Fragment Kinetic Energy Research Articles

Fragmentation of molecules under charge-changing collisions of a few MeV heavy ions

We investigated molecular fragmentation of CO and C2H2 molecules by impact of various fast heavy ions. Fragment ions produced in electron capture and loss collisions of projectile ions were measured in coincidence with final projectile charge states. Data acquisition using position sensitive detection system allows us to obtain 3D momentum imaging of fragment ions and kinetic energy release (KER) in various charge-changing collisions. It was found that the KER spectra show strong dependence on the type of charge-changing collisions. This may be caused by the difference of impact parameters associated with individual charge-changing collisions. Moreover we revealed the different fragmentation pathway between ion impacts and photoionization.

Ionization Chamber for Prompt Fission Neutron Investigations

In this work we report recent achievements in design of twin back-to-back ionization chamber (TIC) for fission fragment (FF) mass and kinetic energy measurement. Correlated FF kinetic energies, their masses and the angle of FF in respect to the axes in 3D Cartesian coordinates can be determined from analysis of the heights and shapes of the pulses induced by the fission fragments on the anodes of TIC. Anodes of TIC were designed as consisting of isolated strips each having independent electronic circuitry and special multi-channel pulse processing apparatus. Mathematical formulae provided for FF angles measured in respect to the coordinate axes. It was shown how the point of fission fragments origin on the target plane may be determined using the same measured data. The last feature made the TIC a rather powerful tool for prompt fission neutron (PFN) emission investigation in event-by-event analysis of individual fission reactions from non- point fissile source. Position sensitive neutron induced fission detector for neutron-imaging applications with both thermal and low energy neutrons was found as another possible implementation of the designed TIC.

Ion–polycyclic aromatic hydrocarbon collisions: kinetic energy releases for specific fragmentation channels

We report on 30 keV He2 + collisions with naphthalene (C10H8) molecules, which leads to very extensive fragmentation. To unravel such complex fragmentation patterns, we designed and constructed an experimental setup, which allows for the determination of the full momentum vector by measuring charged collision products in coincidence in a recoil ion momentum spectrometer type of detection scheme. The determination of fragment kinetic energies is found to be considerably more accurate than for the case of mere coincidence time-of-flight spectrometers. In fission reactions involving two cationic fragments, typically kinetic energy releases of 2–3 eV are observed. The results are interpreted by means of density functional theory calculations of the reverse barriers. It is concluded that naphthalene fragmentation by collisions with keV ions clearly is much more violent than the corresponding photofragmentation with energetic photons. The ion-induced naphthalene fragmentation provides a feedstock of various small hydrocarbonic species of different charge states and kinetic energy, which could influence several molecule formation processes in the cold interstellar medium and facilitates growth of small hydrocarbon species on pre-existing polycyclic aromatic hydrocarbons.

Dynamic fragmentation of natural ceramic tiles: Ejecta measurements and kinetic consequences

Velocity and size measurements of ejecta derived from impacts of railgun-launched projectiles into 10 mm thick gabbro tiles are examined. Fragmentation of the target and the ejecta velocity field are found to be governed by Hertzian fracture and the transfer of kinetic energy to the target. Over 90% of the total kinetic energy is contained above the average ejecta velocity and greater than 95% in angles bounded by Hertzian fracture. Log-normal distributions of the ejecta reveal that the kinetic energy transfer from projectile to ejecta is an organized process that spans over four orders of magnitude. The ejection angle that contains the most total kinetic energy coincides with the primary ejection angle, indicating the importance of larger plate-like fragments in the fragmentation process. Approximately 11–16% of energy to the target (initial − final kinetic energy of the projectile) is converted to the kinetic energy of fragments when the projectile does not perforate the target (≤52 J). The conversion to fragment kinetic energy increases near-linearly to ∼50% at an incoming projectile energy of 305 J. This reveals the importance of this energy conversion mechanism under these experimental conditions.

Investigation of the maximum accessible kinetic energy of fragments in the neutron-induced fission of 238U nuclei

The masses, total kinetic energies (TKE), and emission angles of fragments originating from the fission of 238U nuclei that was induced by 5- and 6.5-MeV neutrons were measured by using digital methods for processing signals. A detailed analysis of the shape of digital signals made it possible to reduce substantially the contribution of fragments whose TKE values were distorted because of a superimposition of signals from recoil protons and from alpha particles produced in the spontaneous decay of uranium. The total statistics exceeded two million events for either neutron energy, and this permitted performing a detailed analysis of fission-fragment yields in the region of the highest attainable TKE values. An analysis of fragment yields made it possible to draw specific conclusions on the structure of the potential surface of fissile nuclei.

Carrier-envelope-phase-dependent dissociation of hydrogen

We studied the dependence of dissociative ionization in H2 on carrier-envelope phase (CEP) of few-cycle (6 fs) near-infrared laser pulses. For low-energy channels, we present the first experimental observation of the CEP dependence of combined dissociation yield (with protons emitted in both directions), as well as the highest degree of asymmetry reported to date (40%). The observed modulations in both asymmetry and combined yield could be understood in terms of interference between different n-photon dissociation pathways—n and (n + 1) photon channels for asymmetry, n and (n + 2) photon channels for yield—as suggested by the general theory of CEP effects (Roudnev and Esry 2007 Phys. Rev. Lett. 99 220406). The yield modulation is found to be π-periodic in CEP, with its phase strongly dependent on fragment kinetic energy (and reversing its sign within the studied energy range), indicating that the dissociation yield does not simply follow the CEP dependence of maximum electric field, as a naïve intuition might suggest. We also find that a positively chirped pulse can lead to a higher dissociation probability than a transform-limited pulse.

A Multiparameter Nuclear-fission Experiment: Can All be Obtained at Once?

Abstract A large variety of experimental works has been done since the discovery of nuclear fission, aimed at studying different aspects of the phenomenon. Yet our comprehension of the fission process is not complete. This is, among others, due to a certain lack in multi-parameter experimental data. An example here is the correlation between fractional independent yields of fission products and neutron and gamma-ray multiplicities. Fragment-gamma-neutron measurements, especially if correlated with fission- fragment kinetic energies, give the complete set of observables and are therefore of interest from the point of view of modeling and understanding of the fission process. A two-arm spectrometer of fission products (STEFF) has been recently built at the Manchester University. In addition to the identification of masses from complementary fission products, by the double energy/double velocity measurement, the spectrometer is capable of delivering information on their nuclear charges, on the event-by-event basis. The spectrometer also comprises an array of NaI and may house a further array of neutron detectors. In such configuration, STEFF has been used at the ILL neutron guide at the benchmark experiment 235U(nth, f). Details on the experiment will be presented, results on the identification of atomic numbers in the light group of fission products will be demonstrated and the perspectives discussed.

Rapid bond rearrangement in core-excited molecular water

The angular anisotropy of fragments created in the dissociation of core-electron excited water molecules is studied to probe the correlation between fragmentation channels, kinematics and molecular geometry. We present fragment kinetic measurements for water molecules where the inner-shell oxygen electron is excited to the unoccupied 4a1 and 2b2 valence molecular orbitals. The kinematics of individual fragmentation channels are measured using fully three-dimensional momentum imaging of fragments. The results show that the geometry of the molecule and the kinetic energy of fragments are strongly coupled in the atomisation process. In addition we identify a fragmentation process arising from bond rearrangement evidenced by the H2(+)-O(+) ion pair which is accessible for resonant excitation of the 1s electron. In all of the two-body fragmentation processes the dissociation takes place along the potential-energy surface, while atomisation reveals both dissociation along the potential surface and Coulomb explosion. The angular distribution of fragments suggests that the bond rearrangement is very rapid; likely on a sub 10 fs time scale.

Highly Charged Ion - Induced Water Cluster Fragmentation

Interaction between slow multiply charged ions and small to medium-size neutral water clusters is investigated. Strong projectile charge dependence is found for all of the fragmentation patterns. A variation of the projectile charge dramatically changes the proportion of multi-electron removal, the fragment kinetic energies as well as the mean size of the emitted fragments. First results for highly charged ion - water cluster ion collisions will also be presented.

Dissociative Photoionization of Methane at the Carbon K-edge

We have used soft X-ray radiation from the Advanced Light Source (ALS) in combination with Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) to examine the correlated momenta of the photoelectron and molecular fragments arising from the dissociative photoionization of methane. In particular, we are studying (1) molecule frame photoelectron angular distributions as a stringent test of theory and (2) the interplay between electronic and nuclear dynamics through the correlated measurement of vibrationational state population and fragmentation branching ratios and kinetic energy release.

Fragment kinetic energy distributions in ion induced CO2 fragmentation

The dissociation of CO3+2 formed in heavy ion induced ionization of CO2 has been studied using the technique of time of flight mass spectroscopy with position sensitive ion detector, with 5 MeV/u Si12+ ions as projectiles. The kinetic energy released in the CO3+2→ C+ + O+ + O+ is measured and compared to theoretical ab initio calculations as well as photoionization results.

Decay of C60 by delayed ionization and C2 emission: Experiment and statistical modeling of kinetic energy release

C(60) molecules highly excited in the nanosecond regime decay following ionization and dissociation by emitting a series of carbon dimers, as well as other small fragments if excitation is strong enough. The fragmentation mass spectrum and kinetic energy release of all charged fragments obtained in these experiments are interpreted within the framework of the Weisskopf theory, using a realistic Monte Carlo procedure in which the rates of all relevant decay channels are modeled using Arrhenius expressions. Comparison between the measurements and the simulated spectra allows the distribution of deposited energy to be accurately estimated. The dependence of the fragment kinetic energies on the laser fluence, found in the simulation but not observed in the experimental results, indicates that the small fragments are not necessarily emitted from small fullerenes resulting from C(60) by sequential decay. Rather, direct multifragmentation of C(60) is invoked to interpret the observed patterns. The possible role of post-ionization of neutral emitted fragments is discussed.

Monte Carlo simulation of prompt neutron emission during acceleration in fission

The possibility of prompt neutron emission during acceleration of fission fragments (FFs) was examined by means of Monte Carlo method and statistical neutron emission model. Multimodal random neck-rapture model was used to describe the initial distribution of mass, charge, and total kinetic energy of the primary fragments. Statistical model was used to simulate the de-excitation process of the fragments from the moment of scission until full acceleration. By random number sampling, the fission process was simulated in order to obtain the basic physical quantities, and their correlations were analyzed to verify the adequacy of the model. It was found that, on the average, ∼10% and ∼16% of prompt neutrons for 235U(n th,f) and 252Cf(sf), respectively, were emitted before reaching 90% of the final fragment kinetic energy.

Dissociation dynamics of diatomic molecules in intense laser fields: A scheme for the selection of relevant adiabatic potential curves

We investigated the nuclear dynamics of diatomic molecular ions in intense laser fields by analyzing their fragment kinetic-energy release (KER) spectra as a function of the pump-probe delay $\ensuremath{\tau}$. Within the Born-Oppenheimer (BO) approximation, we calculated ab initio adiabatic potential-energy curves and their electric dipole couplings, using the quantum chemistry code gamess. By comparing simulated KER spectra as a function of either $\ensuremath{\tau}$ or the vibrational quantum-beat frequency for the nuclear dynamics on both individual and dipole-coupled BO potential curves with measured spectra, we developed a scheme for identifying electronic states that are relevant for the dissociation dynamics. We applied this scheme to investigate the nuclear dynamics in O${}_{2}$${}^{+}$ ions that are produced by ionization of neutral O${}_{2}$ molecules in an ultrashort infrared (IR) pump pulse and dissociate due to the dipole coupling of molecular potential curves in a delayed IR probe laser field.

CONTROLLING ABOVE-THRESHOLD DISSOCIATION BRANCHING RATIOS OF HD+ WITH FEMTOSECOND LASER PULSE TRAIN

Above-threshold dissociation (ATD) process of the molecular ions HD+ steered by a femtosecond laser pulse train (LPT) is investigated theoretically using the time-dependent quantum wave packet method. Energy-dependent distributions of ATD fragments are analyzed by using an asymptotic-flow expression in the momentum space. It is found that fragment kinetic energy spectra shift to low energy region with increasing pulse number of LPT. The photofragment branching ratio between the 1sσg and 2pσu dissociation channels is sensitive to the pulse number of LPT. The momentum distribution of the ATD fragments is discussed in detail.

Role of the Renner-Teller effect after core hole excitation in the dissociation dynamics of carbon dioxide dication

The fragmentation of the doubly-charged carbon dioxide molecule is studied after photoexcitation to the C 1s(1)2π(u) and O 1s(1)2π(u) states using a multicoincidence ion-imaging technique. The bent component of the Renner-Teller split states populated in the 1s→ π* resonant excitation at both the carbon and oxygen 1s ionization edges opens pathways to potential surfaces in highly bent geometries in the dication. Evidence for a complete deformation of the molecule is found in the coincident detection of C(+) and O(2)(+) ions. The distinct alignment of this fragmentation channel indicates rapid deformation and subsequent fragmentation. Investigation of the complete atomization dynamics in the dication leading to asymmetric charge separation shows that the primary dissociation mechanisms, sequential, concerted, and asynchronous concerted, are correlated to specific fragment kinetic energies. The study shows that the bond angle in fragmentation can extend below 20°.

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). II. Velocity map imaging studies

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following excitation in the 4ν(1) region (OH stretch overtone, near 13,600 cm(-1)) was studied using sliced velocity map imaging. A new vibrational band near 13,660 cm(-1) arising from interaction with the antisymmetric CH stretch was discovered for CH(2)OH. In CD(2)OH dissociation, D atom products (correlated with CHDO) were detected, providing the first experimental evidence of isomerization in the CH(2)OH ↔ CH(3)O (CD(2)OH ↔ CHD(2)O) system. Analysis of the H (D) fragment kinetic energy distributions shows that the rovibrational state distributions in the formaldehyde cofragments are different for the OH bond fission and isomerization pathways. Isomerization is responsible for 10%-30% of dissociation events in all studied cases, and its contribution depends on the excited vibrational level of the radical. Accurate dissociation energies were determined: D(0)(CH(2)OH → CH(2)O + H) = 10,160 ± 70 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,135 ± 70 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,760 ± 60 cm(-1).

Theoretical Research of Measuring the Kinetic Energy for Swarm of Fragments with Six-Light-Screen Array

Aimed at the measurement of kinetic energy for swarm of flying fragments, a measuring method of kinetic energy for the flying fragment using six-light-screen array was presented in this paper. When a fragment pass through the six light screens, the six-time sequences, can be gathered in turn. Lots of fragments can generate some sequence of six-times. The algorithm that can distinguish the sequence from a group of sequences of time is put forward in this paper. According to the coordinates of fragment position on a position indicator as well as all the sequences of the fragments, the real time sequence can be distinguished and recognized. Then the real velocity and accurate location of the fragment can be calculated with the algorithm designed, and the kinetic energy of fragments can be measured. The algorithm is proved accurate and effective by the simulation.

Numerical Study on Dispersion Characteristics and Damage Results of Pre-Fragment Warhead

The dispersion characteristics, damage region and destroy results of typical defending materials by pre-fragment warhead using explosively driven were numerical studied by non-linear finite code of LS-DYNA with fluid-structure interaction algorithm, meanwhile, the influence of different detonation types and explosive properties on dispersion characteristics and damage results of kinetic energy rod were analysed. Results show that: 1.fragments disperse symmetrically with greater damage region when center detonation and line detonate are adopted, and the dispersion velocity of fragments reach the maximum when line detonate is used; 2.explosive of higher detonation velocity in favour of improving the initial velocity of fragments, strengthing the damage ability; 3.the penetration kinetic energy of fragments increase with the decrease of axial angle between fragment and damage target.

Applications of Event-by-Event Fission Modeling with FREYA

The recently developed codeFREYA (Fission Reaction Event Yield Algo- rithm) generates large samples of complete fission events, consisting of two receding product nuclei as well as a number of neutrons and photons, all with complete kinematic information. Thus it is possible to calculate arbitrary correlation observables whose behav- ior may provide unique insight into the fission process. We first discuss the present status ofFREYA, which has now been extended to include spontaneous fission. Concentrating on 239 Pu(n th,f), 240 Pu(sf) and 252 Cf(sf), we discuss the neutron multiplicity correlations, the dependence of the neutron energy spectrum on the neutron multiplicity, and the relation- ship between the fragment kinetic energy and the number of neutrons and their energies. We also suggest novel fission observables that could be measured with modern detectors.