Single Channel Electron Multipliers

Straight tube CEMs can not be operated at gains 10 as a result of instability arising from ionization of residual background molecules by fast electrons within the tube and acceleration of the positive ions towards the input end. Some of these ions strike the wall, creating secondary electrons that are multiplied as usual, thus causing spurious output pulses not directly related to the ion current input (i.e. sharp spikes in the detector output). The probability of such events obviously increases with both a rise in background gas pressure and with the 

It is important to note that the gain of a CEM is a function of both channel length and diameter, but not independently. Rather the gain is found to be a function of the aspect ratio, defined in this case as the ratio of the length 

For CEMs the linear dynamic range is limited at the upper end by the maximum count rate capability for versions designed for use in pulse counting mode, or by maximum linear output current for those designed for 

For a CEM designed for pulse counting detection the maximum count rate capability depends not only on bias current, but also on channel capacitance (and thus the response time) and the condition of the channel surface. As the count rate is increased beyond a limiting point the pulse amplitudes begin to decrease, and eventually 

As mentioned above, the ruggedness of a CEM represents an advantage over the older discrete dynode SEMs. However, as for the latter, the lifetime of a CEM can be significantly affected by the environment in which it is operated chemical contamination together with intense electron bombardment can result in a change in surface 

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Pendyala et al. [24] found that the efficiency of single-channel electron multipliers (CEMs) (also called channeltrons or spiraltrons) was about the same for positrons and electrons of the same incident energy. 

Figure 14.4 Schematic diagram of an ESCA instmment with a single channel electron multiplier detector.

The ideal high-throughput analytical technique would be efficient in terms of required resources and would be scalable to accommodate an arbitrarily large number of samples. In addition, this scalability would be such that the dependence of the cost of the equipment to perform the experiments would scale in a less than linear manner as a function of the number of samples that could be studied. The only way to accomplish this is to have one or more aspects of the experimental setup utilize an array-based approach. Array detectors are massively multiplexed versions of single-element detectors composed of a rectangular grid of small detectors. The most commonly encountered examples are CCD cameras, which are used to acquire ultraviolet, visible and near-infrared (IR) photons in a parallel manner. Other examples include IR focal plane arrays (FPAs) for the collection of IR photons and channel electron multipliers for the collection of electrons. 

The quadrupole mass spectrometer acts as a filter, transmitting ions with a preselected mass/charge ratio. The transmitted ions are then detected with a channel electron multiplier. ICP-MS can be operated in two distinctly different modes, i.e. with the mass filter transmitting only one mass/charge ratio, or where the DC and RF values are changed continuously. The former would allow single-ion... 

Fig. 11.14. Diagram of a single-channel (continuous dynode) electron multiplier. Gains of 105 or 106 are achievable with modern electron multipliers.

At the output side, a single metal anode collects the stream of secondary electrons of all the channels and the current is measured as in other types of electron multipliers. In that case, all the microchannels are then connected between them, so as to act as a large single detector. 

Most mass spectrometers measure one m/z value at a time. A single channel ion detector is used for these instruments, either an electron multiplier or a Faraday cup. TOF, ion trap, and FTICR mass spectrometers have the ability to extract ions with many m/z values simultaneously, so simultaneous detection of these ions is desirable. One approach to multiple ion detection has been to use multiple detectors. Multiple detectors are also used for high-resolution magnetic sector MS instruments designed for very precise isotope ratio determination and for quantitative analysis using isotope dilution. Instruments with... 

The effectiveness of the relaxation processes with thermal excitations of electronic states of the matrix ions diminishes as 6xp(-A/A b7 ) with the decrease of temperature. At fairly low temperatures the mechanism of nuclear relaxation via impurity paramagnetic centers, common for dielectrics, comes into effect (Abragam 1961, Khutsishvili 1968, Atsarkin 1980). This is well illustrated in fig. 20 the temperature motion of the nuclear relaxation rate is sharply slowed down for F at T < 5 K and fbr Tm at T < 3 K, and at the lowest temperatures the thulium nuclear moments relax only ten times faster than those of fluorine. This fact clearly shows that the relaxation of different nuclei proceeds by a single channel. The observed factor-of-ten difference is easily obtained, if one multiplies the concentration ratio nxm/nF = 4 by the ratio of the squares of their magnetic moments Thus, the role of 4f electrons is reduced here to the enhancement of dipole-dipole interactions of nuclei of the VV ions with impurity paramagnetic centers. 

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