Electron multiplier operating principles

Figure 7.7 Schematic diagrams of a channel electron multiplier. (a) principle of operation of a CEM for detection of positive ions the electroding at each end consists of a thin band of metal deposited to provide electrical contact, and the potential gradient along the length of the device is developed from the external high voltage supply applied along the intrinsic resistance of the doped lead glass. Reproduced from Wiza, Nucl. Instrum. Methods 162,587 (1979), copyright (1979), with permission from Elsevier, (b) A cross-section of the surface structure of a CEM. Reproduced from literature provided by Burle ElectroOptics Inc, with permission.

The basic principles of discrete-dynode electron multiplier operation are shown schematically in Figure 3.1. When an ion strikes the first dynode of a discrete-dynode electron multiplier (or conversion dynode) it liberates secondary electrons. The electron-optics of the dynodes then accelerates these electrons to the next dynode in the multiplier, which in turn produces a greater number of secondary electrons. This process is repeated at each subsequent dynode, generating a cascade of millions of electrons, which are finally captured (as an output pulse , hence the term pulse counting) at the multiplier output electrode. The gain of an electron multiplier can be defined as the average number of electrons collected at the multiplier s output electrode for each input ion that initiates an electron cascade. Similarly, it can be described as the current measured from the output divided by the input ion current. It should be noted that this second definition includes the ion detection efficiency of the multiplier. 

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

Detecting ions in GC/MS is performed almost exclusively using an electron multiplier. There are two types of electron multipliers the continuous dynode type and the discrete type. Both operate on the principle that ions with sufficient kinetic energy will emit secondary electrons when they strike a metal surface. The discrete type of electron multiplier has a series of... 

A newer and less expensive alternative to the microchannel plate is the microsphere plate (MSP). As illustrated in Figure 3.6, this electron multiplier consists of glass beads with diameters from 20 to 100 pm that are sintered to form a thin plate with a thickness of 0.7 mm. This plate is porous with irregularly shaped channels between the planar faces. The surfaces of the beads are covered with an electron emissive material and the two sides of the plate are coated to make them conductive. The operating principle of this electron multiplier is similar to that of the microchannel plate. A potential difference of between 1.5 and 3.5 kV is applied across the plate, with the output side of the plate at the more positive potential. When particles hit the input side of the microsphere plate, they produce secondary electrons. These electrons are then accelerated by the electric field through the porous plate and collide with other beads. Secondary electron multiplication in the gaps occurs and finally a large number of secondary electrons are emitted from the output side of the plate. 

Molecules with Several Atomic Cores.—From the above discussion it is seen that, in principle, the effective hamiltonian for atomic valence electrons is dependent on the valence state of the atom, this dependence arising from the valence contribution to the all-electron Fock operator F. In practice this dependence is very weak unless the atom is multiply ionized, and can usually be safely neglected, so that a single effective hamiltonian can suffice for many valence states. However, for a molecular system in which there is more than one core region additional approximations must be introduced to maintain a simple form of the effective hamiltonian. For two atomic cores defined in terms of orbital sets and and a valence set < F) the equation equivalent to (21) is... 

MicroChannel Plates. One type of array transducer for mass spectrometry is the electrooptical ion detector (EOID) shown in Figure ll-4a. The key clement in the EOID is the microchannel electron multiplier, or microchannel plate, which is referred to in some optical applications as an image intensilier. The design and basic principles of operation of the microchannel plate are illustrated in Figure 1 l-4b. The plate consists of an array of tiny lubes (diameters as small as 6 pm)... 

Figure 2.26 Basic operating principle of an electron multiplier, (a) Continuous dynode electron multiplier (b) electron multiplier with discrete dynodes. Reproduced with permission of Dr. Paul Gates, School of Chemistry, University of Bristol, from [114].

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