Fast Bursts Research Articles

Experimental Study of Neutron Initiation Probability at CFBR-II with Pulsed Neutrons

Based on a new experimental method implemented for validating neutron initiation probability, a set of burst initiation probability experiments (128 bursts) that were initiated by simultaneously injecting pulsed neutrons just as the reactor achieves the prompt supercritical state of 0.042 $ has been carried out at the CFBR-II (Chinese Fast Burst Reactor–II). The experimental configuration and procedures remained the same throughout the entire set of experiments. Based on the measured data, each burst was tallied by judging whether or not the burst was initiated by the pulsed neutrons. With the injection of pulsed neutrons (the equivalent strength of the neutrons is 1230), the tallies of the burst initiated by pulsed neutrons were 44, and hence, the experimental result of initiation probability is 0.344, which is 27% more than the theoretical calculation result of 0.271. Some factors that influence the experimental results are discussed. The discrepancy is attributed mainly to neutrons that are scattered and returned from the environment during the injection of pulsed neutrons and the statistical deviation.

Catherine Pasquier Memorial Award Lecture (shared): The adventure of detecting subtle signals (such as protein nitrosothiols and superoxide) among cell traffic jams (from proteomics to hypoxia)

Like big human cities, cells are crowded environments with a lot of inhabitants, where a myriad of signals try to orchestrate how each component can do its best among the crowd, and how the whole system respond to changes in the environment. Some signals are robust and stable, so they can be easily studied by life science researchers. However, free radical researchers know well that other molecular signals are short-lived and unstable. I have been involved in several studies that can be seen as adventures searching for those subtle signals, overcoming the difficulties that appear in the way. One is the reliable proteomic identification of protein S-nitrosylation (or S-nitrosation), and of other reversible oxidations of cysteines, in physiological scenarios where they can act as signals (not just in nitrosative, oxidative or similar stresses). Different techniques have been used for that purpose (called “thiol redox proteomics”), with technical developments still ongoing. A related adventure is the study of the functional relevance of these modifications in individual proteins, and of the particularities of these modes of signalling. Another adventure deals with the role of reactive oxygen species production in hypoxia sensing and adaptation, on which a debate has been maintained for a long time. We have measured superoxide production in acute hypoxia, showing there is a fast burst at the first minutes of acute hypoxia, and we are currently studying its molecular mechanism, implying mitochondrial complex I and the sodium/calcium exchanger. This can be a common signal to different cell types that produces diverse effects depending on the signalling cascades and effectors of each cell type. Thiol redox proteomics can help in identifying the specific protein cysteine residues that can be affected by reversible oxidation and take part in those signals.

Test simulation of neutron damage to electronic components using accelerator facilities

The purpose of this work is to demonstrate equivalent bipolar transistor damage response to neutrons and silicon ions. We report on irradiation tests performed at the White Sands Missile Range Fast Burst Reactor, the Sandia National Laboratories (SNL) Annular Core Research Reactor, the SNL SPHINX accelerator, and the SNL Ion Beam Laboratory using commercial silicon npn bipolar junction transistors (BJTs) and III–V Npn heterojunction bipolar transistors (HBTs). Late time and early time gain metrics as well as defect spectra measurements are reported.

Electrosprayed poly(butylene succinate) microspheres loaded with indole derivatives: A system with anticancer activity

Electrospraying of poly(butylene succinate) and its mixture with different indole derivatives was successfully performed using chloroform as solvent and relatively low flow rates and concentrations. Morphology of particles (size, diameter distribution and surface texture) and encapsulation efficiency were dependent on the loaded drug and specifically on the type of substituent (methyl or phenyl) and its position in the indole core. In general, particles showed a raisin-like morphology caused by the shell collapsing of the resulting structurally weak microspheres. Accumulation of electrosprayed particles gave rise to consistent mats and they had a more hydrophobic surface than that determined for smooth films. The increase of hydrophobicity was mainly dependent on the porosity and the hydrophobic nature of the incorporated drugs. Indole derivatives were hardly delivered in a standard phosphate saline buffer due to their scarce solubility in aqueous media but the addition of ethanol caused a drastic change in the release behavior. This was generally characterized by a fast burst effect and followed by the establishment of an equilibrium condition that was dependent on the indole derivative. However, a clearly different behavior was found when the indole was unable to form hydrogen bonds (e.g. 1-methylindole) since in this case a slow and sustained release was characteristic. Microspheres loaded with indole derivatives showed a high antiproliferative activity that was dependent on encapsulation efficiency and the type of loaded drug. The best results were specifically attained for the indole with an aromatic substituent. Interestingly significant differences were found between cancer and immortalized cells, a feature that points out the potential use of such systems for cancer prevention and treatment.

Solar-wind control of plasma sheet dynamics

Abstract. The purpose of this study is to quantify how solar-wind conditions affect the energy and plasma transport in the geomagnetic tail and its large-scale configuration. To identify the role of various effects, the magnetospheric data were sorted according to different solar-wind plasma and interplanetary magnetic field (IMF) parameters: speed, dynamic pressure, IMF north–south component, epsilon parameter, Auroral Electrojet (AE) index and IMF ultra low-frequency (ULF) fluctuation power. We study variations in the average flow speed pattern and the occurrence rate of fast flow bursts in the magnetotail during different solar-wind conditions using magnetospheric data from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission spacecraft and solar-wind data from NASA's OMNIWeb. The time interval covers the years from 2008 to 2011 during the deep solar minimum between cycles 23 and 24 and the relatively quiet rising phase of cycle 24. Hence, we investigate magnetospheric processes and solar-wind–magnetospheric coupling during a relatively quiet state of the magnetosphere. We show that the occurrence rate of the fast (|Vtail| > 100 km s−1) sunward flows varies under different solar-wind conditions more than the occurrence of the fast tailward flows. The occurrence frequency of the fast tailward flows does not change much with the solar-wind conditions. We also note that the sign of the IMF BZ has the most visible effect on the occurrence rate and pattern of the fast sunward flows. High-speed flow bursts are more common during the slow than fast solar-wind conditions.

Kinetic analysis of bypass of abasic site by the catalytic core of yeast DNA polymerase eta

Abasic sites (Apurinic/apyrimidinic (AP) sites), produced ∼50,000 times/cell/day, are very blocking and miscoding. To better understand miscoding mechanisms of abasic site for yeast DNA polymerase η, pre-steady-state nucleotide incorporation and LC–MS/MS sequence analysis of extension product were studied using pol ηcore (catalytic core, residues 1–513), which can completely eliminate the potential effects of the C-terminal C2H2 motif of pol η on dNTP incorporation. The extension beyond the abasic site was very inefficient. Compared with incorporation of dCTP opposite G, the incorporation efficiencies opposite abasic site were greatly reduced according to the order of dGTP>dATP>>dCTP and dTTP. Pol ηcore showed no fast burst phase for any incorporation opposite G or abasic site, suggesting that the catalytic step is not faster than the dissociation of polymerase from DNA. LC–MS/MS sequence analysis of extension products showed that 53% products were dGTP misincorporation, 33% were dATP and 14% were -1 frameshift, indicating that Pol ηcore bypasses abasic site by a combined G-rule, A-rule and -1 frameshift deletions. Compared with full-length pol η, pol ηcore relatively reduced the efficiency of incorporation of dCTP opposite G, increased the efficiencies of dNTP incorporation opposite abasic site and the exclusive incorporation of dGTP opposite abasic site, but inhibited the extension beyond abasic site, and increased the priority in extension of A: abasic site relative to G: abasic site. This study provides further understanding in the mutation mechanism of abasic sites for yeast DNA polymerase η.

Relaxed predation results in reduced phenotypic integration in a suite of dragonflies.

Although changes in magnitude of single traits responding to selective agents have been studied intensively, little is known about selection shaping networks of traits and their patterns of covariation. However, this is central for our understanding of phenotypic evolution as traits are embedded in a multivariate environment with selection affecting a multitude of traits simultaneously rather than individually. Here, we investigate inter- and intraspecific patterns of trait integration (trait correlations) in the larval abdomen of dragonflies as a response to a change in predator selection. Species of the dragonfly genus Leucorrhinia underwent a larval habitat shift from predatory fish to predatory dragonfly-dominated lakes with an associated relaxation in selection pressure from fish predation. Our results indicate that the habitat-shift-induced relaxed selection pressure caused phenotypic integration of abdominal traits to be reduced. Intraspecific findings matched patterns comparing species from both habitats with higher abdominal integration in response to predatory fish. This higher integration is probably a result of faster burst swimming speed. The abdomen holds the necessary morphological machinery to successfully evade predatory fish via burst swimming. Hence, abdominal traits have to function in a tight coordinated manner, as maladaptive variation and consequently nonoptimal burst swimming would cause increased mortality. In predatory dragonfly-dominated lakes, no such strong link between burst swimming and mortality is present. Our findings highlight the importance of studying multivariate trait relationships as a response to selection for understanding patterns of phenotypic diversification.

Computational modeling of seizure dynamics using coupled neuronal networks: factors shaping epileptiform activity.

Epileptic seizure dynamics span multiple scales in space and time. Understanding seizure mechanisms requires identifying the relations between seizure components within and across these scales, together with the analysis of their dynamical repertoire. Mathematical models have been developed to reproduce seizure dynamics across scales ranging from the single neuron to the neural population. In this study, we develop a network model of spiking neurons and systematically investigate the conditions, under which the network displays the emergent dynamic behaviors known from the Epileptor, which is a well-investigated abstract model of epileptic neural activity. This approach allows us to study the biophysical parameters and variables leading to epileptiform discharges at cellular and network levels. Our network model is composed of two neuronal populations, characterized by fast excitatory bursting neurons and regular spiking inhibitory neurons, embedded in a common extracellular environment represented by a slow variable. By systematically analyzing the parameter landscape offered by the simulation framework, we reproduce typical sequences of neural activity observed during status epilepticus. We find that exogenous fluctuations from extracellular environment and electro-tonic couplings play a major role in the progression of the seizure, which supports previous studies and further validates our model. We also investigate the influence of chemical synaptic coupling in the generation of spontaneous seizure-like events. Our results argue towards a temporal shift of typical spike waves with fast discharges as synaptic strengths are varied. We demonstrate that spike waves, including interictal spikes, are generated primarily by inhibitory neurons, whereas fast discharges during the wave part are due to excitatory neurons. Simulated traces are compared with in vivo experimental data from rodents at different stages of the disorder. We draw the conclusion that slow variations of global excitability, due to exogenous fluctuations from extracellular environment, and gap junction communication push the system into paroxysmal regimes. We discuss potential mechanisms underlying such machinery and the relevance of our approach, supporting previous detailed modeling studies and reflecting on the limitations of our methodology.

Development of NIR-II fluorescence image-guided and pH-responsive nanocapsules for cocktail drug delivery

Nanocapsule-based targeted delivery and stimulus-responsive release can increase drug effectiveness, while reducing the side effects of the drug. However, difficulties in the scale-up synthesis, fast burst release, and low degradability, could hamper the translation of drug nanocapsules from lab to clinic. Here we have controllably functionalized the biodegradable and widely available polysuccinimide, in order to obtain an amphiphilic poly(amino acid). Using this polymer, we designed nanocapsules (< 100 nm) for hydrophobic drug delivery, which could facilitate tumor targeting, hydrogen bond-based pH-responsive release, and real-time fluorescence tracking, in the second near-infrared region. This method is versatile, eco-friendly, and easy to scale up at low costs. In addition, this system can carry a cocktail of drugs, obtained by loading multiple anticancer drugs to the same vehicle. Our nanocapsules were observed to be stable in blood vessels (pH = 7.4), and the pH-responsive release (pH = 5.0 in lysosome) was sustained. The chemotherapy results in tumor-xenografted mice suggested that our nanocapsule was safe and efficient, and may be a useful tool for drug delivery.

Live cell imaging of duplex siRNA intracellular trafficking.

Intracellular distribution of siRNA after in vitro transfection typically depends on lipopolyplexes, which must release the siRNA into the cytosol. Here, the fate of siRNAs was monitored by FRET-based live cell imaging. Subsequent to in situ observation of uptake and release processes, this approach allowed the observation of a number of hitherto uncharacterized intracellular distribution and degradation processes, commencing with a burst of endosomal releases, followed, in some cases, by fast siRNA influx into the nucleus. The continued observation of intact siRNA against a background of free fluorophores resulting from advanced degradation was possible by a specifically developed imaging algorithm, which identified populations of intact siRNA in pixels based on FRET. This proved to be essential in the end point definition of siRNA distribution, which typically featured partially degraded siRNA pools in perinuclear structures. Our results depict the initial 4 h as a critical time window, characterized by fast initial burst release into the cytosol, which lay the foundations for subsequent intracellular distribution of siRNA. Combination with a subsequent slower, but sustained release from endosomal reservoirs may contribute to the efficiency and duration of RNAi, and explain the success of lipopolyplexes in RNAi experiments in cell culture.

A Novel Experimental Method for Validating Neutron Initiation Probability

A novel experimental method is introduced for effectively validating neutron initiation probability, through which the delayed neutron influence on the source strength can be neglected—this is the main reason for substantially explaining the burst waiting time experiments performed in the Godiva and Caliban burst reactors. The key idea of the new method is that the burst is initiated by simultaneously injecting a pulse of neutrons with appropriate strength just as the reactor achieves prompt supercritical and tallied by judging whether or not the burst is initiated by the pulsed neutrons based on the measured data. The principle of the method is described using initiation theory. The Chinese Fast Burst Reactor–II (CFBR-II) structure and two sets of configurations for preliminary experiments are then described. Last, those two sets of preliminary experiments are carried out on CFBR-II in the state of 0.042 $ prompt supercritical, and results, including the typical picture and other important measured data, are provided in order to illustrate how the initiation probability can be validated. The initiation probability of 0.43 was determined by preliminary performing of 65 bursts, which is an ˜35% relative difference between the theoretic calculations and the experimental results. Some discussion and suggestions for possible follow-on work are provided.

Functional properties of GABA synaptic inputs onto GABA neurons in monkey prefrontal cortex.

In rodent cortex GABAA receptor (GABAAR)-mediated synapses are a significant source of input onto GABA neurons, and the properties of these inputs vary among GABA neuron subtypes that differ in molecular markers and firing patterns. Some features of cortical interneurons are different between rodents and primates, but it is not known whether inhibition of GABA neurons is prominent in the primate cortex and, if so, whether these inputs show heterogeneity across GABA neuron subtypes. We thus studied GABAAR-mediated miniature synaptic events in GABAergic interneurons in layer 3 of monkey dorsolateral prefrontal cortex (DLPFC). Interneurons were identified on the basis of their firing pattern as fast spiking (FS), regular spiking (RS), burst spiking (BS), or irregular spiking (IS). Miniature synaptic events were common in all of the recorded interneurons, and the frequency of these events was highest in FS neurons. The amplitude and kinetics of miniature inhibitory postsynaptic potentials (mIPSPs) also differed between DLPFC interneuron subtypes in a manner correlated with their input resistance and membrane time constant. FS neurons had the fastest mIPSP decay times and the strongest effects of the GABAAR modulator zolpidem, suggesting that the distinctive properties of inhibitory synaptic inputs onto FS cells are in part conferred by GABAARs containing α1 subunits. Moreover, mIPSCs differed between FS and RS interneurons in a manner consistent with the mIPSP findings. These results show that in the monkey DLPFC GABAAR-mediated synaptic inputs are prominent in layer 3 interneurons and may differentially regulate the activity of different interneuron subtypes.

Putting the brakes on the magnetosphere's bursty bulk flows

In Earth's magnetosphere, fast bursts of plasma have been observed with speeds varying between 200 and 1500 kilometers per second at distances between 20 Earth radii and 6 Earth radii from Earth. These bursty flows have been observed for decades, but it has not been clear what makes the flows come to a stop.

Performance of a 4H-SiC Schottky diode as a compact sized detector for neutron pulse form measurements

4H-silicon carbide (4H-SiC) detectors are desirable for neutron pulse form measurement for their compact size, excellent radiation resistance and hydrogen free composition. The aim of this study is to investigate the use of a 4H-SiC detector to measure the pulse form of a neutron burst. A 4H-SiC detector is fabricated and tested in the pulsed neutron field of the Chinese Fast Burst Reactor II (CFBR II). Important parameters such as the breeding period and the FWHM of the neutron pulse are derived from the experimental result of the 4H-SiC detector. These parameters agree well with those from a plastic scintillator detector. The divergences are only 0.5%, demonstrating that the 4H-SiC detector can yield a high fidelity time profile of the CFBR II pulse. The difference in peak centroid of alpha spectra is negligible for the 4H-SiC detector even after 18 reactor pulses (a neutron fluence of 8.41×1012cm−2), confirming the excellent radiation hardness of the 4H-SiC detector in pulsed neutron field. This study therefore indicates that 4H-SiC detectors can be usable as a compact sized detector to measure neutron pulses.

Use of natural neural scaffolds consisting of engineered vascular endothelial growth factor immobilized on ordered collagen fibers filled in a collagen tube for peripheral nerve regeneration in rats.

The search for effective strategies for peripheral nerve regeneration has attracted much attention in recent years. In this study, ordered collagen fibers were used as intraluminal fibers after nerve injury in rats. Vascular endothelial growth factor (VEGF) plays an important role in nerve regeneration, but its very fast initial burst of activity within a short time has largely limited its clinical use. For the stable binding of VEGF to ordered collagen fibers, we fused a collagen-binding domain (CBD) to VEGF through recombinant DNA technology. Then, we filled the ordered collagen fibers-CBD-VEGF targeting delivery system in a collagen tube to construct natural neural scaffolds, which were then used to bridge transected nerve stumps in a rat sciatic nerve transection model. After transplantation, the natural neural scaffolds showed minimal foreign body reactions and good integration into the host tissue. Oriented collagen fibers in the collagen tube could guide regenerating axons in an oriented manner to the distal, degenerating nerve segment, maximizing the chance of target reinnervation. Functional and histological analyses indicated that the recovery of nerve function in the natural neural scaffolds-treated group was superior to the other grafted groups. The guiding of oriented axonal regeneration and effective delivery systems surmounting the otherwise rapid and short-lived diffusion of growth factors in body fluids are two important strategies in promoting peripheral nerve regeneration. The natural neural scaffolds described take advantage of these two aspects and may produce synergistic effects. These properties qualified the artificial nerve conduits as a putative candidate system for the fabrication of peripheral nerve reconstruction devices.

Fast and slow frequency-drifting millisecond bursts in Jovian decametric radio emissions

We present an analysis of several Jovian Io-related decametric radio storms recorded in 2004−2012 at the Ukrainian array UTR-2 using the new generation of baseband digital receivers. Continuous baseband sampling within sessions lasting for several hours enabled us to study the evolution of multiscale spectral patterns during the whole storm at varying time and frequency resolutions and trace the temporal transformation of burst structures in unprecedented detail. In addition to the well-known frequency drifting millisecond patterns known as S bursts we detected two other classes of events that often look like S bursts at low resolution but reveal a more complicated structure in high resolution dynamic spectra. The emissions of the first type (LS bursts, superposition of L and S type emissions) have a much lower frequency drift rate than the usual quasi linearly drifting S bursts (QS) and often occur within a frequency band where L emission is simultaneously present, suggesting that both LS and at least part of L emissions may come from the same source. The bursts of the second type (modulated S bursts called MS) are formed by a wideband frequency-modulated envelope that can mimic S bursts with very steep negative (or even positive) drift rates. Observed with insufficient time-frequency resolution, MS look like S bursts with complex shapes and varying drifts; MS patterns often occur in association with (i) narrowband emission; (ii) S burst trains; or (iii) sequences of fast drift shadow events. We propose a phenomenological description for various types of S emissions, that should include at least three components: high- and low-frequency limitation of the overall frequency band of the emission, fast frequency modulation of emission structures within this band, and emergence of elementary S burst substructures, that we call “forking” structures. All together, these three components can produce most of the observed spectral structures, including S bursts with apparently very complex time-frequency structures.

CONVECTIVE OVERSTABILITY IN ACCRETION DISKS: THREE-DIMENSIONAL LINEAR ANALYSIS AND NONLINEAR SATURATION

Recently, Klahr & Hubbard (2014) claimed that a hydrodynamical linear overstability exists in protoplanetary disks, powered by buoyancy in the presence of thermal relaxation. We analyse this claim, confirming it through rigorous compressible linear analysis. We model the system numerically, reproducing the linear growth rate for all cases studied. We also study the saturated properties of the overstability in the shearing box, finding that the saturated state produces finite amplitude fluctuations strong enough to trigger the subcritical baroclinic instability. Saturation leads to a fast burst of enstrophy in the box, and a large-scale vortex develops in the course of the next $\approx$100 orbits. The amount of angular momentum transport achieved is of the order of $\alpha \approx 10^{-3}$, as in compressible SBI models. For the first time, a self-sustained 3D vortex is produced from linear amplitude perturbation of a quiescent base state.

Initiation of Persistent Fission Chains in the Fast Burst Reactor Caliban

We provide experimental data on the initiation of persistent fission chains obtained at different supercritical states, using the fast burst reactor Caliban. In many previous papers, theory has been compared mostly with initiation experiments at various superprompt critical states, whereas very few experimental data have been published on delayed supercritical states. To fill the lack of data, we have conducted three studies on the reactor at reactivities far below 0.7 $, which is one of the lowest states ever published for a similar assembly. We give a justification of the use of the gamma function to fit experimental results for the temporal distributions of waiting times and compare experiments with numerical simulations obtained with a punctual zero-dimensional Monte Carlo code and a punctual deterministic initiation code.

Tensile and Impact Behavior and Fracture Characteristics of FGH96 Powder Metallurgy Superalloy

Powder metallurgy (P/M) superalloy has not only been the primary material of turbine disk but also the main material of packing disk, packing ring and baffle plate of the aero-engine. Among these components, some parts are very thin, for example, the thinnest of baffle plate is only 2mm. The thin plate component was subjected to complex load, and the failure mode was synthetical. Some parts of component are subjected to impact load or impact-fatigue load although the nominal load of this component is fatigue load, more than one baffle plates have cracked through fast expansion and burst multiple debris because of the different local load type for the same component. It is very useful to investigate the effect on tensile and impact properties by specimen shape and shot peening. In this article, the different tests were carried out, including different specimen shape, temperature and surfaces. The results indicated that fracture strength σb and elongation δ5 were not affected by shape of specimen of P/M superalloy. Rod or plate specimens may be used to characterize the static properties of material. Shot peening would decrease the elongation,δ5 (at room temperature and 650°C) and impact work, but it did not reduce the fracture strength σb. In order to utilize adequately the component subjected to complicated loads, the surface state of the component may be treated distinctively according to the specific local load.

Effect of the fissile bead's and thermocouple wires' sizes on the response time of a fission couple.

The fission couple is proposed as a fast response miniature neutron detector in the measurement of time dependent energy depositions within the fissile material based on theoretical analysis, but the response time of a fission couple is relatively slow in practice. The time lag originated from heat transfer process was demonstrated to be the dominating factor by theoretical simulations and experimental verification in this paper. The response of a fission couple as a function of the bead size and the thermocouple wires' sizes are simulated using ANSYS workbench. The decrease of wires' diameter results in the decrease of response time, and the increase of bead's diameter leads to a slight increase of response time. During a pulse heating transient in the fuel of Chinese Fast Burst Reactor II with a FWHM of 181 μs, the time lag originated from heat transfer process is about tens of microseconds for the peaks of the change rate of temperature, and is of the order of milliseconds to achieve 85% of the temperature rise for a typical fission couple with a Φ 1 mm fissile bead and two Φ 0.05 mm thermocouple wires. The results obtained provide foundation for the optimization of fission couples.