TCSPC stop signal

Routing was aheady used in classic NIM-based TCSPC setups [56, 57, 58, 59]. Each of the detectors had its own CFD. The CFD output pulses were combined into one common TAC stop signal, and controlled the destination memory block in the MCA. The technique was used to detect the fluorescence simultaneously with the IRF, to detect in two wavelength intervals, and to detect the fluorescence simultaneously under 0° and 90° polarisation. However, because separate CFDs were used for the detectors, the number of detector channels was limited. 

Figure 5.60 illustrates the situation. Multiplexing different wavelengths on a pulse-by-pulse basis is shown left. Two or more lasers are multiplexed, and a reference pulse from one of the lasers is used as a stop signal for the TCSPC channel(s). The TAC range is increased in order to record the time-of-flight distributions for all lasers as a single waveform within the TAC window. 

A third time-conversion technique uses sine-wave signals for time measurement. Two orthogonal sine-wave signals are sampled with the start and the stop pulses. The phase difference between start and stop is used as time information [313]. Currently the sine-wave technique is inferior to the TAC-ADC principle and the TDC principle in terms of count rate. It is not used in single-board TCSPC devices. However, with the fast progress in ADC and signal processor speed the sine wave technique may become competitive with the other techniques. The principle is shown in Fig. 4.16. 

The principle is shown in Fig. 5.100. The investigated light signal is split by a 1 1 beam splitter, and the two light signals are fed into separate detectors. One detector delivers the start pulses, the other the stop pulses of a TCSPC device. The stop pulses are delayed by a few ns to place the coincidence point in the centre of the recorded time interval. The setup delivers a histogram of the time differences between the photons at both detectors. Because separate detectors are used for start and stop, there is no problem with detector dead time. 

The TCSPC-FCS technique can also be used in conjunction with a continuous laser. Of course, in this case the measurement does not deliver a meaningful miero time, and no lifetime data are obtained. Because the TCSPC module needs a synchronisation pulse to finish the time measurement for a recorded photon, an artificial stop pulse must be provided. This can be the delayed detector pulse itself or a signal from a pulse generator see Fig. 5.116. 

Several deteetors are eonneeted via a router to the same TCSPC module. The photon pulses from the seeond deteetor are delayed by more than the dead time of the TCSPC module. More than two detectors ean be used if their delay lines are different by more than the module dead time. The stop pulses for the TCSPC module come from the pulsed laser, or, if a CW laser is used, from an external eloek generator. Due to the different delay of the deteetor signals, photons deteeted simultaneously do not arrive simultaneously at the router inputs. Therefore, photons detected in the same laser pulse period are reeorded at different times and stored in the FIFO data file with a macro time offset. The differenees in the maero times caused by the delay lines in front of the router are known and ean easily be corrected when the photons are correlated. 

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. 

The TAC parameters determine the time scale and the part of the signal that is reeorded. The available parameters may differ for different TCSPC deviees. Espe-eially deviees based on direct time-to-digital conversion (TDC) or sine-wave eon-version may differ eonsiderably from devices using the TAC/ADC prineiple and reversed start-stop, which will be considered below. 

For the discussion above it was assumed that the detected light signal was continuous. However, signals measured by TCSPC are mostly pulsed signals. Moreover, the detection and therefore the dead time is synchronised with the signal period. This synchronisation can lead to a different behaviour than predicted by (7.33). Dead-time-related counting loss in nonreversed start-stop systems is illustrated in Fig. 7.82. 

The simplest and most accurate way to calibrate a TCSPC system is to use the pulse period of a high repetition rate laser as a time standard. The pulse period of Ti Sapphire lasers is between 78 and 90 MHz and accurately known. Diode lasers are usually controlled by a quartz oscillator and have an absolute frequency accuracy of the order of several tens of ppm. The signal is recorded in the reversed start-stop mode with a frequency divider in the reference path. The recorded waveform covers several laser periods, and the time between the pulses can be measured and compared with the known pulse period. 

Delay lines are incorporated into all TCSPC instruments. The need for del lines is easily undentood by recognizing that there are rignificant time delays in all components of the instnmient. A pbouelectnm pulse may take 20 ns to exit a FMT. Electrical signals in a cable travd one foot in about 1 ns. It would be ififScult to match all these delays in the start and stop detector channds. The need for match-... 

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