The population kinetics of a two-level system has been theoretically investigated taking into account the transfer of spontaneous radiation under continuous optical pumping. It is shown that, when ...the photon mean free path in a resonantly absorbing medium is smaller than the characteristic size of this medium or comparable to it, the spontaneous radiation may become an additional pump source. The contribution of spontaneous photons to the kinetics of level has been considered within two approaches. The first approach consists of solution of the radiation transfer equation within the diffusion approximation. The second approach is related to solution of the Biberman–Holstein equation.
The cross sections of nuclear reactions between the radioisotope Be7 and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in big-bang nucleosynthesis, were measured at ...center-of-mass energies between 0.2 and 1.5 MeV. The measured cross sections are dominated by the (d,α) reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5) MeV with a strength of ωγ=1.7(5) keV was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial Li7, but not sufficiently to solve the primordial lithium problem.
We present the results of our experiments at the Luch laser facility in which the processes of the destruction of iron asteroids and cometary nuclei in space by nuclear explosions on their surface ...are simulated based on the principle of physical similarity. We present the results of our numerical simulations of impacts on the surface of iron objects confirming the similarity of the key processes in the full-scale and model cases. The technologies of fabricating miniature mockups with parameters close to those of iron asteroids and cometary nuclei are described. For various mini-mockups 3–12 mm in size at a laser energy from 50 to 450 J we provide an experimental estimate of the energy threshold for the undeniable destruction of mockups and investigate the parameters of their fragmentation. Conclusions about the probable results of the impact of a nuclear explosion on hazardous cosmic objects have been drawn.
State of the art measurements with TexAT Bishop, J; Rogachev, G V; Aboud, E ...
Journal of physics. Conference series,
08/2019, Letnik:
1308, Številka:
1
Journal Article
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In the past several years, there has been a large interest of Time Projection Chambers (TPCs) for use in experimental nuclear physics. This has continued in tandem with the requirement for high ...efficiency detectors with low intensity radioactive ion beams. TexAT is a Active Target TPC (AT-TPC) built at Texas A&M University utilizing MICRO MEsh GASeous (MicroMegas) pads and GET electronics developed specifically for TPCs. This design combines good TPC pixelation with a surrounding shell of Si/CsI telescopes to make an extremely versatile detector capable of a wide range of different experimental techniques with only minor modifications to the electronics setup. Two recent experiments performed at the Cyclotron Institute, Texas A&M University, are detailed here demonstrating versatility beyond the usual Thick Target Inverse Kinematics (TTIK) or transfer reactions that these TPCs are more typically used for. The first, a measurement of the 12N → 12C* → 3α decay demonstrates the capabilities of TexAT as a low-energy detector operating at low pressure (20 Torr) to measure β-delayed particle decay. The second, a direct measurement of the 8B+40Ar fusion cross section shows the advantages of operating in active target mode where the target also functions as the detector gas.
The processes of current flow and reaction initiation in a mixture of Ti–Cu spherical particles were studied in model experiments and in real conditions of SPS processing. Our results allow us to ...conclude that the local overheating during SPS-consolidation of powder materials is caused by the formation of some preferable current pathways within which current density is markedly greater than within the adjacent areas. This leads to the breakdown of thin dielectric layers (oxide films, vacuum nanogaps) at particle-to-particle contacts, the conductivity of this route sharply grows, and the constituent particles become overheated selectively. In turn, local overheating gives start to SHS reaction and formation of reacted areas in the sample bulk. Our results may turn useful to those engaged in the optimization of process charts and process conditions for SPS-consolidation of materials.
In this study, nonstoichiometric hafnium carbonitrides (HfCxNy) were fabricated via short-term (5 min) high-energy ball milling of Hf and C powders, followed by combustion of mechanically induced ...Hf/C composite particles in a nitrogen atmosphere (0.8 MPa). The obtained HfC0.5N0.35 powder exhibited a rock-salt crystal structure with a lattice parameter of 0.4606 nm. The melting point of this synthesized ceramic material was experimentally shown to be higher than that of binary hafnium carbide (HfC). The nonstoichiometric hafnium carbonitride was then consolidated under a constant pressure of 50 MPa at a temperature of 2000 °C and a dwelling time of 10 min, through spark plasma sintering. The obtained bulk ceramic material had a theoretical material density of 98%, Vickers hardness of 21.3 GPa, and fracture toughness of 4.7 MPa m1/2.
We present the results of preliminary experiments at laser facilities in which the processes of the undeniable destruction of stony asteroids (chondrites) in space by nuclear explosions on the ...asteroid surface are simulated based on the principle of physical similarity. We present the results of comparative gasdynamic computations of a model nuclear explosion on the surface of a large asteroid and computations of the impact of a laser pulse on a miniature asteroid simulator confirming the similarity of the key processes in the fullscale and model cases. The technology of fabricating miniature mockups with mechanical properties close to those of stony asteroids is described. For mini-mockups 4–10 mm in size differing by the shape and impact conditions, we have made an experimental estimate of the energy threshold for the undeniable destruction of a mockup and investigated the parameters of its fragmentation at a laser energy up to 500 J. The results obtained confirm the possibility of an experimental determination of the criteria for the destruction of asteroids of various types by a nuclear explosion in laser experiments. We show that the undeniable destruction of a large asteroid is possible at attainable nuclear explosion energies on its surface.
Structure of very exotic nucleus, carbon-9, has been studied using novel detector - Texas Active Target (TexAT). Carbon-9 can be produced by removing three neutrons from the stable carbon-12 isotope, ...which we inhale every few seconds as part of carbon-dioxide gas. The structure of this exotic carbon, carbon-9, is very different from the stable carbon (carbon-12) and much less explored due to experimental difficulties of producing it. Yet, it is important for benchmarking the contemporary, state-of-the-art theories of nuclear structure. In particular, excitation energy of the positive parity states, that indicate an onset of the sd-shell, was not known in 9C. Here, we have conclusively identified the first positive parity state in 9C. It was made possible by the new capabilities provided buy TexAT.
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