Stop PMT

Therefore it is unlikely that a time measurement is started and stopped by two successive 1.27 MeV quanta of the Na decay. The by far most likely start-stop event is the detection of a 1.27 MeV quantum in the start PMT followed by the detection of a positron in the stop PMT. The histogram of these events gives the desired positron lifetime distribution. A typical result is shown in Fig. 5.140. 

Finally, the MCA receives (and collects) the time delays (At) from TAC organizing them in a plot showing the number of times that a single photon is seen by the stop PMT at a given Af. 

The fluorescence of the chlorophyll in a leaf is excited by a picosecond diode laser. The fluorescence is separated from the scattered excitation light by a bandpass filter and detected by a PMT. The photon pulses from the PMT are used as start pulses of the TCSPC module, the reference pulses from the laser as stop pulses. When the laser is switched on, a recording sequence in the TCSPC module is triggered. This is done by connecting a diode from the operating voltage input of the laser to the TTL-compatible experiment trigger input of the TCSPC module. 

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. 

A laser scanning ophthalmoscope can relatively easily be combined with the TCSPC scanning technique (see Sect. 3.4, page 37). The fluorescence light from the retina is split off by a dichroic mirror and detected by a second PMT. The detection wavelength of the PMTs is selected by filters, FI and F2. The photon pulses from the fluorescence channel PMT are fed into the start input of the TCSPC module. The stop pulses come from the diode laser. 

The Na source is placed between two identical samples. Two XP 2020 photomultipliers equipped with scintillators are attached directly to the two samples. The pulses from the photomultipliers are used as start and stop pulses for the TCSPC module. The pulses from PMT 2 are delayed by a few nanoseconds so that a stop pulse arrives after the corresponding start pulse. Eaeh y quantum generates a large number of photons in the scintillator. Therefore, the PMT pulses are multiphoton signals, and the time resolution can be better than the transit time spread of the PMTs. Moreover, the amplitudes of the photomultiplier pulses are proportional to the energy of the particle that caused the scintillation. Therefore the amplitudes can be used to distinguish between the 511 keV events of the positron decay and the 1.27 MeV events from the Na. The discriminator thresholds for start and stop are adjusted in a way that the stop channel sees all, the start channel only the larger Na events. The rate of the Na events is of the order of a few kHz or below. 

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