Time of Flight

Two scintillator paddles, approximately 8 cm x 12 cm and separated by 14 m for initial data runs, formed the basis of the TOF detector. When the detector was at normal incidence to the beam, the downstream TOF paddle was only a few centimetres from the ECal. When the detector was rotated, the paddle closest to the ECal had to be moved upstream, reducing the distance between the paddles. The scintillator paddles were a fishtail design, guiding

scintillation light to a Hamamatsu PMT. The processing of the signal was carried out using NIM electronics. Each scintillator signal was read into a constant fraction discriminator (CFD), which produced a NIM logic pulse if both inputs were ’high’ at the same time. A CFD was used because the cable lengths were of order 20 m, so dispersion of the signal became significant. The discriminated signal was then passed to a coincidence unit, to ensure that a pulse was present in both scintillators before calculating the time between the two pulses. The output from the coincidence unit was also used to send a trigger to the DAQ. A TAC (Time to Amplitude Converter) was used to determine the time between the pulses.

Figure 5.2: Diagram of the NIM modules and logic used to trigger and tag events in the testbeam.

Figure 5.2 shows a schematic of the TOF electronics. The output from the pair of CFDs was ’fanned out’. Each CFD signal was sent to the trigger coincidence unit and to another coincidence unit. The output from the trigger coincidence unit was passed to a gate generator. This opened a gate that was sent to the TAC input coincidence units. As a gate was only opened if both TOF paddles saw a signal, the TAC only started timing if it was guaranteed to receive a stop

signal. The gate had to open before the direct signal from the CFDs arrived at the TAC coincidence unit, so that the time between the output signals from the coincidence units was the same as the time between the signals leaving the CFDs. Therefore, the direct signals from the CFDs to the TAC coincidence units were delayed by a period of time exceeding the sum of the time taken for the signal to pass through the coincidence units, plus the gate generator, plus the maximum expected travel time (that of a low momentum proton). The component latency times were typically measured to be 10-20 ns; however gate generators were significantly larger, typically 30-40 ns.

The output from the TAC was recorded via a modified TFB. The TAC output was a voltage pulse proportional to signal size, whereas the ADCs expected a current pulse of the type from an MPPC. A converter was built to output a current proportional to the voltage from the TAC, allowing the data to be output via the TFB. The output pulse from the TAC was of order 2 µs in length, so the output pulse covered three integration periods. As shown above with the modified cosmic trigger used for the beam, data typically was found in integration period 18. Latencies due to processing in the NIM electronics caused the TOF data to be delayed by around a microsecond, pushing the TOF output to integration window 20.

A rising pulse was observed in integration window 18 as the TAC measured the time difference between the TOF paddles. During the following three integration periods, two peaks were seen in the ADC spectrum, a large peak due to electrons and pions and a smaller peak due to protons. These were seen in both high and low gain ADC channels. Event selection in the TOF is based around checking how close a signal lies to the mean value of a peak. Figure 5.3 shows the ADC spectra for both high and low gain channels in integration

period 21. Electron/pion and proton peaks can be seen in both channels.

Figure 5.3: Measured TOF signal in high gain (right) and low gain (left) ADC channels. In the low gain channel a sharp peak can be seen due to electrons and pions with a broader peak due to protons. In the high gain channel electron/pion and proton events are also visible.

Figure 5.4: Showing the relationship between the high and low gain ADC channels. Two clusters due to electron/pion events and protons can be seen. A third cluster is more clearly separated on the two dimensional plot showing deuterons.

TOF Time Calibration

To measure the time taken for a particle to travel between the paddles, the number of ADC counts per nanosecond was required. To make this measure- ment, a NIM delay box was used to create a series of delayed signals into the two CFDs, simulating particles of different velocities. Time differences created by the delay box were in addition to an unknown baseline that was due to the NIM module latency. The difference between each measurement was used to find the number of ADC counts per nanosecond. Measurements were made at time differences of 1 ns, 3 ns, 7 ns, 15 ns and 31 ns. Each ADC distribution was fitted with a Gaussian function. The mean and the standard deviation were recorded and the results displayed in Table 5.1.

Time Difference/ns Mean/ADC counts Standard Deviation/ADC counts

1 259 4.45

3 303.9 4.29

7 359.8 4.5

15 477.1 5.23

31 706.6 7.86

Table 5.1: Measurements of generated time delay from TOF calibration run.

To remove the effect of the unknown baseline, the differences between times and ADC counts were plotted. The error on each measurement was assumed to be approximately equal to the standard deviation of the fit. Fitting the data in Figure 5.5 gave a value of 7.22_±0.36 ADC

ns . The intercept was compatible

with zero, as would be expected if the effects of the baseline time difference had been removed.

A large uncertainty was associated with applying an absolute time calibration because a large run-to-run spread was observed in the electron/pion peak. The spread of mean values for the electron/pion peak is shown in Figure

Figure 5.5: Showing the difference in time generated by the cable delay gen- erator, as a function of difference in ADC count.

5.6. Only data where the detector was at normal incidence were included, as the downstream TOF paddle was moved when the detector was rotated, introducing a systematic shift in the mean ADC value. When fitted to a Gaussian, the standard deviation was found to be 15.77, approximately four times larger than the standard deviation of a single run.

In practise, the electron/pion peak was trivial to find, as only electrons and pions were present in significant quantities in the beam. This was useful for confirming the identity of the proton peak and helping to identify minor peaks in the data. Figure 5.7 shows the reconstructed mass of the second largest peak in the positive beam momentum data. It was assumed that the largest peak was due to electrons and pions. The data was fitted to a Gaussian, which, with a mean of (0.92_±0.0211) GeV, was compatible with the well established proton mass of 0.93 GeV [5]. This suggested that the calibration had worked well, although it had to be applied on a run-by-run basis due to the drift in the electron/pion peak.

Figure 5.6: Showing the spread of the mean ADC values for the electron/pion TOF peak, for on axis data.

No cooling was applied to the TFB which read data from the trigger detectors, leaving it more vulnerable to changes in ambient temperature. Similarly, there was no cooling for the circuitry converting the TAC and PMT pulses to MPPC style pulses. Each run had a time stamp applied, giving the time the run started in Unix time. In Figure 5.8, the variation in the electron/pion mean ADC value is shown as a function of time. Vertical lines indicate successive 24 hour periods. A strong diurnal behaviour was seen in the data. The x- axis had been offset so zero was the start of the first beam run, sometime before midday. Peaks therefore, corresponded to sometime in the middle of the afternoon, when the ambient temperature was at its highest, while troughs corresponded to the early hours of the morning when the TFB would have been coolest.

Figure 5.7: Reconstructed mass of the second largest peak seen in the TOF spectrum. The value is consistent with that of the proton.