been applied with success to the interpretation of pre- and postscission neutron and light charged particle multiplicities extracted from the fusion-fission reactions; 8 to 16 MeV/A ²⁰Ne+¹⁵⁹Tb and ²⁰Ne+¹⁶⁹Tm. The neutron properties were established with the DEMON neutron array. The experimental observables were compared with statistical model calculations using a new version of GEMINI Monte-Carlo code. These simulations, incorporating simple aspects of the fission dynamics (time-dependent fission width and deformation-dependent transmission coefficients) predicted by the HICOL dynamical code, have enabled us to estimate the ranges of conventional-fission and fast-fission timescales at different compound-nucleus excitation energies. The additional fission dynamical delay times which were required to reproduce our data were found to decrease as the initial excitation energy of the fissioning nucleus increases from 108 to 254 MeV (or for nuclear temperature (T) from 1.3 to 2.3 MeV). This method is shown in simulations, to give times closer to the median lifetime (halflife). The time distributions have extremely long tails covering more than SEVEN orders of magnitude, consistent with previous results obtained with other techniques (crystal blocking, KX-rays and GDR γ-rays). The decrease of median fission lifetimes and subsequent fission transient times with increasing the excitation energies may suggest a decrease in nuclear viscosity above , consistent with the onset of two-body dissipation mechanism.

Neutron and proton-induced fission studies on actinide nuclei, at low and intermediate energies, are of great interest for fundamental understanding of the fission processes and especially for applied nuclear research such as transmutation of long-lived radioactive wastes. As a first step of a long-range research program at Louvain-la-Neuve cyclotron facility, we have recently performed the measurements on the proton-induced fission reactions with ²³⁸U and ²³⁹Pu targets at 26.5 and 62.9 MeV bombarding energies. Some preliminary results are presented. Future measurements on other actinides are foreseen.

Accurate ²³⁴U(n,f) cross section data are needed for various applications like the study of the Th-cycle and the incineration of actinides. Data in the resonance region are scarce and huge discrepancies exist between the values for the thermal cross section reported in the commonly used data files. The ²³⁴U(n,f) reaction has been studied with thermal neutrons at the ILL in Grenoble (France) and is presently under investigation at the linear accelerator of the IRMM (Belgium) for neutron energies between 10 meV and 1 MeV. This paper reports on the results of a measurement campaign on a 30 m flight path, focusing on the resonance region between 100 and 1000 eV.

The cross section for the neutron-induced fission of ²³³Pa has been measured from the threshold at about E_n = 1.0 MeV to E_n = 8.5 MeV, which is just above the threshold for second chance fission. The experimental results are then evaluated in terms of extended statistical model calculations. The obtained data are important for the design of future reactor concepts involving advanced fuel cycles.

The ²³³Pa(n,f) cross section has been obtained using the ²³²Th(³He,p)²³⁴Pa to measure the fission probability of ²³⁴Pa and then multiplying this fission probability by calculated compound nucleus neutron reaction cross section. The validity of the method has been tested with existing neutron induced fission of ²³⁰Th and ²³¹Pa as well as recent direct neutron measurements of the reaction ²³³Pa(n,f).

LIST OF PARTICIPANTS

Author Index