Hotspot Mechanism Research Articles

Two Mechanisms Produce Mutation Hotspots at DNA Breaks in Escherichia coli

SummaryMutation hotspots and showers occur across phylogeny and profoundly influence genome evolution, yet the mechanisms that produce hotspots remain obscure. We report that DNA double-strand breaks (DSBs) provoke mutation hotspots via stress-induced mutation in Escherichia coli. With tet reporters placed 2 kb to 2 Mb (half the genome) away from an I-SceI site, RpoS/DinB-dependent mutations occur maximally within the first 2 kb and decrease logarithmically to ∼60 kb. A weak mutation tail extends to 1 Mb. Hotspotting occurs independently of I-site/tet-reporter-pair position in the genome, upstream and downstream in the replication path. RecD, which allows RecBCD DSB-exonuclease activity, is required for strong local but not long-distance hotspotting, indicating that double-strand resection and gap-filling synthesis underlie local hotspotting, and newly illuminating DSB resection in vivo. Hotspotting near DSBs opens the possibility that specific genomic regions could be targeted for mutagenesis, and could also promote concerted evolution (coincident mutations) within genes/gene clusters, an important issue in the evolution of protein functions.

Intrinsic detection efficiency of superconducting nanowire single-photon detectors with different thicknesses

We evaluate experimentally the intrinsic detection efficiency (IDE) of superconducting NbN nanowire single-photon detectors in the range of wire thicknesses from 4 to 12 nm. The study is performed in the broad spectral interval between near-ultraviolet (wavelength 400 nm) and near-infrared (wavelength 2000 nm) light with plane waves at normal incidence. For visible light the IDE of the thinnest detectors reaches 70%. We use numerically computed absorptance of the nanowire-structures for the analysis of the experimental data. Variations in the detection efficiency with both the wire thickness and the wavelength evidence the red boundary of the hot-spot photon-detection mechanism. We explain the detection at larger wavelengths invoking thermal excitation of magnetic Pearl vortices over the potential barrier at the edges of the wire.

Structure and properties of detonation waves of heterogeneous explosives from the point of view of the hot spots mechanism

It is assumed that the chemical reaction of a detonation transformation, that is, burning, arises first in separate hot spots originated by the explosive compression behind or within the detonation wave front due to heterogeneity of charges, instability of the detonation wave, etc. Then the burning spreads around into the bulk of the explosive and is completed in the Chapman–Jouguet plane, where the detonation product velocity relative to the detonation front is equal to the sound velocity. A simple analytical method proposed early by the authors is used to calculate the expression for the detonation transformation time, from which it follows that there are at least two nondimensional numbers defining the possibility of the realization of the mechanism mentioned above. The first one is the Frank-Kamenetskii number, that is, the relation of the characteristic time of the cooling of the hot spot to the adiabatic induction period at some characteristic temperature of the hot spot. The hot spot mechanism is realized if the Frank-Kamenetskii number is greater than the critical one that determines the minimum possible size of hot spots capable of ignition. The second number is the relation of the detonation transformation time in the presence of hot spots to the reaction time in accordance with classical mechanism. The hot spot mechanism at detonation is realized if this number is less than unity. By means of the numbers proposed, some interpretations of available experimental data concerning the detonation process are given.

Synthesis of Shape Controlled Ferrite Nanoparticles by Sonochemical Technique

Synthesis of magnetic iron oxides/ferrites in the nano scale by sonochemical synthesis has become prominent recently. This technique facilitates the synthesis of magnetic particles in the nano scale attributed to the hotspot mechanism arising due to acoustic cavitation induced chemical reaction. Generally volatile organometallic precursor compounds favoring the formation of fully amorphous particles have been used to synthesize various nano magnetic materials. We report here the synthesis of ultrafine, < 10 nm magnetic iron oxide nanoparticles by sonochemical technique starting with a non-volatile precursor iron salt such as iron citrate which seems to favor the formation of semi crystalline/crystalline particles as the reaction takes place either in the interfacial region or in the bulk solution. Mono dispersed, ultra fine, approximately 4 nm spherical shaped magnetic maghemite particles having a saturation magnetization of 58.2 emu/g and coercivity of 118 Oe were obtained at low values of pH, 10 while higher pH, 11-13 favored the formation of elongated, cylindrical, acicular particles with a reduced magnetization. The coercivity was also found to decrease with increasing pH, with it being 118 Oe at pH 10 and 3 Oe at pH 13. When the ultrasound amplitude/intensity was low, 38% heat treatment of the samples at 300 degrees C (at pH 10) was required to make them crystalline, while application of high intensity ultrasound, 50% amplitude served as a single step mechanism for obtaining crystalline maghemite particles. The maghemite particles obtained at a pH of 10 could find applications in information storage media.

Explosive compositions based on the mechanoactivated metal-oxidizer mixtures

Detonation-like regimes in mechanoactivated energetic composites (MAECs) were experimentally studied. The test MAECs consisted of layers of a metal (Al, Mg) and Teflon mixed at the submicron and nano levels. The systems reacted to form solid final products. MAECs are appreciably superior to ordinary mixtures in chemical transformation rate. The burning of MAECs occurs in an explosive regime at a velocity of 300–400 m/s, with the temperature of the products being as high as 4000 K. When initiated with a HE charge, porous MAECs detonate in the steady regime. Depending on the composition and density of the samples, the detonation velocity varies from 700 to 1300 m/s, values markedly higher than the speed of sound in the initial mixture. Detonation is controlled by a hot spot mechanism, more specifically, by relay reaction propagation by jets of products.

Nonclassical steady-state detonation regimes in pressed TNETB

The reaction zones and the dependence of the velocity of steady-state detonation waves on the initial density of pressed TNETB are studied using a VISAR interferometer. It is shown that, in the range of initial densities of TNETB 1.56–1.77 g/cm3, the propagation of a steady-state detonation wave is possible without the range of elevated pressures (chemical spike) in the reaction zone predicted by the classical theory. The dependence of the detonation velocity on the initial density shows singularities which indicate that a steady-state underdriven regime can occur in this range of initial densities. Based on the well-known theoretical concepts of the hot-spot decomposition mechanism of heterogeneous explosives, it is shown that the possibility of the existence of a steady-state detonation wave without a chemical spike, in particular, underdriven detonation, and the effect of the internal structure of the charge on the detonation regime are explained by the decomposition of explosives at the shock-wave front.

Spectral Sensitivity and Spectral Resolution of Superconducting Single-Photon Detectors

Single photon detectors based on meanders from 5-nm thin B1 niobium nitride nanostrips show moderate spectral resolution along with the spectral cut-off of the quantum efficiency occurring in the near-infrared spectral range. Beyond the cut-off there is a gradual decrease of the quantum efficiency. To describe the response beyond the cut-off we combine the previously developed hot-spot detection mechanism with the photon-induced unbinding of the vortex-antivortex pairs, which appear in the superconducting film below the Kosterlitz-Thouless phase transition. The detector responds to the absorption of a single photon with a voltage pulse whose amplitude depends on the photon energy. We report the observation of a photon-energy dependent statistical distribution of the pulse amplitude that defines the energy resolution of the detector. We analyzed the detector response to continuous and femtosecond-pulse radiation using broadband microwave amplifiers and a fast single-shot oscilloscope as well as integrating SQUID amplifiers. Our analysis suggests that both the amplitude and the duration of the response pulses are jointly controlled via the kinetic inductance of the meander line and the Joule power dissipated in the meander by the bias current.

The number defining the realization of the hot spot mechanism on detonation of heterogeneous explosives

The number defining the realization of the hot spot mechanism on detonation of heterogeneous explosives

Non-Equilibrium Patterns in the Space of the Stock Market Prices

Using a phenomenological approach, we analyse the formation and the propagation of the patterns (or ‘dissipative structures’) in the stock market, the spatial coordinate being the bid-offer spread y, as a function of which the spectrum φ of deals is modelled. The stock market will be considered a distributed active medium that is a set of active elements (the brokers) interacting with others through deals (typically a diffusion process). The physical model used is the reaction-diffusion model. The reactive part of the reaction-diffusion equation is developed from a hot-spot mechanism, with a characteristic jump when φ passes the critical value φc. Solving the stationary equation according to the Dirichlet boundary conditions, we find the ‘hot deals’ regions, meaning regions of speculative transactions which can be considered ‘dissipation’ as they do not contribute to the gross national product.

Fatigue of lx-14 and lx-19 plastic bonded explosives

The DOD uses the plastic bonded explosive (PBX) LX-14 in a wide variety of applications including shaped charges and explosively formed projectiles.1 LX-19 is a higher energy explosive, which could be easily substituted for LX-14 because it contains the identical Estane 5703p binder and more energetic CL-20 explosive. Delivery systems for large shaped charges, such as TOE 2, include the Apache helicopter. Loads associated with vibrations and expansion from thermal excursions in field operations may, even at low levels over long time periods, cause flaws, already present in the PBX to grow. Flaws near the explosive/liner interface of a shaped charge result in reduced performance. Small flaws in explosives are one mechanism (the hot spot mechanism) proposed for initiation and growth to detonation of PBXs like LX-14, PBXN 5, LX-04 and LX-17 among others. Unlike cast-cured explosives and propellants, PBXs cannot usually be compression molded to full density. Generally, the amount of explosive ignited by a shock wave is approximately equal to the original void volume.2 Whether or not these flaws or cracks grow during field operations to an extent sufficient to adversely affect the shaped charge performance or increase the vulnerability of the PBX is the ultimate question this effort could address. Currently the fatigue life of LX-14 under controlled conditions is being studied in order to generate its failure stress as a function of the number of fatigue cycles (S-N curve). Proposed future work, depending on the availability of funding, will address flaw and crack growth and their relationship to hot-spot concentration and explosive vulnerability to shock and/or fragment initiation.

A constitutive model for the non-shock ignition and mechanical response of high explosives

An understanding of the non-shock ignition properties of energetic particulate composite materials, high explosives such as PBX-9501 is an important part of the safety assessments for conventional handling (transportation, storage, etc.) of weapons systems including assembly operations. This paper develops and demonstrates the use of a numerical constitutive model for PBX-9501 that includes viscoelastic response, statistical fracture mechanics, and an ignition hot-spot mechanism. The intent is that this model can be used in safety analyses involving accidents to prevent undesirable dispersion of Pu. The parameters have been determined that will predict the mechanical response and ignition:non-ignition of a set of experiments that have explored the non-shock properties of this material.

On the Energy Evolution in Gaseous Detonation Waves

There is a definite inconsistency between the classical ZND theory of detonation and contemporary experimental observations and attempts to model real detonation waves. Nevertheless, the classical onedimensional model of detonation is still extensively used in interpreting measurements because of its simplicity and physical clarity. This naturally raises the questions, what does actually the classical model represent and how one can relate the real multidimensional wave structure to an effective onedimensional detonation? The answer to these questions is extremely important from the practical point of view, because it defines how simple the solution of the criticality problems in detonation could be (i.e. whether one has to solve the very cumbersome three-dimensional nonsteady gasdynamic problem with detailed kinetics of the chemical reaction comprising a few hundreds of elementary steps to predict such parameters as critical initiation energy, critical distances, and concentrations, or the solution of a simplified onedimensional gasdynamic problem with global chemical kinetic equations would suffice?). The present communication discusses the following topics with the aim of answering partly the above questions. (1) An analysis of the detailed kinetic calculations of the heat evolution rate in some fuel-air mixtures and comparison of these results with experiments reveals that (a) in the majority of detonable mixtures heat release can be described by global equations with an accuracy quite sufficient for assessing the critical detonation parameters, (b) chemical kinetics shows no so-called recombination zones with an appreciable heat evolution that follow the main reaction zone, (c) the ratio between the induction and the explosion (within which the major fraction of the stored energy is evolved) times exceeds unity under the conditions corresponding to detonation waves in fuel-air mixtures, which contradicts the shock tube measurements. All these results can be reasonably explained by the fact that the chemical reaction in detonation waves (in shock waves as well) proceeds in a gas with a highly inhomogeneous distribution of the parameters (temperatures, pressures, and particle velocity) which gives rise to the so-called hot-spot mechanism of chemical reactions in gaseous and two-phase media. It is shown that the hot spot ignition is the main reason of instability of detonation waves. Nonsynchronous mixture ignition at different points produces the effect of extended heat evolution zone and reduces the effective induction zone. Results of measurements of averaged and local parameters behind detonation waves in gases support the idea that the effective heat evolution rate in the onedimensional representation of the wave is much more complicated than that inherent in the chemical reaction alone, because it contains also a significant contribution of the kinetic and thermodynamic energy redistribution within the major reaction zone and downstream of it, which makes the onedimensional structure of detonation waves deviate significantly from the classical pattern leading to nonmonotonic heat release behind the shock front (the possibility of appearance of two or more effective sonic planes and heat rlease maxima). However, a onedimensional analysis of marginal detonation waves has demonstrated that critical parameters can be estimated quite accurately within simplified models, which is attributable to the fact that the criticality is associated with local termination of the reaction within narrow stream tubes where the flow pattern resembles closely the ZND non-CJ detonation wave structure. Unlike the chemical reaction zone in detonations far away from the limit, which is shorter than the detonation cell size, the reaction zone in marginal detonations is comparable with the cell size. The results of onedimensional calculations of critical diameters and minimal energies of direct initiation of detonation are compared with experim

Geochemical studies of Tahiti, Teahitia and Mehetia, Society Island chain

The Society Island chain is located within a region of the South Pacific characterized by anomalously enriched isotopic compositions. The islands of Mehetia and Tahiti, together with the seamount Teahitia, are among the youngest volcanoes in the chain and must be close to the present-day hotspot location. We report here detailed geochemical (major- and trace-element, and isotope) studies of fresh, age-controlled lavas. The data reveal temporal geochemical variations on Tahiti, and close compositional links between Tahiti and both Teahitia and Mehetia. Major-element compositions of the Tahitian lavas define three magmatic fractionation series which coincide with three volcanic phases that have been identified independently. Fractionation-corrected trace-element data also show temporal variations in both absolute concentrations and in the ratios of highly incompatible to moderately incompatible trace elements. Both major- and trace-element results suggest a progressive decrease in the degree of partial melting with time. Mixing of magmas derived from an enriched plume source and from metasomatized lithosphere is consistent with the observed compositional and isotopic variations among the different parental magmas. The earliest recognized (shield-building?) lavas on Tahiti carry the strongest geochemical plume signature, while late-stage lavas were apparently derived partly from lithospheric sources with smaller degrees of partial melting. The similarities in chemical characteristics among lavas from Tahiti, Teahitia and Mehetia suggest common sources and similar magmatic processes for the three volcanoes, consistent with the hotspot mechanism for the origin of these and other islands in the Society chain.