Multichannel Scalers

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))...

The current pulse is electronically amplified, passed through a lower level discriminator, and counted by a multichannel scaler. The lower level dis-... 

Figure 4. Decay of CF2C102 radicals in the reaction, CF2C102 + N02 =s CF2-C102N02. The peroxy radical was monitored by the signal at m/z = 82 (CF202) +, which was accumulated for 4748 flashes. Each channel of the multichannel scaler represents a time increment of 0.1952 ms.

TCSPC is Ulnstrated in Fig. 3a. In addition to a mode-locked laser for pnlsed excitation and a detector with high time resolntion (nsnally a micro-channel plate photomultiplier tube capable of time-resolution of 20-30 ps), the required instrumentation inclndes constant-fraction discriminators to generate electrical pnlses triggered by fluorescence photons and by the reference (the excitation pulse), a time-to-amphtude converter or other device to measnre the time lag between reference and flnorescence connts, and a multichannel scaler to accumnlate... 

A small portion of the mixture leaks out of a pinhole and is formed into a beam (Fig. 2.14). The radicals are ionized by light from a halogen, hydrogen or rare gas resonance lamp, and then mass selected. Temporal ion signals are recorded on a multichannel scaler including a short portion of background signal before the laser is fired. 

Figure 14. Schematic diagram of the CO metastable TOF experimental apparatus is shown. The molecular beam (MB) containing 10% ketene in neon or helium can be placed at any acute angle (0,ab) relative to the flight path, and it is collimated by an electroformed skimmer (not shown). The photolysis laser is an unpolarized excimer (XeCl or XeF), and the probe laser is a pulse dye amplification system whose polarization can be made either parallel (sPR, ) or perpendicular (e ) to the flight path. The metastables pass through a 1-cm orifice and deflector plates and grids (both not shown), and they strike a heated Ni surface. Electrons produced from the Ni surface by the metastables are steered by a plate set at —1500 V onto a stack of 3 MCPs the resulting pulses are then amplified, discriminated against noise from dark current, and counted by a multichannel scaler.

Multichannel Scalers ( Multiscalers ) are the photon-counting equivalent of a digital oscilloscope or a transient recorder. They record the input pulses directly into a large number of consecutive channels of a fast memory. The technical principles behind multiscalers are described below. 

Beeause there is no sealer for the photons (or other trigger events), devices based on event reeording are aetually not multichannel scalers in the original sense. However, waveform reeording is possible by analysing the stream of event times. The result is then the same as for a multiscaler that accumulates the data direetly. 

Due to the finite speed of signal processing, a photon eounter is unable to deteet a seeond photon within a certain dead time after the deteetion of a previous one. For gated photon counters or multichannel scalers, the dead time ean be as short as a nanoseeond. The relatively complicated signal proeessing sequenee in a TCSPC deviee leads to a much longer dead time. Older TCSPC deviees had dead times of the order of 10 ps. Newer, more advanced TCSPC modules are mueh faster but still have a dead time in the range of 100 to 150 ns. 

Multichannel Scaler. The device counts events, e.g. photons, into successive channels of a fast memory. 

The photomultiplier output pulses were amplified, discriminated, and fed into a multichannel scaler, and OH fluorescence decays were signal averaged over 25-250 excimer laser shots. 

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