Multidetector operation

The application of refractive index and differential viscometer detection in SEC has been discussed by a number of authors [66-68]. Lew et al. presented the quantitative analysis of polyolefins by high-temperature SEC and dual refractive index-viscosity detection [69]. They applied a systematic approach for multidetector operation, assessed the effect of branching on the SEC calibration curve, and used a signal averaging procedure to better define intrinsic viscosity as a function of retention volume. The combination of SEC with refractive index, UV, and viscosity detectors was used to determine molar mass and functionality of polytetrahydrofuran simultaneously [70]. Long chain branching in EPDM copolymers by SEC-viscometry was analyzed by Chiantore et al. [71]. 

Multidetector operation Several detectors are used. A router generates a channel information that indicates which of the detectors detected the current photon. With an array of detectors, a multianode PMT, or another position-sensitive PMT spatial resolution or spectral resolution can be obtained. 

Fig. 3.2 Principle of TCSPC multidetector operation. The detectors are receiving different signals originating from the same excitation laser. The photon pulses from both detectors are combined, and the times of the pulses are measured in a single TAC. A routing signal indicates which of the detectors detected the currently processed photon. The TCSPC module puts the photons from different detectors into different memory segments...

Fig. 3.4 TCSPC multidetector operation. By the. .channel signal from the ronter, the photons of the individual detectors are routed into separate memory blocks...

Sequential recording, also known as double kinetic mode [353] or time-lapse recording , adds one or two additional dimensions to the photon distributions recorded by multidetector operation and multiplexing. Controlled by its internal clock oscillator, the sequencer switches through a specified number of memory blocks. Each memory block contains the photon distributions of all detectors and multiplexing channels. Sequential recording in a multidetector system is illustrated in Fig. 3.7. For sake of simplicity, multiplexing has been omitted. 

Another aetive mode, called Scan XY Out sends digital position signals to a seaimer. This simplifies the control of a scan stage considerably. Due to the limited number of signal input and output lines, multidetector operation and multiplexing are usually not applicable. 

The position is calculated by the ratio of the pulse amplitudes (or charge) measured at the outputs of the anode stracture. Timing information is available from the pulse at the last dynode or at the low-voltage side of the multichannel plate (see also Sect. 6.1.3, page 215). Once the position information is available, the task is reduced to multidetector operation in a large number of detector channels. 

With a dead time of 100 ns per TCSPC channel, total useful count rates of the order of 20 MHz can be achieved. All four channels can be used for multidetector operation. The high count rate and the high number of channels make multimodule TCSPC systems exceptionally useful for diffuse optical tomography [34], and high count rate applications in laser scanning microscopy [39]. Details are described under Sect. 5.5, page 97 and Sect. 5.7, page 129. 

Burst-recording in the continuous-flow mode can be eombined with multidetector operation. The detectors can be used to record the fluoreseence in different wavelength intervals or under different angles of polarisation. This multiparame-ter detection technique delivers the lifetime, the angle of polarisation, the fluorescence anisotropy, and the emission wavelength within the individual bursts [442, 500]. 

Fig. 7.80 Multidetector operation reduces pile-up by recognising multiphoton events...

Of course, the multidetector technique does not increase the maximum throughput rate of a TCSPC system. In any TCSPC device there is a small but noticeable loss of photons due to the dead time" of the processing electronics. The dead time of advanced TCSPC devices is of the order of 100 ns, and for count rates above 1 MHz the counting loss becomes noticeable (see Sect 7.9, page 332). The counting loss for a multidetector TCSPC system is the same as for a single detector system operated at the total count rate of the detectors of a multidetector system. 

An alternative to the multidetector technique is parallel operation of several independent TCSPC channels, which increases the total counting capability at the expense of higher system cost. Please see Sect. 3.7, page 45. 

If a multidetector system is being operated, the RI detector must always be connected last in the detector train. This will ensure that the detector cell is not subjected to any back-pressure which may cause the cell to rupture. 

In an experimental campaign, the reaction Mg + " Cm was investigated at five different beam energies [88, 89]. For these experiments, performed at GSI, a heated recoil chamber, which was operated at 400 °C, was constructed in order to optimize the rapid formation of tetroxides. Also, the new cryo thermochromatography detector cryo on-line multidetector for physics and chemistry of transactinides (COMPACT), with an improved detection efficiency, was developed. Version 1 had a detection efficiency for a single a particle of 78% and version 2 of 93%. This way, the probabihty to detect all members of a decay chain was significantly improved. In two experiments, a total of 26 decay chains assigned to Hs isotopes were identified. 

Multidetector capability (radial ejection). Those familiar with the classical QIT operation have realized that half of the ions are lost during ejection because only one end cap is followed by a detector. (Note that some manufacturers have solved this problem.) The LQIT, however, ejects ions radially and thus allows for the use of one or more radial ion detectors. 

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