Electron processes in a liquid-xenon ionization chamber under conditions of high-intensity pulsed irradiation

The results of experimental investigation and simulation of electron processes in a liquid-xenon-filled ionization chamber, which is irradiated with bremsstrahlung-radiation pulses from a synchrotron, are presented. The radiation dose was varied from 10^?6 to 0.11 Gy/pulse. The response of the chamber to an intense pulsed excitation and the recovery of its serviceability after intense irradiation were studied. It has been shown that, at doses >10^?4 Gy/pulse, the space charge produced in the chamber has a noticeable effect on the formation of electron-conductivity pulses. The shape of the current pulse in the chamber, inside which a space charge is produced, is studied, the pulse parameters are measured, and a model is proposed with allowance for the effect of the space charge. It has been shown that, at doses exceeding the critical values, the space charge disappears within ～50–100 μs, the durations of the transient processes in the electric circuits being as long as ～500 μs. Hence, the spectrometric characteristics of the ionization chamber are restored ≤1 ms after the irradiation terminates.     

A subnanosecond pulsed source of electrons and X-rays

We measured the time and dose characteristics of electron and X-radiation of the иMA3-150З tube connected to the subnanosecond megavolt CпиH-2 accelerator. About ∼5×10¹² electrons per pulse are generated when the accelerating-voltage pulse is ≥600 kV high and ≈=0.3 ns long. The current amplitude reaches ∼5 kA, and the radiation dose is ∼5 kGy/pulse. The X-ray dose from the external tantalum target is 0.15 Gy/pulse. The development of the electron-tube and X-ray-tube prototypes with considerably smaller sizes has opened up new fields of application in medicine and engineering.     

The amplitude and current pulse duration of a supershort avalanche electron beam in air at atmospheric pressure

The results of experimental studies of the parameters (amplitude and duration) of a supershort avalanche electron beam (SAEB) generated in air at atmospheric pressure are presented. It is shown that the pulse duration of the beam current behind the foil from the entire area of the anode foil is larger than from small areas and depends on the cathode design. The number of electrons that are detected behind the 10-μm-thick Al foil is ∼6 × 10¹⁰ electrons, which corresponds to a SAEB amplitude of ∼100 A at a FWHM of the current pulse of ∼100 ps. An X-ray exposure dose per pulse of ∼1.8 mR was obtained using a 20-μm-thick copper foil. It was confirmed that the FWHM of a SAEB pulse is within ∼50 ps from small foil areas (with diameters of ∼7 mm or smaller).     

Studying the spectrum of neutrons emitted from the LSDS-100 using a multiwire proportional fission chamber

A fast multiwire proportional fission chamber capable of detecting ∼10⁶ fission fragments per second was produced. Heptane vapor at a pressure of 15 mbar was used as a working gas. The full width at the base of the pulse from a fission fragment was ≤0.2 μs. The spectrum of the neutron flux density on the spectrometer surface was measured by the neutron slowing-down time in the lead of the LSDS-100 at neutron energies ranging from 18 eV to 11.3 keV.     

A silicon position-sensitive detector of charged particles and radiations on the basis of functionally integrated structures with nano-micron active regions

A novel pixel array two-coordinate detector with internal amplification is described. Linear amplification of the primary ionization current is attained by using functionally integrated structures of a p-i-n diode and a bipolar transistor in the detector pixel. It is shown that a 16-fold decrease (from 0.64 to 0.04 μm²) in the emitter area of the functionally integrated structures allows the current gain of pixel bipolar structures to be increased from 10 to 80 at a pixel collector current of ∼1 nA. As a result, it is possible to obtain spectra of radiations weakly interacting with the silicon (e.g., γ rays) and to reduce the response time of the position-sensitive detectors by an order of magnitude, down to 100 ns and less. It is demonstrated that the detector can be used in fast position-sensitive detection of particles with time, coordinate, and energy resolutions of ≤100 ns, ≤25 μm, and ≤12%, respectively.     

A method of controllable pulse-heating for investigation of properties of short-lived liquids

A method of controlled pulse heating of low-inertia thermal probe immersed into the liquid under study with a temperatureT ₀ is described. The control system provides a “temperature plateau”-type heating mode, which consists in a rapid (t ₁∼10 μs) increase in the mass-average probe temperature to a chosen valueT _pl ≫T ₀ and maintains this value for a certain time interval (t ₂∼10³–10² μs) to within 1 K. Thermal effusivity of the substance, in relative units, is determined from the value of its internal heat flux. Sensitivity to changes in the thermal effusivity of a reference substance was 10⁻⁴. Due to the short pulse length and fine tuning of theT _pl value, the method allows one to conduct step-by-step scanning of “instantaneous” thermal properties of a substance in the region of its short-lived states.     

A Hemispherical Fission Chamber for the Neutron-Flux Monitoring

A fission ionization chamber has been designed to monitor neutron fluxes produced by the proton beam of the Van de Graaf accelerator in the ⁷Li(p, n)⁷Be nuclear reaction. A target producing neutrons is placed at the center of the chamber, which has a hemispherical shape. The neutrons escaping from the target are detected within the limits of a space angle a little larger than 2 sr. A ²³⁵U isotope is used as a fissile material. A U₃O₈ layer with a thickness of 300 g/cm² and an area of 80 cm² has already been deposited on the surface of the inner hemisphere using electrolysis. The chamber is filled with a gas mixture of 97% Ar + 3% CO₂. The detection efficiency of the chamber for neutrons with 1- to 150-keV energies is found to be 1.6 × 10^–5. Using this chamber, it is possible to measure (with a statistical accuracy of 1%) the integrated neutron yield in a thick target exposed for 20 min to a proton beam with a current of 3 A and an energy of 1942 keV, which is 60 keV above the threshold of the ⁷Li(p, n)⁷Be reaction. The design of the fission chamber is described, and the test results are presented.     

Measuring the extraction time of negative ions from the ionization chamber of a static mass spectrometer

A technique for measuring the time of extraction of negative ions from the ionization chamber of the ion source of an MИ-1201“B” static mass spectrometer is described. The time of extraction of SF₆ negative molecular ions equal to ∼(8.5 ± 1.0) μs is comparable to the ion time of flight in the tube of the analyzer.     

A system of multiwire drift chambers for wide-aperture spectrometers

A system of wide-aperture drift chambers with an operating area of 2.5×1.9 m² and 1.5×1.5 m² is described. The chamber, operating with two gas mixtures, is studied. The dependence of the chambers efficiency on the high voltages applied to the cathodes and potential wires, and the time distributions are presented. Using the system of drift chambers, a spatial distribution of ∼ 180 μm was achieved for straight tracks in a 8-mm drift gap. The chambers were included in the track systems of the VES and GAMS spectrometers at the Y-70 accelerator of the Institute for High-Energy Physics (Protvino).     

On the DMT theory

The frequent claim that the Tabor parameter μ governs the transition from the DMT theory to the JKR theory is investigated. The change from the simple surface force law σ ∼ A/h ³ of the DMT theory to the Lennard–Jones law σ ∼ A/h ³–B/h ⁹ of the MDT theory and the numerical solutions is noted, and the ‘adhesive force’ is evaluated for both laws. Except in the limit of zero Tabor parameter, when the Derjaguin theories reduce to the rigid-sphere model, the predictions are consistently worse than assuming the sphere to be rigid. A ‘semi-rigid’ sphere model is proposed, which correctly describes the asymptotic behaviour as μ → 0, but leaves a considerable gap before the JKR theory can be applied.     

A source of broad homogeneous beams of low-energy (∼0.5 keV) gas ions

A source of gas ions (argon, oxygen, nitrogen, etc.), the operating principle of which is based on the use of a glow discharge in an electrode system of a wide-aperture hollow cathode and anode in a magnetic field, is described. The exit aperture diameter of the hollow cathode, increased up to a size close to the ion beam diameter (10 cm), ensures the uniform ion emission of the plasma generated in the discharge region near the anode. A decreased angular divergence or increased ultimate ion-beam current density is achieved by a change in the potential drop in the space charge sheath between the plasma and the ion optics. The source generates broad (50 cm²) slightly diverging (ω/2∼3°–5°) ion beams with energies of 300–1000 eV at a beam current density of ∼0.5 mA/cm².     

A light-erosion method for high-pressure dust-gas-plasma flows generation

A light-erosion method for generating high-pressure dust-gas-plasma flows during ultraviolet (λ = 213 nm) laser ablation of a polymeric matrix ((C₂F₄) _n ) containing dust particles (thin-wall borosilicate glass microspheres with a ∼15- to 80-μm diameter d) is described. The carrying-out of the dust particles by ionized vapors of the substance of the target matrix, their space-time localization in the gas-plasma flow in a period Δτ ∼ 15–75 μs after the laser exposure up to the further spatial separation of the vaporized substance of the target matrix and the dust particle cloud is recorded by laser interferometry and shadow photography methods. The importance of certain selection of the matrix-dust system for realizing the light-erosion method for generating dust-gas-plasma flows, in particular laser exposure conditions, is shown. When condensed media with a low ionization potential (Al, Ce) are used as a dust component, the proposed method for generating heterogeneous gas-plasma flows can be efficient for their further heating by coherent radiation.     

A comparative study of switching possibilities of silicon diodes with different axial distribution shapes of recombination centers

Three groups of silicon p ⁺-n-n ⁺ diodes with a small turn-off time ensured by charged particle irradiation were studied in the high-power (3 kA) and short (50 μs) current pulse switching modes. The diodes of the first group were irradiated by electrons with a 550-keV energy, those of the second group were irradiated by protons with a 2.5-MeV energy and then by electrons with a 6-MeV energy, and those of the third group were irradiated by electrons with a 6-MeV energy. The studied diodes have a 16-mm semiconductor element diameter and 3-kV maximum permissible disabled voltage. The radiation doses were selected in such a way as to obtain an approximately equal current carrier lifetime near the p ⁺-n junction (4 μs), measured by the Lacks method, when the forward current density is 1 A/cm². It was determined that, when diodes of the first group change to the OFF position, the peak power of energy losses is two and more times smaller than that of diodes of the second and third groups, when they are switched off. The diodes of the first group also feature a substantially smaller voltage, when the forward current is at maximum (by two times as compared with diodes of the second group and by seven times as compared with diodes of the third group) and significantly smaller (by several times) voltage spike at a sharp forward current increase instant.     

A radiation foil bolometer for measuring the energy losses of fast Z-pinches

A radiation foil bolometer intended for measuring radiation-energy fluxes and plasma flows in high-power pulsed plasma X-ray sources is studied. In the bolometer, the radiation or ion-flow energy is absorbed by a 13-μm-thick niobium foil preliminarily heated to T ≈ 1900 K by a quasi-constant current lasting 2 s. Radiation from the foil was fed through an 8-m-long light guide to a semiconductor photodetector. The sensitivity of the instrument obtained from the results of calibrations is 0.56 J/V cm². The time resolution of this technique (∼2.5 μs) is determined by the foil heating time. In experiments with megaampere Z-pinches, the time resolution attained allows detection of the radiation energy independently of the energy of expanding plasma flows. In this case, the photoelectric effect from X-rays and the conductivity of expanding plasma do not affect the operation of the radiation bolometer. The working capacity of the bolometer was demonstrated at the Angara-5-1 facility in experiments with high-current Z-pinches with a radiation output energy of 50–100 kJ/pulse. The measurements performed have shown that the energy flux density of plasma expanding in the direction perpendicular to the pinch axis is at most 5% of the energy flux density of soft X-rays.     

A high-energy YAG: Nd<Superscript>3+</Superscript> laser with a phase conjugation in an optical fiber

An optical scheme and design of a YAG:Nd³⁺ frequency laser with a two-pass amplifier and a mirror with the phase conjugation by the stimulated Brillouin scattering in an optical fiber is described. The maximal radiation energy is 1000 mJ at a 1.064-μm fundamental wavelength and 500, 200, and 20 mJ at the second, fourth, and fifth harmonics, respectively, when the divergence is ∼5 × 10⁻⁴ rad and the pulse duration is 4.5 ns.     

A Study of the Electronic Properties of Liquid Argon–Deuteromethane Mixtures

The serviceability of a pulse ionization chamber filled with liquefied argon–deuteromethane mixtures was demonstrated. The substances were purified to a level of 1.7 × 10^–9equiv O₂using a Ni/SiO₂adsorbent. The dependences of the ionization yield from relativistic-electron tracks and the drift velocity of ionization electrons on the electric-field strength were measured in the mixtures with deuteromethane concentrations of 2, 5, 10, and 25%. The properties of liquid CH₄and CD₄were compared. An isotopic effect was observed for the electron-drift velocity.     

The Time Resolution of a Vacuum Fission Chamber

The process of the formation of a signal by a vacuum fission chamber (VFC) under the irradiation by a short-duration pulsed neutron flux is considered for the first time. It is shown that the collection time of charge carriers in the VFC with plane–parallel electrodes does not exceed 0.4 ns at a supply voltage of >100 V and that the square of the VFC pulse width is inversely proportional to the supply voltage. The minimum detectable flux densities for fast and thermal neutrons are estimated at 5 × 10¹⁰ and 10⁸ cm^–2 s^–1, respectively, at a noise current of 10^–3 A for the electronic equipment.     

Effect of the Dose Rate and Temperature on Radiation-Induced Charge Separation in Cables with Mineral Insulation under Pulsed Reactor Irradiation

The results of studying the level of charge separation between the core and sheath in cables with mineral insulation and nickel and stainless-steel conductors are presented. The cables were irradiated in a BARS-6 pulsed reactor with dose rates of 3 × 10³ and 4 × 10³ Gy/s at temperatures of 270–650 K. A mechanism for elucidating the complex temperature dependence of the level of temperature-induced charge separation is proposed.     

YPT-0.5 repetitive-pulse nanosecond electron accelerator

An YPT-0.5 repetive-pulse nanosecond electron accelerator designed according to the thyratron-pulse transformer-semiconductor opening switch scheme is described. Its accelerating voltage reaches 0.5 MV, the FWHM pulse duration is 50 ns, and the pulse repetition rate is 200 Hz. A metal-dielectric cathode allows for obtaining an electron beam with a diameter of 30–100 mm at a maximum pulse current density of 40 A/cm². Operating in the bremsstrahlung generator mode, the accelerator provides an absorbed dose rate of 30.4 Gy/min at a distance of 5 cm from the target.     

A nanosecond light source for scintillation-and Cerenkov-detector calibration

A nanosecond blue-light source with increased brightness is described. A light-emitting diode by NICHIA is used. The pulse shaper triggering the light-emitting diode is based on avalanche transistors. The number of photons per pulse is ∼10⁹ at a light-pulse duration of ∼2 ns.