Multianode PMT

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. 

A higher number of channels can be obtained if a multianode PMT is used. In a multianode PMT the combined photon pulses of all channels are available at the last dynode. This makes an external combination of the pulses unnecessary. The noise problem is therefore lessened. Routers for multianode PMTs are combined with the PMT tube into a common detector housing. TCSPC multichannel detector heads now exist for 16 channels. Using this method, devices with 32 channels and even 64 channels appear feasible. In practice the number of channels is limited only by the power dissipation of the routing electronics. 

Parallel image aequisition is, in prineiple, possible by two-dimensional multianode PMTs. The anode elements are eonneeted to a standard routing deviee, and the photons are reeorded as deseribed above under Seet. 3.1, page 29. However, the number of amplifiers and diseriminators in the router inereases with the number of anode elements. Power eonsumption and spaee requirements restriet the number of pixels that ean be obtained this way. The praetieal limit is somewhere between 16 and 64. An array of 64 pixels eannot really be eonsidered an image. 

A more detailed fluorescence spectrum is obtained by using a polychromator (or spectrograph) and recording the spectrum with a multianode PMT. The principle of a multiwavelength fluorescence experiment is shown in Fig. 5.24. 

Fig. 536 Continuous-Flow mixer with TCSPC detection. Left Single-point detection with scanning. Right Multipoint detection with a multianode PMT...

A stopped-flow setup can relatively easily be equipped with multispectral detection. The fluorescence light is emitted from a small spot in the flow channel. Therefore the flow cell can be placed directly in the input slit plane of a poly-chromator. The spectram is detected by a multianode PMT. The photons detected in the spectral channels are recorded simultaneously by a TCSPC device and a router. However, although the implementation is relatively simple, no spectrally resolved TCSPC-based stopped-flow system has yet been described. 

Quaresima et al. used a single TCSPC channel and a multianode PMT to record sequences of time-of-flight curves in eight parallel channels [421]. The acquisition time per step of the sequence was 166 ms. The data of five steps were averaged. Values of p s and Pa were calculated from the averaged data by using a standard model of diffusion theory. 

Often a long-pass filter must be inserted in the polychromator to improve the blocking of scattered laser light. The spechnm at the output of the polychromator is detected by a multianode PMT, and the decay curves in the spectral channels are recorded by multidetector TCSPC. 

A potential application of multimodule systems is high-speed two-photon multibeam scaiming systems [53, 77]. FLIM systems with 4, 8 or even 16 beams and the same number of parallel TCSPC channels appear feasible. The problem is to direct the fluorescence signals from the individual beams to separate PMTs or separate charmels of a multianode PMT. If this problem is solved, two-photon lifetime images can be recorded with unprecedented speed and resolution. 

In combination with advanced TCSPC, the systems can be used to record fluorescence decay curves, dynamic changes of fluorescence decay curves, fluorescence correlation in combination with fluorescence lifetime, and spectrally resolved fluorescence decay profiles. Examples for dynamic lifetime measurements and spectrally resolved lifetime measurements by a multianode PMT with routing are shown under Sects. 5.4.1, page 90, and 5.2, page 84. 

The efficiency of the measurement can be increased by multiwavelength detection. The monochromator is replaced with a polychromator, and a multianode PMT with routing electronics is used to detect the full spectrum. However, despite its obvious benefits, no application of multiwavelength TCSPC to sonoluminescence has yet been published. 

To obtain position sensitivity, the single anode can be replaced with an array of individual anode elements [297, 298] see Fig. 6.4. The position of the corresponding photon on the photocathode can be determined by individually detecting the pulses from the anode elements. Multianode PMTs are particularly interesting in conjunction with the multidetector capability of advanced TCSPC techniques. 

Fig. 6.4 Multianode PMT with metal channel dynodes left) and multianode MCP-PMT right)...

A version of the multianode PMT uses a system of crossed wires as the anode [297]. In principle, individual wires could be connected to a TCSPC device via a router as in the case of the multianode PMT. Another way to obtain the X-Y information is to connect the wires via two resistor chains, as shown in Fig. 6.5, left. The spatial coordinates of the photons are then determined by measuring the pulse amplitudes at the ends of the resistor chains. 

Fig. 6.44 R5900-L16 Multianode PMT routing electronics middle) and complete...

Compared to scanning of a spectrum by a monochromator, a polychromator with a multianode PMT system is more efficient at recording a spectrum the degree of its relative efficiency is proportional to the number of PMT channels. 

Nevertheless, there is a way to reduce the classic pile-up. The light is distributed into several deteetors, or into several channels of a multianode PMT. Two photons arriving within the same signal period are then more likely to hit different detectors than the same one (Fig. 7.80, right). Pulses that appear at the outputs of different deteetors can easily be identified as multiphoton events. The recording of both photons in the TCSPC module can be suppressed and pile-up distortion can be avoided. 

An additional push can be expected from new technical developments in TCSPC itself. The largest potential is probably in the development of new detectors. The introduction of direct (wide-field) imaging techniques is clearly hampered by the limited availability of position-sensitive detectors. In addition the selection of multianode PMTs is still very limited, especially for NIR-sensitive versions. Large-area detectors with 64 or more channels may result in considerable improvements in DOT techniques. Single photon APDs with improved timing stability are urgently required for single-molecule spectroscopy and time-resolved microscopy. 

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