Probabilistic distribution and noise factor of solid state photomultiplier signals,taking into account cross-talk processes

Solid-state photomultipliers (SSPMs) are a new type of photodetectors based on avalanche breakdown of the semiconductor in the Geiger mode, limited by negative feedback. Under such conditions, the avalanche breakdown is characterized by high multiplication factors, 10⁴–10⁶, and low noise, which allows detection of single photons. However, in practice, each primary breakdown in the SSPM can be accompanied by secondary breakdowns due to cross-talk processes which change the probabilistic distribution of the signal and introduce excess noise. In this paper, the effect of cross-talk on signal and noise characteristics of SSPMs is considered based on the generalized Poisson distribution.     

Gamma-ray astronomy by the air shower technique: Performance and perspectives

Summary The techniques for γ-ray astronomy at energies ≥10 TeV using air shower detectors are discussed. The results, based on a number of large arrays, are negative, with no point sources being identified. While the contributions to γ-ray astronomy so far have been only upper limits, these arrays in the future will make significant progress in the understanding of cosmic rays in the energy range 10¹³ eV to 10¹⁶ eV. Also, contributions to solar physics are being made by observations of shape and time dependence of the shadow of the Sun as observed in cosmic rays. For the advancement of γ-ray astronomy a greater sensitivity is required in the energy region of 10 TeV. A number of promising techniques to accomplish a greater sensitivity are discussed. They include the enlargement of the Tibet array at 4300 meters altitude, the array of open photomultipliers at La Palma (AIROBICC), which views the shower by the Cherenkov photons produced in the atmosphere, and the instrumentation of a pond at Los Alamos with photomultipliers (Milagro). Invited talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Adaptive Correction of a High-Power Titanium-Sapphire Laser Radiation

An adaptive optical closed-loop feedback system based on a phase-conjugation algorithm has been developed and investigated. It includes a deformable bimorph mirror and a Shack-Hartmann wavefront sensor. This system was used to improve the radiation focusing in an ultrashort-pulse laser designed for basic research in the field of high-energy physics. With the indicated system, the power density of 10-TW radiation pulses in the focal plane of a Ti:Al₂O₃ laser reached 4 ⋅ 10¹⁹ W/cm². Report at the Fifth International Scientific and Technical Conference “Quantum Electronics,” November 22–25, 2004, Minsk, Belarus. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 5, pp. 678–683, September–October, 2005.     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

Local surface segregations of implanted aluminum in an iron crystal with a low density of defects

The surface distribution of elements is studied by scanning a 3-MeV proton beam along the surface of a bcc-Fe sample implanted with aluminum ions in the dose interval (1–50) · 10¹⁶ cm⁻². Ring-shaped regions, up to 30 μm in diameter, with a high density of aluminum, which appear at implantation doses (5–20) · 10¹⁶ cm⁻², are observed. These regions appear as a result of radiation-stimulated segregation processes. A mechanism based on the existence of a low density of dislocations in the initial crystal is proposed to explain the implanted impurity segregation processes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 86–89 (10 January 1997)     

Estimates of electron-positron pair production in the interaction of high-power laser radiation with high-Z targets

Electron-positron production processes occurring in the interaction of 10¹⁸–10²⁰ W/cm² laser radiation with high-Z targets are examined. Computational results are presented for the pair production and the positron yield from the target with allowance for the contribution of pair production processes due to electrons and bremsstrahlung photons. Monte Carlo simulations using the PRIZMA code confirm the estimates obtained. The possible positron yield from high-Z targets irradiated with 10²–10³ TW laser radiation is estimated to be 10⁹–10¹¹. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 239–244 (25 February 1998)     

Development of a segmented fast neutron spectrometer

We discuss the development of a spectrometer based on full energy absorption using liquid scintillator doped with enriched ⁶Li. Of specific interest, the spectrometer is expected to have good pulse height resolution, estimated to lie in the range 10–15% for 14-MeV neutrons. It should be sensitive to flux rates from 10⁻⁶ cm⁻² s⁻¹ to 10⁶ cm⁻² s⁻¹ above a threshold of 500 keV in an uncorrelated γ background of up to 10⁴ s⁻¹. We have constructed a pilot version of the detector using undoped liquid scintillator, and we report its present status. The detector’s efficiency is determined by the volume of the scintillator (∼1.21) and is estimated to be 0.2–0.5% for 3-MeV neutrons. The good pulse height resolution is achieved by compensation of the nonlinear light yield of the scintillator due to the use of optically separated segments, which collect scintillations from each recoil proton separately. We demonstrate here the response of the detector to neutrons from a Pu-α-Be source, whose energies range up to 10 MeV. Initial testing indicates a low threshold (≈600 keV) and good spectral response after requiring a multiplicity of three segments. Such a spectrometer has applications for low-background experiments in fundamental physics research, characterizations of neutron flux in space, and the health physics community. The text was submitted by the authors in English.     

Dielectric properties of polymer structures of the azomethine base with an inclusion of a metal center

The results of studying the temperature-frequency dependences of the dielectric parameters of metal-polymer films of poly-[CumSalpn-1,3] are presented. The dispersion of polarization characteristics has been established in the ranges of frequencies f = 10²–10⁶ Hz and temperatures T = 293–340 K. The temperature dependence of the parameter of dielectric relaxation time distribution has been obtained. The role of the temperature factor in the charge processes under study has been discussed.     

Formation of narrow low-energy high-intensity electron beams

The process of formation of high-density low-energy (5–10 keV) pulsed electron beams of small diameter (on the order of a few millimeters) in a gun of the “channel-spark” type is studied. It is shown that beams with a rate of rise of the current exceeding 10¹¹ A/s and an amplitude exceeding the Alfvén current by a factor of 1.5–2.0 can be obtained in experiments with intense preionisation of the transport channel combined with a pulsed supply of the accelerating voltage to the cathode. In the optimal pressure mode, the current density at a distance of 2–3 cm from the gun outlet is 40–25 kA/cm², which will ensure ablation of most solid targets.     

A Planck-scale axion and SU(2) Yang–Mills dynamics: present acceleration and the fate of the photon

From the time of CMB decoupling onwards we investigate cosmological evolution subject to a strongly interacting SU(2) gauge theory of Yang–Mills scale, Λ ∼ 10^-4 eV (masquerading as the U(1)_Y factor of the SM at present). The viability of this postulate is discussed in view of cosmological and (astro-) particle physics bounds. The gauge theory is coupled to a spatially homogeneous and ultralight (Planck-scale) axion field. As first pointed out by Frieman et al., such an axion is a viable candidate for quintessence, i.e. dynamical dark energy, being associated with today’s cosmological acceleration. A prediction of an upper limit for Δt_mγ=0, the duration of the epoch stretching from the present to the point where the photon starts to be Meissner massive, is obtained: Δt_mγ=0∼2.2 billion years.     

Physical processes in a laser-greenhouse target: Experimental results, theoretical models, and numerical calculations

The paper is devoted to recent results concerning investigation of physical processes occurring in a “laser greenhouse” target. Results of experimental and theoretical studies of laser-pulse interaction with a low-density absorber of the target, namely, with a porous substance having density close to the plasma critical density, are presented. On the basis of a vast cycle of experiments carried out in a number of laboratories, it is shown that the absorption of the laser radiation in porous media, including those with a density exceeding the critical one by at least a factor of 4 to 6, has a bulk nature and is distributed over the target depth. In particular, the laser-radiation absorption region in a porous substance with density 10⁻³–10⁻² g/cm³ is extended into the target 400–100 μm, respectively. The coefficient of absorption of laser radiation with intensity 10¹⁴–10¹⁵ W/cm² in porous substances, including those of the supercritical density, is 70–90%. Experiments have not shown enhanced (compared to a solid-state target) radiation intensity associated with a possible development of parametric instabilities in an extended laser plasma of low-density porous media, as well as noticeable contribution of fast electrons to the energy balance and their effect on the energy transfer. In this paper, theoretical models are developed explaining features of the laser-radiation absorption and energy transfer in porous media. These models are based on the phenomenon of laser-radiation interaction with solid components of a porous substance and plasma production inside pores and cells of the medium. The efficiency of energy conversion in the vicinity of the ignition threshold for the laser-greenhouse target is investigated in the case of an absorber having the above properties. Numerical calculations have shown that a thermonuclear-gain coefficient of 1 to 2 (with respect to the energy absorbed) is attained for a laser-radiation energy of 100 kJ. Translated from Preprint No. 58 of the P. N. Lebedev Physical Institute, Moscow (1999).     

Formation of SERS-active silver structures on the surface of mesoporous silicon

We have optimized the procedure for preparation of nanostructured silver films on the surface of mesoporous silicon (PSi) to use them as active substrates in surface-enhanced Raman scattering (SERS) spectroscopy. The greatest enhancement of the SERS signal was observed for samples obtained when the silver was deposited on PSi from an aqueous AgNO₃ solution with concentration 1⋅10^–2 M over a 10–15 minute period. The detection limit for rhodamine 6G on SERS-active substrates prepared by the optimized procedure was 1⋅10^–10 M. The enhancement factor for the SERS signal on these surfaces was estimated as ≈2⋅10⁸. We have shown that SERS-active substrates based on mesoporous silicon are promising for detection and study of complex organic compounds, in particular tetrapyrrole molecules. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 2, pp. 298–306, March–April, 2009.     

Plasma-emitter electron accelerators

This paper considers the physical processes associated with the extraction of electrons from the gas discharge plasma in plasma emitters where the emission boundary is stabilized with a fine grid. The ways of improving the uniformity of the emission current density distribution are discussed. Accelerators designed on the basis of plasma emitters are capable of producing pulsed beams of current 10–10³ A, current density 0.1–1.0 A/cm², pulse duration 10⁻⁷–10⁻³ s, and cross-sectional area up to 10⁴ cm². Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 92–96, April, 2000.     

Some aspects of positronium physics

Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps → invisible) ≃ 10⁻⁸–10⁻⁷ are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. The text was submitted by the authors in English.     

Threshold of femtosecond laser-induced damage in transparent materials

The rate at which conduction-band electrons (CBE) absorb laser energy is calculated by both the quantum mechanical and the classical methods. Here fused silica irradiated with a 780-nm femtosecond-pulse laser is used as an example. It is found that the rate obtained by the quantum mechanical method is about one-third of that by the classical method, and it is much less than the direct-current limit. In the flux-doubling model, the avalanche rate in fused silica is 4 I  ps^-1 obtained by the quantum mechanical method, while it is about 13.7 I  ps^-1 by the classical method, where the laser intensity I is in units of TW cm^-2. The differential equation of the evolution of CBE density is solved numerically, and it is found that the combination of CBE–hole recombination, CBE diffusion and initial CBE density (<10¹³ cm^-3) is not important. The dependence of avalanche breakdown threshold on laser-pulse duration is presented. The threshold calculated by the quantum mechanical method agrees well with experimental results, while the threshold obtained by the classical method differs greatly from the experiments. Received: 18 December 2000 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Negative space charge and electric field intensity in polymeric insulators at low temperatures

The effect of negative space charge accumulation due to injection of electrons from cathode microprotrusions on the steady-state and transient electric field distributions in polymer dielectrics is discussed. An isolated microprotrusion is modeled by a spherical capacitor in which an electrode of smaller radius is the cathode. The calculations include the fact that the distribution of negative space charge depends on the rate of capture and liberation of electrons by traps, while the activation energy of this process depends on the electric field intensity. An exponential energy distribution is proposed for the traps. It is shown that significant electrical overvoltages can only appear near the cathode microtips immediately after switching on the voltage. In the course of 10⁻⁶–10⁻⁵ s, the coefficient of electrical overvoltage drops to a few units and approaches its steady-state value. The region of significant electrical overvoltage is localized, and is the same order as the dimensions of the microtip. Fiz. Tverd. Tela (St. Petersburg) 40, 1167–1172 (June 1998)     

Dielectric relaxation in Bi12SiO20 crystals

Anomalies of the dielectric properties of undoped and aluminum-and gallium-doped crystals of Bi₁₂SiO₂₀ are investigated in the frequency and temperature range ν=10²–10⁸ Hz and T=300–800 K. They are shown to be due to Debye relaxation processes and determined by the relaxor parameters. The mechanism of electron thermal polarization is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1223–1229 (July 1997)     

First experimental observations of neutron bursts under thunderstorm clouds near sea level

The connection of short-term neutron bursts near sea level with the electric and geomagnetic atmospheric fields during thunderstorms in 2009–2011 has been experimentally studied. The data from the cosmic-ray spectrograph named after Kuzmin, an electrostatic fluxmeter, and a three-component fluxgate magnetometer in Yakutsk have been analyzed. It has been shown that short-term (no longer than 4 min) neutron bursts are due to negative lightning discharges. The bursts are detected at the ground level 1–3 km below thunderstorm clouds. In this case, the neutron flux is about 4 × 10⁻³ cm⁻² s⁻¹. The minimum energy of the neutrons that are efficiently detected by the monitor is about 10 MeV. It has been found that short-term neutron bursts are detected when the electric field strength reaches a threshold value of −16 kV/m.     

Black hole/string transition for the small Schwarzschild black hole of AdS 5 × S5 and critical unitary matrix models

In this paper we discuss the black hole–string transition of the small Schwarzschild black hole of AdS ₅×S⁵ using the AdS/CFT correspondence at finite temperature. The finite temperature gauge theory effective action, at weak and strong coupling, can be expressed entirely in terms of constant Polyakov lines which are SU(N) matrices. In showing this we have taken into account that there are no Nambu–Goldstone modes associated with the fact that the 10-dimensional black hole solution sits at a point in S⁵. We show that the phase of the gauge theory in which the eigenvalue spectrum has a gap corresponds to supergravity saddle points in the bulk theory. We identify the third order N=∞ phase transition with the black hole–string transition. This singularity can be resolved using a double scaling limit in the transition region where the large N expansion is organized in terms of powers of N^-2/3. The N=∞ transition now becomes a smooth crossover in terms of a renormalized string coupling constant, reflecting the physics of large but finite N. Multiply wound Polyakov lines condense in the crossover region. We also discuss the implications of our results for the resolution of the singularity of the lorenztian section of the small Schwarzschild black hole.