Slow Emission Research Articles

Influence of slow wave structure explosive emission on high-power surface wave oscillator

As the working frequency of a vacuum electron device reaches the terahertz frequency band, the cross section of the surface wave oscillators (SWO) becomes very small, and the micro-fabrication precision of the device cannot be guaranteed, at the same time, because the electromagnetic field of SWO concentrates on the inner surface in slow wave structure, when the working voltage of surface wave oscillator is very high, the explosive emission probability of the slow wave structure increases greatly, and the explosive emission can influence the working characteristic of the device. This paper analyses the distributing property of the electrical field in the slow wave structure of 0.14 THz SWO. Parameters of the SWO under study are as follows: working voltage is 312 kV, explosive emitted current is 1.67 kA, periodic length of the slow wave structure is 0.7 mm, width of the slot is 0.4 mm, and the height of it is 0.3 mm; cold-test results indicate that the amplitude of the electrical field in the slow wave structure varies sinusoidally; the amplitude of the electrical field reaches a maximum value in the middle of the slow wave structure near its inner surface, and the explosive electron emission can occur most possibly in this position, because the electrical field in the slow wave structure varies with very high working frequency. The explosive emitted electron may bombard back the slow wave structure, and the secondary electrons will be emitted at a certain probability, for which the formula proposed by Vaughan is used to compute the secondary emission yield, and this formula is implemented in the self-developed particle-in-cell code UNIPIC; while the code is used to simulate 0.14 THz SWO with explosive emission in the slow wave structure. In the simulation, the slow wave structure multipactor discharge induced by electrons is also considered; the phase space of the electrons emitted from the slow wave structure shows that the energy of secondary electrons is below 5 keV, so the validity for secondary electron yield is affirmed. Numerical simulation results indicate that because the emitted electrons from the slow wave structure change the distribution character of the electrical field in the slow wave structure, especially the amplitude of the electrical field in the middle of the slow wave structure, the beam-wave interaction is weakened, and as a result, output power decreases from about 22.6 megawatts to only 1.89 megawatts.

Stereotactic radiotherapy using tomotherapy for early-stage non-small cell lung carcinoma: analysis of intrafraction tumour motion.

Intrafraction tumour motion in helical tomotherapy was investigated by comparing pre- and mid-fraction CT scans in patients with early non-small cell lung carcinoma (NSCLC) to assess the efficacy of a 7-mm margin around gross tumour volumes (GTVs) in stereotactic body radiation therapy (SBRT). Thirty patients with early-stage NSCLC received SBRT in four or five fractions for a total of 141 treatments. A slow positron emission tomography/CT scan was fused with the simulation CT to determine the GTV. A planning target volume was created by placing an isotropic margin of 7 mm around the GTV. Data were retrospectively analyzed to assess translational tumour positional changes along the x, y and z axes and vector changes in millimeters from the pretreatment megavoltage (MV)-CT to the mid-fraction MV-CT. Average movements for all 141 treatment days along the x, y and z axes were 0.5 ± 2.3, -0.3 ± 3.0 and 0.9 ± 3.0 mm, respectively. Average movements for each patient along the x, y and z axes were 0.5 ± 1.5, -0.2 ± 2.0 and 0.9 ± 1.9 mm, respectively. Average vector displacement was 4.3 ± 2.4 mm for all treatment days and 4.2 ± 1.7 mm for each patient. Of 141 treatments, 137 (97.2%) fell within 7.0 mm in all axes. The addition of a 7-mm margin to the GTV for patients receiving SBRT for NSCLC using tomotherapy is adequate to account for tumour movement. Mid-fraction CT scans proved to be valuable in assessing intrafraction tumour motion.

Energy Transfer and Scintillation Properties of <formula formulatype="inline"><tex Notation="TeX">${\hbox{Ce}}^{3+}$</tex></formula> Doped <formula formulatype="inline"><tex Notation="TeX">${\hbox{(LuYGd)}}_{3}$</tex></formula><formula formulatype="inline"><tex Notation="TeX">${\hbox{(AlGa)}}_{5}{\hbox{O}}_{12}$</tex> </formula> Multicomponent Garnets

Photoluminescence, X-ray excited radioluminescence and photoelectron yield characteristics were studied in multicomponent GdGaYAG:Ce and GdGaLuAG:Ce garnet films grown by liquid phase epitaxy. Nonradiative energy transfer from Gd3+ to activator Ce3+ ions was evidenced. At low Gd doping, , two virtually independent emission centers coexist, which compete for intensity: Ce3+ with fast 5d-4f emission and Gd3+ with slow 4f-4f emission. At higher Gd concentrations, the Ce3+ emission retrieves intensity due to the energy transferred from Gd3+ donors. At Gd concentration ~ 50% and above, the Gd3+ emission is suppressed due to energy migration and transfer to Ce3+ centers. In these samples, the host emission is also completely quenched and the best scintillator performance is achieved. Contrary to melt-grown crystals, no positive role of Ga substitution was found in epitaxial films.

Development of a 2D dosimetry system based on the optically stimulated luminescence of Al2O3

A laser-scanning 2D dosimetry system based on the Optically Stimulated Luminescence (OSL) signal from Al2O3 films was built and demonstrated. The main challenge of using the OSL from Al2O3 for 2D dosimetry by laser scanning is the long lifetime (∼35 ms) of the main luminescence centers in this material (F-centers). In this work, we demonstrated the possibility of performing 2D dosimetry by laser scanning using a combination of the fast UV emission of F+-centers (lifetime <7 ns) and the slow F-center emission of Al2O3:C, and an algorithm to correct for the slow F-center luminescence lifetime. We also investigated the possibility of using Al2O3:C,Mg, to take advantage of its greater F+-center emission compared to Al2O3:C. Results from 6 MV photon beam irradiations from a clinical linear accelerator were compared to radiographic and radiochromic film profiles showing a good qualitative agreement.

Temperature-independent slow carrier emission from deep-level defects in p-type germanium

In the deep-level transient spectroscopy (DLTS) spectra of the 3d-transition metals cobalt and chromium in p-type germanium, evidence is obtained that hole emission from defect levels can occur by two parallel paths. Besides classical thermal emission, we observed a second, slower and temperature-independent emission. We show that this extra emission component allows determining unambiguously whether or not multiple DLTS peaks arise from the same defect. Despite similar characteristics, we demonstrate that the origin of the non-thermal emission is not tunnelling but photoionization related to black-body radiation from an insufficiently shielded part of the cryostat.

In-situ metalorganic chemical vapor deposition and capacitance-voltage characterizations of Al2O3 on Ga-face GaN metal-oxide-semiconductor capacitors

The in-situ metalorganic chemical vapor deposition of Al2O3 on Ga-face GaN metal-oxide-semiconductor capacitors (MOSCAPs) is reported. Al2O3 is grown using trimethylaluminum and O2 in the same reactor as GaN without breaking the vacuum. The in-situ MOSCAPs are subjected to a series of capacitance-voltage measurements combined with stress and ultraviolet-assisted techniques, and the results are discussed based on the presence of near-interface states with relatively fast and slow electron emission characteristics. The in-situ MOSCAPs with Al2O3 grown at 900 and 1000 °C exhibit very small hystereses and charge trappings as well as average near-interface state densities on the order of 1012 cm−2eV−1.

Crossluminescence of Nanosized KYF$_{4}$

A series of KYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nano-powders, undoped and doped with Tb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , with well-crystallized, unaggregated, monodisperse (± 15%) nanoparticles of cubic or hexagonal crystal structure and the size in the range from ~ 10 to ~ 40 nm has been synthesized by microwave-hydrothermal method. It was found that crossluminescence in KYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanopowders is strongly quenched whereas the slow near-defect excitonic emission as well as the Tb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> 4 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> -4 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> luminescence in Tb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> doped KYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanopowders is excited rather efficiently even for small sizes of particles.

Pulsed cathodoluminescence of calcite crystals of various origins

We have studied the luminescence of calcites from phlogopite-calcite veins, marbles, leucogranites, apatite-calcite ores, and carbonatites in the spectral range 300–800 nm under excitation with nanosecond electron pulses at an electron beam current density of ∼10 A/cm2. All of the calcite varieties have a fast emission component, with a broad spectrum in the near-UV and visible spectral regions, whose decay time is comparable to the excitation pulse duration, and a slow emission component, with a peak-emission wavelength of 610–620 nm and decay time of tens of milliseconds. The spectral composition of the emission depends on the nature of the calcite sample. We analyze the evolution of the spectrum after excitation and discuss the luminescence excitation mechanisms and the nature of the emission centers.

Direct radiative recombination in the Se-terminated nanoscale Si porous structure

We prepared vertical porous silicon nanowire (SiNW) arrays using the silver-assisted electroless etching method. The post-selenization treatment was carried out to passivate the surface of the nanostructure with Se-related bonds. Besides the great enhancement of the photoluminescence (PL) intensity and stability, the slow red emission band from the untreated porous SiNWs disappears and the new PL bands blue-shift to the higher energy, with the recombination rate more than three orders of magnitudes faster than that of the red emission band. The lifetime of 0.49 and 2.68ns are attributed to the recombination in the Si nanostructures passivated with SiSe, and SiSeO bonds, respectively. The fast recombination rates indicate that surface modification induced by selenization treatment could lead to the direct radiative recombination of Si nanostructure. Furthermore, a broad near-infrared emission band located at around ∼1300nm is also observed. These results are thought useful in the band gap engineering of Si.

Bright Visible Luminescence in Silica Nanoparticles

We demonstrate that a porous film of silica nanoparticles emits a bright visible luminescence associated with defects stabilized by oxygen chemisorption at oxygen-deficient center sites. Time-resolved spectra excited by a tunable laser allow us to distinguish the luminescence at 1.99 eV, characteristic of the nonbridging oxygen hole center (NBOHC) (≡Si–O)3Si–O•, and a fast and a slow emission: the first (lifetime τ ≈ 25 ns) is peaked at 2.27 eV with an excitation spectrum centered at 5.5 eV; the second (τ ≈ 7.5 μs) is peaked at 2.41 eV and is excited around 3.2 and 5.2 eV. Reaction in an air atmosphere leads to the disappearance of the NBOHC luminescence and of the fast band, whereas the slow one remains stable. On the basis of the comparison with previous experimental and computational works, we discuss the role of the silanone Si═O and of the dioxasilyrane Si(O2) as the emitting defects.

Alkaline phosphatase-labeled macromolecular probe for sensitive chemiluminescence detection of proteins on a solid-phase membrane

In the present study, we synthesized dextran (MW = ca. 2,000 kDa)-based macromolecular probes containing multiple molecules of alkaline phosphatase (ALP) as a signal-trigger enzyme and of biotin as an assembly mediator. The ALP and biotin molecules were covalently attached into the dextran backbone after the formation of aldehyde groups into the macromolecule by periodate oxidation. The synthesized probes contained 27-31 molecules of ALP in their macromolecules when 50-fold molar ratio of ALP to the dextran was used for the synthesis. These probes provided 14-20 times stronger chemiluminescence (CL) than that of the equimolar free ALP adsorbed on a nylon membrane. The velocity of the CL reaction of ALP-catalyzed adamantlyl-1,2-dioxetane substrate was improved from a slower emission (glow type) of CL to a faster one (flash type). The CL signal integrated for 2 min under strongly alkaline conditions (pH 13.0) was about ten times greater than that obtained by the conventional conditions (pH 9.5). Therefore, the synthesized macromolecular probe could be successfully utilized for the high-throughput CL detection of biotin-conjugated anti-rabbit IgG antibody with a lower detection limit of 880 amol per spot on the nylon membrane. This study provides analytical strategy for the rapid, convenient, and sensitive detection of target proteins in immunoassays.

Photophysics of fluorinated benzene. I. Quantum chemistry

The electronic structure of energetically low-lying excited singlet states of fluorobenzene molecules is investigated here. Increasing fluorine substitution alters the nature of the excited electronic states and the so-called perfluoro effect is observed for penta- and hexafluorobenzene. Detailed quantum chemistry calculations are carried out at the equation-of-motion coupled-cluster singles and doubles level of theory to establish the potential energy surfaces of the low-lying electronic states of mono-, di- (ortho- and meta-), and pentafluorobenzene molecules. A sequence of low-energy conical intersections among the electronic potential energy surfaces is established. It is found that increasing fluorine substitution lowers the energy of the pisigma* electronic state and leads to conical intersections between the S(1) and S(2) electronic states of pentafluorobenzene. Existence of numerous conical intersections among the excited electronic states of these molecules forms the mechanistic details underlying their nonradiative internal conversion. In particular, the slow and biexponential fluorescence emission in pentafluorobenzene is attributed to the existence of low-lying S(1)-S(2) conical intersections. The electronic structure data are analyzed in detail and the coupling mechanism among various electronic excited states of mono-, di-, and pentafluorobenzene molecules is established.

ChemInform Abstract: Solar Energy Conversion at the p-InP/Vanadium3+/2+ Semiconductor/ Electrolyte Contact. A Study Based on Differential Capacitance and Current-Voltage Data.

Previously unachieved values are obtained for the open-circuit voltage (0.7 to 0.8 V) and the fill-factor (76%) at the illuminated p-InP/V[sup 3+/2+]-HCl semiconductor/electrolyte contact ( face) coated with a submonolayer amount of silver. Photoelectrochemical solar cell efficiency is 11%, despite significant optical reflection and electrolyte absorption losses. Differential capacitance measurements demonstrate a semiconductor/surface energy barrier height of up to [psi][sub B] = 1.2 eV, independent of the Ag treatment. This value is considerably larger than typically found at solid-state p-InP/Ag contacts. Barrier heights derived from Mott-Schottky data are compared to photovoltages; results indicate that interface recombination and a slow charge-transfer (bare p-InP) and thermionic emission from the metal (Ag-treated p-InP) are important factors limiting the cell efficiency. Flatband potential shifts owing to a slow charge-transfer under photocurrent flow are reduced by the Ag treatment. In a finite potential range of about 400 mV (0 to [minus]400 mV/SCE), both photovoltages and Mott-Schottky data reveal an unpinned Fermi level at bare and Ag-treated electrodes. The unpinned barrier follows the ideal Schottky barrier model for an absolute SCE potential of [minus]5.0 eV/SCE. Reference to related work at InP and GaAs is made.

Further studies of two-phase avalanche detectors: Electron emission and gamma-ray detection properties

Characteristic properties of two-phase avalanche detectors are further studied. Such detectors are relevant in the field of coherent neutrino-nucleus scattering, dark matter search and positron emission tomography (PET). The detectors investigated comprised a liquid Ar, Ar+N2 or Xe layer followed by a multi-GEM multiplier with CsI photocathode operated in saturated vapor. It was found that doping of two-phase Ar with N2 removed the slow electron emission component, existing in two-phase Ar, and enhanced the fast component. The electron emission efficiency was close to 100% both in two-phase Ar and Ar+N2 even at lower electric fields. The detection of 511keV gamma-ray induced signals was demonstrated in two-phase Ar (scintillation signals) and two-phase Xe (ionization signals) avalanche detectors, in connection with PET applications.

Recombination luminescence of CsI(Tl) under electron pulse irradiation

Abstract The luminescence kinetics of CsI(Tl) exposed to an electron pulse irradiation ( E e = 250 keV, t 1/2 = 10 ns, j = 2 ÷ 160 mJ/cm 2 ) has been studied. It has been discovered that the slow emission rise is due to hole V k –Tl 0 recombination luminescence at temperature from 100 to 160 K and electron–V kA recombination, where electrons released from single Tl 0 at temperature from 180 to 300 K. The effect of Tl concentration on both processes has been investigated.

Electron emission properties of two-phase argon and argon-nitrogen avalanche detectors

Electron emission properties of two-phase Ar avalanche detectors are studied. The detectors investigated comprised a liquid Ar or Ar+N2 layer followed by a multi-GEM multiplier operated in the saturated vapour at 84 K. Two components of the electron emission through the liquid-gas interface were observed: fast and slow. In Ar, the slow emission component dominated even at higher fields, reaching 2 kV/cm. In Ar+N2 on the contrary, the fast emission component dominated at higher fields, the slow component being disappeared. This is explained by the electron backscattering effect in the gas phase. The slow component decay time constant was inversely proportional to the electric field, which is compatible with thermionic emission model. The electron emission efficiencies in two-phase Ar and Ar+N2 were estimated to be close to each other.

Luminescence characteristics of LuAG:Pr and YAG:Pr single crystalline films

Emission and excitation spectra and luminescence decay kinetics were studied for Pr 3+-doped Lu 3Al 5O 12 and Y 3Al 5O 12 single crystalline films (SCF) grown by the liquid phase epitaxy method from a PbO-based flux. The influence of lead-induced centers on their scintillation characteristics was clarified. It was found that the influence of single Pb 2+-based centers on the characteristics of Pr 3+ centers due to the Pb 2+ → Pr 3+ energy transfer was weak. However, an overlap of the emission spectra of single and dimer lead-induced centers with the emission spectrum of Pr 3+ ions, and especially a strong overlap of the 4f–5d 1 absorption band of Pr 3+ ions with the slow emission of localized excitons in the 290 nm band had a considerable influence on the scintillation characteristics of the Pr 3+-doped SCF.

Noisy breathers and temperature-dependent luminescence decay

Discrete breathers created in the immediate neighborhood of an impurity were introduced to explain an experimentally observed decay anomaly of the slow emission component in doped alkali halides. We study the effect of temperature on these breathers to find a correlation with the corresponding dependence of the anomaly as seen in experiments. As the temperature increases the decay anomaly weakens and is ultimately extinguished. In the light of the breather explanation this corresponds to temperature-induced breather decay. We present a study of the dissipative effects of temperature on lattice breathers.

Fast and efficient VUV/UV emissions from (Ba,La)F 2:Er crystals

We report observation of fast and efficient VUV/UV luminescence from the mixed (Ba,La)F 2:Er crystals. The broad bands, peaking at 162.5, 181.9, 194.2, 202.8, 216.1, 233.5 and 281.5 nm and decaying, at 10 and 293 K, with time constants of 46 and 35 ns respectively, are due to spin-allowed transitions from the low-spin (LS) state of the 4f 105d configuration. We also observed a weak and slow broad band emission peaking at 170 nm due to the spin-forbidden transition from the high-spin (HS) state of the 4f 105d configuration. While at room temperature the excitation into any of the three identified LS bands ( J=8, 7 and 6) dominating the excitation spectrum yields fast VUV and UV emissions, at 10 K the excitation into higher lying J=7 and 6 bands generates slow and sharp line emissions. The positions of these lines fit energies of transitions originating from the 2G 7/2 multiplet at 66140 cm −1. The emission from the 2G 7/2 multiplet has been never, to the best of our knowledge, observed before. The efficient and fast VUV and UV emissions from the higher (LS, J=8) with almost no contribution from the lower (HS, J=8) level of the 4f 105d configuration are possible because the modified crystal field in (Ba,La)F 2 shifts the level of the (LS, J=8) state below the 2F 5/2 multiplet which, therefore, does not contribute to nonradiative relaxation between the LS and HS levels. We conclude that the 2G 7/2 and 2F 5/2 levels have major impact on VUV and UV emissions from the Er 3+ ion in (Ba,La)F 2 contributing to complex emission pattern described in this report Their key role, elucidated by the VUV and UV luminescence spectroscopy, is consistent with predictions from a simple configuration coordinate model based on experimental results and calculations of the 4f 11 energy levels.

Simulations of Capacitance–Voltage–Temperature Behavior of Metal/Insulator/AlGaN and Metal/Insulator/AlGaN/GaN Structures

The effect of electronic states at insulator/AlGaN and AlGaN/GaN interfaces on capacitance–voltage (C–V) characteristics of a metal/insulator/semiconductor (MIS) capacitor with an AlGaN layer and a metal/insulator/semiconductor heterostructure (MISH) with an AlGaN/GaN heterojunction was studied theoretically taking into account extremely slow electron emission from the deep interface levels. The states at the insulator/AlGaN interface in MIS and at the AlGaN/GaN interface in MISH stretch out the C–V curves as usual, whereas the insulator/AlGaN interface states in MISH only shift the characteristics at room temperature and 300 °C (fixed-charge-like behavior). These effects were explained by the different positions of the Fermi level at the studied interfaces. Theoretical C–V curves were compared with experimental characteristics, which are available in the literature, measured at room and higher temperatures by the authors and others.