Channel Electron Multiplier Research Articles

Triboemission from alumina, single crystal sapphire, and aluminum

The purpose of this paper is to present experimental research work seeking a better understanding of triboemission and its role in tribochemistry. A new instrument is described for measuring the intensity and retarded-energy spectra of electrons and ions triboemitted from various sliding contacts. This instrument was designed to accommodate a variety of contact geometries and it features a wide range of applied loads and sliding speeds under high vacuum or controlled atmosphere conditions. The particle-measurement technique employs a channel electron multiplier in the pulse-counting mode. Experimental results are reported on triboemission intensity from sliding contact of diamond on three related materials: alumina, single crystal sapphire (Al 2O 3), and aluminum. Significant negatively-charged particle triboemission was observed from diamond-on-alumina and diamond-on-sapphire contacts. The retarded-energy spectrum of the negative-particle triboemission also was measured. Emission of positively-charged particles was found to be of relatively low intensity. Lower levels of negatively-charged triboemission, as compared to those from alumina and sapphire, were found from diamond-on-aluminum. These results are discussed with a focus on the possible role of charged particle emission in tribochemical processes and, in particular, on tribopolymerization as an effective mechanism of ceramic lubrication.

Multiplication mechanism in open-type continuous-dynode electron multipliers

The multiplication process in open-type continuous-dynode electron multipliers (channel electron multipliers, microchannel plates) is considered as a Markov process that is continuous in time (channel length) and discrete in space (number of electrons in the channel). As a result of this consideration, the relationships for the mean gain and the probability distributions of the number of electrons at the channel output in the saturated and nonsaturated operating modes were obtained. The relationships are in good agreement with the measured dependences of the gain on the power-supply voltage and the distributions of the number of electrons at the channel output.

Detection efficiency of a channel electron multiplier for low energy incident noble gas ions

The absolute detection efficiency of a continuous channel electron multiplier (DeTech 407-EIC) has been measured for the noble gas ions He+, Ar+, and Xe+, with energies up to 4.9 keV. For all ions, the detection efficiency initially rises linearly as a function of velocity. Our results and a reanalysis of literature data show that the threshold velocity below which a multiplier does not respond to impinging ions cannot be regarded as independent of the ion’s atomic number, Z, as previously reported. Instead, the threshold velocity is proportional to Z−c. The experimentally obtained efficiency-versus-velocity curves for our detector have been analyzed in the region of linear increase. The resulting expression for the detector efficiency, ε, is a function of the ion’s atomic number and velocity, v, and can be written as ε(v,Z)=a(v−b/Zc)Zn, with a, b, c, and n being constants.

Flicker noise of the channel electron multiplier

The spectral density function of the channel electron multiplier noise has been investigated. The existence of flicker noise ( 1/f α ) has been shown. Changes of the exponent α versus the multiplier current have been observed and the relationship between them has been established.

A surface detector for metastable atoms

A surface detector for the detection of metal atoms in metastable states is described. The surface used for de-excitation of the metastable atoms is produced from the condensed vapour of the target metal. The resulting secondary-electron emission is detected by a channel electron multiplier. The design offers the important advantage of a low work function and high selectivity for the secondary electrons. Efficient cooling of the detection surface and the electron multiplier makes it possible to use the detector in studies of metal vapours at temperatures of up to 400 °C. An example of the use of the detector to measure the electron-impact excitation function for inner-shell excited metastable states in potassium is presented.

The solar package on ISS: SOL-ACES

The “solar package” comprises the experiments SOLSPEC (UV/Vis to IR spectral range), SOVIM (total solar radiation) and SOL-ACES to be installed on a Coarse Pointing Device (CPD) of the International Space Station for a 1.5 years mission launched in 2002. The CPD allows for measuring periods of at least 15 minutes per orbit totaling approximately 600 hours per year of solar observations. The Solar Auto Calibrating EUV/UV Spectrometers (SOL-ACES) are currently developed to measure the solar radiation (full disk) in the 17 nm to 220 nm spectral range with four grazing incidence grating spectrometers. To obtain high radiometric accuracy of better than 10%, a double ionization chamber is assigned to each of the spectrometers as primary detector standard. Optical bandpass filters are mounted on a filter wheel to be placed at the entrance apertures of the spectrometers and ionization chambers and thereby will establish the radiometric link between these devices. The spectrometers are designed as scanning monochromators operating at fixed incidence angles. The deflected radiation is monitored by rotating an assembly containing a parabolic mirror, an exit slit and a channel electron multiplier around the grating center. The optical length of the ionization chamber of 0.5 m is divided into two identical electrode sections. In addition, the transmitted radiation is measured by a silicon diode detector located at the end of the absorption path. Detector and electrode signals are recorded as a function of the gas pressure in the chamber, which is increased from zero to a few hectopascals during a single measurement. This data permits the absolute quantification of the radiant solar flux in the wavelength interval transmitted by the bandpass filter and the correction for secondary effects, such as ionization caused by photoelectrons. With these measurements the spectrometer efficiencies at the filter bandpass wavelengths can be recalibrated as required during the mission.

The absolute detection efficiency of vacuum electron multipliers, to keV protons and Ar + ions

The single-ion detection efficiency of two types of electron multipliers is provided for incident energies reaching 10 keV. Absolute efficiencies above 90% were measured for a discrete dynode electron multiplier (DDEM), in response to Ar + ions and protons, above an energy of 5 keV. The efficiency to detect protons is consistently higher than that of Ar + ions, due to a higher secondary electron emission yield. For a channel electron multiplier we have measured a similar maximal detection efficiency for Ar + ions; though strongly varying across the detector surface.

Efficiency calibration of low-energy electron detectors by means of β-ray emitters

The relative efficiency of a channel electron multiplier for 2–9 keV electrons was determined using the β-spectra of vacuum evaporated 241 Pu and 63 Ni sources. The results agreed within ±0.8% (1 σ). Conversion electron measurements of 57 Co and 169 Yb supported the result. The electron scattering and energy losses within the sources were taken into account by means of Monte Carlo simulations of individual elastic and inelastic collisions. The detector was calibrated in the same geometry as it works inside the cylindrical mirror analyzer.

Ionization of Rydberg atoms of sodium by 360–4400 eV alkali-metal ions

Impact ionization by singly charged ions from Rydberg $\mathrm{Na}(24d)$ and $\mathrm{Na}(25s)$ states has been investigated over the reduced velocity region $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=0.5--2.2.$ The ions of ${\mathrm{Na}}^{+},$ ${\mathrm{K}}^{+},$ and ${\mathrm{Li}}^{+}$ were accelerated to energies from 360 to 4400 eV and used to bombard the laser-excited Rydberg atoms. Electrons released in the ionization of Rydberg atoms were collected from the collision volume and registered by a channel electron multiplier. Measured relative ionization cross sections from $\mathrm{Na}(24d)$ and $\mathrm{Na}(25s)$ states show maxima at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}=1.6--1.7$ and agree moderately well with classical-trajectory Monte Carlo calculations.

Static SIMS: ion detection efficiencies in a channel electron multiplier

The energy and spatial counting efficiencies of single channel electron multipliers (CEMs) have been analysed for axially incident argon and xenon ions with energies in the range 0.1 to 9 keV. Theoretical predictions have been compared with data for a Philips X919BL CEM. It is recommended that, to minimise the variation in detection efficiencies between ion species, the ions should impact the detector surface at grazing incidence and at as high an energy as possible, typically 10 to 20 keV. Furthermore, the CEM should be operated in the plateau part of the curve of efficiency as a function of detector voltage determined using ions of high mass.

A new method for measuring the absolute efficiency of channel electron multipliers for electrons

A new method for measuring the absolute efficiency for electron counting with channel electron multipliers (CEMs) is described. The new method is based on the transient time response of the CEMs to pulsed electron beams. It is especially suitable for CEMs with resistance higher than 100 MΩ and it has been used to determine the absolute efficiency of a B419BL (Mullard) and KPE07 (TUG) CEM for detection of electrons in the energy range 50–1500 eV.

Position-sensitive detection of positronium for studying transfer and transfer ionization in e+–Ar collisions

For detecting the positronium (Ps) formed in a positron–atom collision and emitted into a small solid angle, we utilize the impact-induced break-up into a positron–electron pair of which the electron subsequently starts an avalanche of secondary electrons. After exploratory measurements with a channel-electron multiplier (CEM) pivoted around the collision region, we employed a position-sensitive detector consisting of a stack of multi-channel plates (MCPs) and a two-dimensional delay-line anode. This experimental arrangement was used to study differential Ps formation in collision of 75 eV positrons with argon atoms. The two Ps-yielding reaction channels – Ar(e+, Ps)Ar+ (transfer) and Ar(e+, e−Ps)Ar++ (transfer ionization) – were distinguished by the flight time of the argon ion.

Quantum efficiency measurements of an uncoated CEM in the range 0.14 - 160 nm (9 keV - 8 eV)

The quantum efficiency of an uncoated channel electron multiplier (CEM) working in the photon-counting regime has been evaluated over a wide spectral range, which comprises the EUV and soft x-ray regions. Three different experimental set-ups have been used: a Johnson - Onaka monochromator for the 30 - 160 nm region, a grazing incidence monochromator for the 0.3 - 30 nm region and a test facility mounting filters for the 0.14 - 0.3 nm region. As a secondary standard, both silicon and aluminium photodiodes have been used. The efficiency has been evaluated at normal incidence angle. The measured values range from 2% to 15% in the range 0.14 - 100 nm, while a rapid decrease is present over 120 nm. Changes in the efficiency due to carbon contamination in the vacuum system are discussed, and also effects due to variations of the illuminated area on the CEM entrance cone.

Comparison between performances of optical gratings and Si-PIN detectors in soft x-ray (2 - 7 keV) spectroscopy

In soft x-ray spectroscopy the region from 2 to 7 keV is the most critical, because optical gratings are considered to become inefficient and solid-state detectors exhibit a relatively low energy resolution. We investigated the performances of ion-etched blazed gratings and of silicon detectors as components for soft x-ray spectroscopy. The gratings were mounted in a Rowland geometry at angles of incidence varying from to . We used a microfocus soft x-ray as source and as the detector an uncoated channel electron multiplier sliding along the Rowland circle to scan the spectrum. We recorded several spectra between 2 and 7 keV. The spectral resolution obtained shows essentially almost slit limited line widths; the diffraction efficiency ranges from 1 to 3% at and up to 4.2% at . As a comparison, we recorded the same spectra with a Si-PIN diode, cooled at by a Peltier circuit. The diode exhibits good sensitivity and low noise; the resolution obtained is around 280 eV, almost constant at the various energies. This experiment shows that it is possible to extend the classical spectroscopic techniques to the soft x-ray region in order to obtain complementary performances with respect to solid-state detectors.

Low power consumption bias supply for channel electronmultiplier

A low power consumption high voltage bias supply for channel electron multipliers is presented. The design achieves low power consumption by the use of a new integrated circuit developed for the latest generation of battery operated mobile equipment. The power supply is intended to be launched in a sounding rocket as a part of a magnetospheric electron spectrograph experiment.

Quantitative detection of low energy positive and negative ions with a channel electron multiplier

We have constructed ion optics which, when interfaced with a channel electron multiplier (CEM), make possible quantitative measurements of positive and negative ion fluxes. We describe the design and operation of a detection system using these optics and present a calibration of the CEM for low energy (0.2–8.7 keV) positive and negative oxygen ions.

Secondary electron focusing in a metastable surface-detector

A general purpose metastable surface-detector is described which incorporates a novel design feature for focusing ejected secondary electrons from the surface into a channel electron multiplier. The primary aim of the detector is to improve the efficiency of metastable species detection; different surfaces (e.g., Hf, Be - Cu and lead-silicate glass) have been used to illustrate the variation in detector efficiency when detecting the same molecular species, for example .

In situ absolute calibration of a channel electron multiplier for detection of positive ions

A technique has been developed to determine the absolute single particle counting efficiency of a channel electron multiplier (CEM) for positively charged ions at keV energies. The calibration technique is applicable to positive ion detectors in general. Ion beam currents of C2+ were created by charge transfer of C3+ on H2 in the thin target regime. By adjusting the H2 pressure in the beam scattering chamber from 10−10 to 10−6 Torr, C2+ currents could be created either low enough to be detected by the CEM in the particle counting mode or high enough to be measured as a current using the CEM as a Faraday cup. The CEM counting efficiency was determined by comparing the C2+ count rate to the C2+ current, scaling by the change in H2 pressure, and also scaling by the incident C3+ current for each C2+ measurement. This method, which effectively uses the charge transfer cross section as a ‘‘transfer standard,’’ allows the CEM counting efficiency to be determined in situ and to be monitored accurately over extended periods of time. The calibration requires that only the relative, not the absolute, pressure change be known. The technique does not require the value of the charge transfer cross section to be known. The accuracy of the calibration technique for the present results was 10% at a confidence level considered to be equivalent to a statistical 90% confidence level. For the present work, the peak counting efficiency of a Galileo 4039 CEM for 32.5 keV C2+ ions was determined to be 96%.

Conversion electron Mössbauer spectroscopy of 197Au and its first applications

This paper describes the technique of measuring Mössbauer spectra of the 77.35 keV γ-rays of 197Au in very thin gold absorbers, by detecting resonant reemission of low-energy electrons at liquid helium temperature with a channel electron multiplier. The technique is best applicable to specimens of about 10–100 μg Au/cm 2, which is up to two orders of magnitude thinner than required for measurements in transmission mode. The sensitivity limits of the applicability of this technique have been examined. Using strong 197Pt sources in a compact scattering geometry, we were able to obtain the spectra of gold absorbers even as thin as 5 μg/cm 2. Measurements of very thin films of pure gold and gold alloys, as well as the spectra of laser and ion beam-modified surfaces in a sub-μm range of depth are presented as the examples.

Application of the channel electron multiplier to the unipole mass spectrometer

The performance of a channel multiplier, type KPE, applied to the ion detection system of the unipole mass spectrometer was tested. It was found that this type of multiplier is fully useful in ion spectrometer systems in the pressure range 10 −4–10 −12 torr. The amplification characteristic of the ion current vs the pressure of gases reveals a good linearity. The shape of the characteristic indicates that amplification of the electron multiplier for the mixture is dependent only on the amount and kinetic energy of the ions. All three measurement series were taken with the electron multiplier used in the analogue mode of operation.