Intensities of backscatter Mössbauer spectra

The intensities of γ‐ray and x‐ray backscatter Mössbauer spectra of ^(57)Fe nuclei in different matrix materials were studied theoretically and experimentally. A previous analysis by J. J. Bara [Phys. Status Solidi A 58, 349 (1980] showed that negative peak intensities occur in backscatter γ‐ray spectra when the ^(57)Fe nuclei are in a matrix of light elements. We report a confirmation of this work and offer a simple explanation of the phenomenon. The present paper extends Bara’s analysis to the case of conversion x‐ray spectra; expressions for the intensity of conversion x‐ray spectra as a function of absorber thickness and absorber material parameters are presented. We show that negative peak intensities are expected in conversion x‐ray spectra when the ^(57)Fe nuclei are in a matrix of heavy elements

Structure-Function Studies of the cAMP-Dependent Protein Kinase \u3ci\u3eIn Vitro\u3c/i\u3e and in Intact Cells

There are 518 protein kinase genes in the human genome; this constitutes about 1.7% of all human genes. The cAMP-dependent protein kinase (PKA) serves as the prototypic model for the study of kinases because it contains a conserved catalytic core shared with all eukaryotic kinases, it is the simplest kinase, and it is one of the best-characterized serine/threonine kinases. PKA is ubiquitous in mammals and regulates multiple physiological mechanisms such as the cell cycle, apoptosis, cell motility, energy metabolism, and gene transcription through a well-defined intracellular signaling pathway. While PKA clearly has a central physiological role it is still unclear how PKA mediates multiple physiological mechanisms at the cellular level. Four approaches were used to explore this question using two PKA catalytic subunits, Cα and Cγ, which share 83% identity in primary structure but differ in function. The first approach sought to identify differences in primary structure between Cγ and Cα, which may define functional differences between them. To this end chimeras were generated, swapping the carboxyl and amino termini between Cα and Cγ and were evaluated for functionality through CREB-mediated reporter assays. Wild type Cα and Cγ induced CREB-mediated transcriptional activation, but the chimeras failed to exhibit any activity. The second approach sought to characterize phosphorylation differences between purified PKA-Cγ and PKA-Cα that defines their physiological function. Two novel phosphorylation sites were identified on both isoforms by tandem mass spectrometry analysis (Cγ S14 and Cα/Cγ S259). It was also determined that Cγ expressed in Sf9 insect cells, like Cα expressed in mammalian cells, is phosphorylated at T197 and S338 and the modification at T197 is important to the function of both isoforms. The third approach sought to characterize the kinetic mechanism of PKA-Cγ through determination of the rate for the reaction-limiting step, which was found to be 9-times slower than that of Cα. The final approach sought to identify Cγ expression in the cell through the use of a new Cγ-specific antibody. Cγ expression was identified following differentiation of U-937 cells suggesting a novel function for Cγ in the cell

Mechanisms of high-frequency song generation in brachypterous crickets and the role of ghost frequencies

Sound production in crickets relies on stridulation, the well-understood rubbing together of a pair of specialised wings. As the file of one wing slides over the scraper of the other, a series of rhythmic impacts cause harmonic oscillations, usually resulting in the radiation of pure tones delivered at low frequencies (2-8 kHz). In the short winged crickets of the Lebinthini tribe, acoustic communication relies on signals with remarkably high frequencies (> 8 kHz) and rich harmonic content. Using several species of the subfamily Eneopterinae, we characterise the morphological and mechanical specialisations supporting the production of high frequencies, and demonstrate that higher harmonics are exploited as dominant frequencies. These specialisations affect the structure of the stridulatory file, the motor control of stridulation and the resonance of the sound radiator. We place these specialisations in a phylogenetic framework and show that they serve to exploit high frequency vibrational modes pre-existing in the phylogenetic ancestor. In Eneopterinae, the lower frequency components are harmonically related to the dominant peak, suggesting they are relicts of ancestral carrier frequencies. Yet, such ghost frequencies still occur in the wings' free resonances, highlighting the fundamental mechanical constraints of sound radiation. These results support the hypothesis that such high frequency songs evolved stepwise, by a form of punctuated evolution which could be related to functional constraints, rather than by the progressive increase of the ancestral fundamental frequency

Thermal Decomposition of the Murchison CM2 Carbonaceous Chondrite: Implications of Space Weathering Processes for Sample Return Missions

Primitive carbonaceous asteroids are the target bodies for the JAXA Hayabusa2 mission to Ryugu and the NASA OSIRIS-REx mission to Bennu. Both asteroids share spectral characteristics of CI/CM type carbonaceous chondrites. Ryugu, in particular, appears to have undergone thermal processing that has modified its spectral properties. The nature and extent of space weathering processes on the surfaces of Bennu and Ryugu are under active investigation using remote sensing data from the missions [4] and through laboratory studies on analog materials. The analog studies are needed in order to understand the mineralogical and chemical changes that occur in space weathered samples that give rise to the observed optical effects measured by remote-sensing and to prepare for the analysis of returned samples. The space weathering effects of micrometeorite impact and solar wind irradiation on primitive carbonaceous chondrites have been simulated by analog studies on the Murchison CM2 chondrite. We performed a coordinated mineralogical, chemical and spectroscopic study to examine in detail the effects of thermal metamorphism on Murchison samples as an analog to processes that may have occurred on Ryugu. The bulk measurements including X-ray diffraction (XRD), Mssbauer spectroscopy, UV-VIS-NIR spectroscopy, thermogravimetric analysis, and evolved gas analysis are reported in a companion paper. Here we report on our preliminary nanoscale mineralogical and chemical analyses of pre- and post-heated Murchison samples using multiple electron beam techniques to understand how the mineralogical, chemical, and physical characteristics of carbonaceous chondrites change with increasing thermal effects

Children's science learning: a core skills approach

Background: Research has identified the core skills that predict success during primary school in reading and arithmetic, and this knowledge increasingly informs teaching. However, there has been no comparable work that pinpoints the core skills that underlie success in science. / Aims and method: The present paper attempts to redress this by examining candidate skills and considering what is known about the way in which they emerge, how they relate to each other and to other abilities, how they change with age, and how their growth may vary between topic areas. / Results: There is growing evidence that early-emerging tacit awareness of causal associations is initially separated from language-based causal knowledge, which is acquired in part from everyday conversation and shows inaccuracies not evident in tacit knowledge. Mapping of descriptive and explanatory language onto causal awareness appears therefore to be a key development, which promotes unified conceptual and procedural understanding. / Conclusions: This account suggests that the core components of initial science learning are (1) accurate observation, (2) the ability to extract and reason explicitly about causal connections, and (3) knowledge of mechanisms that explain these connections. Observational ability is educationally inaccessible until integrated with verbal description and explanation, for instance, via collaborative group work tasks that require explicit reasoning with respect to joint observations. Descriptive ability and explanatory ability are further promoted by managed exposure to scientific vocabulary and use of scientific language. Scientific reasoning and hypothesis testing are later acquisitions that depend on this integration of systems and improved executive control

Molecular detection of type II polyketide synthase genes in Cuban soils

Molecular detection methods were developed to study the distribution of type II polyketide synthase (PKS) genes in Cuban soils. A PCR based detection method targeting the α and β ketosynthase genes was applied to a number of different total community DNA samples. These genes were detected in 43% of samples tested from a number of different locations. A botanical garden site located in Havana, Cuba, was found to show the greatest distribution of type II PKS genes across the sites tested. It was not possible to amplify type II PKS genes from a pristine island site off the coast of Cuba. Further investigation revealed that actinornycetes containing type II PKS were present in the soil community at a level above the detection limit of the PCR protocol. Further total community DNA cleanup steps failed to allow the detection of type II PKS genes within the DNA samples suggesting PCR inhibition was responsible for negative results. The molecular detection of type II PKS genes in total community DNA was compared to the detection of type II PKS genes in actinomycete isolates. A lack of correlation between these two approaches was observed with the molecular detection limit unable to amplify type II PKS genes in <50% of crop soils tested. Actinomycetes containing type II PKS genes could be isolated from all crop soils tested. No difference was seen in the detection of type II PKS genes between rhizosphere and bulk soil samples. Actinomycetes were isolated using a selective isolation procedure at a level of approximately 10(7) cfu g-1 soil compared to 10(8) cfu g-1 for total bacterial counts. Actinomycetes were isolated from Cuban crop soils and screened for the presence of type II PKS genes. Out of 100 isolates 26 were found to contain the genes of interest. Phylogenetic analysis of these isolates based on 16S rDNA and recA sequence data showed them to be closely grouped within the streptomycetes. Sequence data based on KSα genes from Cuban isolates showed them to be representative of both spore pigment and antibiotic polyketide genes. A representative clone library was constructed containing type II PKS genes amplified from total community DNA. Rhizosphere and bulk soil samples were compared from the same site. Sequences obtained from rhizosphere total community DNA appeared to be widely distributed when compared to published sequences and included examples of both spore pigment and antibiotic polyketide genes. A molecular method was developed to amplify near full length α and β KS genes from type II PKS gene clusters. Expression vectors were constructed to allow these genes to be expressed along with an ACP to give a functional minimal PKS for polyketide chain production. This method was used on total community DNA in an attempt to extract diverse genes from as yet uncultured organisms

The Liquidus Temperature for Methanol-Water Mixtures at High Pressure and Low Temperature, with Application to Titan

Methanol is a potentially important impurity in subsurface oceans on Titan and Enceladus. We report measurements of the freezing of methanol-water samples at pressures up to 350~MPa using a volumetric cell with sapphire windows. For low concentrations of methanol, the liquidus temperature is typically a few degrees below the corresponding ice freezing point, while at high concentrations it follows the pure methanol trend. In the Ice-III regime, we observe several long-lived metastable states. The results suggest that methanol is a more effective antifreeze than previously estimated, and might have played an important role in the development of Titan's subsurface ocean

PassGAN: A Deep Learning Approach for Password Guessing

State-of-the-art password guessing tools, such as HashCat and John the Ripper, enable users to check billions of passwords per second against password hashes. In addition to performing straightforward dictionary attacks, these tools can expand password dictionaries using password generation rules, such as concatenation of words (e.g., "password123456") and leet speak (e.g., "password" becomes "p4s5w0rd"). Although these rules work well in practice, expanding them to model further passwords is a laborious task that requires specialized expertise. To address this issue, in this paper we introduce PassGAN, a novel approach that replaces human-generated password rules with theory-grounded machine learning algorithms. Instead of relying on manual password analysis, PassGAN uses a Generative Adversarial Network (GAN) to autonomously learn the distribution of real passwords from actual password leaks, and to generate high-quality password guesses. Our experiments show that this approach is very promising. When we evaluated PassGAN on two large password datasets, we were able to surpass rule-based and state-of-the-art machine learning password guessing tools. However, in contrast with the other tools, PassGAN achieved this result without any a-priori knowledge on passwords or common password structures. Additionally, when we combined the output of PassGAN with the output of HashCat, we were able to match 51%-73% more passwords than with HashCat alone. This is remarkable, because it shows that PassGAN can autonomously extract a considerable number of password properties that current state-of-the art rules do not encode.Comment: This is an extended version of the paper which appeared in NeurIPS 2018 Workshop on Security in Machine Learning (SecML'18), see https://github.com/secml2018/secml2018.github.io/raw/master/PASSGAN_SECML2018.pd

Weakly Nonlinear Analysis of Electroconvection in a Suspended Fluid Film

It has been experimentally observed that weakly conducting suspended films of smectic liquid crystals undergo electroconvection when subjected to a large enough potential difference. The resulting counter-rotating vortices form a very simple convection pattern and exhibit a variety of interesting nonlinear effects. The linear stability problem for this system has recently been solved. The convection mechanism, which involves charge separation at the free surfaces of the film, is applicable to any sufficiently two-dimensional fluid. In this paper, we derive an amplitude equation which describes the weakly nonlinear regime, by starting from the basic electrohydrodynamic equations. This regime has been the subject of several recent experimental studies. The lowest order amplitude equation we derive is of the Ginzburg-Landau form, and describes a forward bifurcation as is observed experimentally. The coefficients of the amplitude equation are calculated and compared with the values independently deduced from the linear stability calculation.Comment: 26 pages, 2 included eps figures, submitted to Phys Rev E. For more information, see http://mobydick.physics.utoronto.c