Side-window PMTs

Side-window PMTs are rugged and inexpensive, and they often have a somewhat higher cathode efficiency than front window PMTs. Their broad TTS and long SER pulses make them less useful for TCSPC applications. However, side-window PMTs are used in many fluorescence spectrometers, in femtosecond correlators, and in laser scanning microscopes. If instruments like these have to be upgraded with TCSPC, it can be difficult to replace the detector. Therefore, some typical instrument response functions for side-window PMTs are given below. 

For all side-window PMTs the width and the shape of the TCSPC instrument response depend on the size and the location of the illuminated spot on the photocathode. The response for the R931 - a traditional 28-mm diameter side-window PMT - for a spot diameter of 3 mm and different locations on the photocathode is shown in Fig. 6.51. The obtained IRF width is between 315 ps and 650 ps. 

The IRF width can be further redueed by eoncentrating the light on an extremely small spot on the photocathode and reeording pulses in a narrow amplitude window only [81, 268]. A TCSPC response width down to 112 ps FWHM has been reported [81], though with eonsiderable loss in counting efficiency. 

The afterpulse probability for an R931 is shown in Fig. 6.52. As usual, the afterpulse probability depends on the operating voltage. The afterpulses appear within a time interval of about 3.5 ps. 

An IRF of a modem 13-mm side-window tube (Hamamatsu R6350) is shown in Fig. 6.53. The full photocathode was illuminated. The FWHM is 540 ps. 

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The fluorescence lifetime of the fiuorophores commonly used in microscopy is between 1 and 5 ns. Single-exponential lifetimes in this range can reliably be measured with a detector IRF width of 200 to 400 ps. Medium speed detectors, such as the Hamamatsu H7422 0 or the H5773 or H5783 modules are sufficient for these applications. Side-window PMTs are not recommended. The IRF of these detectors depends on the illuminated area of the photocathode and can be between 350 ps and about 1 ns. 

Fig. 14.7 a Front face PMT. The photosensitive surface, the photocathode, is at the front, at the back can be seen the pins used in the circuit to fix the dynode voltages, b A side window PMT. Here the photocathode is behind the grid towards the left centre of the device, c The thermoelectric detector element of a calorimetric laser power meter, the laser pulse is made incident on the thin black disc in the centre of the device, absorption converts the photon energy into thermal energy, which is subsequently measured using the thermoelectric effect in a thermopile immediately behind the disc. When in use the unit shown is held in a thermally isolated chamber which is screwed over the top of the detector element. The 5p UK coin, included in the middle of the photograph to give some idea of scale, has a diameter of 18 mm... 

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... 

The approximate upper fiequem limits of commonly used mTs are listed in Ihble 4.1. These values ate estimated based on our experience and product literature. The upper fiequency limh of the side window R928 is near 200 MHz. Much higher fiequency measurements ste posdbie with MCP PMTs (Section 5.7), but special circuits ate needed for cross coiidation outside of the mr. 

As described in Section 4.6A I fTs can displi a wavelength-dependent time response. These effects can also be present in the PD measurements. In FD measurements the effects are somewhat more difficult to understand. There can be systematic errors in the phase or modulation values, and the direction of the errors is not always intuitively obvious. Fortunately, the color edicts are minor with presently used side-window dynode PMTs. and they ap-... 

A typical single threshold-type aerogel Cherenkov counter (ACC) module looks as shown in Figure 28.12. TMSA tiles are stacked in a thin aluminum box of approximate dimensions 120 x 120 x 120 mm. In order to detect the Cherenkov light effectively, two fine mesh-type PMTs are attached directly to the aerogels at both sides of the box. The inner surface of the box (except for the phototube windows) is lined with a diffuse reflector... 

Standard PMTs have a lateral photocathode (side-on window), spectral response from 200 to 800 nm and a few connected dynodes (typically five out of twelve) for a faster response. The operating voltage V (typically V — —950 —... 

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