In this column, as well as the next installment, we describe two detector systems used in MS — the Faraday cup and position-sensitive array detectors.
The beam current was systematically varied (using all possible combinations of the different condenser apertures and spot sizes - in the plot to the left, spots sizes from 1 through 5 are color coded and occur in groups of four as the condenser aperture grows smaller and smaller) and measured with the Faraday cup.
Array detectors (which can include arrays of miniaturized Faraday cups) are used not only as detectors for dispersive-beam instruments, but also as developmental tools for characterizing the position and cross section of a beam of ions as it traverses an instrument. Commercial FC detectors may have a weak magnetic field to prevent secondary electrons from leaving the Faraday cup (4), and they may operate with a slight positive bias on the impacted surface to reduce secondary electron emission. However, that calibration of counts per electron was ~12 whereas recent work with a Faraday cup indicates that the value should be ~7. As expected, the limit of detection for a Faraday cup depends on the sensitivity of the electrometer in the circuit that it is connected to.


For example, a calibration procedure using a Faraday cup was used to measure the beam current in the EMC's JEOL JEM 3200FS. Such images must be recorded without a specimen in the electron beam path (so no scattering from the sample occurs) and such that the entire beam only covers a portion of the CCD frame (so that the total beam current is recorded in the image, meaning that the Faraday cup's reading can be directly related to counts in the image).
The cup is an element in a circuit; the current flow through the circuit can be very accurately measured and is directly proportional to the number of ions that have been intercepted by the Faraday cup. Since the Faraday cup provides the total electron dose for any combination of spot size and condenser aperture, knowing this fraction of the total screen area would allow a user to estimate the total number of electrons that interacted with the specimen that is recorded in any image. Although such Faraday cup measurements can be used to determine the strength of the electron beam in a given image (provided that the imaging conditions were carefully controlled), a more important question is the electron dose per unit area that the specimen experiences. Coupled with modern electronics, the Faraday cup is singularly useful in producing high precision measurements in isotope ratio mass spectrometers.


A current of 1 nA in the circuit corresponds to the arrival of several billion singly charged ions per second at the Faraday cup. The metal cup (Figure 1 is a photograph and Figure 2 is a schematic) is placed within a vacuum system to intercept a beam of charged particles (electrons or ions). Errors in the current measurement are reduced with the addition of an electron suppressor plate to the cup, as shown in Figure 1.



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