X-ray Photocathodes Research Articles

平响应X射线光阴极的理论设计与计算模拟

平响应X射线光阴极的理论设计与计算模拟

A streaked X-ray spectroscopy platform for rapidly heated, near-solid density plasmas.

A picosecond, time-resolved, x-ray spectroscopy platform was developed to study the thermal line emission from rapidly heated solid targets containing buried aluminum or iron layers. The targets were driven by high-contrast 1ω or 2ω laser pulses at focused intensities up to 1 × 1019 W/cm2. The experimental platform combines time-integrating and time-resolved x-ray spectrometers. Picosecond time resolution was achieved with a pair of ultrafast x-ray streak cameras coupled to high-throughput Hall spectrometers. Time-integrated spectra were collected on each shot to correct the streaked data for variations in x-ray photocathode spectral sensitivity. The time-integrated spectrometer uses three elliptical crystals to disperse x rays with energies between 800 and 2100 eV with moderate (E/ΔE ∼ 450) resolving power. The streaked spectrometers accept four interchangeable conical crystals with higher resolving power (E/ΔE ∼ 650) to measure the brightest thermal lines in the 1300 to 1700 eV spectral range.

Characterization of x-ray photocathode in transmission mode for imaging application

The performance of an Al x-ray transmission photocathode was experimentally characterized by measuring the total electron yield from backsurface photocathodes of varying thickness. It was demonstrated that the backsurface electron yield is proportional to the x-ray photocurrent transmitted through the photocathode for thicknesses thicker than the optimum thickness. The optimum photocathode thickness with the highest conversion efficiency was found to be approximately 70nm at 2.963keV. An escape depth of the secondary electrons was determined to be approximately 13nm from the yield-versus-thickness data fitted with a semiempirical equation. Using this parameter, the dependence of the optimum photocathode thickness on the x-ray energy was calculated for the 30–10000eV range. As one example of imaging applications, different photoemission images in the transmission and reflection modes are also presented.

A transmission x-ray microscope based on secondary-electron imaging

A design for a transmission x-ray microscope with 20 nm transverse spatial resolution is presented. The microscope, which is based on the electron-optical imaging of the photoemitted electrons from an x-ray shadowgraph, consists of a transmission x-ray photocathode coupled to a photoelectron emission microscope (PEEM—also called a PEM for photoelectron microscope). Unlike the conventional PEEM, which produces a surface map of photoelectron yield, this microscope can provide information on the subsurface properties of thin samples. The analysis of the microscope’s electron-optical performance is based on the evaluation of Gaussian focusing properties and third-order aberration coefficients computed using several complementary methods. The electron optical properties of the microscope are examined with an emphasis on issues affecting overall performance and achieving the best possible resolution. Preliminary experimental results using a cesium iodide photocathode are shown.

Thermally annealed soft X-ray photocathodes

We describe the X-ray properties of alkali halide photocathodes subjected to post-evaporation thermal treatments. Measurements made in St. Petersburg on planar CsI photocathodes, over an X-ray energy range 5–10 keV, show up to a five-fold increase in current quantum yield χ c (electrons/photon) after thermal annealing. Measurements were also made at Leicester on a microchannel plate (MCP) detector bearing thermally annealed CsI and KBr photocathodes. The MCP quantum efficiency Q (counts/photon) was measured, before and after annealing, at X-ray energies of 0.28, 1.74 and 2.56 keV. A small increase in quantum efficiency was observed at large angles of incidence for the annealed photocathodes. The heat-treated CsI coated MCP half also exhibited a much enhanced mean gain. Both planar photocathode and coated MCP measurements are interpreted with the aid of a semi-empirical photocathode model and scanning electron microscope (SEM) imagery.

The characterisation of gold X-ray photocathodes

We present synchrotron measurements of the X-ray quantum efficiency (electrons/photon) of gold for grazing angles in the range 10°<θ<90° and for proton energies in a 2.2−8.3 keV band which includes the M-shell absorption edges. Our photoyields are in good agreement with previous measurements at isolated X-ray energies and closely conform to the csc θ scaling law for 20°<θ<90°. We confirm the presence of photocurrent EXAFS structure at energies between the gold MIV and MII edges. Preliminary measurements of the dependence of the grazing incidence photoyield on the linear polarisation state of the incident beam support the existence of an “X-ray vectorial effect” in gold at kilovolt energies.

X-ray image-converter streak diagnostics of laser plasmas

A review is given of the developments in dynamic tests on x-ray streak (image-converter) cameras designed for diagnostics of soft x-ray radiation emitted by laser plasmas. A description is given of the procedures used in the application of these cameras in experimental studies of the interaction of high-power laser radiation with matter. Methods for recording x-ray radiation from laser plasmas using plastic scintillators are presented. Special attention is given to the characteristics of x-ray photocathodes such as the energy distribution of photoelectrons, and the thickness and spectral dependences of the quantum efficiency. An account is given of diagnostic experiments on plasmas formed by subpicosecond laser pulses.

A microchannel plate intensified, subnanosecond, X-ray imaging camera

We have developed a microchannel plate intensified, subnanosecond X-ray detector for X-ray imaging experiments. It consists of an X-ray photocathode, a microchannel plate, electrostatic focusing optics and a subnanosecond phosphor. The detector is used for one dimensional imaging and spectroscopic measurements. Signals are recorded using either a one dimensional Reticon camera or a streak camera. The microchannel plates employ an X-ray photocathode (CuI or CsI) deposited on the front surface of the microchannel plate to enhance their soft X-ray efficiency. Electrostatic focusing of the electrons exciting the microchannel plate also enhances the gain of the detector by an order of magnitude. The electrons are accelerated to 20 kV before striking a fast phosphor (indium doped cadmium sulfide). The detector output is coupled to a Reticon camera or a streak camera with a fiber optic array. We have built and calibrated more than twenty microchannel plate intensified detectors. Efficiencies in excess of 1000 (w/cm2 output per w/cm2 input) have been demonstrated. The time response of the detector is less than 500 ps. The efficiency if the CuI and CsI X-ray photocathodes have been measured from 450 eV to 1300 eV at the Stanford Synchrotron Radiation Laboratory. Data are presented on the efficiency, time response, spectral response and the spatial resolution of the detectors.

The energetic characteristics op the x-ray photocathodes

The energetic characteristics op the x-ray photocathodes

Photoemission polarimeter in soft x-ray astronomy

Measurements in our laboratory have recently shown that the pulse quantum yield (counts/photon) of Csl x-ray photocathodes, measured near grazing incidence, depends on the linear polarization state of the incident beam. These results have implications both for the fundamental understanding ofthe x-ray photoeffect and, since Csl is well known as a cathode material of high (unpolarized) quantum efficiency in the 0.1 to 1.0 keV band, for the development of high-throughput astronomical x-ray polarimetry. This paper, concentrating on the latter topic, describes the practical accommodation of a photoemission polarimeter at (or near) the focus of a satellite x-ray telescope. Problems of device format, instrumental polarization, and data compression are discussed with reference to the Composite X-ray Polarimeter recently proposed for the European Space Agency High Throughput Spectroscopy mission (XMM). Examples of the sensitivity of this instrument are given. In the second part of the paper, some recent laboratory measurements are presented. These concern the variation in the number of x-ray photoelectrons with the polarimetric azimuth of a Csl cathode.

X-ray framing camera for picosecond imaging of laser-produced plasmas

We describe the development and characterization of an ultrafast x-ray framing camera capable of recording images with a temporal resolution of 50 ps and spatial resolution of 22 μm at the image plane. The unique design incorporates an x-ray photocathode directly into a suspended-strip transmission line. The photocathode is gated using a high-voltage (−5-kV) pulse of short duration generated with a photoconductive switch. The photoelectrons are extracted through an unbiased microchannel plate and are directly detected with a charge-coupled device in the electron-bombardment mode. We discuss applications of the camera to investigate the dynamics of laser-produced plasmas.

X-ray-induced secondary-electron emission from solid xenon.

The secondary-electron yield has been measured for solid Xe films excited by (1--10)-keV x rays. High yields are observed due to the long escape depth for secondary electrons (\ensuremath{\sim}1 \ensuremath{\mu}m). The measurements are discussed in terms of a one-dimensional diffusion model. The electron escape depth is found to be sensitive to the temperature at which the film is grown. The electron yield was found to exhibit a time dependence due to the combined effects of charging and radiation damage. A large increase in the yield was observed when an electric field was applied. Finally, the effects of added impurities, ${\mathrm{CO}}_{2}$ and propane, were investigated. It is demonstrated that solid Xe will be useful as a very sensitive x-ray photocathode.

Green light 2ω as an x-ray streak camera fiducial

The fiducial system for the multiple optical and x-ray streak cameras on the two Nova target chambers is intended to allow cross comparison of time-resolved data from laser plasmas to an accuracy of ±50 ps. Development of the system has led to the discovery that Al-coated fiber optics incorporated into the x-ray photocathode can be made to produce photoelectrons when irradiated by ∼1 μJ of 2ω light. The low loss and dispersion of 2ω light in fused silica fibers are significant advantages over our original intention of using 3ω light. Results indicating input and output pulse lengths and required energies to produce photoelectrons will be discussed.

Development of an x-ray framing camera

An ultrafast x-ray framing camera is being developed at Lawrence Livermore National Laboratory to be used as a diagnostic instrument in laser experiments. The framing camera is designed to provide temporal resolution of 50 ps and spatial resolution of 30 μ at the image plane. The unique design incorporates the x-ray photocathode directly into a suspended-strip transmission line. The photocathode is gated using a high-voltage (−5 kV) pulse of short duration generated with a photoconductive switch. The photoelectrons are extracted through an unbiased microchannel plate and are directly detected with a charge-coupled device in the electron-bombardment mode. We report on current progress in the fabrication, characterization, and integration of the various components of the instrument.

Thickness variations in x-ray filters and laser targets

Thickness variation of visually opaque foils used as x-ray and ultraviolet (XUV) photon filters and photocathodes can introduce anomalies in recorded intensities, images, and spectral line profiles. In laser fusion studies, mass thickness variations of laser-accelerated foils can hydrodynamically disrupt their motion. We have used transmitted electrons to obtain quantitative local (area ≤1 μm2) measurements of mass thickness as well as qualitative images of mass thickness variations in free-standing Be, C, and Al foils presently in use in the above areas of investigation. Substantial mass thickness variations, up to a factor of 4 for Al and 1.3 for Be, were found which can produce considerable modulations in transmitted XUV photon intensity.

The stability of cesium iodide X-ray photocathodes

For pulsed X-ray measurements as applied, for example, in high temperature plasma diagnostics, it has been established that the cesium iodide photocathode is very efficient for X-ray diode and streak camera applications. Its quantum yield (electrons/incident photon) is ten to one-hundred times higher than that for the gold photocathode in the 100–10 000 eV region. The width of its secondary electron energy distribution is appreciably less than that for gold, allowing time resolved, streak camera measurements to be extended into the picosecond region. In this note an experimental study is described which demonstrates that the cesium iodide photocathode quantum yield and secondary electron energy distribution can be stable under practical conditions of preparation, handling, storage and short period exposures to the atmosphere.

The characterisation of soft X-ray photocathodes in the wavelength band 1–300 Å: I. Lead glass, lithium fluoride and magnesium fluoride

The use of deposition photocathodes for the efficiency enhancement of microchannel plate X-ray detectors is discussed and experimental data reviewed. A detailed theoretical model of X-ray photoemission is presented. This model will provide the basis for the systematic study of X-ray photocathodes for microchannel plate applications. The predictions of the model are shown to be in good agreement with available LiF and MgF 2 plane photocathode data. This is the first part of a two-part study: the second reports the application of the model to CsI and other insulators of high photoelectric yield.

The characterisation of soft X-ray photocathodes in the wavelength band 1–300 Å: II. Caesium iodide and other insulators of high photoelectric yield

In this, the second part of a study of soft X-ray photocathodes, the predictions of a theoretical model of X-ray photoemission are compared with plane photocathode data for CsI, the alkali chlorides, NaF, Kl and CaF 2. Estimates of photoelectric yield are presented for MgO. The implications of this study for its intended field of application - deposition photocathodes for microchannel plate X-ray detectors - are briefly discussed.

The characterization of x-ray photocathodes in the 0.1–10-keV photon energy region

A method and an instrument are described for the measurement of the absolute quantum yield for front-surface and transmission photocathodes in the 0.1–10-keV photon energy region. The total and the secondary electron photoemission yields have been measured for the Al, Au, CuI, and CsI photocathodes as required for the absolute calibration of the x-ray diode detectors and for the x-ray streak cameras. The relative secondary electron yields have also been measured for the same photocathodes by high resolution electron spectroscopy of the secondary electron energy distributions, which are in good agreement with the absolute yield measurements. The secondary electron yield of CsI is ten to one-hundred times higher than that for Au in the 0.1–10-keV region and with a secondary energy distribution that is appreciably sharper. For these reasons, CsI should be an effective photocathode for sensitive, time-resolved spectroscopy into the picosecond region. It is verified experimentally that the secondary electron quantum yield varies approximately as Em(E), with E as the photon energy and m(E) as the photoionization cross section, and that the primary (fast) electron quantum yield is a small fraction of the total yield and varies approximately as E2m(E). A simple model for x-ray photoemission is described which leads to semiempirical equations for front- and back-surface secondary electron photoemission as based upon an escape depth parameter that may be obtained from yield-versus-photocathode thickness data. The model predictions are in good agreement with experiment.

X-ray photocathodes for image intensifiers and large area thick window detectors

This investigation consists of three parts. In the first one, the photoelectric yield for Pb-doped glass, W, Be, Al, CsI, and for CaF 2 was measured in the spectral region between 0.452 and 4.52 keV. Inthe second part a laboratory model of an X-ray image intensifier (or X-ray image converter, or X-ray image enlarger) utilizing a “Bendix” Channeltron electron multiplier array in proximity focus with an X-ray photocathode, a “Shibaden” vidicon, and a TV monitor, has been evaluated. The system provides both high efficiency and good spatial resolution. Finally, in the third part, a large area thick window detector for soft X-ray radiation has been constructed and tested. In this detector, the window acts as a phohotocathode; the photoelectrons are counted by a “SEM-4219” electron multiplier. The system is ideal for applications where no energy resolution is required and weight and power consumption are restricted.