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Plastic Veto Counters

The outer MWPC is surrounded by a thin plastic veto detector (PV). The PV allows an efficient identification of charged particle induced events with good timing resolution.

Figure: The Plastic Veto detector: it contains 20 modules each of which is read longitudinally at both ends. With a thickness of 3.175 mm, it ensures an efficient identification of charged particles.

The PV has a cylindrical geometry with outer radius of , a thickness of and active length of . It contains 20 modules with interleaved edges as displayed in figure gif and its segmentation is consistent with the geometry of the calorimeter and dictated by the stopping rate. Each of its modules is coupled via a light guide at both ends to a photomultiplier tube.

Figure: Simulation of the response of the Plastic Veto detector to Michel events thrown according to the stopping distribution and originating in the target. The arrow indicates the Minimum Ionizing Particle threshold. The energy deposited is superimposed on the histogram of the light output. Even after accounting for photoelectron statistics and light collection nonuniformity, there is a clear separation between charged and neutral particles as indicated by the arrow.

The PV is designed such that it provides a good separation of minimum ionizing particles, i.e. a separation of neutral and charged tracks as shown in figure gif. Therefore, for the pion beta processes, the PV allows the rejection of charged particle induced events in the calorimeter while it vetoes out the neutrals for the processes.

Bernward Krause
Mon Jan 15 14:57:06 MET 1996