Linear Focused PMTs

The IRF width is 204 ps, 320 ps, and 396 ps for areas of 1 x 1 mm, 6x12 mm, 8x16 mm, respectively. Changes in the focusing voltages change the transit time, the shape of the IRF, and the efficiency distribution over the cathode area. The results may vary eonsiderably for different sizes of the illuminated area and different loeations on the eathode. 

The total transit time of the XP2020 is about 28 ns. This is long compared to the 5 to 6 ns of miniature metal-ehaimel PMTs. Although the long transit time is not directly objectionable, it results in a correspondingly large shift of the 

Autocorrelation curves of the afterpulsing are shown in Fig. 6.50. The afterpulsing is not very strong, but extends over a period of more than 10 ps. 

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Figure 7.33 and Fig. 7.34 show the eount-rate dependent timing drift of the IRF for an XP2020 linear-focused PMT and an H5773-20 photosensor module. The eurves were reeorded with a BHL-600 diode laser of 40 ps pulse width and 650 nm wavelength, and an SPC-140 TCSPC module (both Beeker Hiekl, Berlin). The... 

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

Dynode PMTs cost less than MCP PMTs and are adequate for many TCSPC expmments, especially if the excitation source is a flasMamp. Two types of dynode PMTs are used for TCSPC, the side-window and linear-focused PMTs. Their performance is comparable, but there are minor differences. The side-window tubes are less expensive but can still provide good time resolution. Pulse widths from 112 to 700 ps have been obtained with side-window... 

Fig. 6.2 PMT dynode geometries. Circular dynode arrangement, linear focused, fine mesh, Venetian blind, metal channel type. C Cathode, D dynodes, A anode, F focusing electrode,...

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

The spectral resolution of a detector is defined here as equal to its spatial resolution (in urn ) times the reciprocal linear dispersion of the spectrometer (in nm/um ). It was measured to be 1.5 - 2.5 and 2-4 times poorer for the SPD and SIT, respectively, compared to that of a PMT. All measurements were performed with the same spectometer, utilizing 20 urn slit widths. Because, the proximity focused, microchannel plate (MCP) intensi-fier broadens the line images, the spectral resolution of the ISPD was found to be significantly worse than that of the SPD. Peak widths measured at half maximum intensity were four diodes wide even when only a single diode width was illuminated. 

We have focused ou PMTs as detectors, but we note here the growing usefulness of cluage-coupled devices (CCDs) in fluorescence spectroscopy. CCDs are imaging detectors with remarkable sensitivity and linear dynamic range. CCDs typically contain aboirt 500,000 pixels, but versions with more than 10 pixels are available. Each pixel acts as rut acrnrmulating detector, that is, charge accumulates in... 

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