Hot Fragments Research Articles

Fragmentation dynamics of the doubly charged aniline: the source of kinetically excited CnH3+ ions.

The goals of this work are to attempt to decipher if an aniline dication can isomerize to a picoline dication in a given astrochemical environment and if the dissociation of such dications could be a source of kinetically hot fragment ions, some of which could be of significance in the interstellar medium. Toward this purpose, the VUV-induced dication dissociation was investigated experimentally using ion-ion coincidence and computationally by optimizing various pathways. Contrary to previous reports, we show here that the dication of aniline is structurally too weak to retain its ring structure while following the dissociation pathways. A fragile open ring structure could lead to all the experimentally observed pathways of noticeable intensity. The significance of this, especially in terms of molecular dynamics, can be assessed by the fact that all the transformations were facilitated by specific hydrogen migration. A clear selectivity is seen where the dication of aniline was found to prefer a rearrangement of hydrogen within the ring rather than transferring from nitrogen to the ring, which is conventionally expected and has to do with the charge state and charge localization.

Swelling and permeability effects during propellant cookoff

Large rocket motors may violently explode when exposed to accidental fires. Even hot metal fragments from a nearby accident may penetrate the propellant and ultimately cause thermal ignition. A mechanistic understanding of heated propellants leading to thermal runaway is a major unsolved problem. Here we show that thermal ignition in propellants can be predicted using a universal cookoff model coupled to a micromechanics pressurization model. Our model predicts the time to thermal ignition in cookoff experiments with variable headspace volumes. We found that experiments with headspace volumes are more prone to deformation which distorts pores and causes increased permeability when the propellant expands into this headspace. Delayed ignition with larger headspace volume correlates with lower headspace pressures during decomposition. We found that our predictions matched experimental measurements best when the initial propellant was impermeable to gas flow rather than being permeable. Similar behavior is expected with other energetic materials with rubbery binders. Our model is validated using data from a separate laboratory. We also present an uncertainty analysis using Latin Hypercube Sampling (LHS) of thermal ignition caused by a steel fragment embedded in the propellant.

On the Growth of Wildland Fires from a Small Ignition Source

ABSTRACT Wildland and Wildland-Urban-Interface (WUI) fires are an important problem that may have major consequences in terms of safety, air quality, and damage to buildings, infrastructure, and the ecosystem. It is expected that with climate change, the wildland fire and WUI fire problem will only intensify. Wildland fires are often initiated by small ignition sources caused either by human intervention (hot metal fragments or burning biomass) or by natural events (lighting or sun heating). Once the wildfire or structural fire has been ignited and grows, it can spread rapidly through ember spotting, where pieces of burning materials are lifted by the plume of the fire and then transported forward by the wind, landing where they can start spot fires downwind. The ignition mechanisms for all of these fires have the common characteristic of a small and localized area of origin, with the subsequent spread of the fire to wider and larger areas. Because of the three-dimensional characteristics of this type of propagating fire, its rate of spread has an initial acceleration phase leading to an equilibrium rate of spread when the fire reaches a certain size, which is referred to as a “line-fire” type of spread. Most of the studies on wildland fire propagation have been conducted with line fires and have been concerned with characterizing the equilibrium rate of spread rather than the initial spread from a small ignition source. In this paper, some of the studies conducted to date on the subject of wildland fire growth from a small ignition source, and the physics supporting the mathematical expressions that are used to describe the growth of the fire are discussed. An attempt is also made to provide support for these works through a more fundamental approach to model the problem.

Momentum Distribution Studies of Projectile Fragments from Peripheral Collisions Below the Fermi Energy

This paper presents our recent studies of multinucleon transfer in peripheral collisions in reactions below the Fermi regime. Our current focus is the study of the mass, angular and momentum distributions of the projectile-like fragments from the reaction of an 86Kr beam at 15 MeV/nucleon with a target of 64Ni. Experimental data from our previous work with the MARS spectrometer at the Cyclotron Institute of Texas A&M University were compared with model calculations. The dynamical stage of the reaction is described with either the Deep-Inelastic Transfer Model (DIT) or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot projectile-like fragments is performed with the GEMINI model. The momentum distributions are characterized by a quasi-elastic peak and a deep-inelastic peak. Two-body kinematics was employed to extract the total excitation energies of these regions. Through the thorough study of peripheral reactions in the Fermi energy regime we expect to gain valuable information that could lead to the understanding of how the rare isotopes in regions such as the r-process path and the neutron drip line are formed and the reaction mechanism(s) that take place.

In-situ formation of halite in the Sidi El Habib 001 (H5) ordinary chondrite: Implications for hydrothermal alteration in ordinary chondrite parent bodies

The microstructures and chemistry of secondary feldspars and phosphates in equilibrated ordinary chondrites (OCs) suggest that fluids were involved in the formation of these phases, challenging the conventional view that secondary alteration of equilibrated OCs occur under water-absent conditions. The newly discovered Sidi El Habib 001 (SEH 001), a halite-bearing H5 OC, provides a unique opportunity to further probe the role of fluids during thermal metamorphism on the OC parent bodies. Here we report a petrographic and mineralogic study of SEH 001, with the aim of understanding the origins of halite grains and their implications for the alteration histories of equilibrated OCs. Our investigation reveals a main halite-bearing lithology and a halite-free lithology, both of which show equilibrated textures. Except for halides, no significant textural or compositional differences were observed between halite-bearing and -free lithologies. Halite occurs at all spatial scales in the main lithology and shows clear textures of replacing albitic plagioclase and Cl-apatite. Chlorapatite grains in SEH 001 are Cl-rich and many of them contain elevated amounts of “other” anions.Our observations suggest that halite grains in SEH 001 formed in situ on the parent body via precipitation from an aqueous fluid. The replacement of plagioclase and Cl-apatite by halite and the equilibrated textures of halite-bearing and halite-free lithologies point to a hydrothermal alteration history where halite formed during advanced thermal metamorphism before the fluid was completely lost. The two lithologies were likely affected by fluids with different Cl concentrations that resulted from heterogeneous distribution of HCl hydrate. Based on comparison to experimental data, halite in SEH 001 could have survived peak metamorphism because of its relatively high thermal stability. Collisional disruption of its original parent body could also facilitate the preservation of halite via release of heat. In the rubble pile model of the OC parent body formation, subsequent accretion of hot fragments into a rubble pile body could have resulted in the blurred boundaries between halite-free and -bearing lithologies now observed in our sample. The occurrence of halite in SEH 001 is clear evidence that aqueous fluids were involved in the alteration of equilibrated OCs.Combined with previous reports of hydrous minerals (such as phyllosilicates) and other related aqueous products in unequilibrated OCs, our study further suggests that S-type asteroids, the parent bodies of OCs, could be more hydrated than previously thought and might serve as a potential source of water for terrestrial planets in the inner solar system. Nevertheless, whether the proposed hydrothermal history of SEH 001 can be extrapolated to other equilibrated OCs needs to be tested. The in-situ formation origin of halite in SEH 001 contrasts with the exogeneous origin of halite in Monahans (1998) and Zag, suggesting that halites with different origins occurred on the OC parent bodies. The rarity of halite in OCs could be attributed to the heterogeneous distribution of HCl hydrate in the OC parent bodies, although the fragile nature of halite in terrestrial and laboratory environments also increases the likelihood of halite being destroyed in OC samples.

Automated identification and calculation of prompt effects in kinetic mechanisms using statistical models

The kinetics of prompt dissociation involves rovibrationally excited species (generally formed by exothermic reactions) which may dissociate or isomerize prior to thermalization via collisions with the bath gas. Treating such rovibrationally excited species (so-called “hot” species) with standard kinetic phenomenology may result in incorrect macroscopic representation of their reactivity. This work presents the first fully automated methodology for the calculation of prompt effects of a chosen species in a kinetic mechanism, including (i) reaction selection; (ii) theoretical calculation of rate constants and prompt branching fractions; and (iii) final rate constant fitting. The energy partition between hot fragments is estimated using a variety of statistical models, including a new physically sound microcanonical statistical model based on the rovibrational density of states of the fragments. The methodology is validated against literature data for the prompt dissociations of HCO and C3H7 radicals. The microcanonical statistical model is in better agreement with trajectory simulations for larger species and is thus applicable for practical systems that typically involve large molecules, for which direct dynamics calculations are impractical. The automated workflow is applied to the evaluation of the effects of prompt dissociation for two isomeric radicals C4H71-3 (1-methylallyl) and C4H71-4 (3-buten-1-yl). Twelve H-atom abstraction reactions are selected and the corresponding rate constants are computed with first principles theory. The microcanonical statistical model predicts that prompt dissociations of C4H71-3 and C4H71-4 are already significant at 1000 K, resulting in differences of up to an order of magnitude at 2000 K with respect to the phenomenological thermal rate constants. To illustrate the effects of prompt dissociation on simulations of experimental data, the calculated prompt rate constants are implemented in both CRECK and C3MechV3.3 kinetic mechanisms. Simulations of experimental flame data illustrate the noticeable impact of prompt dissociation kinetics on the high-temperature combustion reactivity of C4H8-1 and C4H8-2.

Study of Multinucleon Transfer Mechanisms in 86Kr-Induced Peripheral Reactions at 15 and 25 MeV/nucleon

This paper presents our recent studies of the multinucleon transfer in peripheral collisions in reactions near the Fermi regime. Specifically, the reactions of a 86Kr beam at 15 MeV/nucleon with targets of 124Sn ,112Sn, 64Ni and 58Ni and reactions of a 86Kr beam at 25 MeV/nucleon with targets of 124Sn and 112Sn. The experimental data were obtained from the previous work of our group with the MARS spectrometer at the Cyclotron Institute of Texas A&M University. Our current focus is the thorough study of the experimental mass and momentum distributions of the projectile-like fragments. The momentum distributions are characterised by a narrow quasi-elastic peak and a broader deep-inelastic peak. We employed two-body kinematics to characterize the excitation energies of these reactions. We also compared the data with model calculations. The dynamical stage is described with either the Deep-Inelastic Transfer Model (DIT), or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot projectile-like fragments is performed with the Gemini model. Studying these reactions will provide us with a better understanding as to how the energy of the beam and the different targets affect the mechanism of the multinucleon transfer reactions. With the recent work, our continued efforts in the study of peripheral reactions in the Fermi energy regime delineate new opportunities to elucidate the reaction mechanism(s) of rare isotope production and may effectively contribute to the study of unexplored regions of the nuclear chart toward the r-process and the neutron drip line.

Constraints from isoscaling on the source size in energetic heavy ion collisions

In the framework of the statistical multifragmentation model, the nuclear isoscaling analysis is extended to constrain the ratio between the sizes of the decaying sources formed in a collision between two heavy ions. It is found that the ratio between the probabilities of observing $n$ fragments in each event, for each of the sources, follows a scaling law, similar to the traditional nuclear isoscaling. However, the corresponding slope is sensitive to the source sizes. This property is explained analytically using the grand-canonical ensemble. The extent to which our findings are affected by finite size effects and by the deexcitation of the hot primary fragments is investigated. The scaling turns out to be robust and weakly affected by effects implied by these two aspects. We also find that the Poisson distribution is a fairly good approximation to the above mentioned probabilities, associated with both the primordial fragments, produced at the breakup stage, and the final ones, found at the end of the fragment deexcitation process.

Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation.

While common molecular anions show a strong propensity to undergo electron detachment upon UV excitation, this process often occurs in competition with molecular ion dissociation. The factors that affect the balance between these two major possible decay pathways have not been well understood to date. Laser photodissociation spectroscopy of the deprotonated forms of the UV filter molecules, Homosalate (HS) and Octyl Salicylate (OS), i.e. [HS − H]− and [OS − H]−, was used to acquire gas-phase UV absorption spectra for [HS − H]− and [OS − H]−via photodepletion from 3.0–5.8 eV. No photofragmentation (i.e. dissociation of the ionic molecular framework) was observed for either [HS − H]− and [OS − H]− following photoexcitation, revealing that electron loss entirely dominates the electronic decay pathways for these systems. High-level quantum chemical calculations were used to map out the excited states associated with [HS − H]− and [OS − H]−, revealing that the minimum-energy crossing points (MECPs) between the S1 and S0 states are located in elevated regions of the potential energy surface, making internal conversion unlikely. These results are consistent with our experimental observation that electron detachment out-competes hot ground state molecular fragmentation. More generally, our results reveal that the competition between molecular dissociation and electron detachment following anion photoexcitation can be determined by the magnitude of the energy gap between the excitation energy and the MECPs, rather than being a simple function of whether the excitation energy lies above the anion's vertical detachment energy.

Improving in-memory file system reading performance by fine-grained user-space cache mechanisms

Nowadays, as the memory capacity of servers become larger and larger, distributed in-memory file systems, which enable applications to interact with data at fast speed, have been widely used. However, the existing distributed in-memory file systems still face the problem of low data access performance in small data reading, which seriously reduce their usefulness in many important big data scenarios. In this paper, we analyze the factors that affect the performance of reading in-memory files and propose a two-layer user space cache management mechanism: in the first layer, we cache data packet references to reduce frequent page fault interruptions (packet-level cache); in the second layer, we cache and manage small file data units to avoid redundant inter-process communications (object-level cache). We further design a fine-grained caching model based on the submodular function optimization theory, for efficiently managing the variable-length cache units with partially overlapping fragments on the client side. Experimental results on synthetic and real-world workloads show that compared with the existing cutting-edge systems, the first level cache can double the reading performance on average, and the second level cache can improve random reading performance by more than 4 times. Our caching strategies also outperform the cutting-edge cache algorithms over 20% on hit ratio. Furthermore, the proposed client-side caching framework idea has been adopted by the Alluxio open source community, which shows the practical benefits of this work. • A packet-level cache policy for reducing page faults during sequential data reading. • An in-memory cache layer to cache the variable-length hot file fragments. • Proved that the variable-length cache problem is a submodular optimization problem theoretically. • Comprehensive performance evaluation of the client-side cache strategies.

Nuclear temperature and its dependence on the source neutron-proton asymmetry deduced using the Albergo thermometer

Albergo thermometers with double isotope, isotone, and isobar yield ratio pairs with one proton and/or neutron difference are investigated. Without any extra sequential decay correction, a real temperature value of $4.9\ifmmode\pm\else\textpm\fi{}0.5$ MeV is deduced from the yields of the experimentally reconstructed primary hot intermediate mass fragments (IMFs) from $^{64}\mathrm{Zn}+^{112}\mathrm{Sn}$ collisions at 40 MeV/nucleon using the Albergo thermometer for the first time. An experimental sequential decay correction from the apparent temperatures to the real ones for 12 other reaction systems with different neutron-proton ($N/Z$) asymmetries in the same experiment, $^{70}\mathrm{Zn}$, $^{64}\mathrm{Ni}$ on $^{112,124}\mathrm{Sn}$, $^{58,64}\mathrm{Ni}$, $^{197}\mathrm{Au}$, and $^{232}\mathrm{Th}$ at 40 MeV/nucleon, is performed using an empirical correction factor approach of Tsang et al. [Phys. Rev. Lett. 78, 3836 (1997)] with the deduced 4.9-MeV temperature value. The dependence of nuclear temperature on the source $N/Z$ asymmetry is further investigated using these deduced real source temperature values from the present 13 systems. It is found that the deduced real source temperatures at the present source $N/Z$ range show a rather weak dependence on the source $N/Z$ asymmetry. By comparison between our previous results and those from other independent experiments, a consistent description for the $N/Z$ asymmetry dependence of nuclear temperature is addressed.

Identification of Hot Plasma Anomalies in Solar Wind Using Fe Ion Charge Distributions

Abstract A presence of high Fe charge states in the ionic charge state distributions of the solar wind (SW) plasma, commonly characterized by the mean charge Q Fe, provides valuable information on heating processes in the SW sources. We study the relationship between the parameter Q Fe and the charge state distributions of Fe ions using the Solar TErrestrial RElations Observatory/PLAsma and SupraThermal Ion Composition data on the beginning of the 24th Cycle (2010 January–2011 July). We find that the Fe charge state distributions related to SW with Q Fe ≈ 8–10 have an uni-modal shape peaked around Fe8+–Fe9+. When the Q Fe value increases, the distributions change: at first, the profile extends to higher charge states and then transforms into a bi-modal shape with a second maximum around Fe16+ and a minimum around Fe12+. We discuss possible reasons for such bi-modality through the example of the interplanetary coronal mass ejection (ICME) event on 2011 February 24–26, where it was related to the heating of an eruptive prominence. For such an analysis, it is informative to have a special measure of the fraction of highly charged ions for the Fe ion charge distribution in SW. In addition to Q Fe, we introduce a parameter q12 equal to a fraction of Fe ions with charges Q ≥ 12 and show that this parameter can be applied for identifying both the large-scale hot plasma enhancements associated with ICMEs and small hot fragments of SW plasma, which may be associated with small-scale solar activity in various coronal structures.

Vibrational predissociation in the bending levels of the à state of C3Ar.

Vibrational predissociation (VP) has been observed in 16 bands of the C3Ar van der Waals complex near the 0 v2 0 - 000 (v2 = 2-, 4-, 2+) and 0 2- 2 - 100 bands of the Ã1Π-X̃1Σ+ g system of C3. New higher resolution wavelength-resolved emission (WRE) spectra covering a wider spectral range have been recorded for many of these C3Ar bands, which show that most of the features observed in fluorescence must be reassigned as emission from the C3 fragment. Two types of VP processes have been recognized. The first type gives rise to vibrationally hot C3 fragments, mostly following |Δv| = 1, |ΔP| = 1 propensity rules, where P is the vibronic angular momentum of C3. The second type gives vibrationally cooled fragments. The VP processes can change abruptly from one type to the other with comparatively small differences in vibrational energy. Although the initial states are associated with both orbital components of the C3, Ã1Πu state, most of the VP fragments belong to the lower orbital component. A dipole-induced dipole model has been used to interpret the observed ΔP- propensities. Ab initio calculations of the binding energies of the ground and excited electronic states of C3Ar have been carried out; the calculated values are consistent with estimates of ≤144 cm-1 and 164 cm-1, respectively, given by the WRE spectra.

Evidence for early fragmentation-reassembly of ordinary chondrite (H, L, and LL) parent bodies from REE-in-two-pyroxene thermometry

Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3–6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with cooling rate in the parent body. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (∼500–200 °C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Köhler [1990]) and Ca-in-olivine thermometry (TCa-Ol; Köhler and Brey [1990]), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected within an onion shell structure, we obtain fast cooling at rates ≳0.3 °C/y from peak temperatures of ∼900 °C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (∼700 to 800 °C) at ∼10−2 to 10−1 °C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (≤500 °C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.

0190 Impact of Menopause-Related Sleep Fragmentation on Daytime Sleepiness and Neurobehavioral Performance: Results of an Experimental Model

Abstract Introduction Cognitive performance may be adversely affected during the menopause transition from hot flash-induced sleep fragmentation even without changes in sleep duration. We examined the effects of experimentally-induced sleep fragmentation without shortened sleep duration on daytime sleepiness and neurobehavioral performance in women in a high and low estradiol (E2) state. Methods Seven pre-menopausal women (29.4 ± 3.8 years) participated in two 6-day inpatient studies repeated in a high-E2 (mid-to-late follicular phase) then low-E2 state (gonadotropin-releasing hormone agonist-induced E2 suppression - similar to levels during menopause) ~6 weeks apart. Sleep was uninterrupted on nights 1–2 [8-h time-in-bed (TIB)] and fragmented on nights 3–5 (9-h TIB) using an auditory stimulus delivered every 15 min that sustained wake for 2 minutes, producing 1-h total wake after sleep onset. Wakefulness was confirmed by event-markers during polysomnographically-recorded sleep episodes. Daytime subjective sleepiness (Karolinska Sleepiness Scale; KSS) and neurobehavioral performance (Psychomotor Vigilance Task; PVT) were assessed every 2–3 hours on study days 2–5. The effects of study day and E2 state on KSS scores and PVT measured reaction time (RT) and attentional failures (RT>500ms) were examined using linear mixed models. Results Participants reported feeling sleepier (+10%), had longer RTs (+22ms), and more attentional failures (+53%) after sleep fragmentation than after uninterrupted sleep (all p<0.001). While there was no main effect of E2 state, there was a differential effect of sleep fragmentation by E2 state on PVT, but not sleepiness, such that the increase in RT and attentional failures in response to sleep fragmentation was only observed in the high-E2 state (p<0.001). Conclusion Eight hours of total sleep time may not be sufficient to maintain subjective sleepiness and PVT performance levels when sleep is not consolidated. These findings have important implications for understanding the role of sleep and E2-modulated cognitive impairment during the menopause transition. Support This work was supported by the NIH: 5R01 AG053838-02 (HJ) and K24-HL105664 (EBK).

Probing the deuteron breakup and linking the cross sections of residue production between the neutron- and deuteron-induced spallation at 500 MeV/nucleon

Background: Cross section data of neutron-induced spallation are requisite in order to study the transmutation of the long-lived fission products from the spallation. However, those data are very scarce at present due to the difficulty of providing a neutron source in the GeV region and the impossibility of the inverse dynamics.Purpose: The present work is an attempt to probe the dynamic process of deuteron breakup in deuteron-induced spallation and discuss the possibility of measuring indirectly the cross section in neutron-induced spallation.Method: The isospin-dependent quantum molecular dynamics model is applied to simulate the spallation process until the excitation energy of the hot fragment is less than 2 MeV/nucleon. The statistical model gemini based on the Hauser-Feshbach formalism is used to describe the deexcitation of the hot fragments.Results: By comparing the calculations to the data, the theoretical framework is proved to be reasonable when predicting the neutron production in spallation. By investigating the deuteron breakup, deuteron-induced spallation is divided into deuteron absorbing, neutron stripping, proton stripping, and elastic breakup. It is found that the proton stripping plays the dominant role in the production of high-energy neutrons, while the neutron stripping is responsible for the emission of high-energy protons. Due to the weak binding of the deuteron, it is suggested that the combination of neutron stripping and elastic breakup in deuteron-induced spallation is equivalent to neutron-induced spallation.Conclusions: Using the proton recoiling from the deuteron as the trigger signal, deuteron-induced spallation may be applied to measure indirectly the cross section in neutron-induced spallation.

Mesoscale modeling of hypervelocity impacts using the CTH shock physics code

Material fragmentation after a hypervelocity impact is important to predictive electro-optical and infrared (EO/IR) modeling. Successful comparisons with data require that hot, submicron fragments are generated in such impacts; however, experimental data has so far been unable to produce fragments of this scale. The purpose of this work was to investigate how modeling assumptions of macro-scale, bulk materials might influence the generation of debris in hypervelocity impacts and ultimately the predicted EO/IR signatures of these debris clouds. Sphere-on-plate impact simulations simplified the comparison of different modeling approaches. In one set of simulations, materials were modeled with the traditional, bulk approach. Those results were compared to simulations run with the mesoscale material grain structure explicitly modeled. This study focused on the comparison of two parameters that are tied directly to the EO/IR signature: strain rate at failure (a proxy for debris fragment size) and material temperature. Interfaces between grains, here due to void insertion, resulted in the most notable change in both the strain rate at failure and material temperature. Shock reflections from grain-void interfaces induced higher strain rates and material temperatures, and it is expected that similar effects may be produced from inclusions or dislocations in real materials. Thus, interfaces within a material may play an important role in producing smaller hot debris fragments that support the EO/IR predictive models of hypervelocity impacts.

Post breakup dynamical evolution of fragments produced in nuclear multifragmentation

The deexcitation of the primary hot fragments, produced in the breakup of an excited nuclear source, during their propagation under the influence of their mutual Coulomb repulsion is studied in the framework of a recently developed hybrid model. The latter is based on the Statistical Multifragmentation Model (SMM), describing the prompt breakup of the source, whereas the particle emission from the hot fragments, that decay while traveling away from each other, is treated by the Weisskopf-Ewing evaporation model. Since this treatment provides an event by event description of the process, in which the classical trajectories of the fragments are followed using molecular dynamics techniques, it allows one to study observables such as two-particle correlations and infer the extent to which the corresponding observables may provide information on the multifragment production mechanisms. Our results suggest that the framework on which these treatments are based may be considerably constrained by such analyses. They imply that information obtained from these model calculations may provide feedback to the theory of nuclear interferometry. In particular, our results do not support the hypothesis that the total momentum of the selected pair of particles used in the two-particle correlations remains constant during the post-breakup dynamics. Furthermore, the correlations may be appreciably affected by this fact. We also found that neutron deficient fragments should hold information more closely related to the breakup region than neutron rich ones, as they are produced in much earlier stages of the post breakup dynamics than the latter.

Effect of liquid drop model parameters on nuclear liquid-gas phase transition

The phenomenon of liquid-gas phase transition occurring in heavy ion collisions at intermediate energies is a subject of contemporary interest. In statistical models of fragmentation, the liquid drop model is generally used to calculate the ground state binding energies of the fragments. It is well known that the surface and symmetry energy of the hot fragments at the low density freeze out can be considerably modified. In addition to this, the level density parameter also has a wide variation. The effect of variation of these parameters is studied on fragmentation observables which are related to the nuclear liquid gas phase transition. The canonical thermodynamical model which has been very successful in describing the phenomenon of fragmentation is used for the study. The shift in transition temperature owing to the variation in liquid drop model parameters has been examined.

Rare Isotope Production in peripheral heavy-ion collisions at beam energy of 15 MeV/nucleon

This paper presents our recent study on the production of neutron-rich rare isotopes with heavy-ion beams in the energy region of 15 MeV/nucleon. We present calculated production cross sections of neutron-rich nuclides from collisions of a 86Kr (15 MeV/nucleon) beam with 238U targets. Our calculations are based on a two-step approach: the dynamical stage of the collision is described with either the phenomenological Deep-Inelastic Transfer model (DIT), or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot heavy projectile fragments is performed with the Statistical Multifragmentation Model (SMM). We also performed calculations with a radioactive beam of 92Kr (15 MeV/nucleon) with a target of 238U and observed that the multinucleon transfer mechanism leads to very neutron-rich nuclides toward and beyond the astrophysical r-process path. In the future, we plan to experimentally investigate such reactions in the KOBRA spectrometer at the RISP facility in Korea. We conclude that the reaction mechanism at beam energies below the Fermi energy involving periph- eral nucleon exchange, constitutes a novel and effective route to access extremely neutron-rich isotopes toward the r-process path and the neutron drip-line.