Effect of Surface Potential Barrier on the Electron Energy Distribution of NEA Photocathodes

By calculating the energy distribution of electrons reaching the photocathode surface and solving the Schrodinger equation that describes the behavior of an electron tunneling through the surface potential barrier,we obtain an equation to calculate the emitted electron energy distribution of transmission-mode NEA GaAs photocathodes. Accord- ing to the equation,we study the effect of cathode surface potential barrier on the electron energy distribution and find a significant effect of the barrier-Ⅰ thickness or end height,especially the thickness,on the quantum efficiency of the cath- ode. Barrier Ⅱ has an effect on the electron energy spread, and an increase in the vacuum level will lead to a narrower electron energy spread while sacrificing a certain amount of cathode quantum efficiency. The equation is also used to fit the measured electron energy distribution curve of the transmission-mode cathode and the parameters of the surface barri- er are obtained from the fitting. The theoretical curve is in good agreement with the experimental curve.

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Untersuchung von NEA-Photokathoden mittels zeitlich hochauflösender Vermessung von Intensitäts- und Spinpolarisationsverteilungen

Gegenstand dieser Arbeit ist die Untersuchung von Photokathoden mit negativer Elektronenaffinität (NEA) mittels zeitlich hochauflösender Vermessung der emittierten Ladungs- und Spinpolarisationsverteilungen nach Anregung mit einem ultrakurzen Laserpuls. Untersucht wurden uniaxial deformierte GaAsP-Photokathoden mit dünnen emittierenden Schichten (≤150nm), sowie undeformierte GaAs-Photokathoden mit unterschiedlichen Schichtdicken. Die Untersuchungen wurden an einer 100keV-Elektronenquelle durchgeführt, wie sie am Mainzer Mikrotron (MAMI) zur Erzeugung eines Spinpolarisierten Elektronenstrahls verwendet wird. Mit der Apparatur konnte eine Zeitauflösung von 2,5ps erreicht werden. Es zeigte sich, dass die tatsächliche Antwortzeit der Photokathoden die erreichte Zeitauflösung noch unterschreitet. Eine Depolarisation in den kurzen, wegen der Zeitauflösung auf 2,5ps begrenzten, Elektronenpulsen konnte aber nachgewiesen werden. Weiterhin wurde gezeigt, dass der Polarisationsverlust der emittierten Elektronen bei dünnen Schichten im Wesentlichen auf eine energiekorrelierte Depolarisation beim Durchqueren der Bandbiegungszone zurückzuführen ist. Als weiteres Resultat wird, für die GaAsP-Photokathoden mit einer Schichtdicke von 150nm, eine Obergrenze für die mittlere Emissionszeit von ≤1,25ps angegeben. Daraus ergibt sich nach dem hier verwendeten Diffusionsmodell eine Untergrenze für die Oberflächenrekombinationsgeschwindigkeit an der Bandbiegungszone von S≥1,2·10^7 cm/s.

Study of electron focusing in thick GEM based photon detectors using semitransparent photocathodes

The detection efficiency of a GEM based UV sensitive gaseous photomultiplier (GPM) depends on the focusing of electrons from the drift gap to the GEM aperture. We have studied the effect of drift parameters on the efficiency of electron focusing into Thick GEM (THGEM) holes in a GPM with semitransparent UV photoconverter. This study comprises simulation of electron focusing into THGEM holes using GARFIELD for different Ar and Ne based gas mixtures and experimental investigations of the same with P10 gas mixture. (c) 2013 Elsevier B.V. All rights reserved.

Carbon nanotube based photocathodes for high frequency amplifiers

The combination of high frequency, high power, high efficiency capabilities is a feature of vacuum tube technology. For most of applications, large bandwidths are required, and therefore the modulation method should also allow large bandwidth operation. Optically modulated cold cathodes, avoiding the use of resonant cavities, should satisfy this requirement. This is the reason why we have developed carbon nanotube based photocathode.© 2009 IEEE.

利用分子束外延研究GaAs半导体NEA光电发射材料

Submitted by zhangdi (zhangdi@red.semi.ac.cn) on 2009-04-13T11:45:31Z

Stability and photoemission characteristics for GaAs photocathodes in a demountable vacuum system

The stability and photoemission characteristics for reflection-mode GaAs photocathodes in a demountable vacuum system have been investigated by using spectral response and x-ray photoelectron spectroscopy measurements at room temperature. We find that the shape of the spectral response curve for the cathode changes with time in the vacuum system, but after applying fresh cesium to the degraded cathode, the spectral response can almost be restored. The change and restoration of curve shape are mainly attributed to the evolution of the surface barrier. We illustrate the evolution and analyze the influence of the barrier on the spectral response of the cathode. (C) 2008 American Institute of Physics.

Stability of GaAs photocathodes under different intensities of illumination

The photocurrent curves of reflection-mode GaAs photocathodes as a function of time, when were illuminated by white light with an intensity of 0, 33 and 100 Ix, respectively, were measured using a multi-information measurement system. The calculated lifetimes of cathodes are 320, 160 and 75 min, respectively, showing that the stability of cathodes degraded with the increase of light intensity. The lifetime of cathode, illuminated by white light with an intensity of 100 Ix, while no photocurrent was being drawn during the illumination, was 100 min. Through comparison, we found that the influence of illumination on cathodes stability is greater than that of photocurrent. The quantum-yield curves of cathodes as a functions of time, when illuminated by white light with an intensity of 33 Ix, were measured also. The measured results show that the shape of the yield curves changes with increasing illumination time due to the faster quantum-yield degradation rate of low energy photons. Based on the revised quantum-efficiency equations for the reflection-mode cathodes, the variation of yield curves are analyzed to be due to the intervalley diffusion of photoelectrons and the evolution of the surface potential barrier profile of the photocathodes during degradation process.

High-Integrated-Photosensitivity Negative-Electron-Affinity GaAs Photocathodes with Multilayer Be-Doping Structures

The effect of changing Be doping concentration in GaAs layer on the integrated photosensitivity for nega- tive-electron-affinity GaAs photocathodes is investigated. Two GaAs samples with the monolayer structure and the muhilayer structure are grown by molecular beam epitaxy. The former has a constant Be concentration of 1 × 10^19 cm^-3, while the latter includes four layers with Be doping concentrations of 1 × 10^19, 7 × 10^18, 4 × 10^18, and 1 × 10^18 cm^-3 from the bottom to the surface. Negative-electron-affinity GaAs photocathodes are fabricated by exciting the sample surfaces with alternating input of Cs and O in the high vacuum system. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathode with the muhilayer structure enhanced by at least 50% as compared to that of the monolayer structure. This attributes to the improvement in the crystal quality and the increase in the surface escape probability. Different stress situations are observed on GaAs samples with monolayer structure and muhilayer structure, respectively.

Physical and dynamical characterisation of low ΔV NEA (190491) 2000 FJ10

Aims: We investigated the physical properties and dynamical evolution of near-Earth asteroid (NEA) (190491) 2000 FJ10 in order to assess the suitability of this accessible NEA as a space mission target. Methods: Photometry and colour determination were carried out with the 1.54 m Kuiper Telescope (Mt Bigelow, USA) and the 10 m Southern African Large Telescope (SALT; Sutherland, South Africa) during the object's recent favourable apparition in 2011-12. During the earlier 2008 apparition, a spectrum of the object in the 6000-9000 Angstrom region was obtained with the 4.2 m William Herschel Telescope (WHT; Canary Islands, Spain). Interpretation of the observational results was aided by numerical simulations of 1000 dynamical clones of 2000 FJ10 up to 106 yr in the past and in the future. Results: The asteroid's spectrum and colours determined by our observations suggest a taxonomic classification within the S-complex although other classifications (V, D, E, M, P) cannot be ruled out. On this evidence, it is unlikely to be a primitive, relatively unaltered remnant from the early history of the solar system and thus a low priority target for robotic sample return. Our photometry placed a lower bound of 2 h to the asteroid's rotation period. Its absolute magnitude was estimated to be 21.54 ± 0.1 which, for a typical S-complex albedo, translates into a diameter of 130 ± 20 m. Our dynamical simulations show that it has likely been an Amor for the past 105 yr. Although currently not Earth-crossing, it will likely become so during the period 50-100 kyr in the future. It may have arrived from the inner or central main belt >1 Myr ago as a former member of a low-inclination S-class asteroid family. Its relatively slow rotation and large size make it a suitable destination for a human mission. We show that ballistic Earth-190491-Earth transfer trajectories with ΔV <2 km s-1 at the asteroid exist between 2052 and 2061. Based on observations made with the Southern African Large Telescope (SALT).

Dos obras de teatro sobre la historia contempor??nea

Estas dos obras de teatro, pensadas para el ??mbito escolar, pretenden que los alumnos se acerquen a los temas hist??ricos de una manera l??dica y al mismo tiempo did??ctica, que se origine en ellos un inter??s por la ??poca y los personajes de los que se trata, que les lleve a profundizar en el estudio de los mismos. Adem??s, el interpretar ellos mismos estos personajes les llevar?? a conocerlos mucho mejor, produci??ndose un proceso de empat??a. Se pretende, en fin, que la historia en general se vea mucho m??s cercana y eso les anime a su estudio. En la primera obra, Las abdicaciones de Bayona, se escenifica la crisis del reinado borb??n al principio del XIX, los pactos con Napole??n, la renuncia de Carlos IV y el inicio de la guerra de la Independencia. En la segunda obra, El sue??o de una siesta de verano, se trata la figura del general Franco en cuanto personaje que justifica sus iniquidades, acciones, silencios y secretos ante personajes del momento como su mujer Carmen Polo, Aza??a, Alfonso XII, Primo de Rivera y Dolores Ib??rruri.

Diversidad en la Europa mediterr??nea

Resumen basado en el de la publicaci??n

Diferencias en la fluidez sangu??nea cortical en los subtipos de TDAH : un estudio piloto

Resumen tomado de la publicaci??n