Collinear Cluster Tri-partition Research Articles

Sequential decay analysis of $^{235}$U(n$^{th}$,f) reaction using fragmentation approach

Abstract Numerous experimental and theoretical observations conclude that the probability of the three fragment emission (ternary fission) or the binary fission increases when one proceeds towards the heavy mass region of nuclear periodic table. The collinear cluster tripartition (CCT) channel of $^{235}$U(n$^{th}$,f) reaction is studied and it was observed that the CCT may be a sequential process or a simultaneous emission phenomena. Till now, different approaches are introduced to study the CCT process as a simultaneous process or sequential process, but the decay dynamics of these modes is not fully explored. It will be of interest to identify the three fragments of the sequential process and to explore their related dynamics using some excitation energy dependent approach. Hence, in present work, an attempt is made to study the sequential decay mechanism of $^{235}$U(n$^{th}$,f) reaction using quantum mechanical fragmentation theory (QMFT). The decay mechanism is considered in two steps, where initially the nucleus splits into an asymmetric channel. In the second step, the heavy fragment obtained in the first step divides into two fragments. Stage I analysis is done by calculating the fragmentation potential and the preformation probability for the spherical and deformed choice of the decaying fragments. The most probable fragment combination of stage I are identified in view the dips in the fragmentation structure, and the corresponding maxima's of the preformation probability ($P_0$). The excitation energy of the decay channel is calculated using an iteration process. The obtained excitation energy of the identified heavy fragments is further used for the fragmentation analysis, and the subsequent binary fragments of the sequential process are obtained. The identified three fragments of the sequential process are in agreement with the experimental observation and are found nearby the neutron or proton shell closure. Finally, the kinetic energy of the observed fragments is calculated and the middle fragment of the CCT mechanism is identified.

Cluster aspects of collinear cluster tri-partition (CCT)

New results demonstrating the so-called Ni-bump being the most populated mode of the collinear cluster tripartition (CCT) of 252Cf(sf) are presented. The physical scenario of this effect is discussed. It is tested by calculations of potential energy surfaces for the fission of the intermediate fragments formed after first rupture of the mother nucleus. Fission barriers are extracted and mass asymmetries at saddles are compared with the masses of the fission fragments that take part in the Ni-bump.

Ternary fission analysis of Fm242,258 nuclei using equatorial and collinear cluster tripartition configurations

The quantum mechanical fragmentation theory--based three cluster model is employed to investigate the ground-state ternary fission of two Fm isotopes nuclei having atomic mass ${A}_{P}=242$ and 258. The mass asymmetry coordinate and the relative separation among the decaying fragments play a crucial role for the estimation of the fragmentation structure and related barrier penetration process. First, the choice of third fragment (${A}_{3}$) is fixed by minimizing the probable ${A}_{3}$ fragments having different proton neutron configurations. Further, the fission fragment combinations (${A}_{1}+{A}_{2}+{A}_{3}$) are identified for the fixed third fragments by selecting the channel of lower ternary fragmentation potential and higher relative fission yield. Two type of tripartition of radioactive nuclei are considered such as equatorial cluster tripartition (ECT) and collinear cluster tripartition (CCT). A comparative analysis of ternary fragmentation potential and relative fission yield within ECT and CCT geometrical arrangement is carried out for different choices of third fragment, i.e., ${A}_{3}=1$ to ${A}_{P}$/3. The choice of most probable fragments suggest that the proton and neutron magic shell closures play essential role in the ternary mass division. Finally, a relative analysis of binary and ternary fragmentation is worked out for better insight of dynamics involved.

A new radioactive decay mode, true ternary fission, the decay of heavy nuclei into three comparable fragments

The ternary cluster decay of heavy nuclei has been observed in several experiments with binary coincidences between two fragments using detector telescopes (the FOBOS-detectors) with very large solid angles and placed on the opposite sides from the source of fissioning nuclei. The binary coincidences at a relative angle of $$180^{\circ }$$ correspond to binary fission or to the decay into three cluster fragments by registration of two of them in coincidences with missing nuclei of different masses (e.g. $$^{132}\hbox {Sn}$$ , $$^{52-48}\hbox {Ca}$$ , $$^{68-72}\hbox {Ni}$$ ). This marks a new step in the physics of fission-phenomena of heavy nuclei. The new decay mode has been observed more then 10 years ago by the FOBOS group in JINR (Dubna) Russia. These experimental results for the collinear cluster tri-partition, refer to the decay into three clusters of comparable masses. The observation of several other fission modes in the same system can be predicted by the potential energy surface, which show pronounced minima and valleys, they correspond to a variety of collinear ternary fission (multi-modal) decays. In the present work we discuss the various aspects of this ternary fission mode, with different mass partitions. The question of collinearity is analyzed on the basis of recent publications.

Manifestations of pear-shaped clusters in collinear cluster tri-partition of 252Cf

Manifestations of pear-shaped clusters in collinear cluster tri-partition of 252Cf

Clusterization in heavy cold nuclei

A cluster is broadly understood to be an object which keeps its identity in a larger scale system. Manifestations of clustering can be revealed in very different size systems from exotic nuclei to galaxies. In our work we search for cluster effects in low energy fission of heavy nuclei. In the series of experiments carried out with the spectrometers based both on the gas filled (FOBOS spectrometer and its modifications) detectors and mosaics of solid-state detectors (setups at the beams of alpha-particles and deuterons, COMETA spectrometer) using the “missing mass” method and with the direct detection of three decay partners, we have discovered a new type of the ternary decay of heavy nuclei dubbed by us “collinear cluster tri-partition (CCT)”. The most interesting aspects of both the original methodic used and the physics of the effect observed are discussed in the presented report.

Examining the pros and cons of the interpretation of the collinear cluster tri-partition phenomenon

Features of a new phenomenon---so-called collinear cluster tri-partition---are analyzed from various viewpoints. First, limitations on the angular distributions which are imposed by the general principles of quantum mechanics are considered. Second, these limitations together with the experimental data published earlier are used to estimate the effect as a whole. Third, some confirmation of the large yield of the lightest component of the tri-partition arising from geochemical data is presented.

Equatorial, collinear trajectories in the ternary fission of 252Cf for various third fragments

To understand the dynamics of the ternary fission process, trajectory calculations play an important role. Within the classical physics concept, the trajectories of the fragments in a spontaneous ternary fission of Cf are studied. Taking into account the conservation laws, the force acting between the fragments dictates the future path. A good set of parameters needed for the initial configuration can be designed from the known experimental information on the angular and energy distributions of the ternary fission fragments. Else, various initial parameters can be tuned to obtain the desired final results to match with the experimental angular and energy distribution of the fragments. Initial parameters such as the distance between the fission fragments, velocity and position of the fragments are chosen from the earlier theoretical and experimental studies. The kinetic energy and position of the fragments are calculated numerically for different ternary fission fragments. The results indicate that the final angle of the light third fragment is almost around 90°, and it is independent of the initial angle having a preference for equatorial and/or orthogonal emission of third fragments with respect to the other two fission fragments. For heavier third fragments, a strong dependence on the initial angle is seen in the final emission direction, which for very low angles results in collinear type trajectories, supporting the experimental observation of collinear cluster tripartition.

All possible tripartitions of $${}^{236}$$ 236 U isotope in collinear configuration

Using the recently proposed unified ternary fission model (UTFM), the tripartition of $$^{236}\hbox {U}$$ isotope was studied for all possible fragmentations, in which the interacting potential barrier is taken as the sum of the Coulomb and proximity potentials with fragments in collinear configuration. The highest yield is obtained for the fragmentation $$^{48}\hbox {Ca}{+}^{58}\hbox {Ti}{+}^{130}\hbox {Sn}$$ and next highest yield is found for $$^{58}\hbox {Cr}{+}^{46}\hbox {Ar}{+}^{132}\hbox {Sn}$$ , which stress the importance of doubly magic or near doubly magic nuclei in the tripartition of $$^{236}\hbox {U}$$ isotope. The formation of $$^{68}\hbox {Ni}$$ and $$^{70}\hbox {Ni}$$ as the edge fragments linking the doubly magic nucleus $$^{132}\hbox {Sn}$$ by the isotope of Si is in good agreement with experimental and theoretical studies, in the collinear cluster tripartition of $$^{236}\hbox {U}$$ isotope which reveals the reliability of our model (UTFM) in ternary fission.

Scission Point Calculations and Physical Treating of the 'Ni-bump' in 252Cf (sf)

In a series of the experiments at different time-of-flight spectrometers of heavy ions we have observed manifestations of a new at least ternary decay channel of low excited heavy nuclei. Due to specific features of the effect, it was called collinear cluster tri-partition (CCT). The experimental results obtained initiated a number of theoretical articles dedicated to different aspects of the CCT. In the report we compare the theoretical predictions with our experimental data, only partially published so far. The developed model of one of the most populated CCT modes that gives rise to the so called “Ni-bump” is discussed.

Examination of evidence for collinear cluster tri-partition

In a series of the experiments at different time-of-flight spectrometers of heavy ions we have observed manifestations of a new at least ternary decay channel of low excited heavy nuclei. Due to specific features of the effect, it was called collinear cluster tri-partition (CCT). The experimental results obtained initiated a number of theoretical articles dedicated to different aspects of the CCT. We compare theoretical predictions with our experimental data, only partially published so far. The model of one of the most populated CCT modes that gives rise to the so called "Ni-bump" is discussed. Detection of the 68-72Ni fission fragments with a kinetic energy E<25 MeV at the mass-separator Lohengrin is proposed for an independent experimental verification of the CCT.

Collinear cluster tri-partition – the brightest observations and their treating

Careful studies of the fission fragments mass correlation distributions let us to reveal specific linear structures in the region of a big missing mass. It became possible due to applying of effective cleaning of this region from the background linked with scattered fragments. One of the most pronounced structure looks like a rectangle bounded by the magic nuclei. The fission events aggregated in the rectangle show a very low total kinetic energy. We propose possible scenario of forming and decay of the multi-cluster prescission configuration decisive for the experimental findings. This approach is valid as well for treating of another rare decay modes discovered in the past.

Collinear cluster tri-partition: Kinematics constraints and stability of collinearity

A new mode of nuclear fission has been proposed by the FOBOS collaboration, called Collinear Cluster Tri-partition (CCT), suggesting that three heavy fission fragments can be emitted perfectly collinearly in low-energy fission. It is surprising that CCT escaped observation for so long given the relatively high reported yield, of roughly 0.5% relative to binary fission. These claims call for an independent verification with a different experimental technique. Verification experiments based on direct observation of CCT fragments with fission fragment spectrometers require guidance with respect to the allowed kinetic energy range, which we present in this paper. We discuss corresponding model calculations which, if CCT is found in such verification experiments, could indicate how the breakups proceed. We also study the intrinsic stability of collinearity. Three different decay models are used, which span together the timescales of three-body fission. These models are used to calculate the possible kinetic energy ranges of CCT fragments in 235U(n,f) and 252Cf(sf). We use semi-classical trajectory calculations with a Monte-Carlo method to study the intrinsic stability of collinearity. CCT has a high net Q-value, but in a sequential decay, the intermediate steps are energetically and geometrically unfavorable or even forbidden. Moreover, perfect collinearity is extremely unstable, and broken by the slightest perturbation. According to our results, the central fragment would be very difficult to detect due to its low kinetic energy, raising the question of why previous experiments could not detect a missing-mass signature corresponding to CCT. We find that a realization of CCT would require an unphysical fine-tuning of the initial conditions. Our results enable independent experimental verification and encourage further critical theoretical studies of CCT.

Oblique projectors in image morphology

At the previous ICPPA (2015) we presented the report [1] where we discussed the results of the solution of the problem of estimation of statistical reliability of linear point structures, obtained from the experiments at the FOBOS spectrometer [2] dedicated to study of the spontaneous fission of the 252Cf nucleus in the mass correlation distribution of fission fragments. These new unusual structures bounded by magic clusters were interpreted as a manifestation of a new exotic decay called collinear cluster tri-partition (CCT) [3]. The reliability of these structures was estimated on the basis of methods of morphological image analysis [1], [4], [5]. To improve the quality of revealing and further estimation of linear structures statistical reliability in the mass correlation distribution of fission fragments we used the formalism of oblique projecting [6] and subjective modeling [7], [8].

Computer analysis of nuclear track emulsion exposed to thermal neutrons and Cf source

Application of the nuclear track emulsion technique (NTE) in radioactivity and nuclear fission studies is discussed. It is suggested to use a HSP-1000 automated microscope for searching for a collinear cluster tri-partition of heavy nuclei implanted in NTE. Calibrations of α-particles and ion ranges in a novel NTE are carried out. Surface exposures of NTE samples to a Cf-252 source started. Planar events containing fragments and long-range α-particles as well as fragment triples only are studied. Splittings induced by thermal neutrons are studied in boron-enriched emulsion. Use of the image recognition program ”ImageJ” for obtaining characteristics of individual events and for events from the large scan area is presented.

Recent applications of nuclear track emulsion

Application of the nuclear track emulsion technique (NTE) in radioactivity and nuclear fission studies is discussed. It is suggested to use a HSP-1000 automated microscope for searching for a collinear cluster tri-partition of heavy nuclei implanted in NTE. Calibrations of α -particles and ion ranges in a novel NTE are carried out. Surface exposures of NTE samples to a Cf-252 source started. Planar events containing fragments and long-range α -particles as well as fragment triples only are studied. NTE samples are calibrated by ions Kr and Xe of energy of 1.2 and 3 A MeV. Use of the image recognition program “ImageJ” for obtaining characteristics of individual events and for events from the large scan area is presented.

Estimation of reliability of linear point structures revealed in two-dimensional distributions of experimental data

In the experiments at the FOBOS spectrometer [1] dedicated to study the spontaneous fission of the 248Cm and 252Cf nuclei in the mass correlation distribution of fission fragments new unusual structures bounded by magic clusters were observed for the first time. The structures were interpreted as a manifestation of a new exotic decay called collinear cluster tri-partition (CCT). These pioneer results were confirmed and detailed later in the series of experiments at different time-of-flight spectrometers [2]. Interpretation of the results obtained needs estimation of the statistical reliability of the structures mentioned above. The report presents the results of the solution to the problem of statistical reliability estimation on the basis of morphological image analysis [3].

Multi-modal fission in collinear ternary cluster decay of 252Cf(sf, fff)

We discuss the multiple decay modes of collinear fission in 252Cf(sf, fff), with three fragments as suggested by the potential energy surface (PES). Fission as a statistical decay is governed by the phase space of the different decay channels, which are suggested in the PES-landscape. The population of the fission modes is determined by the minima in the PES at the scission points and on the internal potential barriers. The ternary collinear decay proceeds as a sequential process, in two steps. The originally observed ternary decay of 252Cf(sf) into three different masses (e.g. 132–140Sn, 52–48Ca, 68–72Ni), observed by the FOBOS group in the FLNR (Flerov Laboratory for Nuclear Reactions) of the JINR (Dubna) the collinear cluster tripartition (CCT), is one of the ternary fission modes. This kind of “true ternary fission” of heavy nuclei has often been predicted in theoretical works during the last decades. In the present note we discuss different ternary fission modes in the same system. The PES shows pronounced minima, which correspond to several modes of ternary fragmentations. These decays have very similar dynamical features as the previously observed CCT-decays. The data obtained in the experiments on CCT allow us to extract the yields for different decay modes using specific gates on the measured parameters, and to establish multiple modes of the ternary fission decay.

First Results on Observation of New Shape Isomers

In our previous publications devoted to the collinear cluster tri-partition (CCT) of the low excited nuclei [1,2] we have discussed the role of scattering medium in the registration of the CCT products. This decay mode has been called by us “collinear cluster tri-partition” (CCT) in view of the observed features of the effect, that the decay partners fly apart almost collinearly and at least one of them has magic nucleon composition. Briefly, even if initially two CCT partners fly in the same direction perfectly collinearly they get some angular divergence after passing the scattering medium on the flight pass due to the multiple scattering. Thanks to such effect they can be registered independently in the “stop” mosaic detector. Actually even thin backing of the radioactive source provides the observable effect.

Toward an Automated Analysis of Slow Ions in Nuclear Track Emulsion

Application of the nuclear track emulsion technique (NTE) in radioactivity and nuclear fission studies is discussed. It is suggested to use a HSP-1000 automated microscope for searching for a collinear cluster tri-partition of heavy nuclei implanted in NTE. Calibrations of α-particles and ion ranges in a novel NTE are carried out. Surface exposures of NTE samples to a 252Cf source started. Planar events containing fragments and long-range α-particles as well as fragment triples only are studied. NTE samples are calibrated by ions Kr and Xe of energy of 1.2 and 3 A MeV.