Position-Sensitive PMTs

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. 

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. 

Virtually continuous X-Y information can be obtained from an MCP-PMT with a resistive anode [311, 312, 361] (Fig. 6.5, right). TCSPC operation of these detectors is described under Sect. 3.5, page 39. 

The drawback of any resistive network used at the MCP output is that it introduces additional noise into the position signals. The thermal noise current is proportional to the reciprocal square root of the resistance. To obtain X-Y resolution of the order of 1,000 x 1,000 pixels, the resistance is usually kept in the 100 kf2 range. However, the high resistance reduces the signal bandwidth, and consequently the useful count rate. 

Noise from a resistive output structure is avoided in the wedge-and-strip anode. The structure is shown in Fig. 6.6. 

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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. 6.5 Position-sensitive PMT with crossed-wire anode leji) and resistive anode (right)...

Preclinical imaging is essential key tool for many research purposes in nuclear medicine. Since small animal imaging requires high resolution imaging modality, it has been realized that conventional systems do not meet the requirements in spatial resolution and sensitivity for this type of imaging [1]. Dedicated systems based on position sensitive PMT coupled to pixelated detector would provide excellent resolution sensitivity tradeoff in small objects. 

Another, yet more complicated way to record the spectrally split signal is position-sensitive detection by a delay-line-anode PMT [510], or a resistive-anode PMT [262]. 

Of special interest for time-correlated single photon counting are the linear fo-cused dynodes, which give fast single electron response and low transit-time jitter, and the fine mesh and metal channel types, which offer position sensitivity when used with an array of anodes. Moreover, PMTs with fine-mesh and metal channel dynodes can be made extremely small, which results in low transit time, low transit-time jitter, and a fast single-electron response. 

Fig. 6.7 Position-sensitive MCP-PMT with delay-line anode...

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. 

CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x 18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity and large dynamic range and which are free from spectral interferences. Each subarray is comprised of pixels. The pixels are photosensitive areas of silicon and are positioned on the detector atx -y locations that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The emission lines are detected by means of their location on the chip and more than one line may be measured simultaneously. The detector can then be electronically wiped clean and the next sample analysed. The advantages of such detectors are that they make available as many as ten lines per element, so lines which suffer from interferences can be identified and eliminated from the analysis. Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower dark current. 

The photomultiplier tube (PMT) is similar in construction to the phototube but is significantly more sensitive. Its photocathode is similar to that of the phototube, with electrons being emitted on exposure to radiation. In place of a single wire anode, however, the PMT has a series of electrodes called dynodes, as shown in Figure 25-13. The electrons emitted from the cathode are accelerated toward the first dynode, which is maintained 90 to 100 V positive with respect to the cathode. Each accelerated photoelectron that strikes the dynode surface produces several electrons, called secondary electrons, that are then accelerated to dynode 2, which... 

On the positive side, upconversion allows efficient detection of red wavelengths (> 650 nm) where direct detection spectrometers are insensitive. For example, for our spectrometer 700 nm fluorescence is upconverted to 422 nm. This latter wavelength is in a region where our pmt and array detectors are quite sensitive. 

Combination of assembled microcell [134] or Au chip electrode [132] with a positive-intrinsic-negative (PIN) photodiode detector lead to advancement in the miniaturization of ECL instrumentation for DNA quantification. Though, noise level and sensitivity of sihcon PIN diode are less sensitive than a PMT [134]. A highly sensitive DNA biosensor, fabricated by self-assembling the ECL probe of thiolated hairpin DNA tagged with ruthenium complex on the surface of a gold electrode works on a switch-off mode and ECL intensity decreases with an increase of the concentration of target DNA, and a detection limit of 9 x 10 M... 

Figure 2 Properties of ideal turbidimeter and spectrophotometers. For a clear sample, the position of the detector (PMT, photomultiplier tube) is unimportant (A). If a turbid sample is examined in a spectrophotometer where the PMT is far removed from the cuvette (B), the instrument will behave as a sensitive turbidimeter because it will detect little of the scattered light. If a turbid sample is examined in a spectrophotometer where the PMT is close to the cuvette (C). the instrument will detect absorbance even though much of the light is scattered.

With numerous maxima and minima, we used an expanded light beam with photographic film as the light distribution detector. For weak Kerr constant materials we improve measurement sensitivity by using a narrow light beam with a photomultiplier tube (PMT) for field measurement as a function of time at a single position x. 

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