Time Measurement Block

There are a number of different time measurement teehniques applicable to TCSPC. The TAC-ADC principle of the elassic TCSPC teehnique has been upgraded with a fast, error-cancelling ADC technique. Integrated eireuits for direct time-to digital conversion (TDCs) have been developed, and a sine-wave time-eonversion teehnique has been introdueed. 

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The timing pulse from the multichannel plate is processed in the usual way in the time-measurement block. The pulses from the anode structure of the detector, A1 through A4, are converted by four ADCs on the TCSPC board. The position of the photon is calculated in a digital arithmetic unit. The unit has to deliver one X Y data pair within 100 ns or less to keep the dead time within the common standard of advanced TCSPC. The solution to the problem is pipelining ... 

The structure in the time-tag mode is shown in Fig. 3.15. It contains the channel register, the time-measurement block, a macro time" clock, and the FIFO buffer for a large number of photons. It has some similarity to the multidimensional TCSPC described in the paragraphs above. In fact, many advanced TCSPC modules have both the photon distribution and the time-tag mode implemented, and the configuration can be changed by a software command [25]. The sequencer then turns into the macrotime clock, and the memory turns into the FIFO buffer. 

When a photon is detected, the micro time" in the signal period is measured by the time-measurement block. Simultaneously the deteetor ehannel number for the eurrent photon and often a number of additional bits from external experiment eontrol deviees are written into the channel register. The macro time" clock delivers the time of the photon from the start of the experiment. All these data are written into the FIFO. 

The time measurement block of a TCSPC device working in the reversed start-stop mode is shown in Fig. 7.69. 

Fig. 7.69 TAC control parameters in the time measurement block of TCSPC.

Figure 4.14 shows the first few iteration steps in the evolution of the spatial measure block-entropy of rule R122 for blocks with size B < 5. Although the irreversibility of this rule predictably leads to a decrease of entropy with time, there nonetheless appears to be a relaxation to equilibrium values. Observe also... 

Entropies The individual spac e and time measures introduced above may also be generalized to space-time blocks of size B x T ... 

A company sent a set of gage blocks to NBS at regular intervals to be calibrated. NBS dutifully calibrated the blocks and provided a calibration certificate and data report. The company kept their current NBS calibration certificate on file to prove to their auditors that they were traceable to NBS for dimensional measurements. The problem was that each time the blocks returned to NBS, our people found the seal unbroken. The gage blocks had never been used, yet the company satisfied the auditors that they were traceable to NBS as required. 

Long-range radar devices use FMCW with an indirect flight-time measurement. Fig. 7.7.2 shows a very simple block diagram of a FMCW radar that utilizes the Doppler shift to detect moving targets. 

The block diagram in the last section shows the spectrometer with the CAMAC modules identified. Several different experiments requiring different pulse sequences can be performed easily with such a system. A moderately complicated example is a spin-lattice relaxation time measurement in the time domain on a poly crystalline intermetallic sample containing 1=3/2 nuclei. Since a non-cubic 1=3/2 system has unequally spaced levels, special techniques must be used for relaxation time measurements (see III.C.3.) and we adopt the procedure of Avogadro and Rigamonti (1973) to initialize the populations before the magnetization recovery. 

Figure 3 Evolution of reaction products with time measured in the molecular beam reactor. At 423K(filled circles) hydrogen is evolved and stabilised acetate build-up on the surface, eventually blocking it to further reaction, whereas at 473K (squares) the acetate is unstable and the reaction proceeds at steady state on the c(2x2)-C layer...

Figure 3.4 shows how the router works in concert with the TCSPC module. The CFD of the TCSPC module receives the single-photon pulse from the router, i.e. the amplified pulse of the detector that detected the photon. When the CFD detects this pulse, it starts a normal time measurement sequence for the detected photon. Furthermore, the output pulse of the CFD loads the channel information from the router into the channel register. The latched channel information is used as a dimension in the multidimensional recording process. In other words, it controls the memory block in which the photon is stored. Thus, in the TCSPC memory separate photon distributions for the individual detectors build up. In the simplest case, these photon distributions are single waveforms. However, if the sequencer is used, the photon distributions of the individual detectors can be multidimensional themselves. 

Figure235 Block diagram of apparatus for transit-time measurements of sound velocity in pulse-echo mode. (Adapted from Schreiber et al., 1973, reproduced courtesy of McGraw-Hill, New York.)...

Reliability and precision of the measurements were assessed. For a test set-up sample, we use an aluminum block was used which circumvents time dependent property changes associated with the drying of the green ceramics. Using this test sample the following parameters were varied to determine their effect on transit time measurements ... 

In the Crucible Test a well, drilled or moulded in a block of the refractory to be tested is filled with the selected slag and the block heated to the required temperature, for a specified time. The block is then removed, cooled, sectioned and the slag/refractory interface examined. The test is simple, but is static, and does not allow a temperature gradient to be studied. The reaction layer built up at the interface may produce erroneous results. The Pill Test is similar - a pellet of slag can be measured. In the Drip Test a stream of slag pellets falls on to a refractory block... 

Fig. 3.2. A lAA flux into basal agar receivers from 10-mm sections of corn coleoptiles apically supplied with a 5-min pulse of " C-IAA (5 pM in agar). At the time indicated by each column, the sections (30 per transport block) were transferred to new receivers. (Data from Hertel and Flory 1968). B Distribution of mobile auxin in oat coleoptiles apically supplied with a 10-min pulse of " C-IAA (0.7 pM in agar). The mobile auxin concentration was estimated from the radioactivity in basal agar receivers taken from samples of nine coleoptiles subdivided serially into 1-mm sections at the times measured from the start of donor application. Note the differences in translocation of the peak, indicated by arrows, in different regions of the coleoptile. (Data from Newman 1970). C Distribution of radioactivity within 20-mm sections of corn coleoptiles apically supplied with a 15-min pulse of C-IAA (10 pM in agar). The pulse donors were replaced by blocks containing unlabeled lAA at an identical concentration. The sections were cut into successive 2-mm pieces at the times indicated, and the level of tissue radioactivity was measured. Note the differences in translocation of the peak, indicated by arrows, during the two 30-min transport periods. (Data from Goldsmith 1967b)...

This works well at low to moderate count rates but is limited at high count rate. Dead time is discussed further in Chapter 14 in this connection. For now, it is sufficient to say that, in general, high dead times are to be avoided. Each laboratory has its own arbitrary limit. My own was about 30 %, but many laboratories have much lower limits. At very high dead times, measurement of hve time may be inaccurate because of differences in shape between the detector pulses and the live time clock pulses, which are not blocked in the same way by the input gate. Inaccurate live time measurement does not prevent nuclide identification but does affect the quality of quantitative nuchde measurements. 

The operator did not know that the PORV had failed. He believed the RCS depressurization was due either to the fully open pressurizer spray valve or to the feedwater flow to the steam generators. He closed the spray valve and the PORV block valve as precautionary measures. But subsequent analyses showed that the failed PORV was responsible for the rapid RCS depressurization. Two minutes later, the reactor operator opened the PORV block valve to ensure that the PORV was available. Fortunately, the PORV had closed by itself during the time the block valve was closed. The failed PORV was the ninth abnormality that had occurred within 15 minutes after reactor trip. 

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