Detectors channeltron

Modern XPS spectrometers employ a lens system on the input to the CHA, which has the effect of transferring an image of the analyzed area on the sample surface to the entrance slit of the analyzer. The detector system on the output of the CHA consists of several single channeltrons or a channel plate. Such a spectrometer is illustrated schematically in Fig. 2.6. 

Soft X-ray absorption measurements are done at low-energy synchrotron X-ray facilities such as the UV ring at NSLS or the Advanced Photon Source (APS) at Lawrence Berkeley National Laboratory (LBNL). The beam size is typically 1 mm in diameter. The electron yield data are usually obtained in the total electron yield (EY) mode, measuring the current from a channel electron multiplier (Channeltron). Sometimes a voltage bias is applied to increase surface sensitivity. This is referred to as the partial electron yield (PEY) mode. Huorescence yield (EY) data are recorded using a windowless energy dispersive Si (Li) detector. The experiments are conducted in vacuum at a pressure of 2 X 10 torr. 

FIG. 35. Vertical cross section of the reaction chamber equipped with the mass spectrometer system. Indicated are QMF. the quadmpole mass filter ESA. the electrostatic analyzer CD, the channeltron detector DE, the detector electronics DT, the drift tube lO, the ion optics TMP, the turbomolecular pump PR, the plasma reactor and MN. the matching network. 

Fig. 2. Schematic diagram of a high resolution He time-of-flight spectrometer. N-nozzle beam source, SI, 2-skimmers, Al-5 - apertures, T - sample, G - gas doser, CMA - Auger Spectrometer, IG - ion gun, L - LEED, C -magnetically suspended pseudorandom chopper, QMA-detector, quadrupole mass analyzer with channeltron.

The detectors used in mass spectrometers for atmospheric applications are essentially the same as for other MS applications and are commonly electron multipliers, either channeltrons or multichannel plate... 

Figure 16.22—MS detectors, a) Multiple stage electron multipliers (reproduced by permission of ETP Scientific Inc.) b) channeltron the conical shape of the cathode allows the detection of ions with slightly different trajectories c) electron multiplication within a channeltron d) entrance of a multichannel plate detector (microchanneltron).

In contrast, aPs was determined by lowering the voltage on the grid in front of the channeltron to zero and so allowing all scattered positrons to reach the detector except those neutralized by forming positronium. To a good approximation, it could be assumed that only those positrons which had formed positronium would then be lost from the beam, and erPs could be obtained from... 

Figure 1.17 An experimental set-up for electron spectrometry with synchrotron radiation which is well suited to angle-resolved measurements. A double-sector analyser and a monitor analyser are placed in a plane perpendicular to the direction of the photon beam and view the source volume Q. The double-sector analyser can be rotated around the direction of the photon beam thus changing the angle between the setting of the analyser and the electric field vector of linearly polarized incident photons. In this way an angle-dependent intensity as described by equ. (1.55a) can be recorded. The monitor analyser is at a fixed position in space and is used to provide a reference signal against which the signals from the rotatable analyser can be normalized. For all three analysers the trajectories of accepted electrons are indicated by the black areas which go from the source volume Q to the respective channeltron detectors. Reprinted from Nucl. Instr. Meth., A260, Derenbach et al, 258 (1987) with kind permission of Elsevier Science—NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

It depends on the geometry of the analyser (sector or 2n) and is related to the dimensions of the acceptance source volume.) The transmitted electrons are then detected by a common channeltron detector placed behind this dispersion... 

A critical part of the electron spectrometer is the detector which registers the energy-analysed electrons. Channeltrons or channelplates are very convenient detectors, and they will now be discussed with respect to their performance characteristics, the use of channelplates as position-sensitive detectors and their detection efficiencies. 

The names of both detectors reflect that these devices are channels which act as continuous dynode electron multipliers. If there is one channel, it is called a channeltron (channeltron electron multiplier, CEM), if many microchannels are used to form a plate it is called a microchannel electron multiplier plate (in short a microchannelplate, MCP, or channelplate), see Fig. 4.17. A comprehensive description of these devices is given in [Wiz79]. 

Figure 4.19 Time-dependent current i(f) and voltage U(t) signal produced by an electron avalanche in the channeltron/channelplate. The shaded area of the current pulse represents the total charge Q of the avalanche collected during the time rcoll on the detector s capacitance, thus producing l/mM on this capacitor.

Figure 4.20 Equivalent circuit for the channeltron/channelplate as the current source i(t) shown together with the main components of the external circuit. CD is the capacitance of the detector which is depicted by the dotted lines in order to indicate that - in contrast to actual electronic components - it is a property inherent to the detector HV is a high voltage source, Ra the load resistor, CK the high voltage coupling capacitor and Amp the...

From the function of a channeltron/channelplate detector it is obvious that high gains are desirable. However, ion feedback and space charge effects limit the gain with increasing charge of the electron avalanche, electron impact ionization with molecules of the residual gas or molecules desorbed under electron bombardment from the channel surface occurs more frequently. The ions produced are then accelerated towards the channel input. If such an ion hits the surface at the channel entrance, it may release an electron which again can start an avalanche of practically the same size, i.e., it causes after-pulses. 

The same arguments hold for the detection efficiency of a channelplate detector as for the channeltron, but in addition the ratio ropenarea of the area channel openings to the total plate area has to be included also. As a rough estimate one gets... 

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