MCP-PMTs

Because of the low timing-jitter (down to 25 ps) TCSPC-based systems are often equipped with a MCP-PMT at detriment of acquisition speed (<106 counts per second). On the other hand, a TG-SPC system equipped with four gates and a fast PMT (10 MHz) could be slower than a TCSPC at low count-rates (<100 kHz), because of a lower photon-economy. However, already at 1 MHz, the former would be almost three times faster and more the one order of magnitude faster at 10 MHz. 

Time-Correlated Single-Photon Counting. For the application of TCSPC in the picosecond time domain, lasers with pulses whose half-widths are 20 ps or less are used. For better time resolution, the combination of a microchan-nel plate photomultiplier tube (MCP-PMT) and a fast constant fraction discriminator (CFD) are used instead of a conventional photomultiplier tube (PMT). A TCSPC system with a time response as short as 40 ps has at its core a Nd YLF (neodymium yttrium lithium fluoride) laser generating 70-ps, 1053-nm pulses at... 

As an example. Fig. 1.4 shows the single-electron response measured with a high-speed oscilloscope and the transit-time distribution for a Hamamatsu R3809U MCP PMT measured by TCSPC. 

The effective resolution of a TCSPC experiment is characterised by its instrument response function (IRF). The IRF contains the pulse shape of the light source used, the temporal dispersion in the optical system, the transit time spread in the detector, and the timing jitter in the recording electronics. With ultrashort laser pulses, the IRF width at half-maximum for TCSPC is typically 25 to 60 ps for microchannel-plate (MCP) PMTs [4, 211, 547], and 150 to 250 ps for conventional short-time PMTs. The IRF width of inexpensive standard PMTs is normally... 

Almost continuous position information can be obtained from an MCP-PMT with a resistive anode or with a micromachined wedge-and-strip geometry [247, 248, 262, 312] (see also Sect. 6, page 213). The detector principles are shown in Fig. 3.11. 

The third limitation, the counting capability of the detector, depends on the detector type used, the voltage divider design, and the requirements for IRF stability, long-term gain stability, and detector lifetime. For conventional PMTs the practical limit for TCSPC is of the order of 5 to 10 MHz. For MCP-PMTs the maximum count rate is 200 kHz to 2 MHz, depending on the MCP gain used. 

The Hamamatsu R3809U MCP PMTs deliver an IRF width of 25 to 30 ps in conjunction with Ti Sapphire lasers, and of 50 to 100 ps in conjunction with diode lasers. A typical result is shown in Fig. 5.6. The IRF width is 24.5 ps, FWHM, and the fluorescence lifetime 80 ps. A colour shift of the monochromator of 5.7 ps was corrected in the data. 

Typical results obtained in a filter-based setup with diode-laser exeitation are shown in Fig. 5.12. The sample was an indocyanine green solution in ethanol in a 1 cm cuvette. The sample was excited by a diode laser of 650 nm emission wavelength, about 50 ps pulse width, and 50 MHz repetition rate. The IRF and the fluorescence recorded at 830 nm are shown for an H5773-20 PMT module (left) and for an R3809U MCP-PMT (right). 

Fig. 5.12 Fluorescence decay curves of indocyanine green (ICG). A in ethanol, B in water, C instrument response function. Time scale 400 ps / div, 1-cm cuvette, diode laser 650 nm, 50 MHz. Left Detected by a cooled H5773-20 photosensor module. Right Detected by an R3809U MCP PMT...

Fig. 5.14 Le/f Scattering solutions of different scattering intensity. Right IRFs recorded with the solutions. 10 mm cuvette, 90° illumination, light detected from full width of cuvette, 650 nm diode laser, R3809U MCP PMT, SPC-730 TCSPC module, time scale 50 ps/div...

With a femtosecond laser and an MCP PMT cr is about 10 ps. For a total number of recorded photons of A = 10 both in the IRF and the fluorescence, which is certainly a conservative assumption, the standard deviation of a short lifetime is 14 fs. This value is surprisingly low. 

Applications of multiwavelength TCSPC to laser scanning microscopy have been demonstrated in [35, 60]. Spectrally resolved detection in diffuse optical tomography is described in [23]. A multianode MCP PMT and an SPC-330 TCSPC module were used to resolve the luminescence of alkali halides under N, Ar, Kr, and Xe ion irradiation [266]. 

The main rack contains all components apart from the laser source and MCP-PMT cooler unit. Most of the electronics and the control PC are housed on the front of the 19-inch rack. Four variable optical attenuator boxes containing eight... 

The detectors, especially MCP-PMTs, can be severely overloaded by daylight leaking into the detection path. Moreover, the halogen or mercury lamp of the microscope may be a source of detector damage. Therefore, an NDD FLIM system must protect the detectors from overload. Detector protection by suitably controlled shutters is described under Sect. 7.3, page 302. 

An NDD FLIM detector upgrade kit for the Zeiss LSM 510 NLO is shown in Fig. 5.77. The kit is available with one or two R3809U MCP PMTs or one or two H5773-based detector modules. 

Like conventional PMTs, channel PMTs and MCP PMTs can also be used for the detection of single electrons or ions. The particles are fed directly into the input of the multiplication channels. 

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

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. 

Ratio calculation is avoided by using a delay line as an anode of an MCP-PMT [247, 248] (Fig. 6.7). The location along the delay line is obtained by measuring the delay between the outputs at both ends of the delay line. 

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

Due to the random nature of the detector gain, the amplitude of the single-photon pulses of PMTs and MCP-PMTs varies from pulse to pulse. The pulse height distribution ean be very broad, up to 1 5 to 1 10. Figure 6.11 shows the SER pulses of an R5600 PMT recorded by a 1-GHz oscilloscope. 

The transit time between the absorption of a photon at the photocathode and the output pulse from the anode of a PMT varies from photon to photon. The effeet is called transit time spread", or TTS. There are three major TTS components in conventional PMTs and MCP PMTs - the emission at the photoeathode, the transfer of the photoelectron to the multiplieation system, and the multiplication process in the dynode system or mieroehannel plate. The total transit time jitter in a TCSPC system also contains jitter indueed by amplifier noise and amplitude jitter of the SER. 

The dark count rate decreases by a factor of 3 to 10 for a 10 °C decrease in temperature. Cooling is therefore the most efficient way to keep the dark count rate low. Figure 6.18 shows the dark count rate versus temperature for different cathode versions of the Flamamatsu R3809U MCP PMT [211]. 

The TTS of conventional PMTs and miniature PMTs with metal channel dyn-odes can be measured with satisfactory accuracy using picosecond diode lasers. These lasers deliver pulses as short as 30 to 50 ps FWHM. However, the pulses may have a tail or a shoulder, especially at higher power. The diode driving conditions for clean pulse shape with minimum tail are usually not the same as for shortest FWHM. The TTS of MCP PMTs ean be reasonably measured only by a Ti Sapphire laser or a similar femtoseeond or picosecond laser system. 

MCP-PMTs [298] are currently the fastest commercially available detectors for TCSPC. A typical representative of this detector type, the Hamamatsu R3809U [211], is described below. The detector is shown in Fig. 6.29. 

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