Signal Transit Time

The signal transit time is the time from the absorption of a photon at the photocathode to the corresponding pulse at the anode of a PMT. The transit time is almost entirely determined by the transit time of the electrons through the PMT. It is therefore proportional to the reciprocal square root of the supply voltage. Typical transit times for a number of frequently used PMTs are shown in the table below. 

The transit time has to be taken into account when choosing the cable length in the detector and reference signal path of a TCSPC system. More important than the transit time itself is the variation of the transit time with the detector supply voltage. To keep the transit time of a conventional PMT stable within 1 ps a stability of the operating voltage of the order of 0.05% to 0.01% is required. 

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Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

The flow velocity is thus proportional to the difference in transit time between the upstream and downstream directions and to the square of the speed of sound in the fluid. Because sonic velocity varies with fluid properties, some designs derive compensation signals from the sum of the transit times which can also be shown to be proportional to C. 

In most ultrasonic tests, the significant echo signal often is the one having the maximum ampHtude. This ampHtude is affected by the selection of the beam angle, and the position and direction from which it interrogates the flaw. The depth of flaws is often deterrnined to considerable precision by the transit time of the pulses within the test material. The relative reflecting power of discontinuities is deterrnined by comparison of the test signal with echoes from artificial discontinuities such as flat-bottomed holes, side-drilled holes, and notches in reference test blocks. This technique provides some standardized tests for sound beam attenuation and ultrasonic equipment beam spread. 

Figure 12-26. Time-of-lighl signals I m hole nioiion in DPOP-PPV ai different temperatures and electric fields. The sample thickness was 4 (/in Arrows indicate the transit time (Ref. 83 ).

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

When time-dependent signals are to be measured by a photomultiplier, the time sensitivity is usually limited by the inhomogeneous transit time. The transit time is the time taken by electrons generated in the cathode to arrive at the anode. If all of the emitted electrons had the same transit time, then the current induced in the anode would display the same time dependence as the incoming light, but delayed in time. However, not all of the electrons have the same transit time. This produces some uncertainty in the time taken by electrons to arrive at the anode. There are two main causes of this dispersion ... 

Figure 3.20 shows the effect of the transit time dispersion on the measurement of an ideal light pulse. Since photoelectrons spend some time traveling from the photocathode to the anode (transit time), the photomultiplier signal is delayed in time with respect to the incident pulse. Furthermore, due to the transit time dispersion, the... 

The IFTOF technique requires the current mode of operation in which the various Resistance-capacity product (RC) time constants in the bridge are much smaller than the transit time tj. If the resistance Ri is larger than Rj, then the signal across the amplifier will be simply Rj i(t), where i(t) is the transient photocurrent. 

The transit time tj is more clearly defined by the break in the current signal. Thus, it is preferable to operate the apparatus in the /-mode. Figure 4.12 shows transient... 

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