Electron multiplier pulse counting

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

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. 

Figure 5.6. Diagram of a low-energy, high-angle electron-impact spectrometer. (A) Electron gun (B) monochromator (180° spherical electrostatic energy selector) (C) electron optics (D) scattering chamber (E) analyzer (180° spherical electrostatic energy selector) (F) electron multiplier (G) amplifier and pulse discriminator (H) count-rate meter (I) multichannel scaler (J) X Y recorder (K) digital recorder. (After Kupperman et a/.<42))...

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 ... 

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... 

Figure 9 Magnetic sector pulse-counting mass spectrometer (a) ion source, (b) ion lens, (c) source defining slit, (d) collector slit, (e) electron multiplier. (From Ref. 47.) (Courtesy of R H. Hemberger, Los Alamos National Laboratory.)...

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. 

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