Asymmetric Fission Research Articles

A Suggestion Complementing the Magic Numbers Interpretation of the Nuclear Fission Phenomena

Ideas, solely related on the nuclear shell model, fail to give an interpretation of the experimental central role of 54Xe in the asymmetric fission of actinides. The same is true for the β-delayed fission of 180Tl to 80Kr and 100Ru. The representation of the natural isotopes, in the Z-Neutron Excess plane, suggests the importance of the of the Neutron Excess evolution mode in the fragments of the asymmetric actinide fission and in the fragments of the β-delayed fission of 180Tl. The evolution mode of the Neutron Excess, hinged at Kr and Xe, is directed by the 50 and 82 neutron magic numbers. The present isotope representation offers a frame for the interpretation of the post fission evaporation of neutrons, higher for the AL compared to the AH fragments, a tenet in nuclear fission. Further enlightened is the functional meaning of the 50 proton magic number, marking the start of the yield rise of the AH fragments in actinide fission.

Spontaneous fission and competing ground state decay modes of actinide and transactinide nuclei

Based on the preformed cluster model, we have carried out a comprehensive theoretical study on the decay paths of ground state actinide and transactinide nuclei, specifically from $^{232}\mathrm{U}$ to $^{264}\mathrm{Hs}$ exhibiting the phenomenon of spontaneous fission (SF). This is an extension of our earlier studies on $\ensuremath{\alpha}$ decay, exotic cluster emission, and heavy particle radioactivity, where an effort is made to identify the most probable fragments in the SF process. These observations in turn, could provide a testing ground for future SF half-life measurements. To obtain a clear picture of the dynamics involved, the variations of fragmentation potential, preformation factor, and decay barrier height have been examined. The calculated potential energy surfaces show a change from a predominantly asymmetric fission to a symmetric fragmentation with the increase in the N/Z ratio of parent nuclei. In addition, an exclusive analysis of SF with $\ensuremath{\alpha}$ and other possible cluster emissions for the $^{232,234,236,238}\mathrm{U}$ parents was made to have better insight of nuclear structure information. In other words, the comparative nature of $\ensuremath{\alpha}$, cluster, heavy fragment, and SF decay paths is analyzed in view of shell closure property of the decay fragments. Interestingly, the calculated decay half-lives for the $^{82}\mathrm{Ge}$ heavy cluster are in fact shown to lie within the limits of experiments, thereby presenting themselves as exciting new possibilities which may be validated via future experiments.

Transitions between symmetric and asymmetric modes in the region of heavy actinides

Using the improved scission-point model, the mass and charge distributions of fragments resulting from the fission of californium, fermium, and nobelium isotopes are calculated and compared with the available experimental data. For the fissioning Fm and No nuclei, the transitions from a two-peaked to a single-peaked mass distributions are predicted. The transition between asymmetric and symmetric fission modes with the variation of neutron number is shown to be related to the change of the potential energy surface at scission point.

Aspects of charge distribution measurement for Cf252 (sf)

Measurements of charge distributions of fragments in the spontaneous fission of $^{252}\mathrm{Cf}$ have been performed using a unique detector setup consisting of a typical grid-ionization chamber coupled with a $\mathrm{\ensuremath{\Delta}}E\ensuremath{-}E$ charged particle telescope. We find that the fragment mass dependency of the kinetic-energy-averaged width of the charge distribution shows a systematically decreasing trend with obvious fluctuations. The variation of the widths of the charge distribution with kinetic energy shows a pan-like shape. This is due to the large number of neutrons emitted at the high excitation energies and cold fragmentation at the low excitation energies. Deviation of the kinetic-energy-averaged most probable charge ${Z}_{p}$ from the unchanged charge distribution (UCD), $\mathrm{\ensuremath{\Delta}}Z$, as a function of the mass number of primary fragments, ${A}^{*}$, changes from negative for mass asymmetric fission to positive near the symmetric fissions. Concerning the kinetic energy dependence of ${Z}_{p}$ given primary mass number ${A}^{*}$, obvious increasing tendencies of ${Z}_{p}$ with increasing kinetic energy are observed.

Delayed fission and multifragmentation in sub-keV C60 – Au(0 0 1) collisions via molecular dynamics simulations: Mass distributions and activated statistical decay

We have recently observed, both experimentally and computationally, the phenomenon of postcollision multifragmentation in sub-keV surface collisions of a C60 projectile. Namely, delayed multiparticle breakup of a strongly impact deformed and vibrationally excited large cluster collider into several large fragments, after leaving the surface. Molecular dynamics simulations with extensive statistics revealed a nearly simultaneous event, within a sub-psec time window. Here we study, computationally, additional essential aspects of this new delayed collisional fragmentation which were not addressed before. Specifically, we study here the delayed (binary) fission channel for different impact energies both by calculating mass distributions over all fission events and by calculating and analyzing lifetime distributions of the scattered projectile. We observe an asymmetric fission resulting in a most probable fission channel and we find an activated exponential (statistical) decay. Finally, we also calculate and discuss the fragment mass distribution in (triple) multifragmentation over different time windows, in terms of most abundant fragments.

Corrupted Colonic Crypts Bordering Regenerating Mucosal Ulcers in Ulcerative Colitis.

Histology in protracted ulcerative colitis (UC) discloses high numbers of chronic inflammatory cells and crypts with architectural distortions. In severe cases, ulcerations are frequently found. The histogenesis of colonic crypts with architectural distortions in UC remains elusive. A recent review of colectomy specimens from patients with UC revealed crypts surrounding mucosal ulcerations exhibiting severe architectural distortions. They were called corrupted colonic crypts, CCCs. Archival hematoxylin and eosin (H&E)-stained sections from three colectomies having several mucosal ulcers were selected for the study. The mucosa bordering mucosal ulcers was particularly scrutinized. The review of 49 sections (mean=16.3, range=14-20) in the three colectomies revealed 60 ulcers (mean=20, range=13-27). The following CCC phenotypes were found bordering mucosal ulcers: with asymmetric lateral fission (n=11), with dual or three-foiled corrupted fission (n=19), with cystic dilatations (n=3), L-shaped crypts (n=7), T-inverted crypts (n=6), shoe-shaped crypts (n=3), horizontal crypts (n=14), multi-lobate crypts (n=2), and/or inter-connecting crypts (n=5). The regeneration of ulcers in UC seems to proceed with neo-formation of corrupted crypts. In the same colectomies, none to occasional CCCs were found in large areas of the mucosa having severe chronic inflammation. Importantly, none of the occasional CCCs were found in other diseases of the colonic mucosa with chronic inflammation or in unspecific ulcers of the colon. Since neither chronic mucosal inflammation per se, nor unspecific ulcers of the colon are central for the formation of CCCs, it is suggested that crypt distortions of the non-ulcerated colonic mucosa in patients with UC might mirror formerly healed mucosal ulcerations.

Binary fragmentation based studies for the near super-heavy compound nucleus 256Rf

Binary fragmentation of the near super-heavy compound nucleus 256Rf has been studied through the reaction 48Ti + 208Pb at a bombarding energy well above the Coulomb barrier. For a better understanding of its reaction dynamics, the mass distribution, mass-energy distribution and mass-angle distribution of the fission fragments produced from 256Rf have been investigated thoroughly. The masses and kinetic energies of the fission fragments were reconstructed event-by-event from their measured velocities and emission angles. From the mass-energy analysis, a sizeable contribution from the asymmetric fission was observed on the edges of symmetric mass distribution. Evidence of asymmetric fission was also clued from the observed correlation between the masses and emission angles of the fission fragments. Contribution of the quasi-fission products has also been estimated by performing the theoretical dinuclear system calculations.

Are Corrupted Non-dysplastic Colonic Crypts the First Histological Event in Experimental Colonic Carcinogenesis?

The colonic crypts in rats replicate by symmetric fission at the base of the crypts, and proceeds upwards, to generate two separate identical crypts. Recently, anomalous crypts (called corrupted colonic crypts, CCCs) were found in the colonic mucosa of Sprague-Dawley rats treated with the carcinogen dimethylhydrazine. Here it was investigated whether CCCs develop in the colonic mucosa of another rat strain, treated with a different carcinogen. Archived Swiss-roll colon sections from 25 male Fisher-344 rats treated with the mutagen 2-amino-6-methyldipyrido imidazole (GLU1) were reviewed. Non-dysplastic and dysplastic CCCs were regarded as those exhibiting asymmetric fission, asymmetric lateral sprouting/lateral fission, basal dilatations, or spatial aberrations of the normal (vertical) axis. Colonic adenomas were found in three out of the 25 specimens. In the entire colonic mucosa of the 25 GLU1-treated rats, 130 non-dysplastic CCCs were recorded amongst 357 non-dysplastic crypts with fission (36.4%). The mean number of non-dysplastic CCCs per animal was 5.2 (range=2-12). These numbers only mirror events taking place at a particular time (i.e. at sacrifice). Considering the high cell production rate of the colonic crypts, the actual number of CCCs/rat occurring during the usual mucosal turnover time of 72 hours might be substantial. In the three adenoma specimens, non-dysplastic CCCs were found underneath CCCs with dysplasia. For many years, the development of crypt dysplasia and adenoma have been considered the initial histological events in colonic carcinogenesis. This study demonstrates that non-dysplastic CCCs also develop in GLU 1-treated Fisher-344 rats. Non-dysplastic CCCs were found underneath CCCs with dysplasia. Non-dysplastic CCCs might act as scaffolds at the time of top-down cell replacement/transformation of the crypts by dysplastic cells. It is submitted that non-dysplastic CCCs might be the initial histological recordable event in experimental colonic carcinogenesis.

Rayleigh limit and fragmentation of multiply charged Lennard-Jones clusters: Can charged clusters provide clues to investigate the stability of electrospray droplets?

The fragmentation of multiply charged clusters composed of N≤1000 Lennard-Jones particles augmented with electrostatic interactions is explored by classical Monte Carlo and molecular dynamics simulations with the stated goal of establishing possible analogies with electrospray droplets. Clusters with few charge carriers are shown to be only subject to particle ejection and their Rayleigh limit can be estimated by quantifying the loss of charged particles. On the contrary, uniformly charged clusters can both evaporate particles and undergo fission, making them better candidates to model electrospray droplets. Critical charges delimiting regions of instability of these clusters are defined from the calculation of lower order multipole moments and asymmetry parameters based on the knowledge of moments of inertia. The first discontinuity of quadrupole moments and asymmetry parameters is related to cluster elongation before twofold fission and the corresponding charge is deemed to be a good estimate of the Rayleigh limit. Octopole moments are negligible about this charge, their discontinuities arising at higher charges when threefold fissions are allowed. The size dependence of these critical charges is qualitatively predicted from Rayleigh's formula and the expression of surface energy advocated in liquiddrop models. Deviations below 15% are commonly achieved when comparing Rayleigh limits extracted from experimental data with theoretical predictions based on Monte Carlo simulations or liquiddrop models for a set of eleven atomic and molecular liquid clusters. Although manifold fission of uniformly charged clusters is unlikely close to the Rayleigh limit, successive asymmetric fissions are found to occur in conjunction with other fragmentation mechanisms, including the expansion of ring-shaped structures, at charges more than twice as large as the Rayleigh limit.

The Histogenesis of the Third Pathway of Colonic Carcinogenesis in Rats.

Conventional (tubular or villous) adenomas, and the more recently described serrated adenomas, are non-invasive neoplasias that precede colon carcinomas in carcinogen-treated rats. In contrast, the histological steps antedating carcinomas in gut-associated lymphoid tissue (GALT) in rats, i.e. the third pathway of colonic carcinogenesis, remain unidentified. Aim of the study was to investigate the histological changes preceding colonic GALT carcinomas in Sprague-Dawley (SD) rats. Archived sections from previous experiments showing GALT mucosal domains in 292 rats were re-evaluated: 276 were injected with 1,2-dimethylhydrazine (DMH) suspended in ethylenedia-minetetra-acetic acid (EDTA), and 16 were controls (8 EDTA-treated, and 8 untreated). A total of 402 colonic GALT mucosal domains were found in the 292 rats: 382 in 276 DMH-treated, 10 in eight EDTA-treated, and 10 in eight-untreated rats. In DMH-treated rats, corrupted crypts (CCS; i.e. with asymmetric fission or abnormal crypt-alignment) were recorded in 50% of the GALT domains (15% had no dysplasia and 35% had epithelial dysplasia). Adenomas on top of GALT domains were found in 7%, and GALT carcinomas in 53%. Histology of the 146 colonic GALT carcinomas revealed highly differentiated carcinomas or signet-ring cell carcinomas. EDTA-treated and untreated animals showed no dysplastic CCS, or other neoplasia. This study demonstrated that GALT mucosal domains in carcinogen-treated rats often develop dysplastic CCS. Non-dysplastic CCS appear to act as scaffolds for the top-down replacement/transformation by dysplastic cells. Importantly, highly differentiated carcinomas were seen to evolve from dysplastic CCS and from adenomas, and signet-ring cell carcinomas from dysplastic goblet cells present at the base of crypts. This is the first study showing that non-invasive neoplastic lesions (dysplastic CCS and adenomas) antedate colonic GALT carcinomas in DMH-treated SD rats. The DMH-SD paradigm permits detailed study of the histological events preceding GALT carcinoma under standard laboratory conditions.

Corrupted colonic crypt fission in carcinogen-treated rats.

BackgroundThe colonic crypts in rats reproduce themselves by symmetric fission at the base of the crypts, and proceeding upwards, generate two separate identical crypts. Recently we reported corrupted colonic crypt fission (CCCF) in rats with colonic carcinoma. Here we investigated whether CCCF also occurred in the colonic mucosa without carcinoma in carcinogen-treated rats.MethodsFiled Swiss-roll sections from 35 male rats (25 treated with 1,2-dimethyhydrazine (DMH) suspended in EDTA solution, and 10 EDTA-treated) were reviewed. CCCF were regarded those with either asymmetric basal fission, asymmetric lateral sprouting/lateral fission, basal dilatations, or spatial aberrations of the normal (vertical) axis.Results202 CCCF (38%) were recorded amongst 533 crypts with fission in DMH-treated rats, and only one CCCF (0.1%) was found amongst 571 crypts with fission in EDTA-treated rats (p<0.05). The basal aspect of four adenomas included in Swiss roll sections exhibited CCCF lined either with indigenous (non-dysplastic) epithelium or with dysplastic epithelium.ConclusionIt was demonstrated that CCCF without dysplasia develop in carcinogen-treated SD rats. As judged by the figures presented, the possibility that the epithelium in those corrupted crypts was successively replaced by top-down growing dysplastic cells, could not be totally rejected. This is the first report showing that non-dysplastic CCCF may antedate the very early stages of colonic carcinogenesis in SD rats.

Experimental fission study using multi-nucleon transfer reactions

It is shown that the multi-nucleon transfer reactions is a powerful tool to study fission of exotic neutron-rich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multi-nucleon transfer channels of the reactions of 18 O+232 Th, 18 O+238 U and 18 O+248 Cm are used to study fission for various nuclei from many excited states. Identification of fissioning nuclei and of their excitation energy is performed on an event-by-event basis, through the measurement of outgoing ejectile particle in coincidence with fission fragments. Fission fragment mass distributions are measured for each transfer channel. Predominantly asymmetric fission is observed at low excitation energies for all studied cases, with a gradual increase of the symmetric mode towards higher excitation energy. The experimental distributions are found to be in general agreement with predictions of the fluctuation-dissipation model. Role of multi-chance fission in fission fragment mass distributions is discussed, where it is shown that mass-asymmetric structure remaining at high excitation energies originates from low-excited nuclei by evaporation of neutrons.

Study of fission using multi-nucleon transfer reactions

It is shown that multi-nucleon transfer reaction is a powerful tool to study fission of exotic neutronrich actinide nuclei, which cannot be accessed by particle-capture or heavy-ion fusion reactions. In this work, multi-nucleon transfer channels of the reactions of 18O+232Th, 18O+238U, 18O+248Cm, and 18O+237Np were used to measure fission-fragment mass distribution for each transfer channel. Predominantly asymmetric fission is observed at low excitation energies for all the studied cases, with an increase of the symmetric fission towards high excitation energies. Experimental data are compared with predictions of the fluctuation-dissipation model, where effects of multi-chance fission (neutron evaporation prior to fission) was introduced. It is shown that mass-asymmetric structure remaining at high excitation energies originates from low-excited and less neutronrich excited nuclei due to higher-order chance fissions.

Fission product yield measurements using monoenergetic photon beams

Measurements of fission products yields (FPYs) are an important source of information on the fission process. During the past couple of years, a TUNL-LANL-LLNL collaboration has provided data on the FPYs from quasi monoenergetic neutron-induced fission on 235 U, 238 U, and 239 Pu and has revealed an unexpected energy dependence of both asymmetric fission fragments at energies below 4 MeV. This peculiar FPY energy dependence was more pronounced in neutron-induced fission of 239 Pu. In an effort to understand and compare the effect of the incoming probe on the FPY distribution, we have carried out monoenergetic photon-induced fission experiments on the same 235 U, 238 U, and 239 Pu targets. Monoenergetic photon beams of E γ = 13.0 MeV were provided by the HIγS facility, the world's most intense γ-ray source. In order to determine the total number of fission events, a dual-fission chamber was used during the irradiation. These irradiated samples were counted at the TUNL's low-background γ-ray counting facility using high efficient HPGe detectors over a period of 10 weeks. Here we report on our first ever photofission product yield measurements obtained with monoenegetic photon beams. These results are compared with neutron-induced FPY data.

Physical Origin of the Transition from Symmetric to Asymmetric Fission Fragment Charge Distribution

Using an improved scission-point model, we describe the charge distribution of fission fragments produced in the electromagnetic-induced fission of even-even thorium isotopes. The transition from a symmetric fission mode to an asymmetric one is explained. A three-peaked charge distribution is predicted for several fissioning nuclei at low excitation energy.

Unexpected asymmetry of the charge distribution in the fission of Th222,224 at high excitation energies

Using the improved scission-point model, the isotopic trends of the charge distribution of fission fragments are studied in induced fission of even-even Th isotopes. The calculated results are in good agreement with available experimental data. With increasing neutron number the transition from symmetric to asymmetric fission mode is shown to be related to the change of the potential energy surface. The change of the shape of mass distribution with increasing excitation energy is discussed for fissioning $^{A}\mathrm{Th}$ nuclei. At high excitation energies, there are unexpected large asymmetric modes in the fission of neutron-deficient Th isotopes considered.

Evidence of Staphylococcus Aureus Deformation, Proliferation, and Migration in Canaliculi of Live Cortical Bone in Murine Models of Osteomyelitis

Although Staphylococcus aureus osteomyelitis is considered to be incurable, the major bacterial reservoir in live cortical bone has remained unknown. In addition to biofilm bacteria on necrotic tissue and implants, studies have implicated intracellular infection of osteoblasts and osteocytes as a mechanism of chronic osteomyelitis. Thus, we performed the first systematic transmission electron microscopy (TEM) studies to formally define major reservoirs of S. aureus in chronically infected mouse (Balb/c J) long bone tissue. Although rare, evidence of colonized osteoblasts was found. In contrast, we readily observed S. aureus within canaliculi of live cortical bone, which existed as chains of individual cocci and submicron rod-shaped bacteria leading to biofilm formation in osteocyte lacunae. As these observations do not conform to the expectations of S. aureus as non-motile cocci 1.0 to 1.5 μm in diameter, we also performed immunoelectron microscopy (IEM) following in vivo BrdU labeling to assess the role of bacterial proliferation in canalicular invasion. The results suggest that the deformed bacteria: (1) enter canaliculi via asymmetric binary fission; and (2) migrate toward osteocyte lacunae via proliferation at the leading edge. Additional in vitro studies confirmed S. aureus migration through a 0.5-μm porous membrane. Collectively, these findings define a novel mechanism of bone infection, and provide possible new insight as to why S. aureus implant-related infections of bone tissue are so challenging to treat. © 2016 American Society for Bone and Mineral Research.

Proton induced fission of 232Th at intermediate energies

The mass-energy distributions and cross sections of proton-induced fission of 232Th have been measured at the proton energies of 7, 10, 13, 20, 40, and 55 MeV. Experiments were carried out at the proton beam of the K-130 cyclotron of the JYFL Accelerator Laboratory of the University of Jyvaskyla and U-150m cyclotron of the Institute of Nuclear Physics, Ministry of Energy of the Republic of Kazakhstan. The yields of fission fragments in the mass range A = 60–170 a.m.u. have been measured up to the level of 10−4%. The three humped shape of the mass distribution up has been observed at higher proton energies. The contribution of the symmetric component grows up with increasing proton incident energy; although even at 55 MeV of proton energy the shoulders in the mass energy distribution clearly indicate the asymmetric fission peaks. Evolution of shell structure was observed in the fission fragment mass distributions even at high excitation energy.

Challenging fission dynamics around the barrier: The case of 34S + 186W

The current status of fission dynamics studies in heavy-ion collisions around the Coulomb barrier is illustrated with the 34S + 186W reaction. The fission-fragment mass and total kinetic energy were measured at the ALTO facility at IPN Orsay, France, with a dedicated set-up using the (v, E) approach. The measurement reveals the presence of an asymmetric fission component on top of a predominantly symmetric distribution. The asymmetric structure, pointed out for the first time, is discussed along with results of previous experiments studying the same reaction. While these analyses suggested the contribution from either quasi-fission or pre-equilibrium fission, we offer an alternative interpretation, in terms of shell-driven compound-nucleus fission. The present measurement demonstrates the critical influence of resolution when addressing puzzling cases, situated at the crossroads of the various channels opened in a heavy-ion collision. Current status in the field clearly calls for innovative measurements involving manifold correlations and new observables. The outcome of the attempt done in this work in this direction, based on the coincident measurement of prompt \( \gamma\)-rays is reported, and encouraging perspectives are discussed.

Vertically- and horizontally-transmitted memories - the fading boundaries between regeneration and inheritance in planaria.

ABSTRACTThe Weismann barrier postulates that genetic information passes only from the germline to the soma and not in reverse, thus providing an obstacle to the inheritance of acquired traits. Certain organisms such as planaria – flatworms that can reproduce through asymmetric fission – avoid the limitations of this barrier, thus blurring the distinction between the processes of inheritance and development. In this paper, we re-evaluate canonical ideas about the interaction between developmental, genetic and evolutionary processes through the lens of planaria. Biased distribution of epigenetic effects in asymmetrically produced parts of a regenerating organism could increase variation and therefore affect the species' evolution. The maintenance and fixing of somatic experiences, encoded via stable biochemical or physiological states, may contribute to evolutionary processes in the absence of classically defined generations. We discuss different mechanisms that could induce asymmetry between the two organisms that eventually develop from the regenerating parts, including one particularly fascinating source – the potential capacity of the brain to produce long-lasting epigenetic changes.