Pulse counting mode

Detection systems. Prior to the past decade, most instruments used for uranium-series analysis were single-collector instruments, for which ion beams of the various isotopes are cycled onto a single low-intensity detector, usually with electronics operating in pulse counting mode (Chen et al. 1986 Edwards et al. 1987 Bard et al. 1990 Goldstein et al. 1989 Volpe et al. 1991 Pickett et al. 1994), in order to measure the low-intensity ion beams of °Th, Pa, Pa, Ra and Ra. Daly detectors and... 

Electron multipliers can also be operated in analog mode as current detectors. In this mode, they have a lower gain and measure higher signals than in pulse-counting mode. This... 

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

Fig. 9. X-Ray diffraction data of a-iron oxide samples in Fig. 8 collected by pulse-counting mode using MoK, radiation for samples of average crystallite size of (A) 14.5 nm, (B) 25 nm, (C) 7.5 nm, and (D) 9.5 nm. For spectrum C, the region from 13 to 16° was counted for 1000 sec/step. From ref. 56, reprinted with permission, copyright 1984 by the American Chemical Society.

Principle of TOF mass spectrum acquisition with TDC in pulse counting mode. Data from JEOL documentation. 

These saturation effects, resulting from the loss of ions due to the TDC dead time, can be statistically corrected by applying a correction factor [4], However, there is no effective correction when the quantity of ions increases and several ions arrive at the detector simultaneously. In conclusion, detection systems that operate in pulse counting mode are well suited to detect small quantities of ions by accumulation for a long period of time and when detection of individual ion events is important in order to obtain a good signal-to-noise ratio. 

The entrance optics to the analyser is usually chosen so that it can be tuned to nearly constant transmission over a large energy range. The gun optics is also usually chosen so that the beam can be focussed at the same position with the same image size over a wide energy range. This is necessary in order to avoid distortion of peak shapes and resonance features and inaccuracies in the cross-section measurements. After transmission through the analyser a simple lens transfers the electrons to the surface of an electron multiplier, usually a channeltron, which is operated in the pulse count mode. 

The tandem MS includes a quadrupole mass filter, an octopole ion guide," a second quadrupole mass filter, and an ion detector. The ions from the flow tube are focused through electrostatic lenses into the first quadrupole, where a particular reactant ion is selected. These ions are then focused into the octopole, which passes through a cell that contains the collision gas. From the octopole, the dissociated and unreacted ions are focused into a second quadrupole for mass analysis. The detector is an electron multiplier operating in pulse-counting mode. 

Variants of the EM include the discrete dynode, channeltron (or continuous dynode), the microchannel plate, and the Daly detector. The EM can detect ion currents below 1 ion/s. EM detectors are often operated in the pulse counting mode, in which the pulse produced by a single ion is amplified and detected as an individual events. Pulse counting offers excellent signal-to-noise ratio, high sensitivity, and insensitivity to change in EM gain. 

Whether an electron multiplier has a discrete-dynode or continuous-dynode design, it can be operated in both analog and pulse-counting modes. For analog detectors. 

This represents the practical limit for such detection schemes, so Faraday Cup detectors are relatively insensitive compared with electron multipliers (see below) especially when used in pulse counting mode. 

For CEMs the linear dynamic range is limited at the upper end by the maximum count rate capability for versions designed for use in pulse counting mode, or by maximum linear output current for those designed for... 

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