New Applications of Nuclear Fission

The following sections are included:

• PREFACE

• CONTENTS

Heavy particle radioactivity predicted in 1980 was experimentally confirmed since 1984. The obtained until now data on half-lives and branching ratios relative to α-decay of ¹⁴C, ^18,20O, ²³F, ^{22, 24−26}Ne, ^28,30Mg and ^32,34Si; radioactivities are in good agreement with predicted values within the analytical superasymmetric fission (ASAF) model. The strong shell effect may be further exploited to search for new cluster emitters.

The following sections are included:

• Introduction

• Spallation target

• Sub-critical core

• Containment building

• Objectives

• Conclusions

• References

Dissipation in nuclear collective motions is testified through anomalous neutron multiplicities emitted prior fission. Its importance is also shown in fusion of massive systems and thereby in the synthesis of the superheavy elements. It is recommended to keep in mind the important role of the dissipation and its associated fluctuation in nuclear collective motions such as nuclear fission.

Quasifission and fusion are treated with master equations describing the evolution of the dinuclear system in the charge and mass asymmetries. Agreement of the calculated results with experimental data of fusion reactions leading to superheavy nuclei is found.

Progress on the use of the double-folding method for calculations of microscopic real potentials for complex particles is reported as well as their use within nuclear data evaluation of actual interest for radiation damage estimation and radioactive waste transmutation projects. The effective nucleon-nucleon interaction involved for optical-potential calculation and analysis has been used also within multistep direct calculations.

Ternary fission of ²⁵²Cf was studied at Gammasphere with using eight ΔE×E particle telescopes. The 3368 keV gamma transition from the first excited state in ¹⁰Be was undoubtedly found. The ratio of the population probabilities for these two levels was estimated as 0.160±0.025. The nuclear temperature of the neck region near the scission point was estimated as 1.0±0.2 MeV. No evidence was found for 3368 keV γ rays emitted from a triple molecular state.

Fission fragments of heavy nuclei (Z > 90) are neutron-rich isotopes of the elements from Zn (Z = 30) to Nd (Z = 60) with a neutron number of 45 – 90. The large neutron excess in the fission fragments under study (in some cases there are 10 – 15 more neutrons, than in the nuclei situated in the β-stability valley) could lead to an essential change in their structure and radioactive decay characteristics. The abnormal ratio of protons and neutrons in such nuclei reflects on spin-orbit interactions and can lead to another order of nucleon shell filling. This change will manifest itself in the appearance of new magic numbers of protons or neutrons, new regions of deformation, of new islands of isomerism. A striking example of such phenomena is found in light neutronrich nuclei of ³¹Na and ³²Mg at the magic number N = 20. Contrary to our knowledge about nuclear structure, these nuclei are strongly deformed [1,2]. The same situation could occur in the case of very neutron-rich isotopes of Cu and Zn near N = 50 as well as Ag and Cd near N = 82…

Target and projectile charge densities are treated as free parameters in the calculation of the deformation energy. Different charge density paths are proposed as a result of geometrically related law of variation of the number of protons in the non-overlapped volumes of the two partners. As a result of minimization along the distance between the two centers fusion barriers differences reach up to 4 MeV for light nuclei and 8 MeV for superheavy synthesis.

The properties of the parent quasimolecular states are deduced from the general properties of the exotic resonant states found by the Riemann surface approach to S-matrix poles.

Fission fragment properties and cross-sections have been investigated for ^235,238U(n,f), ²³⁹Pu(n,f), ²³⁷Np(n,f), ²⁵²Cf(SF) and ²³³Pa(n,f). Interpretation of the experimental results was done within the multi-modal fission model. The three most dominant fission modes were considered, the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric superlong (SL) mode. For the evaluation the statistical model was extended to include the concept of multi-modality of the fission process. Fission mode deconvoluted fission cross-sections, neutron multiplicites and spectra for the S1, S2 and SL modes were calculated for the first time in the incident neutron energy range from 0.01 to 5.5 MeV for the isotopes mentioned above, except ²⁵²Cf and ²³³Pa. Good agreement was found with the experimental data. Additionally, branching ratios were deduced for the different modes giving the possibility to calculate fission fragment mass yield distributions at incident neutron energies, where no experimental data exist. Last but not least, the first ever direct fission cross-section measurements of ²³³Pa were performed. The new evaluation resulted in reduced fission cross-sections as compared to evaluated libraries.

A reliable modeling of GeV proton-induced spallation reactions is indispensable for the design of the spallation module and the target station of future accelerator driven hybrid reactors (ADS) or spallation neutron sources (ESS), in particular, to provide precise predictions for the neutron production, the radiation damage of materials (window), and the production of radioactivity (³H, ⁷Be etc.) in the target medium. Detailed experimental nuclear data are needed for sensitive validations and improvements of the models, whose predictive power is strongly dependent on the correct physical description of the three main stages of a spallation reaction: (i) the Intra-Nuclear-Cascade (INC) with the fast heating of the target nucleus, (ii) the de-excitation due to pre-equilibrium emission including the possibility of multi-fragmentation, and (iii) the statistical decay of thermally excited nuclei by evaporation of light particles and fission in the case of heavy nuclei. Key experimental data for this endeavor are absolute production cross sections and energy spectra for neutrons and light charged-particles (LCPs), emission of composite particles prior and post to the attainment of an equilibrated system, distribution of excitation energies deposited in the nuclei after the INC, and fission probabilities. Systematic measurements of such data are furthermore needed over large ranges of target nuclei and incident proton energies. Such data has been measured with the NESSI detector. An overview of new and previous results will be given.

The aim of this work is to experimentally investigate transient effects in fission. In order to simplify the theoretical description, we have chosen peripheral heavy-ion collisions at relativistic energies to reduce the side effects and to produce highly excited fissioning systems with well-defined initial properties. Thanks to an experimental setup specially conceived for fission studies in inverse kinematics, we could determine two new observables very sensitive to transient effects in fission. Quantitative values for transient effects are deduced from the comparison of these observables with a nuclear-reaction code where dissipation effects in fission are modeled in a highly realistic way.

One of the main focus of the work at the K-130 cyclotron in Jyväskylä has been the investigation of properties of neutron-rich nuclei, which are produced in fission reactions. As the production yields of nuclei far off the β-stability become increasingly smaller new, more efficient techniques for producing and selecting these nuclei have to be employed. Therefore a triple-trap setup has been installed at the IGISOL facility in Jyväskylä. This new facility not only improves the experimental condition for decay-spectroscopy and collinear laser spectroscopy it also enables mass-measurements as an additional tool for studying neutron-rich nuclei.

The fission probability of ²³⁶U has been measured as a function of the excitation energy with high energy resolution. Rotational band structures have been observed, with moments of inertia corresponding to hyperdeformed nuclear shapes. From the level density of the rotational bands the excitation energy of the ground state in the third minimum was determined. The excitation energy of the lowest hyperdeformed transmission gave an upper limit for the height of the inner fission barrier. The predicted effects of the clusterization in hyperdeformed states have been studied by measuring the mass and total kinetic energy distribution of the fission fragments in case of ²³²Th.

We analyze the α-decay along N – Z chains in heavy and superheavy nuclei. The α-particle preformation amplitude is estimated within the pairing model, while the penetration part by the deformed WKB approach. We show that for N > 126 the plateau condition is not fulfilled along any α-chain, namely the logarithmic derivative of the Coulomb function changes much faster in comparison with that of the preformation factor. We correct this deficiency by considering an α-cluster factor in the preformation amplitude, depending upon the Coulomb parameter. For superheavy region an additional dependence upon the number of interacting α-particles indicates a clustering feature connected with a larger radial component.

The recently introduced critical point symmetries in the nuclear phase transition between spherical and deformed shapes have produced a large interest in the search for their empirical realizations. A review of this search in different regions of nuclear chart, including the neutron rich nuclei obtained in nuclear fission, is presented.

The fission barriers standing in the quasi-molecular shape path have been determined within a generalized liquid drop model taking into account the nuclear proximity energy, the mass and charge asymmetry and an accurate nuclear radius. The barrier heights agree with the experimental symmetric and asymmetric fission barrier heights. The half-lives of the alpha and light nucleus decay and cluster radioactivity are reproduced within a tunneling process through these barriers. Rotating highly deformed states exist in this path. The entrance and exit channels governing the superheavy nucleus formation and decay have been investigated.

A recent experiment with a new array detector aiming the investigation of halo neutron pair pre-emission in Si(¹¹Li, fusion) is described. A new approach for testing the true n-n coincidences against cross-talk has been worked out. An experimental evidence for residual correlation of the pre-emitted neutrons is presented. The results obtained in building the n-n correlation function by using the available denominators are discussed. A recent iterative method for calculation of the intrinsic correlation function was also applied. An experiment for precise measurement of the intrinsic correlation function is proposed.

In unitary manner, all kind of binary partitions, including the alpha- and cluster-decays, are treated as fission processes in a wide range of mass-asymmetries. In this spirit, the whole nuclear system is characterized by some collective coordinates which vary and manage the evolution of the system. The decaying system provides a time dependent single-particle potential in which the nucleons move independently. Investigations concerning the fine structure of alpha– and cluster–decay together with the dissipation in fission are realized.

Several examples of recent results obtained with SPIRAL beams on the properties of nuclei in the vicinity of drip-lines and/or magic numbers are presented. The future plans of the GANIL/SPIRAL facility related to the SPIRAL 2 project are shortly described.

The decay of the proton drip-line nucleus ⁴⁵Fe has been studied at the SISSI-LISE3 facility of GANIL, after projectile fragmentation of a ⁵⁸Ni primary beam. Fragment-implantation events have been correlated with radioactive decay events in silicon telescopes on an event-by-event basis. The decay-energy spectrum of ⁴⁵Fe implants shows a distinct peak consistent with a two-proton ground-state decay of ⁴⁵Fe. None of the events in the peak were found to be in coincidence with γ radiation or with β particles which were searched for in the telescope-surrounding detectors. The decay energy for ⁴⁵Fe agrees nicely with several theoretical predictions for two-proton emission. These predictions strongly support the observed decay to be a simultaneous two-proton ground-state decay.

Two ways of production of radioactive beams using uranium carbide targets are taken into consideration: fission induced by fast neutrons and by bremsstrahlung radiation. For the SPIRAL 2 project, the fission of the uranium carbide target will be induced by a neutron flow created by bombarding a carbon converter with a 40 MeV high intensity primary deuteron beam. Calculations and design of the target in order to reach 10¹³ fission events per second with good release have been done. The second way is the photofission using an electron beam. In 2004 the ALTO project (Accélérateur Linéaire Auprès du Tandem d'Orsay) will give a 50 MeV/l0μA electron beam. This facility will allow more than 10¹¹ fissions/s. In this case, the electron beam hits the target without converter. Calculations are realized in order to estimate the production and to choose the best target shape.

The PARRNE 2 device allows the production of neutron-rich isotopes beams using the ISOL method on a thick ²³⁸U target. With fast neutrons produced by 26 MeV / 1 μA deuteron beam, 10⁹ fission/s are induced in the UC_x target. In order to improve this production, it has been decided to use the photofission method. A 50 MeV electron accelerator connected with the PARRNE 2 separator is now under construction at IPN.

Note from Publisher: This article contains the abstract and references only.

The calculations of the isomeric ratios σ_m/σ_g and excitation functions for the reactions ¹⁰⁹Ag(α, 3n)^{110 mg}In, ¹⁰⁷Ag(^6,4He, n, 3n)^110mgIn and ^{128, 130}Te(^8,6He, 3n, n) ^133mgXe in the framework of the statistical theory of nuclear reactions were performed. The model of Hauser-Feshbach, the exciton Griffin model and the recently developed statistical model code EMPIRE-2.18¹ with full angular momentum coupling were used. For the reactions induced by α-particles we used the Gilbert-Cameron level densities and the Multi-Step Compound approach. The fusion cross sections of the reactions induced by ^6,8He projectiles were calculated in terms of the simplified coupled channels method (CCFUS code). The new results of the calculations ICSR for the reactions ¹³⁰Te(⁴He,n)^133mgXe (1) and ¹²⁸Te(⁶He,n)^133mgXe (2) on the statistical code EMPIRE-18 are shown in the Table 1. The results for the reactions ¹²⁸Te(⁸He,3n)^133mgXe (3) and ¹³⁰Te(⁶He,3n)-^133mgXe (4) are shown in Table 2. The results are in a good agreement with the experimental data for α-particles induced reactions obtained ² at Moscow State University (INP)…

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In order to investigate the influence of the initial β-vibrational motion of the fissioning nucleus on the scission configuration, we have measured the fragment mass distribution of the ²⁴⁰Pu fission following the β-vibrational resonance¹. The β-vibrational state (K^π = 0⁺ state²) is formed on the second minimum ( superdeformed(SD) minimum ) of the double-humped fission barrier. This state is observed below the threshold energy in the form of an enhanced fission cross section due to a resonance tunneling induced when the excitation energy (E_ex) of the compound nucleus matches the level…

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A microcanonical multifragmentation model is used to investigate different aspects related to a possible liquid-gas phase transition taking place in excited nuclei. It is shown that due to the finite size of the system properties generally specific to the critical point manifest is a wide region of the phase diagram. Is is shown that the scaling of isotopic yields is affected by finite size effects. Using a simplified one component Lennard-Jones fluid phase diagrams of different size systems are calculated. The result is that the critical point of the system is size dependent.

We have done a systematic analysis of α-decays to excited 2+ states. Collective excitations were considered within the Quasiparticle Random-Phase Approximation. As a residual two-body force we used the surface-delta interaction. The only free parameter is the ratio of the isovector and isoscalar strengths. This approach is able to explain the general features of electromagnetic and α-transitions along isotope chains. We conclude that the α-decay to excited 2⁺-state is induced mainly by the effective residual interaction between proton and neutron systems. We also predicted α-decay hindrance factors of 2⁺-states with respect to the ground state and B(E2) values for even-even nuclei.

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Nuclear fission of heavy nuclei is a process which allows the production of neutron rich isotopes. Exploiting this property, a source of intermediate energy nuclei near the stability limit can be conceived using the ISOL (Isotope Separation On-Line) technique and the photo-fission process. In a first step, an electron beam of 30–60 MeV is focused onto a W converter or onto the uranium carbide target (UCx) itself to deliver a bremsstrahlung radiation. In the second step, the gamma rays induce the fission of the ²³⁸U. Large fission events yields are expected due to the huge photo-fission cross section traduced by a broad peak of about 160 mb height for 15 MeV gamma rays…

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The theoretical analysis of fission processes is limited by the use of one-center shell models. The simplest way to generalize the Nilsson model for such purposes is to use a two-center shell model. A numerical code¹ for a superasymmetric two-center shell model was developed recently in our Institute in order to study axially-symmetric disintegration processes in a wide range of mass-asymmetries, including the alpha-decay². This work was based on the Frankfurt model³ for the two-center oscillator potential. However, the formalism used in this context is not appropriate for the study of near-symmetric fission. The main difference lies in the fact that the system of eigenvectors of the basic two-center oscillator for reflection symmetric systems is characterized by two good quantum numbers, the parity and the projection of the intrinsic spin Ω. In the case of mass-asymmetries, the levels are characterized by only one good quantum number, that is Ω. Accordingly, the mathematical formalism differs between the two kinds of parameterization. The analytical formulas are based on the formalism described in Ref. 4…

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A master equation for the deformed quantum harmonic oscillator interacting with a dissipative environment, in particular with a thermal bath, is obtained in the microscopic model, using perturbation theory. The coefficients of the master equation depend on the deformation function. The steady state solution of the equation for the density matrix in the number representation is derived and the equilibrium energy of the deformed harmonic oscillator is calculated in the approximation of small deformation.

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The decommissioning of Research Nuclear Reactor VVR-S Magurele – Bucharest involves the removal of the radioactive and hazardous materials to permit the facility to be released without representing a further risk to human health and the environment [1-3]. A very important aspect of decommissioning is the analyze, justification and selection of the decommissioning strategy. Two strategies: DECON (Immediate Dismantling) and SAFSTOR (Safe Enclosure) are in study (see Table 1)…

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K-shell ionization cross sections and mean vacancy sharing probabilities measured in the near-symmetric collision systems Fe and Co + Cr at 0.2 -1.75 MeV/u energies are reported. The cross section values have been corrected for multiple ionization in the outer (L, M) shells by using the energy and yield shift method. Mean ionization probabilities per electron in the outer shells have been estimated. Calculations in the Briggs model, using SCA with relativistic hydrogenic wave functions and binding correction, could explain the 2pσ -molecular orbital ionization cross sections at higher energies (≥1 MeV/u). The vacancy sharing results show that predictions of a two-state coupling Meyerhof-Demkov model is less fulfilled at higher energies (≥1.5 MeV/u).

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In the reference neutron spectrum ΣΣ [1,2,3], some yields for fission reactions of ²³⁵U, ²³⁹Pu and ²³⁸U have been measured…

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The selection of neutron fields allowing the integral neutron data validation as well as the calibration of the instrumentation used in the absolute measurements, was a continuous task of the experts in the area. The choosing of the neutron spectrum is based on the hypothesis of correct description of the spectral shape by calculation methods as well as integral and differential measurements. Several standards were proposed to cover the energy range between 0 and 18 MeV…

The atmospheric flux of muons and neutrinos originates from the decay of charged pions and kaons produced by cosmic rays in the atmosphere. Muons have a relatively large lifetime and they decay in electrons and neutrinos [1]. The muon charge ratio represents the ratio of positive to negative atmospheric muons. The measurements are performed using a small compact device, WILLI (Weakly Ionizing Lepton Lead Interactions), by detecting the lifetime of the muons in different materials [2], Avoiding the difficulties of measurements with magnetic spectrometers, this method gives precise results on muon charge ratio especially in the low energy where the influence of magnetic field is stronger. In the present configuration the detector is construct as a rotatable device which permits measurements on different inclinations and azimuthal directions [3,4]. Measurements of the muon charge ratio performed in the East – West directions with the detector inclined at 45° shows a pronounced Eas-West effect for the muon momentum range 0.35-0.50 GeV/c.

The following sections are included:

• Guidelines

Dissipative processes have been investigated experimentally in several light heavy-ion systems, using a complex detector which has as main components two position sensitive ionization chambers. Experimental evidence and comparison with theoretical calculations suggest a mechanism similar to deep inelastic processes in heavy and medium systems, even in the case of completely damped events.

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In present work we propose an approach to reduce the problem of particle emission rates of spheroidal nuclei to the solution of homogeneous and inhomogeneous systems of coupled differential equations arising from the Schrodinger equation. We show that some of the previous development in the question carried out so far in the Wigner R-matrix formalism can be restated in a slightly different form within the Feshbach formalism of resonance nuclear reactions which has advantages of unified treatment of the structure and nuclear dynamics and of emphasis on the more essential mathematical steps. Additionally, the treatment of the decay rates for radioactive states associated with resonances in deformed field is more general and inherently less dependent on arbitrary assumptions than other available R-matrix treatments…

The following sections are included:

• LIST OF PARTICIPANTS