Experiment Proposal - opera - Infn

Figure 151: Prototype scintillator strip modules at the assembly site.
• location accuracy with a prototype scintillator strip tracker and a dummy brick target.
The modules were stacked, with their planes perpendicular to the beam, on a remotely controlled
platform which could be moved vertically (along the Y axis) and horizontally (along the X axis) across
the beam. This allowed to scan the sensitive area of the modules and measure their response at different
positions across and along the strips.
The beam spot had a full width of about 3 × 5 cm 2 . The trigger telescope includes scintillator
counters and four multi-wire beam profile chambers (BPCs). Only non-interacting beam particles with
less than 2 mip deposit in each trigger counter are selected for the analysis. Additional cuts are imposed
on the beam incidence angle and impact position, as measured by the BPCs.
The module PMT’s are calibrated with the LED pulser tuned so as to measure the response of each
pixel to a single photoelectron. Single photoelectron signals are clearly separated from the pedestals
(Fig. 152). This allows us to express raw strip signal pulse-heights (PH) in photoelectrons, defined for
each strip as the PH at the maximum of its single photoelectron peak. The effect of pixel-to-pixel gain
variations is eliminated.
Due to pixel-to-pixel crosstalk in a PMT, small signals (≤ 1 p.e.) appear in pixels adjacent to the one
coupled to the hit strip. Therefore, we associate a hit in a mip event with the strip producing the largest
pulse-height (PH max ). Fig. 153 (left) shows the average PH max as a function of the corresponding strip
number in one module. The strip response fluctuates by 6% due to variations in scintillator strip and WLS
fibre light yield, light attenuation in fibres, strip-fibre and fibre-PMT coupling, light trapping efficiency,
etc., and also due to the photocathode disuniformity. Fig. 153 (right) shows the overall PH max distribution
measured at the far end of one module (at 84 cm from the PMT). The mean value of this distribution is
7.7 p.e. for events with PH max > 1 p.e. Asinglep.e. signal is still recognisable in the PH max distribution.
208