Faraday-cup

The least-expensive ion detector is the Faraday cup, a metal or carbon cup that serves to capture ions and store the charge. The resulting current of a few microamperes is measured and amplified. The cup shape decreases the loss of electrons from the metal due to ion impact. The Faraday cup is an absolute detector and can be used to calibrate other detectors. The current is directly proportional to the number of ions and to the number of charges per ion collected by the detector. Unlike dynode-based detectors, the Faraday cup does not exhibit mass discrimination. The detector does have a long response time, which limits its utility. The Faraday cup detector is used for making very accurate measurements in isotope-ratio MS, where the ion currents do not change rapidly. The Faraday cup detector has no gain associated with it, unlike dynode-based detectors. This limits the sensitivity of the measurement. 

This low sensitivity can be an advantage when combined with a high-sensitivity discrete dynode secondary EM (SEM) detector. The SEM detector can cover a linear dynamic range of nine orders of magnitude, but at a concentration range equivalent to ppq to ppm. If concentrations of elements 

For some applications where ultratrace detection limits are not required, the ion beam from the mass analyzer is directed into a simple metal electrode or Faraday cup. With this approach, there is no control over the applied voltage (gain), so the Faraday cup can only be used for high ion currents. Their lower working range is 

The use of a particular detection- and recording-unit mainly depends on the intensity and stability of the ion beam. The three major types of ion detectors include Faraday cup, electron multiplier and photographic plate. 

The first mass spectrometers used photographic plates located behind the analyser as detectors. Ions sharing the same m/z ratio all reach the plate at the same place and the position of the spots allows the determination of their m/z values after calibration. The darkness of the spots gives an approximate value of their relative abundance. This detector, which allows simultaneous detection over a large m/z range, has been used for many years but is obsolete today. 

Because the charge associated with an electron leaving the wall of the detector is identical to the arrival of a positive ion at this detector, secondary electrons that are emitted when an ion strikes the wall of the detector are an important source of errors if they are not suppressed. In consequence, the accuracy of this detector can be improved by preventing the escape of reflected ions and ejected secondary electrons. Various devices have been used to capture ions efficiently and to minimize secondary electron losses. For instance, 

The disadvantages of this simple and robust detector are its low sensitivity and its slow response time. Indeed, the sensitivity of such detectors is limited by the noise of the amplifiers. Furthermore, this detector is not well adapted to ion currents that are not stable in the same time as during the scanning of the analyser because of its slow response time. These detectors are nevertheless very precise because the charge on the cylinder is independent of the mass, the speed and the energy of the detected ions. 

The Faraday cup was widely used in the beginning of mass spectrometry but all the characteristics of this detector mean that it is now generally used in the measurement of highly precise ratios of specific ion species as in isotopic ratio mass spectrometry (IRMS) or in accelerator mass spectrometry (AMS). To obtain a highly accurate ratio in such relative abundance measurements, the intensities of the two stable beams of specific ions are measured simultaneously with two Faraday cups. 

Inorganic Mass Spectrometry Principles and Applications J. S. Becker 2007 John Wiley Sons, Ltd 

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Ions traveling at high speed strike the inside of the metal (Faraday) cup and cause secondary electrons to be ejected. This production of electrons constitutes a temporary flow of electric current as the electrons are recaptured. 

An ion beam causes secondary electrons to be ejected from a metal surface. These secondaries can be measured as an electric current directly through a Faraday cup or indirectly after amplification, as with an electron multiplier or a scintillation device. These ion collectors are located at a fixed point in a mass spectrometer, and all ions are focused on that point — hence the name, point ion collector. In all cases, the resultant flow of an electric current is used to drive some form of recorder or is passed to an information storage device (data system). 

By placing a suitable detector at the focus (a point detector), the arrival of ions can be recorded. Point detectors are usually a Faraday cup (a relatively insensitive device) or, more likely, an electron multiplier (a very sensitive device) or, less likely, a scintillator (another sensitive device). 

Arrival of ions, which have a positive or negative charge, causes an electric current to flow either directly (Faraday cup) or indirectly (electron multiplier and scintillator detectors). 

Faraday cup (or cylinder) collector. A hollow collector, open at one end and closed at the other, used to measure the ion current associated with an ion beam. 

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

As an electrolyte, Nafion 112 (Du Pont, Inc) membrane was pretreated using H2O2, H2SO4 and deionized water before ion beam bombardment. The prepared membranes with a size of 8 X 8 cm were mounted on a bombardment frame with a window size of 5 x 5 cm, equal to the active area of the test fuel cells, and dried up at 80 C for 2 hr. Then, the mounted membrane was brought in a vacuum chamber equipped with a hollow cathode ion beam source as described in the previous study [1]. Ion dose was measured using a Faraday cup. Ion density... 

All MC-ICPMS instruments are equipped with a multiple Faraday collector array oriented perpendicular to the optic axis, enabling the simultaneous static or multi-static measurement of up to twelve ion beams. Most instruments use Faraday cups mounted on motorized detector carriers that can be adjusted independently to alter the mass dispersion and obtain coincident ion beams, as is the approach adopted for MC-TIMS measurement. However, some instruments instead employ a fixed collector array and zoom optics to achieve the required mass dispersion and peak coincidences (e.g., Belshaw et al. 1998). 

Amplifier noise on large peaks measured via Faraday cups ... 

The specimen chamber (or target chamber) may contain a number of samples and standard specimens mounted on the same sample holder. Also here are the X-ray detection system and a Faraday cup which monitors the proton current incident... 

The height of a given X-ray peak is a measure of the amount of the corresponding element in the sample. The X-ray production cross-sections are known with good accuracy, the beam current can be measured by, for example, a Faraday cup (Figure 4.1) and the parameters of the experimental set-up are easily determined so that the sample composition can be determined in absolute terms. 

M. Klein, D.J.W. Mous and A. Gottdang, Fast and accurate sequential injection AMS with gated Faraday cup current measurement, Radiocarbon 46, 77 82 (2004). 

Fig. 2 Schematic diagram of a hydrogen depth profiling setup using a high efficiency BGO detector. A cooled sample holder is placed close to the front surface of the BGO scintillator in ultra-high vacuum. The sample holder can be moved perpendicular to the plane of the figure to bring different samples into the 15N beam and is surrounded by a Faraday cup arrangement to ensure accurate measurement of the analyzing beam dose.

The. experimental procedure was to use a BeO-B sample to adjust the total system such that the 10B4+ beam was maximized and the accelerator voltage was well-stabilized on the 9Be4+ beam. The mass -10 Faraday cup was then replaced by an energy-telescope preceded by a 16 mg/cm2 absorber, which prevented any 10B and other, higher-mass adventitious ions from over-loading the detectors. (Since the mass-10 cup is rotated into position in front of the detectors, the switch can be done easily, and it is possible to easily check the system periodically.) The irradiated BeO sample with 10Be/9Be ratio of 10 9 was arbitrarily selected as a standard and measurements of all other samples were made with respect to this standard. 

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