Development of high-gain gaseous photomultipliers for the visible spectral range

We summarize the development of visible-sensitive gaseous photomultipliers, combining a semitransparent bi-alkali photocathode with a state-of-the-art cascaded electron multiplier. The latter has high photoelectron collection efficiency and a record ion blocking capability. We describe in details the system and methods of photocathode production and characterization, their coupling with the electron multiplier and the gaseous-photomultiplier operation and characterization in a continuous mode. We present results on the properties of laboratory-produced K$_2$CsSb, Cs$_3$Sb and Na$_2$KSb photocathodes and report on their stability and QE in gas; K$_2$CsSb photocathodes yielded QE values in Ar/CH$_4$(95/5) above 30% at wavelengths of 360-400 nm. The novel gaseous photomultiplier yielded stable operation at gains of 10$^5$, in continuous operation mode, in 700 Torr of this gas; its sensitivity to single photons was demonstrated. Other properties are described. The successful detection of visible light with this gas-photomultiplier pave ways towards further development of large-area sealed imaging detectors, of flat geometry, insensitive to magnetic fields, which might have significant impact on light detection in numerous fields.Comment: 22 pages, 12 figures, for submission to JINS

Further progress in ion back-flow reduction with patterned gaseous hole-multipliers

A new idea on electrostatic deviation and capture of back-drifting avalanche-ions in cascaded gaseous hole-multipliers is presented. It involves a flipped reversed-bias Micro-Hole & Strip Plate (F-R-MHSP) element, the strips of which are facing the drift region of the multiplier. The ions, originating from successive multiplication stages, are efficiently deviated and captured by such electrode. Experimental results are provided comparing the ion-blocking capability of the F-R-MHSP to that of the reversed-bias Micro-Hole & Strip Plate (R-MHSP) and the Gas Electron Multiplier (GEM). Best ion-blocking results in cascaded hole-multipliers were reached with a detector having the F-R-MHSP as the first multiplication element. A three-element F-R-MHSP/GEM/MHSP cascaded multiplier operated in atmospheric-pressure Ar/CH4 (95/5), at total gain of ~10^{5}, yielded ion back-flow fractions of 3*10^{-4} and 1.5*10^{-4}, at drift fields of 0.5 and 0.2 kV/cm, respectively. We describe the F-R-MHSP concept and the relevance of the obtained ion back-flow fractions to various applications; further ideas are also discussed.Comment: 17 pages, 11 figures, published in JINS

Dynamic model of elastoplastic normal collision of spherical particles under nonlocal plasticity

The problem of normal collision of a spherical particle with a half-space is considered with allowance for nonlocal plastic deformation in the case where the strength limit depends on the contact radius, as well as for the strengthening effect in the deformed material. The dimensionless coefficient of normal velocity restitution has been calculated numerically as a function of the initial velocity of the spherical particle. The obtained data coincide well with experimental results available in the literature

Multifractal analysis of stress time series during ultrathin lubricant film melting

Melting of an ultrathin lubricant film confined between two atomically flat surfaces is we studied using the rheological model for viscoelastic matter approximation. Phase diagram with domains, corresponding to sliding, dry, and two types of $stick-slip$ friction regimes has been built taking into account additive noises of stress, strain, and temperature of the lubricant. The stress time series have been obtained for all regimes of friction using the Stratonovich interpretation. It has been shown that self-similar regime of lubricant melting is observed when intensity of temperature noise is much larger than intensities of strain and stress noises. This regime is defined by homogenous distribution, at which characteristic stress scale is absent. We study stress time series obtained for all friction regimes using multifractal detrended fluctuation analysis. It has been shown that multifractality of these series is caused by different correlations that are present in the system and also by a power-law distribution. Since the power-law distribution is related to small stresses, this case corresponds to self-similar solid-like lubricant.Comment: 22 pages, 10 figures, 41 reference

Heat protective coatings on niobium alloys

Розглянуто різноманітні склади і технології отримання теплозахисних покриттів. Аналіз виявив, що при плазмо-дифузійному нанесенні на поверхні ніобієвого сплаву формувалось багатошарове покриття. Високопористий плазмо-напилений шар силіцидумолібдену має значний розкид по товщині (h=100...350 мкм, H20=6880 МПа). При дослідженні мікроструктури зразків з плазмо-дифузійним покриттям після випробувань виявлено, що тріщини в покритті зароджуються в процесі повзучості здебільшого на границі розділу плазмового і дифузійного шарів покриття. Осередком їх зародження є окремі несуцільності в дифузійному шарі в початковому стані. Поширення тріщин відбувається як в плазмовий, так і в дифузний шари покриття. Гальмування зростання тріщин в плазмовому шарі відбувається за рахунок округлого характеру пор і підвищеної пластичності цього шару. Зростання тріщин в глиб зразка, як правило, гальмується боридним підшаром. Перевага плазмо-дифузійної технології забезпечила підвищену пластичність покриття, наявність тонких бар'єрних підшарів, не суцільну структуру покриття, наявність легкоплавких з'єднань, що сприяють заліковуванню дефектів в покритті, підвищенню його корозійної стійкості і опору термовтомлювальному руйнуванню. Поєднання цих властивостей дозволило забезпечити підвищення довговічності в порівнянні з силіцидними і боросиліцидними покриттями в умовах ізотермічної повзучості на повітрі (1400оС, 50МПа) 1,9...3,7 рази і в умовах термоциклічної повзучості (1400 - 250оС, 50 МПа) в 6,8...8,5 раз. Визначено, що застосування дискретної структури дозволить збільшити товщину шару покриттів та забеспечити підвищення їх робочих властивостей.The article shows that during plasma-diffusion deposition, a multilayer coating was formed on the surface of the niobium alloy. A highly porous plasma-sprayed layer of molybdenum silicide has a significant spread in thickness (h=100...350 m, H20=6880 MPa). When studying the microstructure of samples with a plasma-diffusion coating after testing, it was found that cracks in the coating originate in the process of creep, mostly at the interface between the plasma and diffusion layers of the coating. The source of their origin is individual discontinuities in the diffusion layer as delivered. Crack propagation occurs both into the plasma and diffusion layers of the coating. Crack growth in the plasma layer is inhibited due to the rounded nature of the pores and the increased plasticity of this layer. The growth of cracks deep into the sample is, as a rule, inhibited by a boride sublayer. The advantage of plasma-diffusion technology provided an increased plasticity of the coating, the presence of thin barrier sublayers, a discontinuous coating structure, the presence of low-melting compounds that contribute to the healing of defects in the coating, an increase in its corrosion resistance and resistance to thermal fatigue destruction. The combination of these properties made it possible to provide an increase in durability compared to silicide and borosilicide coatings under conditions of isothermal creep in air (1400 °C, 50 MPa) 1.9...3.7 times and under conditions of thermal cyclic creep (1400-250 °C, 50 MPa) in 6.8...8.5 times. It has been determined that the use of a discrete structure will increase the thickness of the coating layer and ensure an increase in their working properties.Рассмотрены различные составы и технологии получения теплозащитных покрытий. Анализ выявил, что при плазменно-диффузионной нанесении на поверхности ниобиевых сплава формировалось многослойное покрытие. Высокопористый плазменно-напыленный слой силицидамолибдена имеет значительный разброс по толщине (h=100...350 мкм, H20=6880 МПа). При исследовании микроструктуры образцов с плазменно-диффузионным покрытием после испытаний обнаружено, что трещины в покрытии зарождаются в процессе ползучести большей частью на границе раздела плазменного и диффузионного слоев покрытия. Очагом их зарождения являются отдельные несплошности в диффузионном слое в состоянии поставки. Распространение трещин происходит как в плазменный, так и в диффузионный слои покрытия. Торможение роста трещин в плазменном слое происходит за счет округлого характера пор и повышенной пластичности этого слоя. Рост трещин вглубь образца, как правило, тормозится боридным подслоем. Преимущество плазменно-диффузионной технологии обеспечило повышенную пластичность покрытия, наличие тонких барьерных подслоев, не сплошную структуру покрытия, наличие легкоплавких соединений, способствующих залечиванию дефектов в покрытии, повышению его коррозион-ной стойкости и сопротивлению термоусталостному разрушению. Сочетание этих свойств позволило обеспечить повышение долговечности по сравнению с силицидными и боросилицидными покрытиями в условиях изотермической ползучести на воздухе (1400 оС, 50 МПа) 1,9...3,7 раза и в условиях термоциклической ползучести (1400 - 250 оС, 50 МПа) в 6,8...8,5 раз. Определено, что применение дискретной структуры позволит увеличить толщину слоя покрытий и обеспечить повышение их рабочих свойств

Fast photoprocesses in a symmetric indotricarbocyanine dye (hitc) in solutions

Spectral-kinetic and photochemical properties of HITC dye with iodide and perchlorate counterions have been studied in environments where the dye molecules exist in different ionic forms. In ethanol, the dye molecules exist as free ions; in dichlorobenzene, as contact ion pairs. Superfast transformation of non-stationary spectra in an HITC dye bleaching band is found. The observed effects are interpreted within the framework of concepts on "burning out" a notch in the contour of a non-uniformly widened vibronic band of S0 → S1-absorption. Qualitative differences in recorded absorption spectra from the dye excited electronic states for weakly and highly polar solvents are found. It is shown that the observed differences are caused by superfast charge transfer in the contact ion pairs that results in the formation of free radicals

Towards THGEM UV-photon detectors for RICH: on single-photon detection efficiency in Ne/CH4 and Ne/CF4

The article deals with the detection efficiency of UV-photon detectors consisting of Thick Gas Electron Multipliers (THGEM) coated with CsI photocathode, operated in atmospheric Ne/CH4 and Ne/CF4 mixtures. We report on the photoelectron extraction efficiency from the photocathode into these gas mixtures, and on the photoelectron collection efficiency into the THGEM holes. Full collection efficiency was reached in all gases investigated, in some cases at relatively low multiplication. High total detector gains for UV photons, in excess of 10^5, were reached at relatively low operation voltages with a single THGEM element. We discuss the photon detection efficiency in the context of possible application to RICH.Comment: 13 pages, 9 figures, 1 table. Presented at MPGD09, Crete, June 2009; to be published in JINST-proceeding

Hysteresis phenomena during melting of an ultrathin lubricant film

The influence of a deformational defect of the shear modulus on the melting of an ultrathin lubricant film was investigated in the framework of the Lorenz model used for describing a viscoelastic medium. It was established that the film can undergo both stepwise and continuous melting. Analysis of the lubricant behavior revealed that there are three modes corresponding to a zero shear stress, a Hookean portion in the loading diagram, and a plastic-flow portion. The hysteresis in the dependences of the stationary shear stress on the strain and the friction surface temperature is examined. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/1621