Asymmetric Fragmentation Research Articles

Local even–odd effect based on the number of configurations of pre-formed and formed fragmentations in a fissioning nucleus

The present paper proposes a modeling of the local even–odd effect based on the number of configurations in a nucleus undergoing fission at two stages along its fission path. One is the fissioning nucleus stage just after passing through the outer saddle point when the fragments are considered as pre-formed and the intrinsic energy is not yet shared. The other stage is at the end of the fission path when the scission is imminent. Then the intrinsic energy is already partitioned and the fragments are completely formed. The probability that a pre-formed fragmentation arrives at the end of the fission path (i.e. at scission) when the fragmentation is completely formed is expressed by the ratio of the number of configurations of the formed fragmentation to the one of pre-formed fragmentation. The local even–odd effect is defined as half of the difference between these normalized ratios corresponding to even-Z and odd-Z fragmentations.Both numbers of configurations in the fissioning nucleus, in which the fragments are pre-formed and completely formed, are calculated using level densities described by the constant temperature function (justified by the small values of the intrinsic energy before scission).The obtained local even–odd effect results describe well the experimental data, including the increase at asymmetry values corresponding to fragmentations in which one of the fragments is magic or double magic (i.e. fragmentations in which ZH=50 and/or NH=82 and very asymmetric fragmentations in which ZL=28).

Even–odd effects in prompt emission of spontaneously fissioning even–even Pu isotopes

Abstract The available experimental Y ( A , TKE ) data for 236,238,240,242,244 Pu(SF) together with the Zp model prescription with appropriate parameters allows the investigation of even–odd effects in fragment distributions. The size of the global even–odd effect in Y ( Z ) is decreasing from 244 Pu(SF) to 236 Pu(SF) confirming the general observation of a decrease of the even–odd effect with the fissility parameter. Charge polarizations (Δ Z ) and root-mean squares (rms) as a function of A of 236–244 Pu(SF) were obtained for the first time. In the asymmetric fission region both Δ Z ( A ) and rms ( A ) exhibit oscillations with a periodicity of about 5 mass units due to the even–odd effects. The total average charge deviations 〈 Δ Z 〉 (obtained by averaging Δ Z ( A ) over the experimental Y ( A ) distribution) are of about |0.5| for all studied Pu(SF) systems. The comparison of the calculated Δ Z ( A ) and rms ( A ) of 240 Pu(SF) with those of 239 Pu ( n th , f ) reported by Wahl shows an in-phase oscillation with a higher amplitude in the case of 240 Pu(SF), confirming the higher even–odd effect in the case of SF. As in the previously studied cases ( 233,235 U ( n th , f ) , 239 Pu ( n th , f ) , 252 Cf(SF)) the even–odd effects in the prompt emission of 236–244 Pu(SF) are mainly due to the Z even–odd effects in fragment distributions and charge polarizations and the N even–odd effects in the average neutron separation energies from fragments 〈 Sn 〉 . The size of the global N even–odd effect in 〈 Sn 〉 is decreasing with the fissility parameter, being higher for the Pu(SF) systems compared to the previously studied systems. The prompt neutron multiplicities as a function of Z , ν ( Z ) , exhibit sawtooth shapes with a visible staggering for asymmetric fragmentations. The size of the global Z even–odd effect in ν ( Z ) exhibits a decreasing trend with increasing fissility. The average prompt neutron multiplicities as a function of TKE show an increase of the even–odd effect with increasing TKE, with global effect sizes close to each other (a decrease of the effect for heavier fissioning nuclei is not observed here). The amounts of the global even–odd effect in Y ( Z ) and of the N even–odd effect in 〈 Sn 〉 of 240 Pu(SF) are larger compared to 239 Pu ( n th , f ) . This fact affects the prompt emission leading to a lower Z even–odd effect in the prompt neutron multiplicity of 240 Pu(SF) compared to 239 Pu ( n th , f ) .

Even–odd effects in prompt emission in fission

Abstract Compared to the extensively studied proton even–odd effects in fission fragment distributions, the even–odd effects in prompt emission received less attention. In this context the present paper is focusing on the even–odd effects (in both Z and N) appearing in prompt neutron and gamma ray emission. The study of even–odd effects in the prompt emission of four fissioning nuclei with even Z and even N, 233,235U( n th , f ), 239Pu( n th , f ) and 252Cf(SF), shows that these effects are mainly due to the Z even–odd effects in fragment distributions and charge deviations and the N even–odd effect in average neutron separation energies. The most pronounced even–odd effect in prompt emission is for the prompt neutron multiplicity. The average multiplicity as a function of Z, ν ( Z ) , has a sawtooth shape and exhibits a visible staggering for asymmetric fragmentations. The size of the global even–odd effect in prompt neutron multiplicity shows a decrease with increasing fissility. The N even–odd effect in average neutron separation energy influences the global even–odd effect in prompt neutron multiplicity often in an opposite way compared to the Z even–odd effect. The average prompt neutron multiplicity as a function of TKE shows an increase of the Z even–odd effect with increasing TKE and this effect is decreasing for heavier fissioning systems. The Z even–odd effects in the average prompt γ-ray energy and the average prompt neutron energy have comparable sizes and are lower than the even–odd effects in prompt neutron multiplicity. Different average prompt emission quantities (as a function of A, of Z, of TKE and total average ones)—being dependent on fragment distributions—are affected by the even–odd effects in Y ( A , Z , TKE ) . The N even–odd effect in neutron separation energy and the Z even–odd effect in the charge deviation enter the primary multi-parametric matrices of prompt emission quantities (e.g. ν ( A , Z , TKE ) ) which are not depending on fragment distributions.

Shell effects in fission, quasifission and multinucleon transfer reaction

Results of the study of mass-energy distributions of binary fragments for a wide range of nuclei with Z= 82-122 produced in reactions of ions located between 22Ne and 136Xe at energies close and below the Coulomb barrier are reported. The role of the shell effects, the influence of the entrance channel asymmetry and the deformations of colliding nuclei on the mechanism of the fusion-fission, quasifission and multinucleon transfer reactions are discussed. The observed peculiarities of the mass and energy distributions of reaction fragments are determined by the shell structure of the formed fragments. Special attention is paid on the symmetric fragment features in order to clarify the origin of these fragments (fission or quasifission). The influence of shell effects on the fragment yield in quasifission and multinucleon transfer reactions is considered. It is noted that the major part of the asymmetric quasifission fragments peaks around the region of the Z=82 and N=126 (double magic lead) and Z=28 and N=50 shells; moreover the maximum of the yield of the quasifission component is a mixing between all these shells. Hence, shell effects are everywhere present and determine the basic characteristics of fragment mass distributions.

Analysis of fragment distribution and associated effects in 12,13C induced reactions

The decay of 220 Ra * nucleus formed in two different entrance channels 12 C +208 Pb and 13 C +207 Pb is investigated over a wide range of incident energies using the dynamical cluster decay model (DCM). The DCM is a non-statistical model used to account for the decay of hot and rotating nuclei formed in low energy heavy ion reactions. The excitation functions are calculated by considering quadrupole (β2) deformations with optimum orientations [Formula: see text] of decaying fragments. The DCM-based cross-sections for evaporation residue (ER), fusion–fission, αxn and neutron decay processes find nice agreement with the reported experimental data over wide range of incident energies. The cross-sections corresponding to different decay mechanism are worked out within DCM by fitting neck length parameter (ΔR). The entrance channel and angular momentum effects are investigated in reference to the above-mentioned reaction channels. In addition to this, the fragment mass distribution is worked out by colliding 13 C weakly bound stable projectile with a variety of target nuclei resulting in 13 C +159 Tb , 13 C +181 Ta and 13 C +207 Pb reactions. At comparable projectile energies, the increase in target mass is shown to favor asymmetric fragmentation in the fissioning region. Besides this, the incomplete fusion (ICF) contribution is worked out for 12 C and 13 C channels by applying necessary energy corrections in the framework of DCM.

ChemInform Abstract: Enantioselective Direct α‐Amination of Aldehydes via a Photoredox Mechanism: A Strategy for Asymmetric Amine Fragment Coupling.

ChemInform Abstract: Enantioselective Direct α‐Amination of Aldehydes via a Photoredox Mechanism: A Strategy for Asymmetric Amine Fragment Coupling.

β-delayed fission andαdecay of178Tl

A detailed nuclear-decay spectroscopy study of the neutron-deficient isotope 178Tl has been performed using the highly selective Resonance Ionization Laser Ion Source and ISOLDE mass separator (CERN), which allowed a unique isobarically pure beam of 178Tl to be produced. The first identification of the $\beta$-delayed fission of this isotope was made and its probability P$\beta$DF(178Tl) = 0.15(6)\% was determined. An asymmetric fission fragment mass distribution of the daughter isotope 178Hg (populated by the $\beta$ decay of 178Tl) was deduced based on the measured fission fragment energies. The fine-structure $\alpha$-decay pattern of 178Tl allowed the low-energy states in the daughter nucleus 174 Au to be studied.

Enantioselective direct α-amination of aldehydes via a photoredox mechanism: a strategy for asymmetric amine fragment coupling.

The direct, asymmetric α-amination of aldehydes has been accomplished via a combination of photoredox and organocatalysis. Photon-generated N-centered radicals undergo enantioselective α-addition to catalytically formed chiral enamines to directly produce stable α-amino aldehyde adducts bearing synthetically useful amine substitution patterns. Incorporation of a photolabile group on the amine precursor obviates the need to employ a photoredox catalyst in this transformation. Importantly, this photoinduced transformation allows direct and enantioselective access to α-amino aldehyde products that do not require postreaction manipulation.

Coulomb Fission in Dielectric Dication Clusters: Experiment and Theory on Steps That May Underpin the Electrospray Mechanism

A series of five molecular dication clusters, (H2O)n(2+), (NH3)n(2+), (CH3CN)n(2+), (C5H5N)n(2+), and (C6H6)n(2+), have been studied for the purpose of identifying patterns of behavior close to the Rayleigh instability limit where the clusters might be expected to exhibit Coulomb fission. Experiments show that the instability limit for each dication covers a range of sizes and that on a time scale of 10(-4) s ions close to the limit can undergo either Coulomb fission or neutral evaporation. The observed fission pathways exhibit considerable asymmetry in the sizes of the charged fragments, and are associated with kinetic (ejection) energies of ~0.9 eV. Coulomb fission has been modeled using a theory recently formulated to describe how charged particles of dielectric materials interact with one another (Bichoutskaia et al. J. Chem. Phys. 2010, 133, 024105). The calculated electrostatic interaction energy between separating fragments accounts for the observed asymmetric fragmentation and for the magnitudes of the measured ejection energies. The close match between theory and experiment suggests that a significant fraction of excess charge resides on the surfaces of the fragment ions. The experiments provided support for a fundamental step in the electrospray ionization (ESI) mechanism, namely the ejection from droplets of small solvated charge carriers. At the same time, the theory shows how water and acetonitrile may behave slightly differently as ESI solvents. However, the theory also reveals deficiencies in the point-charge image-charge model that has previously been used to quantify Coulomb fission in the electrospray process.

The asymmetric fragmentation operator applied to meteo-limnological time series in a tropical reservoir

Ecological data are sometimes difficult to analyze due to the intrinsic complexity of natural systems containing nonlinear equations. Efficient methods are required to better understand the information extracted from complex processes and to support the development of ecological prediction models. In this context, two techniques were used in this work to analyze meteo-limnological time series from a tropical reservoir: the Wavelet Transform (WT) and the Asymmetric Fragmentation Operator (AFO). The in situ data is based on a dataset acquired from an Eulerian system deployed in Manso Reservoir, Brazil, in 2007. The results indicated that the AFO method outperformed the WT method in the analysis of ecological parameters. The pioneering use of AFO in this sort of study proved to be a useful tool to study the variability of water quality parameters.

Strategy for the Rational Design of Asymmetric Triply Bridged Dinuclear 3d-4f Single-Molecule Magnets.

Three triply bridged M(II)-Dy(III) dinuclear complexes, [Ni(μ-L)(μ-OAc)Dy(NO(3))(2)] 1, [Zn(μ-L)(μ-OAc)Dy(NO(3))(2)] 2, and [Ni(μ-L)(μ-NO(3))Dy(NO(3))(2)]·2CH(3)OH 3 were prepared with a new and flexible compartmental ligand, N,N',N″-trimethyl-N,N″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylene triamine (H(2)L), containing N(3)O(2)-inner and O(4)-outer coordination sites. These complexes have diphenoxo/acetate (1 and 2) or diphenoxo/nitrate (3) asymmetric bridging fragments. Compounds 1 and 3 exhibit ferromagnetic interaction between Ni(2+) and Dy(3+) ions and frequency dependence of the out-of-phase (χ″(M)) alternating current (ac) susceptibility signal characteristic of single-molecule-magnet behavior. The energy barriers Δ/k(B) for compound 3 under zero and 1000 Oe applied direct current (dc) magnetic fields were estimated from the Arrhenius plots to be 7.6 and 19.1 K, respectively.

The More the Merrier?

The trade-off between traits in life-history strategies has been widely studied for sexual and parthenogenetic organisms, but relatively little is known about the reproduction strategies of asexual animals. Here, we investigate clonal reproduction in the freshwater planarian Schmidtea mediterranea, an important model organism for regeneration and stem cell research. We find that these flatworms adopt a randomized reproduction strategy that comprises both asymmetric binary fission and fragmentation (generation of multiple offspring during a reproduction cycle). Fragmentation in planarians has primarily been regarded as an abnormal behavior in the past; using a large-scale experimental approach, we now show that about one third of the reproduction events in S. mediterranea are fragmentations, implying that fragmentation is part of their normal reproductive behavior. Our analysis further suggests that certain characteristic aspects of the reproduction statistics can be explained in terms of a maximum relative entropy principle.

BREAKING OF AXIAL AND REFLECTION SYMMETRIES IN SPONTANEOUS FISSION OF FERMIUM ISOTOPES

The nuclear fission phenomenon is a magnificent example of a quantal collective motion during which the nucleus evolves in a multidimensional space representing shapes with different geometries. The triaxial degrees of freedom are usually important around the inner fission barrier, and reduce the fission barrier height by several MeV. Beyond the inner barrier, reflection-asymmetric shapes corresponding to asymmetric elongated fragments come into play. We discuss the interplay between different symmetry breaking mechanisms in the case of even-even fermium isotopes using the Skyrme HFB formalism.

Communication: Delayed asymmetric Coulomb fission of molecular clusters: Application of a dielectric liquid-drop model

Delayed asymmetric Coulomb fission in size-selected molecular dication clusters has been recorded for the first time. Observations on (NH(3))(n)(2+) clusters show that fragmentation accompanied by charge separation can occur on a microsecond time scale, exhibits considerable asymmetry, and involves a kinetic energy release of ∼0.9 eV. The fission process has been modeled by representing the fragments as charged dielectric spheres and the calculated maximum in the electrostatic interaction energy between the fragments gives a good account of the measured kinetic energy release. A simple kinetic model shows that instrumental factors may contribute to the observation of asymmetric fragmentation.

Effect of proline position on symmetric versus asymmetric fragmentation of doubly-protonated tryptic-type peptides

The fragmentation reactions of the doubly-protonated tryptic-type peptides APAAAAR, AAPAAAR, AAAPAAR and AAAAPAR have been studied using an electrospray/quadrupole/time-of-flight (QqTo F) mass spectrometer. The tendency to cleave amide bonds N-terminal to proline (P) is in competition with the tendency to cleave the second amide bond, counting from the N-terminus; the result of such competition depends on the position of the Pro residue. When the Pro residue is remote from the Arg (R) residue a strong proline effect is observed resulting in formation, to a large extent, of a doubly-charged y species and a neutral fragment, so-called asymmetric amide bond cleavage. By contrast, when the Pro residue approaches the C-terminal Arg residue the proline effect is reduced with respect to cleavage of the second amide bond; in both cases formation of singly-charged y and b ions, so-called symmetric bond cleavage, increases significantly in importance. The results are discussed in terms of relative energetics for symmetric and asymmetric bond cleavage as revealed by approximate proton affinities of the b species and the singly-charged y species.

Octupole deformation properties of the Barcelona-Catania-Paris energy density functionals

We discuss the octupole deformation properties of the recently proposed Barcelona-Catania-Paris (BCP) energy density functionals for two sets of isotopes, those of radium and barium, where it is believed that octupole deformation plays a role in the description of the ground state. The analysis is carried out in the mean field framework (Hartree- Fock- Bogoliubov approximation) by using the axially symmetric octupole moment as a constraint. The main ingredients entering the octupole collective Hamiltonian are evaluated and the lowest lying octupole eigenstates are obtained. In this way we restore, in an approximate way, the parity symmetry spontaneously broken by the mean field and also incorporate octupole fluctuations around the ground state solution. For each isotope the energy of the lowest lying $1^{-}$state and the $B(E1)$ and $B(E3)$ transition probabilities have been computed and compared to both the experimental data and the results obtained in the same framework with the Gogny D1S interaction, which are used here as a well established benchmark. Finally, the octupolarity of the configurations involved in the way down to fission of $^{240}$Pu, which is strongly connected to the asymmetric fragment mass distribution, is studied. We confirm with this thorough study the suitability of the BCP functionals to describe octupole related phenomena.

Determination with Matrix-Assisted Laser Desorption/Ionization Tandem Time-of-Flight Mass Spectrometry of the Extensive Disulfide Bonding in Tarantula Venom Peptide Psalmopeotoxin I

Psalmopeotoxin I (PcFK1) is a 33-residue peptide isolated from the venom of the tarantula Psalmopoeus cambridgei. This peptide specifically inhibits the intra-erythrocyte stage of Plasmodium falciparum in vitro. It contains six cysteine residues forming three disulfide bridges and belongs to the superfamily of natural peptides containing the inhibitor cystine knot (ICK) fold. We produced the wild-type and mutated forms of the recombinant peptide to examine the mechanism of action of PcFK1. The purified toxins were consistently produced as two isobaric peptides (r-PcFK1-1 and r-PcFK1-2) with different retention properties but identical anti-plasmodial -biological activity. Comparison of (15)N-NMR heteronuclear single quantum correlation spectra revealed that although rPcFK1-1 was highly structured, rPcFK1-2 does not have a stable three-dimensional structure. We used high-energy collision-induced fragmentation of the peptides with a matrix-assisted laser desorption/ionization tandem time-of- flight mass spectrometer to further investigate the structure of the native peptides in its natural form and produced in E. coli. The fragmentation spectra of the native peptides were very complex due to the occurrence in the spectrum of ions resulting from (1) cross-linking of fragments through a disulfide bridge and (2) asymmetric fragmentations of the disulfide bridges and (3) multiple neutral losses. The tandem mass spectrometry fragmentation pattern of r-PcFK1-1 was similar to that of the natural peptide isolated from crude venom, but r-PcFK1-2 had a clearly distinct fragmentation pattern, more closely resembling the fragmentation spectra of reduced and alkylated peptides. Observed ions could be attributed to specific fragments by comparing spectra between the wild-type and selected variants with point mutations (Y11W, R20T, Y26W, K28V). The disulfide connections in r-PcFK1-2 differed from those of the native peptide and showed a rare disulfide bridge between vicinal cysteine residues. The r-PcFK1_(R20T) variant showed a very limited fragmentation pattern when analyzed in positive mode but displayed much more fragmentation in negative mode pointing out the importance of the R20 residue in the fragmentation of PcFK1. Using the reductive matrix 1,5-diaminonaphtalene promoted strongly in source decay fragmentation of the peptides in MS mode. Our findings illustrated the critical role of the electronic environment around the central Cys(18)-Cys(19) doublet in PcFK1 in internal fragmentation of the peptide.

Identification of Disulfide-Containing Chemical Cross-Links in Proteins Using MALDI-TOF/TOF-Mass Spectrometry

Cross-linking can be used to identify spatial relationships between amino acids in proteins or protein complexes. A rapid and sensitive method for identifying the site of protein cross-linking using dithiobis(sulfosuccinimidyl propionate) (DTSSP) is presented and illustrated with experiments using murine cortactin, actin and acyl-CoA thioesterase. A characteristic 66 Da doublet, which arises from the asymmetric fragmentation of the disulfide of DTSSP-modified peptides, is observed in the mass spectra obtained under MALDI-TOF/TOF-MS conditions and allows rapid assignment of cross-links in modified proteins. This doublet is observed not only for linear cross-linked peptides but also in the mass spectra of cyclic cross-linked peptides when simultaneous fragmentation of the disulfide and the peptide backbone occurs. We suggest a likely mechanism for this fragmentation. We use guanidinylation of the cross-linked peptides with O-methyl isourea to extend the coverage of cross-linked peptides observed in this MALDI-MS technique. The methodology we report is robust and amenable to automation, and permits the analysis of native cystines along with those introduced by disulfide-containing cross-linkers.

Structure properties ofTh226andFm256,258,260fission fragments: Mean-field analysis with the Gogny force

The constrained Hartree-Fock-Bogoliubov method is used with the Gogny interaction D1S to calculate potential energy surfaces of fissioning nuclei $^{226}\mathrm{Th}$ and $^{256,258,260}\mathrm{Fm}$ up to very large deformations. The constraints employed are the mass quadrupole and octupole moments. In this subspace of collective coordinates, many scission configurations are identified ranging from symmetric to highly asymmetric fragmentations. Corresponding fragment properties at scission are derived yielding fragment deformations, deformation energies, energy partitioning, neutron binding energies at scission, neutron multiplicities, charge polarization, and total fragment kinetic energies.

Centrifugal fragmentation of a dinuclear system in the process of its evolution toward a compound nucleus

The centrifugal fragmentation of a dinuclear system in the process of evolution toward a compound nucleus is examined. If the angular momentum in the collision of primary nuclei is quite high, centrifugal forces become dominant at the final stage of the evolution of the dinuclear system formed, causing the decay of this dinuclear system to two strongly asymmetric nuclear fragments. Experimental data in which this specific nuclear process manifests itself are presented. Centrifugal fragmentation makes it possible to reveal the cluster facet of the evolution of a dinuclear system toward a compound nucleus. The possibility of this fragmentation process is a logical consequence of the concept of a dinuclear system for the complete fusion of nuclei.