Faraday detector

Unlike the photoplate, the Faraday detector (or Faraday cup) is still very much in use today. The main reasons for its lasting popularity are accuracy, reliability, and mgged construction. The simplest form of Faraday detector is a metal (conductive) cup that collects charged particles and is electrically connected to an instrument that measures the produced current (Fig. 2.21b). Faraday cups are not particularly sensitive and the signal produced must in most applications be significantly amplified. An important application for Faraday detectors is precise measurements of ratios of stable isotopes [278]. See, for example, Section 2.2.7 and Chapter 11 for examples of applications and methods in which Faraday detectors are utilized. 

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Thorium. Multiple-collector measurement protocols by TIMS for thorium isotopic analysis typically involve the simultaneous measurement of Th and °Th (for silicate rocks), or Th and °Th, then Th and Th (for low- Th samples), using an axial ion counter and off-axis Faraday collector (Table 1). Various methods are used to correct for the relative gain between the low-level and Faraday detectors and 2a-uncertainties of l-5%o are typically obtained (Palacz et al. 1992 Cohen et al. 1992 McDermott et al. 1993 Rubin 2001). Charge-collection TIMS protocols enable Th, °Th and Th to be monitored simultaneously on a multiple-Faraday array and can achieve measurement uncertainties at the sub-permil level (Esat et al. 1995 Stirling et al. 1995). 

The measurement is always relative, that is, in 14C measurements the amount of 14C is always related to that of 13C or 12C. The more abundant isotope is detected by a Faraday detector while the less abundant isotope is detected by a specialized device... 

Figure 4.8 Multiple ion collector array from MC-ICP-MS NEPTUNE and MC-TIMS TRITON (Thermo Fisher Scientific, Bremen). Miniaturized ion counters identical in size to Faraday detectors are mounted in the high mass range to detect, e.g., low-abundance uranium isotopes. (Reproduced by permission of Thermo Fisher Scientific, Bremen.)...

Second, the source of ions is geometrically broad compared with that in TIMS. Therefore, the extracted ions need to be spatially restricted and direction-ally focused at the source (defining) slit with minimal loss of transmission, in order for the mass resolved ion beams to fit easily within the openings of the multiple Faraday detectors (buckets) and produce excellent peak shape. 

Regelous et al have reported ou the use of the isotope dilutiou techuique (using a Pa spike with a half-life of 26.97 days) for the quantitative measurement of 20 fg of protactinium in silicate rocks after chemical separation of the actinide from the rock matrix by MC-ICP-MS (Neptune, Thermo Fisher Scientific, Bremen - equipped with uiue Faraday detectors, oue secondary electron multiplier and a retarding potential quadrupole for high abundance sensitivity measurements). 

The determination of differences in isotopic ratios requires very precise measurements. The combustion step for the sample preparation is usually carried out immediately prior to the injection into the MS. There exist instruments which associate in line a gas chromatograph, a tubular combustion oven, containing copper oxide heated to 800 °C, and a low-resolution MS equipped with several Faraday detectors, each collecting the signal corresponding to a specific mass. A calibration compound is co-injected with the product to be studied. 

Before the connection to the laser system, the MIC system was optimized using a pure Pb standard solution (SRM-981) at 0.1 ng/mL. Th and U were added to this solution for peak center optimization, specifically the five counters (IC2, IC3, IC4, IC5, IC6) and the two Faraday cups (H3 and H4) see Table 31.1. The Th and U concentrations were adjusted to achieve 100-mV signals on the Faraday detectors. The test solution was run to determine the most efficient instrumental settings. The peak shape and the peak superposition of aU masses were optimized by adjusting the optic zoom lenses. In the low mass range, the MIC system consists of two movable blocks IC2, IC3, L4 and IC4, IC5, IC6, L3, the ion counters being fixed relative to each other in each block (Table 31.1). The dark noise was measured... 

The ions impinging on a detector produce an electric current that is converted into a voltage and amplified. The simplest detector is the Faraday detector that is a flat metal plate that gets charged when ions strike it or the Faraday cup that has a concave configuration and serves a similar function (Figure 1.26a). A more sensitive detector is an electron multiplier made of a series of dynodes. When an ion strikes the surface of the first dynode, a number of electrons are emitted (Figure 1.26b). 

Figure 3.1 Multi-collector array in the Thermo Scientific Neptune multi-collector ICP-MS instrument. Miniaturized ion counters identical in size with Faraday detectors are mounted on the high-mass side (for U) and low-mass side (for Pb). Connections to the miniaturized ion counters are completely independent from the Faraday cup signal lines, protecting the integrity of the signals.

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