Critical Reaction Rate Research Articles

Understanding globular cluster abundances through nuclear reactions

Globular clusters contain multiple stellar populations, with some previous generation of stars polluting the current stars with heavier elements. Understanding the history of globular clusters is helpful in understanding how galaxies merged and evolved and therefore constraining the site or sites of this historic pollution is a priority. The acceptable temperature and density conditions of these polluting sites depend on critical reaction rates. In this paper, three experimental studies helping to constrain astrophysically important reaction rates are briefly discussed.

Experimental study of theSi30(He3,d)P31reaction and thermonuclear reaction rate ofSi30(p,γ)P31

[Background] Abundance anomalies in some globular clusters, such as the enhancement of potassium and the depletion of magnesium, can be explained in terms of an earlier generation of stars polluting the presently observed ones. It was shown that the potential range of temperatures and densities of the polluting sites depends on the strength of a few number of critical reaction rates. The reaction has been identified as one of these important reactions. [Purpose] The key ingredient for evaluating the thermonuclear reaction rate is the strength of the resonances which, at low energy, are proportional to their proton width. Therefore the goal of this work is to determine the proton widths of unbound 31P states. [Method] States in 31P were studied at the Maier-Leibnitz-Laboratorium using the one-proton transfer reaction. Deuterons were detected with the Q3D magnetic spectrometer. Angular distribution and spectroscopic factors were extracted for 27 states, and proton widths and resonance strengths were calculated for the unbound states. [Results] Several unbound states have been observed for the first time in a one-proton transfer reaction. Above 20 MK, the reaction rate is now entirely estimated from the observed properties of states. The reaction rate uncertainty from all resonances other than the resonance has been reduced down to less than a factor of two above that temperature. The unknown spin and parity of the resonance dominates the uncertainty in the rate in the relevant temperature range. [Conclusion] The remaining source of uncertainty on the reaction rate comes from the unknown spin and parity of the resonance which can change the reaction rate by a factor of ten in the temperature range of interest.

Feature engineering using homogenization theory with multiscale perturbation analysis for supervised model-based learning of physical clogging condition in seepage filters

Aquifer recharge and recovery systems (ARRS), which can broadly be analysed as seepage depth filters, in natural or engineered aquifers are gaining attention worldwide. Engineering predictions of their complex physical clogging behavior, however, continue to be challenging which has hindered the predictive maintenance of these systems for energy and materials savings. To address this problem statement, we leverage the homogenization theory with the multiscale perturbation analysis as the feature engineering step to reduce the complexity of the physical clogging behavior in ARRS. The analytical approach systematically derives a unique homogenized representation which quantifies the clogging condition at the macroscale. A series of physical parameters are identified from the derived homogenized representation to build a pre-processed input layer into our own multi-layered neural network (NN) architecture for predictive analysis. Measured data extracted from the literature is then used to train and verify the NN model. The trained model yields an average error deviation of 20% between the model's predictions and the respective measurements for an optimized set of hyperparameters tested. We then discuss quantitatively how the model can be adhered to predict the timing for a concerned ARRS to reach its breakthrough stage for a range of operational conditions. Finally, we also demonstrate how the homogenized representation can be useful to determine an arbitrary filter's critical reaction rate and diffusion coefficient responsible for its breakthrough stage.

Chemical-Reaction-Controlled Phase Separated Drops: Formation, Size Selection, and Coarsening.

Phase separation under nonequilibrium conditions is exploited by biological cells to organize their cytoplasm but remains poorly understood as a physical phenomenon. Here, we study a ternary fluid model in which phase-separating molecules can be converted into soluble molecules, and vice versa, via chemical reactions. We elucidate using analytical and simulation methods how drop size, formation, and coarsening can be controlled by the chemical reaction rates, and categorize the qualitative behavior of the system into distinct regimes. Ostwald ripening arrest occurs above critical reaction rates, demonstrating that this transition belongs entirely to the nonequilibrium regime. Our model is a minimal representation of the cell cytoplasm.

The origin of gas-phase HCO and CH3O radicals in prestellar cores

The recent unexpected detection of terrestrial complex organic molecules in the cold (~ 10 K) gas has cast doubts on the commonly accepted formation mechanisms of these species. Standard gas-phase mechanisms are inefficient and tend to underproduce these molecules, and many of the key reactions involved are unconstrained. Grain-surface mechanisms, which were presented as a viable alternative, suffer from the fact that they rely on grain surface diffusion of heavy radicals, which is not possible thermally at very low temperatures. One of the simplest terrestrial complex organic molecules, methanol is believed to form on cold grain surfaces following from successive H atom additions on CO. Unlike heavier species, H atoms are very mobile on grain surfaces even at 10 K. Intermediate species involved in grain surface methanol formation by CO hydrogenation are the radicals HCO and CH3O, as well as the stable species formaldehyde H2CO. These radicals are thought to be precursors of complex organic molecules on grain surfaces. We present new observations of the HCO and CH3O radicals in a sample of prestellar cores and carry out an analysis of the abundances of the species HCO, H2CO, CH3O, and CH3OH, which represent the various stages of grain-surface hydrogenation of CO to CH3OH. The abundance ratios between the various intermediate species in the hydrogenation reaction of CO on grains are similar in all sources of our sample, HCO:H2CO:CH3O:CH3OH ~ 10:100:1:100. We argue that these ratios may not be representative of the primordial abundances on the grains but, rather, suggest that the radicals HCO and CH3O are gas-phase products of the precursors H2CO and CH3OH, respectively. Gas-phase pathways are considered and simple estimates of HCO and CH3O abundances are compared to the observations. Critical reaction rate constants, branching ratios, and intermediate species are finally identified.

TheHubble Space TelescopeUV Legacy Survey of Galactic Globular Clusters – V. Constraints on formation scenarios

We build on the evidence provided by our Legacy Survey of Galactic globular clusters (GC) to submit to a crucial test four scenarios currently entertained for the formation of multiple stellar generations in GCs. The observational constraints on multiple generations to be fulfilled are manifold, including GC specificity, ubiquity, variety, predominance, discreteness, supernova avoidance, p-capture processing, helium enrichment and mass budget. We argue that scenarios appealing to supermassive stars, fast rotating massive stars and massive interactive binaries violate in an irreparable fashion two or more among such constraints. Also the scenario appealing to AGB stars as producers of the material for next generation stars encounters severe difficulties, specifically concerning the mass budget problem and the detailed chemical composition of second generation stars. We qualitatively explore ways possibly allowing one to save the AGB scenario, specifically appealing to a possible revision of the cross section of a critical reaction rate destroying sodium, or alternatively by a more extensive exploration of the vast parameter space controlling the evolutionary behavior of AGB stellar models. Still, we cannot ensure success for these efforts and totally new scenarios may have to be invented to understand how GCs formed in the early Universe.

Evaluating free vs bound oxygen on ignition of nano-aluminum based energetics leads to a critical reaction rate criterion

This study investigates the ignition of nano-aluminum (n-Al) and n-Al based energetic materials (nanothermites) at varying O2 pressures (1–18 atm), aiming to differentiate the effects of free and bound oxygen on ignition and to assess if it is possible to identify a critical reaction condition for ignition independent of oxygen source. Ignition experiments were conducted by rapidly heating the samples on a fine Pt wire at a heating rate of ∼105 °C s−1 to determine the ignition time and temperature. The ignition temperature of n-Al was found to reduce as the O2 pressure increased, whereas the ignition temperatures of nanothermites (n-Al/Fe2O3, n-Al/Bi2O3, n-Al/K2SO4, and n-Al/K2S2O8) had different sensitivities to O2 pressure depending on the formulations. A phenomenological kinetic/transport model was evaluated to correlate the concentrations of oxygen both in condensed and gaseous phases, with the initiation rate of Al-O at ignition temperature. We found that a constant critical reaction rate (5 × 10−2 mol m−2 s−1) for ignition exists which is independent to ignition temperature, heating rate, and free vs bound oxygen. Since for both the thermite and the free O2 reaction the critical reaction rate for ignition is the same, the various ignition temperatures are simply reflecting the conditions when the critical reaction rate for thermal runaway is achieved.

化学反应速率对爆轰特性的影响研究—基于离散Boltzmann模型

基于我们课题组以前提出的高速可压LBGK模型[Gan, Xu, Zhang, Yang, EPL 103 (2013) 24003]，发展了一个适用于模拟爆轰问题的离散Boltzmann模型。化学反应率模型采用只保留增长项的Lee模型。基于新构建的模型，模拟了不同反应速率的爆轰情形，找到了一个临界反应速率。当反应速率等于临界反应速率时，模拟的结果与CJ理论值符合较好。当反应速率低于临界反应速率时，爆轰波的波结构中会出现von-Neumann峰，之后从峰值点过渡到稳态，稳态为CJ爆轰状态。当反应速率高于临界反应速率时，爆轰波会以高于CJ爆轰波速的速度向前传播，稳态为弱爆轰状态。 Based on the high-speed compressible model proposed in our group [Gan, Xu, Zhang, Yang, EPL 103 (2013) 24003], a new discrete Boltzmann model for detonation is presented. A new reaction rate function is adopted which comes from Lee’s model but only the growth term is used. Based on the new model, several kinds of detonations with different reaction rates are simulated and a critical reaction rate is found. In the case where the value of reaction rate equals to the critical value, the simulation results coincide well with CJ theory. In the cases where the reaction rates are lower than the critical value, the von-Neumann peak will appear firstly and then steady state is reached behind the detonation wave. The steady states in those cases are in the CJ detonation states. In the cases where reaction rates are higher than the critical rate, the detonation wave propagates at a speed faster than that of CJ detonation and the steady states in those cases are in the weak detonation states.

Biased random walks and propagation failure

The critical value of the reaction rate able to sustain the propagation of an invasive front is obtained for general non-Markovian biased random walks with reactions. From the Hamilton-Jacobi equation corresponding to the mean field equation we find that the critical reaction rate depends only on the mean waiting time and on the statistical properties of the jump length probability distribution function and is always underestimated by the diffusion approximation. If the reaction rate is larger than the jump frequency, invasion always succeeds, even in the case of maximal bias. Numerical simulations support our analytical predictions.

Modeling the effects of a Staphylococcal Enterotoxin B (SEB) on the apoptosis pathway

BackgroundThe lack of detailed understanding of the mechanism of action of many biowarfare agents poses an immediate challenge to biodefense efforts. Many potential bioweapons have been shown to affect the cellular pathways controlling apoptosis [1-4]. For example, pathogen-produced exotoxins such as Staphylococcal Enterotoxin B (SEB) and Anthrax Lethal Factor (LF) have been shown to disrupt the Fas-mediated apoptotic pathway [2,4]. To evaluate how these agents affect these pathways it is first necessary to understand the dynamics of a normally functioning apoptosis network. This can then serve as a baseline against which a pathogen perturbed system can be compared. Such comparisons can expose both the proteins most susceptible to alteration by the agent as well as the most critical reaction rates to better instill control on a biological network.ResultsWe explore this through the modeling and simulation of the Fas-mediated apoptotic pathway under normal and SEB influenced conditions. We stimulated human Jurkat cells with an anti-Fas antibody in the presence and absence of SEB and determined the relative levels of seven proteins involved in the core pathway at five time points following exposure. These levels were used to impute relative rate constants and build a quantitative model consisting of a series of ordinary differential equations (ODEs) that simulate the network under both normal and pathogen-influenced conditions. Experimental results show that cells exposed to SEB exhibit an increase in the rate of executioner caspase expression (and subsequently apoptosis) of 1 hour 43 minutes (± 14 minutes), as compared to cells undergoing normal cell death.ConclusionOur model accurately reflects these results and reveals intervention points that can be altered to restore SEB-influenced system dynamics back to levels within the range of normal conditions.

A Review of Recent Experiments and Calculations Relevant to the Kinetics of the HF Laser

A review of rate coefficients relevant to HF laser kinetics modeling is presented. The literature has been surveyed from the last published review in 1983 to the present. Updated HF Einstein emission coefficients are tabulated. Rate coefficients are categorized according to their role in the HF laser model: HF generation, reactive quenching, self-relaxation, V–V energy transfer, vibrational relaxation by atoms and molecules, F2 dissociation, and F atom recombination. In addition, a review of recent experiments and theoretical calculations relevant to the role of rotational nonequilibrium in HF lasers is presented. A list of recommended temperature dependent expressions for critical reaction rate coefficients is given.

Gas-phase modeling of homogeneous boron/oxygen/hydrogen/carbon combustion

Boron has practical applications as an advanced fuel in propulsion systems due to its high energy content. The combustion of boron in the presence of hydrocarbon fuels is a complex problem involving heterogeneous particle oxidation followed by gas-phase kinetics of the volatilized boron species. In this study, we have modeled the high-temperature gas-phase combustion chemistry of the B/O/H/C system. We have examined the effects of recent experimental gas-phase kinetic measurements of several of the critical reaction rates and theoretical thermodynamic and transition state calculations on the previous model of boron combustion. Additional reactions that critically affect the combustion efficiency are identified for future experimental and theoretical study. The role of boron oxyhydrides, which are metastable species, is discussed.

Kinetics of high-temperature B/O/H/C chemistry

Homogeneous gas phase boron-oxygen-hydrogen-carbon (B/O/H/C) combustion chemistry is studied to characterize general mechanistic behavior of high-temperature (1800 K < T < 3000 K) B/O/H/C reacting systems and to identify critical reaction rate constants for future experimental evaluation. The chemistry is numerically modeled with a reaction mechanism comprising 19 chemical species and 58 forward and reverse elementary steps. Reaction flux/pathway techniques and sensitivity analysis theory of isothermal systems are used to identify the important reactions of the mechanism. The results include the effects of mixture temperature, pressure, oxygen content, hydrogen content, and carbon content on the reaction dynamics of B, BO, BO 2, B 2O 2, B 2O 3, HBO, and HBO 2. For example, hydrogen addition is observed to accelerate both the oxidation of intermediates and the heat release rate, as well as to alter the dominant suboxides and reaction products. From the sensitivity analysis calculations, a group of 14 key boron-containing reactions are identified and suggested for future elementary reaction studies. To date, only one of these reactions has been studied experimentally. Lastly, several important conclusions are drawn about the combustion process of particulate boron. For example, the current homogeneous calculations have shown that the sensitivity of the species concentration profiles to the chosen initial species speciation (for a fixed number of moles of each element) is nearly independent of this speciation for reaction times greater than a few microseconds, suggesting that the identity of the species evolving from a reacting boron particle is not eritical to the surrounding gas-phase combustion process. Furthermore, as noted above, hydrogen containing species have a significant impact on accelerating the gas-phase combustion. However, larger quantities of hydrogen promote the formation of HBO 2, which is thermodynamically favored over gaseous B 2O 3 as the temperature is lowered. Consequently, as the combustion gases are cooled, gas-phase boron can be trapped as HBO 2.