Optimization of the Cherenkov signal by changing the fibre angle

The Cherenkov effect in optical fibres is highly directional and there is an optimum angle for maximizing the collection efficiency [151] (cf. section 2.3.2). In this experiment, the angle of the fibre was changed by rotating the stage using 3 steps of the stepper motor

Figure 5.6: Output signal recorded before and after the passage of the accelerator beam on the fibre. In absence of beam the YAG screen does not scintillate (a. top) and the only signal is the detector dark noise of about 10-15 mV (a. bottom). When the beam is on, a spot is visible on the YAG screen (b. top). Removing the screen the SiPM response at the passage of the electron beam through the fibre (b. bottom), with a fibre angle of 56◦ with respect to the e−beam and bunch charge of 25 pC is of 300 mV.

(corresponding to 5.4◦ to each turn) in both clockwise and anticlockwise directions. The detector response was recorded on the oscilloscope and the behavior of the signal amplitude with the rotation angle is shown in Fig. 5.7.

The black line is the theoretical curve shown in Fig. 5.7; it is obtained by integrating over the full range of impact parameters. Since the fibre cross section is much smaller than the beam size, the whole cross section is impacted by the electrons of the beam; therefore the integration was carried out considering the ALICE beam as consisting of electrons impinging on the fibre with every possible value of the impact parameter. The

Figure 5.7: Maximization of the Cherenkov angle as a function of the signal amplitude in both directions (red line with data points marked as triangles) compared with the theoretical curve (black line). A discrepancy between the theoretical and experimental curves between -30◦ and +30◦ is visible. The theoretical curve exhibits two peaks for -5◦ and +5◦ and zero signal in between these peaks (where there is a sharp drop to zero, because the angle of incidence of the photon on the wall of the fibre is less than the critical angle). Instead the experimental curve shows a small peak because of beam interactions with the metallic posts that support the fibre.

experimental points, shown in Fig. 5.7, exhibit a maximum in the signal amplitude at an angle of 49◦ in both directions, corresponding to the maximum collection efficiency

of Cherenkov photons. The error on the measurements is 1◦ due to a parallax error

The data are in good agreement with the theoretical results shown previously, that indicate the same angle for the maximum collection efficiency. A discrepancy between the theoretical and experimental curves between -30◦ and +30◦ is visible in Fig. 5.7. The theoretical curve exhibits two peaks for -5◦ and +5◦ and zero signal in between these peaks, where there is a sharp drop to zero, because the angle of incidence of the photon on the wall of the fibre is less than the critical angle. However, the experimental curve shows a small peak: this is probably because of beam interactions with the metallic posts that support the fibre. Scattering off the metallic posts produces a multidirectional shower of additional secondary electrons detected by the sensor.

5.3.4 Sensor response as a function of the bunch number

For testing the detector response in different situations, the number of bunches was

changed by modifying the ALICE injector laser shutter aperture time. The mode

locked laser produces electron bunches with a time spacing of 12 ns from each other. By changing the shutter time, the number of bunches was changed from 0 to 100, and the detector response was observed. In Fig. 5.8 a recorded sequence of oscilloscope traces shows the detector response for 2, 3 and 40 bunches. It can be seen from Fig. 5.8 that there is a one to one relation between the number of bunches and pulses generated on the signal. Since the beam bunches were separated by 12 ns, the fact that each bunch produces a well distinguishable peak indicates that the rise time of the detector is less than 12 ns. However, whilst it is possible to count the number of bunches, it is not possible to measure their charge. To explain this statement, it is noted that the peak height is proportional to the Cherenkov light impinging on the SiPM, which is in turn proportional to the charge crossing the fibre. Moreover, the peak height should be measured from the difference between the voltage at which the peak starts and the voltage reached at the peak maximum elevation. Hence, with reference to Fig. 5.8, it is seen that the first peak (which starts at 0V) shows the largest height; peak height decreases then with the second peak and the third, reaching a steady value beyond this number. Therefore, after the first peak, peak height is no more longer proportional to the impinging Cherenkov light on the SiPM, but also depends on the history of the signal, preventing the measurement of bunch charge which is instead possible with the first peak. This phenomenon is explained by the SiPM saturation and SiPM cells dead time. When a cell fires, creating an avalanche, it takes about 35 ns [152] for the cell to

Figure 5.8: SiPM response to 2, 3 and 40 bunches passing across the optical fibre. Note that the voltage and time scales are different in the second picture. The increasing over- shoot of the signal is due to the amplifier capacitance that is not optimized for increasingly longer signal recovery times.

recharge its capacitance and be ready to fire again. If a photon impinges on the fibre within this dead time, either no avalanche is created, or a smaller avalanche is created, which produces a smaller signal (i.e. shallower peak). This is what happens when two bunches closer than 35 ns hit the fibre in sequence: the light from the second bunch is not fully detected, as a fraction of the photons hit cells still within the dead time. Light from subsequent bunches hitting the fibre will be confronted with even fewer active cells, until a dynamic equilibrium is established between the recovery time of the cells and the number of impinging photons, as can be seen beyond the third bunch shown in Fig. 5.8.