Bulletin of the Lebedev Physics Institute最新文献

The Horizon-T installation at the Tien Shan High-Altittude Scientific Station of the Lebedev Physical Institute is designed to detect extensive air showers (EAS’s) of ultrahigh energies. The capabilities of the installation allow the signal measaured at the scintillation detector points to be digitized with a resolution of 2 ns and the fine time structure of EAS’s, in particular, the so-called multimodal events, to be studied. This paper examines a possible contribution of low-energy (10¹² eV ≤ E ≤ 10¹⁶ eV) showers to the detected signal. Background events measured in the Horizon-T experiment, which can be accidentally included in the recorded EAS event, as well as low-energy artificial events simulated using the CORSIKA-77500 code, are considered. A conclusion is made about the optimality of the simulation parameters and the contribution of random low-energy showers to the time sweeps recorded from high-energy EAS’s.

Currents and voltages are measured in a short vacuum gap at the initial stage of an arc discharge initiated by an auxiliary spark discharge on the dielectric surface. It is found that a cloud of a rarefied plasma in the vacuum gap is formed due to ionization of the residual gas by electrons emitted from the auxiliary discharge. The passage of a cathode electron beam and an ion beam in the vacuum gap is registered. It is shown that electrons in the corpuscular flux directed from the cathode to the anode carry a charge that is an order of magnitude larger than that carried by ions. An acceleration mechanism in a cathode plasma associated with plasma polarization and the formation of explosive electron emission centers is experimentally identified.

The generation of a highly-ionized cathode plasma in millimeter air gaps at pressures of about 100 Torr is investigated. It is shown that this plasma appears explosively at once in the moment of the electric breakdown of a discharge gap, with its characteristic velocity of expansion into a hemispherical region relative to the approximate explosion center on the cathode surface being of ~10⁵ to 10⁶ cm/s. The plasma initiation occurs almost always on the lateral surface of the pointed cathode slightly below its tip. It is established that the observed plasma has a high electron density of around (1–6)×10¹⁹ cm⁻³ almost tenfold exceeding the plasma electron concentration produced by complete dissociation and single ionization of air particles at a pressure of ~100 Torr. The parameters of the explosion of the cathode substance, the composition of the cathode plasma, and the fundamental and applied aspects of the observed phenomenon are discussed.

The OKA Collaboration has obtained a new upper limit on the K⁺ → π⁰π⁰π⁰e⁺ν decay probability which is 65 times lower than the one currently listed by PDG. However it is still about 10⁴ times worse than the theoretical prediction. It was suggested that production of pionium A_2π in the final state of the semileptonic K⁺ decay with subsequent A_2π → π⁰π⁰ decay will increase the value of theoretical branching ratio due to the absence of five-particle phase space suppression. We demonstrate that despite the gain in the phase space the decay K⁺ → A_2ππ⁰e⁺ν appears to be strongly suppressed because of the smallness of the pionium wave function at zero π mesons separation.

Modern gamma-ray spectrometers should exhibit high spectrometric characteristics (energy resolution, efficiency, radiation resistance, operating temperature range, and energy consumption), which provide a possibility of their use in various scientific studies, including nuclear medicine. This paper is devoted to the development of a well-type xenon gamma-ray spectrometer (WT-XGRS) for measuring the gamma activity of pharmaceuticals and determining their isotopic composition. The general design of this device is developed using the parametric computer-aided design modeler (FreeCAD). Using the Monte Carlo method (Geant4 package), model experiments are performed to determine the main physical characteristics of the fabricated equipment. The radioisotope Ba-133 is used as a gamma-ray source. The dependence of the registration efficiency of gamma quanta on their energy for a WT-XGRS, as well as on the location of the point radioactive pharmaceutical inside the well, is obtained. The calculated results are analyzed and the possibility of using this equipment in the field of nuclear medicine is considered.

We synthesized by mechanical alloying high-quality polycrystalline samples of CaRbFe₄As₄ and EuCsFe₄As₄ stoichiometric iron-based superconductors with single phase composition confirmed by XRD phase analysis. The superconducting transition temperatures were 35 and 34 K, and the transition widths were 5.4 and 2.5 K for CaRbFe₄As₄ and EuCsFe₄As₄, respectively. According to our magnetic measurements, we found only one superconducting transition in the samples.

We study the oscillation of three-micron lasers based on Er³⁺:YAG, Er³⁺:Cr³⁺: YSGG, and Ho³⁺:Yb³⁺:Cr³⁺:YSGG crystals under optical pumping by pulsed diode arrays with a total peak power of up to 15 kW at a wavelength of 970 nm and a pulse repetition rate of up to 50 Hz. In a diode-pumped Er³⁺:Cr³⁺:YSGG laser, the maximum output energy of more than 0.8 J was achieved for the first time in the free lasing regime. In the case of active g-switching, the peak power of the output laser pulses with a duration of 55 ns exceeded 1 MW with an energy stability (standard deviation) of less than 3%. Using the developed laser systems as sources for stable pulsed pumping of a Fe²⁺:ZnSe single-crystal laser with a short optical cavity, pulses with a duration of less than 2 ns at a wavelength in the vicinity of 4.3 μm were obtained.

We propose designs of laser frequency shifters providing a large separation angle between direct and diffracted beams. Two variants of frequency shifters are analysed, differing in the mutual arrangement of ordinary and extraordinary beams with respect to the crystal optical axis. An 80 MHz acousto-optic frequency shifter developed on a TeO₂ crystal for laser light with a wavelength of 532 nm provided a separation of direct and diffracted beams by ~12° for both ordinary and extraordinary input beams.

Based on the numerical solution of the time-dependent Schrödinger equation, we study the interaction of attosecond electromagnetic pulses with a particle in a one-dimensional deep rectangular quantum well. It is shown that when a unipolar pulse affects this system, the change in the level populations occurs more efficiently and faster than under the action of bipolar multicycle pulses with a zero electric area.

Dynamic characteristics of adaptive optics systems operating with radiation transmitted through a turbulent atmosphere are analysed on the basis of analytical calculations of the Strehl ratio using the theory of optical wave propagation in randomly inhomogeneous media and the model of an active mirror-corrector. A traditional adaptive optics system with a finite response time is described as a dynamic system with a constant delay. An expression is obtained that relates the most important parameters of the system, such as the error and frequency of the wavefront sensor, the aperture size of the optical system, and atmosphere parameters (the Fried parameter and wind velocity), with an attainable value of the Strehl ratio. It is confirmed that the admissible time delay in such a system is comparable with the time of transfer of turbulent inhomogeneities over a distance of the order of the coherence radius at an average wind velocity. Differences between open and closed loops in adaptive optics tracking systems are analysed. The possibility of using a “predictive” algorithm for adaptive correction is demonstrated.