Channel Electron Multiplier Detector Research Articles

Development of high-speed vacuum ultraviolet spectroscopy using a modified Seya-Namioka monochromator and channel electron multiplier detector in the HL-2A tokamak.

A high-speed vacuum ultraviolet monochromator is developed for the HL-2A tokamak through the introduction of a novel channel electron multiplier in a modified Seya-Namioka spectrometer. A good signal to noise ratio of above 2000 is attained in the development phase of the system with typical operating parameters for observing routine HL-2A plasmas. The wavelength calibration is performed using characteristic line emissions from a hollow cathode light source with helium and argon discharges. The first measurement result of the monochromator at a sample rate of 60 kHz is presented in comparison with the visible Dα signals.

Comparative Response of Microchannel Plate and Channel Electron Multiplier Detectors to Penetrating Radiation in Space

Channel electron multiplier (CEM) and microchannel plate (MCP) detectors are routinely used in space instrumentation for measurement of space plasmas. Our goal is to understand the relative sensitivities of these detectors to penetrating radiation in space, which can generate background counts and shorten detector lifetime. We use 662 keV γ-rays as a proxy for penetrating radiation such as γ-rays, cosmic rays, and high-energy electrons and protons that are ubiquitous in the space environment. We find that MCP detectors are ~ 20 times more sensitive to 662 keV γ-rays than CEM detectors. This is attributed to the larger total area of multiplication channels in an MCP detector that is sensitive to electronic excitation and ionization resulting from the interaction of penetrating radiation with the detector material. In contrast to the CEM detector, whose quantum efficiency εγ for 662 keV γ-rays is found to be 0.00175 and largely independent of detector bias, the quantum efficiency of the MCP detector is strongly dependent on the detector bias, with a power law index of 5.5. Background counts in MCP detectors from penetrating radiation can be reduced using MCP geometries with higher pitch and smaller channel diameter.

Momentum mapping spectrometer for probing the fragmentation dynamics of molecules induced by keV electrons

We describe a new experimental setup for studying the fragmentation dynamics of molecules induced by the impact of keV electrons using the well-known technique of recoil ion momentum spectroscopy. The apparatus consists of mainly a time- and position-sensitive multi-hit particle detector for ion analysis and a channel electron multiplier detector for detecting the ejected electrons. Different components of the setup and the relevant electronics for data acquisition are described in detail with their working principles. In order to verify the reliable performance of the setup, we have recorded the collision-induced ionic spectra of the CO2 molecule by the impact of keV electrons. Information about the ion pairs of CO+:O+, C+:O+ and O+:O+ resulting from dissociative ionizing collisions of 20 and 26 keV electrons with a dilute gaseous target of CO2 molecules has been obtained. Under conditions of the present experiment, the momentum resolutions of the spectrometer for the combined momenta of CO+ and O+ ions in the direction of the time-of-flight axis and perpendicular to the direction of an electron beam are found to be 10.0 ± 0.2 and 15.0 ± 0.3 au, respectively.

Background in channel electron multiplier detectors due to penetrating radiation in space

Penetrating radiation in the Earth's space environment can induce substantial backgrounds in electron multiplier detectors used on space instruments. Background count rates from the Magnetospheric Plasma Analyzer (MPA) on LANL satellites in geosynchronous orbit are calculated and compared to the ambient penetrating particle radiation environment to develop a basis for estimating and predicting detector background count rates throughout the magnetosphere. For several solar energetic proton events and, separately, relativistic electron events, it is shown that background counts in the largely unshielded channel electron multiplier (CEM) detectors on MPA are highly correlated with fluxes of greater than 50 MeV protons or greater than 1.1 MeV electrons. The energetic proton and electron measurements used to characterize the penetrating radiation come from the EPS instrument on the GOES satellites and the SOPA instrument on the LANL satellites, respectively. The high-energy spectrum at geosynchronous orbit during these events can be compared with the AP/AE-8 models of the inner magnetosphere fluxes. Based on the correlation between CEM background and ambient penetrating radiation at geosynchronous, we extrapolate the background response to the inner magnetosphere, a highly relevant instrument design parameter for future missions to this region.

Neutral particle analyzer diagnostics on the TCV tokamak

Experimental apparatus and data analysis techniques used in neutral particle analyzer (NPA) diagnostics on the Tokamak à Configuration Variable (TCV) are described. Two NPAs are used on TCV to measure the energy spectrum of neutral particle fluxes from the plasma. The “five-channel energy analyzer of atomic particles” used in double electrical analysis mode with fast voltage sweeping detect particles without atomic mass discrimination in the energy range of 0.6–8.0keV with a time resolution of 0.5–2.0ms and an energy resolution of 7%–20%. The 28-channel “compact neutral particle analyzer” (CNPA) is an EIIB spectrometer with mass and energy separations designed for medium sized fusion machines featuring a carbon neutral stripping foil, a permanent magnet for dispersion, and channel-electron multiplier detectors. The CNPA simultaneously detects two mass species [hydrogen (H) and deuterium (D) or D and helium (He)] in the 0.5–50keV energy range with a resolution of 60%–10% and a time resolution of 0.5–4.0ms. The CNPA views the plasma across the path of the diagnostic neutral beam and can perform active charge-exchange NPA measurement. Data analysis procedures and numerical algorithms developed for NPA measurement are routinely used on TCV to obtain information on the plasma ion temperature, ion energy distribution function, plasma isotope ratios, and other plasma characteristics.

The Plasma Ion and Electron Instruments for the Genesis Mission

The Genesis Ion Monitor (GIM) and the Genesis Electron Monitor (GEM) provide 3- dimensional plasma measurements of the solar wind for the Genesis mission. These measurements are used onboard to determine the type of plasma that is flowing past the spacecraft and to configure the solar wind sample collection subsystems in real-time. Both GIM and GEM employ spherical-section electrostatic analyzers followed by channel electron multiplier (CEM) arrays for detection and angle and energy/charge analysis of incident ions and electrons. GIM is of a new design specific to Genesis mission requirements whereas the GEM sensor is an almost exact copy of the plasma electron sensors currently flying on the ACE and Ulysses spacecraft, albeit with new electronics and programming. Ions are detected at forty log-spaced energy levels between ∼ 1 eV and 14 keV by eight CEM detectors, while electrons with energies between ∼ 1 eV and 1.4 keV are measured at twenty log-spaced energy levels using seven CEMs. The spin of the spacecraft is used to sweep the fan-shaped fields-of-view of both instruments across all areas of the sky of interest, with ion measurements being taken forty times per spin and samples of the electron population being taken twenty four times per spin. Complete ion and electron energy spectra are measured every ∼2.5 min (four spins of the spacecraft) with adequate energy and angular resolution to determine fully 3-dimensional ion and electron distribution functions. The GIM and GEM plasma measurements are principally used to enable the operational solar wind sample collection goals of the Genesis mission but they also provide a potentially very useful data set for studies of solar wind phenomena, especially if combined with other solar wind data sets from ACE, WIND, SOHO and Ulysses for multi-spacecraft investigations.

A compact mono-energetic positron beam for re-emitted positron measurement

A compact mono-energetic positron beam system has been constructed at The University of Hong Kong in order to perform positron re-emission measurements, after fast positrons from a radioactive 22Na source moderated by a variety of samples. The positron re-emission measurements are performed at a vacuum of 10 −8 Torr. Re-emitted slow positrons are guided by longitudinal magnetic fields provided by three field coils located outside the vacuum tank and are detected by a gamma photon insensitive channel electron multiplier (CEM). The radioactive source, the sample, the focusing grid, the retarding-field energy analyzer (RFA) and the CEM detector were put into a compact assembly having a length of 33 cm and a diameter of 6 cm. Positron re-emission from W(1 1 1), GaN/SiC, Ga/GaN/SiC, 4H and 6H–SiC surfaces have been studied. The moderator conversion efficiencies of these samples are reported.

Time-of-flight scattering and recoiling spectrometer (TOF-SARS) for surface analysis

A UHV spectrometer system has been designed and constructed for time-of-flight scattering and recoiling spectrometry (TOF-SARS). The technique uses a pulsed primary ion beam and TOF methods for analysis of both scattered and recoiled neutrals (N) and ions (I) simultaneously with continuous scattering angle variation over a flight path of ≊1 m. The pulsed ion beam line uses an electron impact ionization source with acceleration up to 5 keV; pulse widths down to 20 ns with average current densities of 0.05–5.0 nA/mm2 have been obtained. Typical current densities used herein are ≊0.1 nA/mm2 and TOF spectra can be collected with a total ion dose of <10−3 ions/surface atom. A channel electron multiplier detector, which is sensitive to both ions and fast neutrals, is mounted on a long tube connected to a precision rotary motion feedthru, allowing continuous rotation over a scattering angular range 0°<θ<165°. The sample is mounted on a precision manipulator, allowing azimuthal δ and incident α angle rotation, as well as translation along three orthogonal axes. The system also accommodates standard surface analysis instrumentation for LEED, AES, XPS, and UPS. The capabilities of the system are demonstrated by the following examples: (A) TOF spectra versus scattering angle θ; (B) comparison to LEED and AES; (C) surface and adsorbate structure determinations; (D) monitoring surface roughness; (E) surface semichanneling measurements; (F) measurements of scattered ion fractions; and (G) ion induced Auger electron emission.

High-resolution duo-multichannel soft x-ray spectrometer for tokamak plasma diagnostics

A high-resolution, time-resolving soft x-ray multichannel spectrometer (SOXMOS) that permits the simultaneous measurement of emission in two different spectral ranges has been developed and tested extensively for tokamak plasma diagnostics. The basic instrument is a high-resolution, interferometrically adjusted, extreme grazing incidence Schwob–Fraenkel duochromator. The instrument is equipped with two multichannel detectors that are adjusted interferometrically and scan along the Rowland circle. Each consists of an MgF2 coated, funneled microchannel plate, associated with a phosphor screen image intensifier that is coupled to a 1024-element photodiode array by a flexible fiber-optic conduit. The total wavelength coverage of the instrument is 5–340 Å with a measured resolution (FWHM) of about 0.2 Å when equipped with a 600-g/mm grating, and 5–85 Å with a resolution of about 0.06 Å using a 2400-g/mm grating. The simultaneous spectral coverage of each detector varies from 15 Å at the short wavelength limit to 70 Å at the long wavelength limit with the lower dispersion grating. The minimum readout time for a full spectral portion is 16 ms, but several individual lines can be measured with 1-ms time resolution by selected pixel readout. Higher time resolution can be achieved by replacing one multichannel detector with a single channel electron multiplier detector. Examples of data from the PLT and TFTR tokamaks are presented to illustrate the instrument’s versatility, high spectral resolution, and high signal-to-noise ratio even in the 10-Å region.

Channel electron multiplier detector for conversion electron Mössbauer spectroscopy at temperatures between 4 and 300 K

A technique is presented for measuring conversion electron Mössbauer spectra at temperatures between 4 K and 300 K, using a cryogenic channel electron multiplier detector in combination with a continuous-flow helium cryostat. The described setup is characterized by a very high signal-to-noise ratio. In particular, for 119Sn, a resonant effect of up to 2000% was measured, likely the largest ever observed in Mössbauer spectroscopy. Other advantages are top-loading capability, easy sample access, high cooling rate, capability of simultaneous Mössbauer transmission measurements, and adaptability to on-line experiments. Some observations of the performance of channel electron multipliers at low temperatures are also described. The setup presented in this work can be used both for CEMS measurements as a function of temperature and as a highly efficient resonant detector for measurements with weak Mössbauer sources or for detecting weak resonant signals.

Investigation of a laser-produced plasma VUV light source

An investigation was conducted on the VUV radiation from laser-produced plasmas using a channel electron multiplier detector and a 1.5-m grazing incidence spectrometer. High-resolution quantitative spectra from 8 to 40 nm were obtained from the plasmas generated by a 0.5-J Nd:YAG laser focused on nine different target materials. The effects on the plasma emission of laser energy and focus were measured.

Electron capture to the continuum from atomic hydrogen

The first known measurement of the differential cross section for electron capture to the continuum (ECC) from atomic hydrogen is presented. A 12 MeV beam of C 6+ ions traversed a static target of atomic hydrogen produced by an electron impact heated dissociation oven. The resulting ECC spectrum was obtained with a channel electron multiplier detector mounted at the exit of a 160° spherical sector electrostatic spectrometer with an angular acceptance of 2°. The ECC spectrum clearly shows the asymmetry generally associated with ECC spectra from gaseous targets. The ratio of the singly differential cross section of H to that of H 2 was found to be 0.80.

A two-dimensional scanner for radiochromatograms employing an electron multiplier detector in vacuo

A step scanner for the measurement of both one- and two-dimensional distributions of radionuclides in paper or thin-layer chromatography has been constructed and tested. Different versions of a miniaturized channel electron multiplier have been applied as detectors. A description is given of the principle of operation for this detector type, the vacuum scanner, and the way of data presentation. Performance tests have been made with calibrated standards of 3H and 14C in the form of poly(methyl methacrylate) pieces with known electron emission defined as the number of electrons emitted per second per cm 2. Several thin-layer media have been used, mostly with directly applied spots of [ 3H]- or [ 14C]glucose solutions. Upon drying, spots of activities down to 5 nCi could easily be detected (spot areas were approximately 0.7 cm 2). A resolution of 1 mm could easily be achieved in the one-dimensional scans. Some more results of a somewhat preliminary nature are reported. Some possible alternative for the recording of measurements which use the channel electron multiplier detector are mentioned. The use of scanning by electron multipliers as a method for radiochromatography has the advantages that the sample is neither damaged nor contaminated compared with conventional techniques which, for example, employ the admixture of wet (or dry) scintillators. It is felt that these advantages for certain applications more than compensate for the fact that the technique a vacuum of 5 ·10 −4 torr.