Decay Of Fragments Research Articles

Implications of two-body fragment decay for the interpretation of emission chronology from velocity-gated correlation functions

From velocity-gated small-angle correlation functions, the emission chronology can be deduced for nonidentical particles, if the emission is independent. This is not the case for nonidentical particles that originate from two-body decay of fragments. Experimental results may contain contributions from both independent emission and two-body decay, so care is needed in interpreting the velocity-gated correlation functions. It is shown that in some special cases, it is still possible to deduce the emission chronology, even if there is a contribution from two-body decay.

On the Stability of IrCl63- and Other Triply Charged Anions: Solvent Stabilization versus Ionic Fragmentation and Electron Detachment for the IrCl63-·(H2O)n n = 0−10 Microsolvated Clusters

The intrinsic gas-phase stability of the IrCl(6)(3-) trianion and its microsolvated clusters, IrCl(6)(3-).(H(2)O)(n) n = 1-10, have been investigated using density functional theory (DFT) calculations. Although IrCl(6)(3-) is known to exist as a stable complex ion in bulk solutions, our calculations indicate that the bare trianion is metastable with respect to decay via both electron detachment and ionic fragmentation. To estimate the lifetime of IrCl(6)(3-), we have computed the electron tunneling probability using an adaption of the Wentzel-Kramer-Brillouin theory and predict that the trianion will decay spontaneously via electron tunneling on a time scale of 2.4 x 10(-13) s. The global minimum structure for IrCl(6)(3-).H(2)O was found to contain a bifurcated hydrogen bond, whereas for IrCl(6)(3-).(H(2)O)(2), two low energy minima were identified; one involving two bifurcated water-ion hydrogen bonds and a second combining a bifurcated hydrogen bond with a water-water hydrogen bond. Clusters based on each of these structural motifs were obtained for all of the n = 3-10 systems, and the effect of solvation on the possible decay pathways was explored. The calculations reveal that solvation stabilizes IrCl(6)(3-) with respect to both electron detachment decay and ionic fragmentation, with the magnitude of the repulsive Coulomb barrier for ionic fragmentation increasing smoothly with sequential solvation. This study is the first to compare the propensity for electron detachment versus ionic fragmentation decay for a sequentially solvated triply charged anion.

Inverse-beta-decay reaction in the antineutrino flux from the fragments of 232Th and 233U fission

The energy spectra of antineutrinos produced in the beta decay of fragments originating from 233U and 232Th fission induced by neutrons are calculated. The relevant cross sections and the spectra of positrons produced in inverse beta decay are found. This study was motivated by the hypothesis (discussed over the past decade)t hat a self-sustained chain reaction proceeds at the center of the Earth (“georeactor”). According to the author of this hypothesis, the georeactor provides energy necessary for maintaining the Earth’s magnetic field. It is 235U and, probably, 232Th and 233U that serve as a nuclear fuel in this reactor. Data obtained in the present study can be guidelines in future experiments aimed at testing the hypothesis of the georeactor and at estimating the composition of its nuclear fuel within the development of geophysical and astrophysical investigations based on the observation of antineutrino fluxes in nature.

Quantum molecular dynamics approach to estimate spallation yield fromp +208Pb reaction at 800 MeV

The spallation yield of neutrons and other mass fragments produced in 800 MeV proton induced reaction on208Pb have been calculated in the framework of quantum molecular dynamics (QMD) model. The energy spectra and angular distribution have been calculated. Also, multiplicity distributions of the emitted neutrons and kinetic energy carried away by them have been estimated and compared with the available experimental data. The agreement is satisfactory. A major contribution to the neutron emission comes from statistical decay of the fragments. For mass and charge distributions of spallation products the QMD process gives rise to target-like and projectile-like fragments only.

Fragmentation and electronic decay of vacuum-ultraviolet-excited resonant states ofmolecular CsCl

Electronic and nuclear relaxation of molecular CsCl following photoexcitation inthe vacuum ultraviolet region is investigated. From the total ion yield and theCs+ andCl+ partial ion yield spectra covering the 8–22 eV photon energy range, a number of resonating excitedstates are identified. The fragmentation patterns of the excited and ionized CsCl molecules and alsothe Cs2Cl2 dimers are studied in detail from ion time-of-flight mass spectra recorded at and around theresonances. Molecular dissociation following the resonant excitation is discussed and anionic model is applied to explain the observations, emphasizing some principal differencesfrom other alkali chloride molecules (KCl). The valence photoelectron spectra are found toshow strong enhancement and pronounced new structures at resonant photon energies.These effects are assigned to a ‘cross-ionic’ autoionization decay process and are found tobe due to a lifetime-dependent interplay between the electronic and nuclear relaxation.

Excitation and decay of projectilelike fragments formed in dissipative peripheral collisions at intermediate energies

Projectilelike fragments (PLF, $15<~Z<~46)$ formed in peripheral and midperipheral collisions of ${}^{114}\mathrm{Cd}$ projectiles with ${}^{92}\mathrm{Mo}$ nuclei at $E/A=50\mathrm{MeV}$ have been detected at very forward angles, $2.1\ifmmode^\circ\else\textdegree\fi{}<~{\ensuremath{\theta}}_{\mathrm{lab}}<~4.2\ifmmode^\circ\else\textdegree\fi{}.$ Calorimetric analysis of the charged particles observed in coincidence with the PLF reveals that the excitation of the primary PLF is strongly related to its velocity damping. Furthermore, for a given ${V}_{\mathrm{PLF}*},$ its excitation is not related to its size, ${Z}_{\mathrm{PLF}*}.$ For the largest velocity damping, the excitation energy attained is large, approximately commensurate with a system at the limiting temperature.

Hydrogen exchange studies on Alzheimer's amyloid-beta peptides by mass spectrometry using matrix-assisted laser desorption/ionization and electrospray ionization.

The conformation and aggregation behavior of synthetic Alzheimer's amyloid peptides (Abeta) has been investigated using hydrogen-deuterium exchange measured by electrospray ionization mass spectrometry and matrix-assisted laser desorption/ionization mass spectrometry. Mass spectrometric fragmentation of deuterated Abeta peptides was carried out by collision-induced dissociation, inlet fragmentation, and post-source decay. In contrast to the C-terminally truncated peptides Abeta(1-40) and Abeta(1-36) showing full hydrogen-deuterium exchange, Abeta(1-42) and the pyroglutamyl peptide Pyr(3)-Abeta(3-42) produced more complex signal patterns resulting from the formation of beta-sheet-structured oligomers having 18-20 strongly protected protons. Using mass spectrometric fragmentation the results show that the reduced isotope exchange of Abeta(1-42) can be attributed to the central part of the chain comprising residues 8-23. This confirms involvement of the hydrophobic binding domain LVFFA in the course of Abeta aggregation and demonstrates that hydrogen-deuterium exchange in combination with mass spectrometry is well suited for structural analysis of monomeric and reversibly associated amyloid peptides using picomole quantities of material.

State-selected dissociation of dehydrovincamine alkaloid stereo-isomers

We have obtained results on alkaloid stereoisomers that show strongly stereoselective fragmentation processes. An original technique for such molecules, i.e. photoionisation by synchrotron radiation in the VUV range coupled to the photoelectron–photoion coincidence technique, is used to prepare state-selected parent ions with a very wide internal energy range. Mass spectra obtained after photoionisation, and metastable spectra obtained after electron impact ionisation, exhibit stereospecific fragmentations leading either to water loss and subsequent fragmentations in dehydrovincamine, or to carbomethoxy and ethyl loss in epi-dehydrovincamine. The results are in agreement with previous measurements obtained on the protonated samples, or on the similar vincamine molecule. Appearance energies for major fragmentations were recorded as well as a state-selected breakdown diagram for the epi-dehydrovincamine sample. They show that thresholds for the fragmentation processes are almost identical in the two isomers, indicating very similar thermodynamics. Very slow fragmentation decay for the first dissociation channels leads to the coexistence of parent and fragment ions in a very wide internal energy range of the breakdown diagram. The results show that the fragmentation of the system is most probably driven by strong steric factors. Further modelling with statistical theories should give the precise transition states and lead to a better understanding of this stereospecificity.

Weak and electromagnetic nuclear decay signatures for neutrino reactions in Super-Kamiokande.

We suggest the study of events in the Super-Kamiokande neutrino data due to charged- and neutral-current neutrino reactions followed by weak and/or electromagnetic decays of struck nuclei and fragments thereof. This study could improve the prospects of obtaining evidence for tau production from nu(mu)-->nu(tau) oscillations and could augment the data sample used to disfavor nu(mu)-->nu(sterile) oscillations.

Hot fragmentation of nuclei

Today, we have a variety of reactions at hand that can be used to multi-fragment nuclei. In many of these reactions even several sources of fragments can be discerned and characterized. There is overwhelming evidence that these sources of fragments are hot. It is already less clear whether heat by itself is sufficient to initiate the fragment decay. What causes fragmentation, and when and how are the fragments (pre)formed? These questions have remained as much a challenge as the complementary class of questions to which they are related: What observations derive their significance from the liquid-gas phase behavior of extended nuclear matter? And, can we observe a phase transition in finite nuclei? Recent developments, largely coming from complex analyses of data sets measured in 4-pi-type experiments as well as from calculations based on advanced theoretical concepts, will be discussed.

State-Resolved Dynamics of Dissociation of Triplet Acetaldehyde: Rate of Appearance of Fragment HCO and Decay of Excited States of Parent Molecule

We studied the state-resolved dynamics of acetaldehyde in electronically excited state A1A‘ ‘ with high resolution, 0.025 cm-1. Near a threshold for dissociation into CH3 + HCO that is 1500−1750 cm-1 above the vibrational ground state of acetaldehyde in A1A‘ ‘, the vibrational levels are investigated with laser-induced fluorescence. The rotational structures of four bands in this region are partially assigned. These assignments allow us to determine the dependence of relaxation of individual rovibrational levels on energy as well as rotational quantum number J. The measured lifetimes vary with J as a result of an increased number of nuclear hyperfine T1 states that couple with a selected vibrationally excited state. For these four vibrational levels the rate constants of fragment HCO appearance are greater than decay for initially prepared excited states; this deviation implies that only a fraction of initially prepared states couple effectively to dissociation.

Statistical model for fragmentation ofC60

A simple statistical model is formulated to predict inclusive distributions and correlations of fragments from highly excited ${\mathrm{C}}_{60}.$ Sequential decay of excited fragments produced in an initial binary fragmentation are taken into account. A qualitative account of results from a recent experiment is achieved.

Towards exotic nuclei via binary reaction mechanism

Assuming a binary reaction mechanism, the yield of isotopes near the heaviest $N=Z$ neutron-deficit nucleus ${}^{100}\mathrm{Sn}$ is studied with a microscopic transport model. The large influence of nuclear shell structure and isotope composition of the colliding nuclei on the production of exotic nuclei is demonstrated. It is shown that the reaction ${}^{54}{\mathrm{F}\mathrm{e}+}^{106}\mathrm{Cd}$ seems to be most favorable for producing primary exotic Sn isotopes which may survive if the excitation energy in the entrance reaction channel is less than about 100 MeV. In the case of large differences in the charge (mass) numbers between entrance and exit channels the light fragment yield is essentially fed from the decay of excited primary heavier fragments. The existence of optimal energies for the production of some oxygen isotopes in the binary mechanism is demonstrated for the ${}^{32}{\mathrm{S}+}^{197}\mathrm{Au}$ reaction.

Caloric curve and conditional moments: Effects of secondary fragment decay

We establish, within the framework of the statistical multifragmentation model, the connection between the caloric curve and the analysis of conditional moments. In particular, we show that the conditional moments of fragment charge distributions peak at the region where the curve temperature versus excitation energy shows a plateau assumed to be a signature of a phase transition. Furthermore, we show that the slopes of the moments at the peak are not influenced by secondary decay after the first breakup of the nucleus. @S0556-2813~98!04901-2#

ToF-SIMS analyses of polystyrene and dibenzanthracene: evidence for fragmentation and metastable decay processes in molecular secondary ion emission

To understand the sputtering processes in unsaturated polymers, dibenzanthracene and polystyrene samples were bombarded by 15 keV, Ga + ions and the secondary ions were mass- and energy-analyzed by means of a time-of-flight spectrometer. The influence of various phenomena, which may play an important role in the secondary ion emission from polystyrene surfaces, is investigated: (1) emission of “original” fragments, reflecting the polymer structure; (2) emission with fragmentation, rearrangements or hydrogen losses; (3) formation of molecular ions by hydrogen or proton capture; and (4) unimolecular dissociation of the secondary ions. The dibenzanthracene molecule (C 22H 14) was chosen as a model compound for the study of the emission and metastable decay of polyaromatic hydrocarbon (PAH) ions under static SIMS analysis conditions. The kinetic energy distribution (KED) of most of the secondary ions sputtered from dibenzanthracene exhibits up to three peaks. The peaks corresponding to a negative apparent energy are due to the metastable decomposition of larger ions in the acceleration section of the spectrometer. Their origin, i.e. delayed H and H 2 loss reactions, is discussed in detail. The kinetic energy distributions of the dibenzanthracene secondary ions are compared with those of the PAH ions emitted from polystyrene, especially in the region m z = 100–200 of the PS mass spectrum. It turns out that similar interpretations can be proposed for polystyrene. To confirm the hydrogen loss reaction in the case of polystyrene, the comparison is also made with the KED of similar but deuterated PAH ions sputtered from deuterated PS. On the basis of these hydrogen loss reactions, the formation times of the daughter ions are calculated.

Transition state rates and complex fragment decay widths.

Experimental excitation functions for the emission of complex fragments from compound nuclei are analyzed to search for atomic number {ital Z} and energy {ital E} dependent deviations from transition-state-method predictions. No {ital Z} and/or {ital E} dependent effects that could be attributed to an increased collectivity with increasing mass (charge) of the emitted fragment and associated with transient or stationary solutions of Kramers` diffusion equation are visible. Over seventy excitation functions, for complex fragments from four different compound nuclei, can be collapsed into a single universal straight line that is rigorously consistent with the transitions-state predictions.

Molecular dynamics model of heavy ion collisions at medium energies

Molecular dynamics methods for describing medium-energy heavy-ion collisions are discussed on the basis of some recent studies of fragmentation reactions by QMD and AMD. Importance of the proper treatment of initialization and the decay of primordial fragments is emphasized, and some advantageous characters of AMD are explained. Fragment production mechanisms are discussed, and exclusive study of fragment and nucleon flow is shown to give important information on EOS and in-medium two-nucleon cross section.

On the breakup of excited 16O projectiles into four alphas at intermediate energies

Mechanisms of the breakup into four alpha particles of 16O projectiles excited from peripheral collisions with 197Au target nuclei have been investigated at 50 and 70 MeV/N. The experimental distributions of relative angles between alpha particles in the rest frame of the primary oxygen nucleus are compared to theoretical predictions based on prompt fragmentation and binary sequential decay. The results are compared to similar studies for that channel at loweer energy, 32.5 MeV/N, where the breakup of 16O was shown to be consistent with sequential emission of alpha particles.

Particle-particle correlations: Independent particle emission versus sequential decay of heavy fragments.

Particle-particle coincidence measurements for both large and small separation angles are reported for the reactions 140 MeV $^{16}\mathrm{O}$${+}^{27}$Al\ensuremath{\rightarrow}p-p, p-d, d-d, \ensuremath{\alpha}-\ensuremath{\alpha}, d-\ensuremath{\alpha}, and p-\ensuremath{\alpha}. Two clusters of seven detectors were used at \ifmmode\pm\else\textpm\fi{}50\ifmmode^\circ\else\textdegree\fi{}, on both the right and left sides of the beam axis. Coincidence data were compared for nearest-neighbor and across-beam geometries. For p-p, p-d, and d-d pairs, the results of this symmetry test are consistent with independent particle evaporation. For the \ensuremath{\alpha}-\ensuremath{\alpha}, d-\ensuremath{\alpha}, p-\ensuremath{\alpha} pairs, the results indicate significant contributions from breakup of preformed fragments. These results suggest that small-angle correlations for the former pairs are useful as probes of the space-time extent of the emitters, but that the latter pairs are compromised for this task.

Fragment formation studied with antisymmetrized version of molecular dynamics with two-nucleon collisions.

A new microscopic simulation method of heavy-ion collisions is formulated by incorporating the two-nucleon collision process into the antisymmetrized version of molecular dynamics. This method can describe quantum-mechanical features such as shell effects. The fragment mass distribution of the $^{12}\mathrm{C}$${+}^{12}$C reaction at 28.7 MeV/nucleon is shown to be reproduced very well by this new method combined with the treatment of statistical cascade decays of excited fragments, which verifies the usefulness of the new method.