Cold Fission Research Articles

Ruptura de pares y efectos coulombianos en la fisión fría de las reacciones 233U(NTH, F), 235U(NTH, F) Y 239PU(NTH, F)

En este trabajo se muestra los efectos de la ruptura de parejas de nucleones y de la interacción electrostática entre fragmentos sobre la distribución de masa y energía cinética en la fisión fría de la fisión de 233U, 235U y 239Pu inducida por neutrones térmicos. El proceso de fisión termina en el punto de escisión, cuando se forman los fragmentos complementarios 1 y 2, con masas y A1A2. Luego, esos fragmentos se alejan entre sí y adquieren energías cinéticas y E1E2. Los fragmentos emiten neutrones erosionando la distribución de masa y energía. Para evitar estas perturbaciones, se estudia la fisión fría i.e., fisión sin emisión de neutrones. Contrario a lo que se esperaba, en estas ventanas de energía cinética se observa un débil efecto par-impar sobre la distribución de masa de fragmentos. Por otro lado, en la fisión fría se observa que el valor mínimo de la energía cinética total (X) que alcanzan los fragmentos, en función de la masa, está correlacionada con el “exceso de energía electrostática” definido como la diferencia entre la energía de interacción electrostática de los fragmentos complementarios (asumidos esféricos) en el punto de escisión y la energía disponible de la reacción. Esta correlación explica también las fluctuaciones de la energía cinética máxima en función de la masa. El efecto crece con la asimetría de las fragmentaciones de carga. En suma, los datos experimentales sobre la fisión fría sugieren que las configuraciones de escisión exploran todas las posibilidades que permite la energía disponible para la fisión.

(A = 10)-Accompanied spontaneous ternary fission of californium isotopes

The ternary fission of 238-256Cf even-A isotopes is studied in the framework of the three-cluster model within the spherical approximation. The study is confined to those decays in which the accompanied light charged particle is of mass A = 10. The most probable ternary fission path is obtained as the one that has a peak in the Q-value and a minimum in the driving potential, with respect to the mass and charge asymmetries. Assuming both equatorial and collinear configurations of the different emissions of the light charged particles with A = 10, the ternary fission of 252Cf is found to be most favored with 10Be. The predicted favorable fragmentation channels of the 10Be-accompanied ternary fission of 238-256Cf, even-A, isotopes are discussed in detail. The closed-shell structure of the produced heaviest fragment is found to play a key role for the detected most favorable channels. A nucleus with a closed neutron shell or even doubly closed shell always appears as the heaviest nucleus in the favored channel of the ternary fission of all mentioned isotopes. As the isospin asymmetry of the isotope increases, the width of its cold fission valley around the symmetric mass region decreases, with more shifting towards the centre of the symmetric region. Neither the most favorable ternary emissions nor the behavior of the ternary fragmentation potential changed with the type of nuclear interaction potential (Yukawa, folding potential with Migdal force, and proximity 77). Also, almost no change is observed according to the method of emission of the fragments (equatorially or collinearly) or when the calculations are performed without inserting the nuclear interaction potential.

Spectroscopy of neutron rich nuclei using cold neutron induced fission of actinide targets at the ILL: The EXILL campaign

One way to explore exotic nuclei is to study their structure by performing γ-ray spectroscopy. At the ILL, we exploit a high neutron flux reactor to induce the cold fission of actinide targets. In this process, fission products that cannot be accessed using standard spontaneous fission sources are produced with a yield allowing their detailed study using high resolution γ-ray spectroscopy. This is what was pursued at the ILL with the EXILL (for EXOGAM at the ILL) campaign. In the present work, the EXILL setup and performance will be presented.

Even-odd Effects of Fragment Yields in Low Energy Fission

Abstract Since the discovery of fission even-odd staggering of proton, neutron and mass yields of the fragments and their energies has been a topic of ongoing research. In the present survey only low energy fission is covered, i.e. spontaneous fission and thermal neutron induced fission of actinides. From studies of fragment yields and their even-odd staggering in cold fission the lesson is drawn that in models of adiabatic fission, where breaking of nucleon pairs is attributed to non- adiabatic neck rupture, two parameters have to be taken into account: neck size and speed of neck rupture.

Role of energy cost in the yield of cold ternary fission of 252 Cf

The energy costs in the cold ternary fission of 252Cf for various light charged particle emission are calculated by including Wong’s correction for Coulomb potential. Energy cost is found to be higher in cold fission than in normal fission. It is found that energy cost always increases with decrease in experimental yield in all the light charged particle emissions. The higher ground state deformation of the fragments, the odd–even effect and the enhanced yield in the octupole region observed in cold fission are found to be consistent with the concept of energy cost.

Half-life and inner penetrability in the cold fission of 248Cm

Using the Coulomb and proximity potential model (CPPM) with the nuclear surface tension improved by Reisdorf, the various fragmentations in the cold fission of 248Cm are studied as a cluster decay process within a fission model. The computed half-life values are compared with the values reported by Sandulescu et al. using the double-folding M3Y nuclear-nuclear potential model and are in agreement with CPPM values. The diffuseness width used in the proximity potential is found to be sensitive to the half-life. The inner penetrability is computed based on the formalism of Poenaru et al. (J. Phys. G: Nucl. Part. Phys. 17, 443 (1991)) and found that in the cold fission the contribution of the inner part (overlap region) of the barrier is significant compared to cluster decays. The variation of the inner penetrability with fragment mass is studied which emphasizes the fact that cold fission is an extension of cluster decay.

Strong dissipation regime in nuclear disintegrations

The coupling of collective degrees of freedom with the microscopic ones causes dissipation and a modification of the adiabatic potential. The term dissipation usually refers to exchange of energy (either linear or angular momentum) by all kind of damping from collective motion to intrinsic heat. A measure of the dissipated energy can be obtained solving the time dependent pairing equations. In this presentation, a generalization of the time dependent pairing equations is presented by including the Landau-Zener effect in the superfluid model. These new equations allows a mixing of seniority one configurations that allows us to obtain a ground state at the end of the process. An application concerning the 14C emission is offered and its fine structure is explained. These new equations are furthermore used to evidence a dynamical pair breaking effect that could explain the fine odd-even effect in cold fission. Finally, the time dependent pairing equations are used to deduce a model for non-adiabatic cranking inertia.

Dissipative regime in low energy fission

Abstract In this presentation, a generalization of the time dependent pairing equations is presented by including the Landau-Zener effect in the superfluid model. These new equations allows a mixing of seniority one configurations that allows us to obtain a ground state at the end of the process. An application concerning the C-14 emission is offered and its fine structure is explained. These new equations are furthermore used to evidence a dynamical pair breaking effect that could explain the fine odd-even effect in cold fission. The partition of the dissipated energy between the two fission fragments will be also modeled. Finally, the time dependent pairing equations are used to deduce a model for non-adiabatic cranking inertia.

Microscopic cold fission yields ofCf252

We show that the sharp maximum corresponding to $^{107}\mathrm{Mo}$ in the fragment distribution of the $^{252}\mathrm{Cf}$ cold fission is actually a Sn-like radioactivity, similar to other decay processes in which magic nuclei are involved, namely $\ensuremath{\alpha}$ decay and heavy cluster emission, also called Pb-like radioactivity. It turns out that the mass asymmetry degree of freedom has a key role in connecting initial Sn with the final Mo isotopes along the fission path. We investigate the cold rearrangement of nucleons within the framework of the two-center shell model in order to compute the cold valleys in the charge equilibrated fragmentation potential. The fission yields are estimated by using the semiclassical penetration approach. We consider 5 degrees of freedom, namely the interfragment distance, the shapes of fragments, the neck parameter, and mass asymmetry. We found an isomeric minimum between the internal and external barriers. It turns out that the inner cold valley of the total potential energy is connected to the doubly magic isotope $^{132}\mathrm{Sn}$.

FISSION POTENTIAL ENERGY SURFACES IN TEN-DIMENSIONAL DEFORMATION SPACE

Various fission processes are described in terms of high-dimensional potential energy surface in the frame of the Strutinsky shell correction method for actinide region. The complete deformation space is necessary to study the potential energy minima responsible for the cluster radioactivity, cold fission and cold multi-fragmentation valleys. The nuclear shape families for the different fission configurations are obtained without any specific change of the parameters. The coordinate system based on the Cassini ovaloids makes it possible to increase the number of independent deformation parameters without divergence. The higher orders of the deformation are shown to play an important role in the description of the potential energy surface structure.

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus 226Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in 18O + 208Pb interaction induced by projectile oxygen ions of energy in the range E lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, Mγ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence Mγ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E lab, the gamma-ray multiplicity Mγ as a function of the total kinetic energy (TKE) of fragments, Mγ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ∼Q max.

Cold valleys in the radioactive decay of 248–254Cf isotopes

Based on the concept of cold valley in cold fission and fusion, we have investigated the cluster decay process in 248–254Cf isotopes. In addition to alpha particle minima, other deep minima occur for S, Ar and Ca clusters. It is found that inclusion of proximity potential does not change the position of minima but minima become deeper. Taking Coulomb and proximity potential as interacting barrier for post-scission region, we computed half-lives and other characteristics for various clusters from these parents. Our study reveals that these parents are stable against light clusters and unstable against heavy clusters. Computed half-lives for alpha decay agree with experimental values within two orders of magnitude. The most probable clusters from these parents are predicted to be 46Ar, 48,50Ca which indicate the role of doubly or near doubly magic clusters in cluster radioactivity. Odd A clusters are found to be favorable for emission from odd A parents. Cluster decay model is extended to symmetric region and it is found that symmetric fission is also probable which stresses the role of doubly or near doubly magic 132Sn nuclei. Geiger-Nuttal plots were studied for various clusters and are found to be linear with varying slopes and intercepts.

FIFTY YEARS OF DISCOVERIES IN NUCLEAR STRUCTURE

The complementary nature of the Vanderbilt experimental work and the theoretical work of the Frankfurt group on the rotation-vibration model, the generalized collective model, shape coexistence and cluster radioactivity is described. Recent discoveries in neutron-rich nuclei γ-vibrational bands compared with RVM and GCM are presented. Ultimate examples of cluster radioactivity in cold binary and ternary fission are discussed. Finally, we present evidence for the evolution from axial symmetry in 101 Y to maximum triaxialilty in 113 Rh and for chiral doublet bands in neutron-rich triaxial even-even nuclei between A =106 - 112.

Indication for hyperdeformed cluster states in 233Th

An activation technique was used to investigate relative yields of fission products from the reaction 232Th(n, f ) for neutron energies between 1.3 and 1.8MeV covering the region around the first hyperdeformed resonances. Intensities of characteristic γ-ray transitions were analyzed to search for changes in the mass distribution for neutron energies corresponding to the resonances and below the resonances. Relative increases in yield between 7 and 23% are observed for A ≈ 100 and 132 in the resonance region around 1.6MeV. It is proposed that the yield enhancement of daughter nuclei of the preformed fragments 132Sn and 101Zr arises from cold fission of a di-cluster configuration. The experimental results support theoretical predictions for the existence of hyperdeformed octupole shapes based on the di-nuclear configuration 132Sn + 101Zr.

Production of noble gas isotopes by proton-induced reactions on lead and bismuth

We measured integral thin target cross-sections for the proton-induced production of He-, Ne-, Ar-, Kr-, and Xe-isotopes from lead and bismuth from the respective reaction threshold up to 2.6 GeV. The production of noble gas isotopes from lead and bismuth is of special importance for design studies of accelerator driven nuclear reactors and/or energy amplifiers. For all experiments with proton energies above 200 MeV a new mini-stack approach was used instead of the stacked-foil technique in order to minimise influences of secondary particles. The phenomenology of the determined excitation functions enables us to distinguish between the different reaction modes fragmentation, hot and cold symmetric fission, asymmetric fission, and deep spallation. For lead more than 420 cross-sections for 23 nuclear reactions have been measured. While the lead data have already been published, here we present first results for the production of noble gas isotopes from bismuth. The experimental data are compared to results from the theoretical nuclear model code INCL4/ABLA. This comparison clearly indicates that experimental data are still needed because the predictive power of nuclear model codes, though permanently improving, does still not allow to reliably predict the cross-sections needed for most applications and irradiation experiments remain indispensable.

Production of noble gas isotopes by proton-induced reactions on lead

We measured integral thin target cross sections for the proton-induced production of He-, Ne-, Ar-, Kr- and Xe-isotopes from lead from the respective reaction threshold up to 2.6 GeV. The production of noble gas isotopes from lead is of special importance for design studies of accelerator driven nuclear reactors and/or energy amplifiers. For all experiments with proton energies above 200 MeV a new mini-stack approach was used instead of the stacked-foil technique in order to minimise the influences of secondary particles on the residual nuclide production. About 420 cross sections for 23 nuclear reactions were determined. The phenomenology of the determined excitation functions enables us to distinguish between the different reaction modes fragmentation, hot and cold symmetric fission, asymmetric fission and deep spallation. Cross sections for the production of 21Ne and 38Ar measured below 100 MeV and 200 MeV, respectively, enable us to study nuclide production below the nominal Coulomb-barrier. The experimental data are compared to results from the theoretical nuclear model code INCL4/ABLA. While the model describes the production of 4He reasonably well, it underestimates the cross sections for Ne- and Ar-isotopes produced via deep spallation and/or multifragmentation by up to two orders of magnitude. For the Kr- and Xe-isotopes the agreement between modelled and measured data strongly depends on the reaction mechanisms. While INCL4/ABLA describes the production of n-poor Kr-isotopes via hot-symmetric fission and the production of Xe-isotopes via asymmetric fission reasonably well, i.e. within a factor of 2, the discrepancies between modelled and measured cross sections for the n-rich Kr-isotopes produced via cold symmetric fission are significantly larger. For the Xe-isotopes produced via spallation, i.e. at energies higher than about 600 MeV, the model completely fails to describe the experimental data. Therefore, the comparison of measured and modelled thin target cross sections clearly indicates that experimental data are still needed because the predictive power of nuclear model codes, though permanently improving, does still not allow to reliably predict the cross sections needed for most applications and irradiation experiments remain indispensable.

Complex fission phenomena

Complex fission phenomena are studied in a unified way. Very general reflection asymmetrical equilibrium (saddle point) nuclear shapes are obtained by solving an integro-differential equation without being necessary to specify a certain parametrization. The mass asymmetry in binary cold fission of Th and U isotopes is explained as the result of adding a phenomenological shell correction to the liquid drop model deformation energy. Applications to binary, ternary, and quaternary fission are outlined.

Half-life predictions for decay modes of superheavy nuclei

We applied the effective liquid drop model to predict the alpha-decay, cluster emission and cold fission half-life values of nuclei in the region of superheavy elements (SHE). The present calculations have been made in the region of the ZN plane defined by 155 ⩽ N ⩽ 220 and 110 ⩽ Z ⩽ 135. Shell effects are included via the Q-value of the corresponding decay case. We report the results of a systematic calculation of the half-life for the three nuclear decay modes in a region of the ZN plane where superheavy elements are expected to be found. Results have shown that, among the decay modes investigated here, the alpha decay is the dominant one, i.e. the decay mode of smallest half-lives. Half-life predictions for alpha-decay, cluster emission and cold fission for the isotopic family of the most recent SHE detected of Z = 115 and for the isotopic family of the already consolidated SHE of Z = 111 are presented.

Spectroscopic factors of cluster decays in an algebraic cluster model

We propose a new ansatz for the cluster spectroscopic factor in an algebraic cluster model. The parameters of the ansatz are fitted to the experimental and Blendowske–Walliser values of the spectroscopic factor for light emitted clusters and then applied to predict the spectroscopic factors for heavier clusters up to the spontaneous cold fission in two equal fragments. We show that due to the cluster and daughter shell structure, the calculated spectroscopic factors are strongly deviating from the empirical law for increasing cluster number. The observed gap between the alpha-like and cluster-like fission on one side and the spontaneous cold fission on the other side for actinides is explained by the moderate increase of spectroscopic factors, as computed in the present Letter, combined with the simultaneous decrease of the barriers when approaching symmetric mass division. This conclusion is inferred by computing the half-lives of cluster emission and spontaneous cold fission for 234U.

Orientation Windows in the Cold Synthesis and Decay of Heavy Elements

Due to the multipolar ground state nuclear deformations (β 2, β 3, β 4,...) the cold synthesis of heavy and superheavy elements is dependent on the mutual orientation of target and projectile. When going from lower bombarding energies to higher bombarding energies, windows for orientations are occurring and others are disappearing. The cold fission is also strongly dependent on the deformation but is more selective in respect to orientation of fragments at scission. The measured cold fission yields of 252Cf show that only one orientation window is active. I propose a qualitative receipt based on the orientation windows of the driving potential which serves as a guidance in understanding the fission and quasifission mass distributions in the synthesis of heavy and superheavy elements.