Channel electron multipliers

Discrete dynode and channeltron detectors have many similar characteristics, which is not surprising given the similarity of their operating mechanisms. Discrete dynode detectors generally have longer lifetimes and higher detection sensitivities than channeltrons because they possess a larger active surface area. The discrete dynode detector is also more physically robust than a channeltron but the latter can be obtained in a more compact form and is usually cheaper than a discrete dynode detector. 

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The most connnon detectors used for TOF-SARS are continuous dynode channel electron multipliers which... 

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. 

The analyser will always be preceded by some form of collection optics, and followed by an ion detector (usually a channel electron multiplier which converts ions into electron showers). There are three types of analyser for use in SIMS spectrometers, the magnetic sector instrument, the quadrupole analyser and time-of -flight (TOF) systems. 

Channel electron multipliers, 24 105-106 Channel-forming systems, crown ethers in, 24 59... 

Fig. 4.58. Schematic of linear channel electron multipher (a) and curved channel electron multiplier (b). By courtesy of Waters Corporation, MS Technologies, Manchester, UK..

Channel electron multipliers (CEM) are fabricated from lead-silica glass (Fig. 1.32) and can have curved or straight forms. In a channel electron multiplier, when the charged particles (positive or negative) hit the surface of the electrode, electrons are produced from the surface which then generate the current. 

Fig. 1.32 Straight channel electron multipliers (CEM) are typically used in quadrupole-type mass spectrometers.

Fig. 1.33 Cu rved channel electron multiplier with conversion dynode. The conversion dynode acts as a post acceleration device of the ions before they hit the surface of the channel electron multiplier.

The lifetime of channel electron multipliers is ca. 1-2 years. Neutrals or photons hitting the detector also increase the noise of the detection. 

The advantage of the photomultiplier compared to the electron multiplier is the longer lifetime (several years). Channel electron multiplier and photomultiplier are mostly used in quadrupole instruments or ion traps. 

Figure 4.5 Channel electron multiplier (CEM) working in the analogue and pulse-counting mode. (Reproduced by permission of GV Instruments Ltd.)...

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

The atomic beam was formed by a multichannel capillary array, placed perpendicular to the positron beam, with a 2.5 mm2 effusing area and a length-to-diameter ratio of 25 1. The head pressure behind the array was kept at 9 torr (ss 103 Pa) in the initial measurements. An annealed tungsten moderator was used to provide a beam of more than 105 positrons per second at 200 eV. A much more intense beam of electrons could also be obtained by reversing the electrostatic potentials on the various elements which made up the transport system. Channel electron multipliers (CEM1 and CEM2 respectively) were used to monitor the incident and scattered beams. In later versions of the apparatus, a third... 

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. 

Figure 4. Micro channel electron multiplier plate. Round approach 14 pm diameter, 17 pm center-to-center. Square Approach 16.3 pm square, 20 pm cen-...

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