Channelplate

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 4.17 (a) Channeltron with entrance cone (from [Gal77]) and (b) channelplate, cutaway view (b) is reprinted from Nucl. Instr. Meth., 162, Wiza, 587 (1979) with kind permission of Elsevier Science - NL, Sara Burgehartstraat 25, 1055 KV Amsterdam, The... 

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. 

Two channelplates in a Chevron mounting can be used as a position-sensitive detector (see Fig. 4.18) provided the collector anode is able to preserve the position information which then has to be transferred to suitable electronics. Position-sensitive detectors are used in electron spectrometry mainly for four different applications ... 

Figure 4.24 Extraction of one-dimensional position information from a channelplate detector by using a discrete multianode with an RC line. For an explanation of the strips and the dashed circular area see the caption of Fig. 4.23. Each strip is connected to the RC line indicated by the resistor and capacitor symbols. The total charge Q of the electron avalanche (shaded area) incident on the multianode flows to both ends of the RC line, giving Qi and Q2 in amounts proportional to the distances (P, L) and (0, P), respectively. The two preamplifiers in which these charges are collected are indicated by triangles. From...

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... 

Since the full photoelectron line is detected with the large-scale channelplate detector, each photoelectron contributes to the single counting rate /t which, therefore, becomes independent of the instrumental resolution and is given by... 

In contrast to the simple result of equ. (10.64c), the experimental determination of At[ue is rather cumbersome. A considerable simplification occurs if one of the analysers (e.g., analyser 1 for the photoelectrons) is capable of recording the transmitted electrons with equal transmission and detection efficiencies, independent of their actual kinetic energies Ekinl (large channelplate detector, see Section 5.6). This means that the whole photoline is recorded at once, and the requirement of energy conservation is always fulfilled. In other words, no scanning over the photoline is necessary, only a scanning over the Auger line. Mathematically, this corresponds to... 

F(channelplate, pass2) 2s,ep = j Gsp2( kin2, pass2) r( kin2 kin2> d kin2... 

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