Pulse counting

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

DC amplification and pulse counting (often inaccurately called photon counting ) are two types of signal amplifications often used. 

In the pulse counting method, each photoelectron pulse arriving at the phototube anode is processed. The pulses are amplified and then used to trigger a pulse generator. The output pulses from the generator are integrated and displayed on a recorder. 

A scintillation ion detector, described in detail elsewhere (41), detected virtually every ion which entered the detector chamber. Pulse counting techniques were used. 

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... 

The instruments include an ionization chamber, the charcoal-trap technique, a flow-type ionization chamber (pulse-counting technique), a two-filter method, an electrostatic collection method and a passive integrating radon monitor. All instruments except for the passive radon monitor have been calibrated independently. Measurements were performed... 

Flow-Type Ionization Chamber PFC flow rate 1-2 /min, continuously measured, current measurement and a-pulse counting... 

Fig. 8. The relative uncertainty in the measured intensity of various detectors as a function of the exposure level. A solid straight line indicates an ideal detector. A dashed line indicates a pulse-counting detector of 10 % efficiency. O and indicate the IP system for MoKa and CuKp, respectively. A and A indicate Kodak DEF-5 X-ray film for MoKa and CuKp, respectively. The munber of X-ray photons required to obtain a certain accuracy in intensity measurements can be compared...

Schematic diagram of an electron multiplier showing and (top) electrical connections for analogue and pulse counting and (bottom) how a single ion gives rise to a large...

Another commonly used detector is the Faraday cup. This detector is an analogue detector and so has poorer sensitivity than a pulse counting electron multiplier. However, it has the advantage of simplicity (it is essentially only a metal plate used to measure ion current), and it does not suffer from burn-out like an electron multiplier (which must be periodically replaced). 

Q. What are the main differences between an electron multiplier operated in pulse counting and analogue mode ... 

Also important is the effect of detector dead time. When ions are detected using a pulse counting (PC) detector, the resultant electronic pulses are approximately 10 ns long. During and after each pulse there is a period of time during which the detector is effectively dead (i.e. it cannot detect any ions). The dead time is made up of the time for each pulse and recovery time for the detector and associated electronics. Typical dead times vary between 20 and 100 ns. If dead time is not taken into account there will be an apparent reduction in the number of pulses at high count rates, which would cause an inaccuracy in the measurement of isotope ratios when abundances differ markedly. However, a correction can be applied as follows ... 

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.)...

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... 

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.

The experimental apparatus consists of eight main parts an ultraviolet flashlamp capable of repetitive flashing at about 5 Hz, a purge flow reactor with either pinhole or molecular beam sampling, an ion source, a mass filter, an ion detector, pulse-counting electronics, computer data aquisition, and a vacuum system. A diagram of the apparatus is shown in Figure 1. 

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