Setup of a Drift Tube Muon Tracker and Calibration of Muon ...

plastic scintillators. To avoid all TDC channels from counting for each event, onlythose channels which receive a signal from a drift tube hit start time counting atthe time the signal arrives. This is referred to as the start signal. The countingstops when the trigger board sends a stop signal to all TDCs at a fixed time period∆ after the trigger. The stop signal is distributed via the crate’s back plane to allTDCs. With t c being the time counted by a TDC channel, the drift time is hencegiven byt d = ∆ − t c (3.2)meaning that larger values of t c mean smaller drift distances. The maximum measurableTDC value hence arises from tracks hitting the anode wire directly and isexpected to be at t c = ∆.The TDC uses a 12 bit register for the time measurement. The resulting 4096channels are used over the measuring range of 3200 ns, leading to a channel width of2532ns. However, neither the knowledge of the absolute drift times in seconds nor theexact value of ∆ are important for the determination of the drift distance since r canbe directly correlated to the TDC channel. This will be described in Section 3.5.3.When the TDC receives a trigger signal, measurements are stopped for all channels.A 2 bit hit classification is now done. For a normal drift time measurement thisis simply set to 0. If the channel did not receive a start signal earlier, it is set to 1,meaning no hit. If the TDC channel starts counting independently of any event (e.g.because of an untriggered particle penetrating a tube), an overflow is registered. Ifa real event occurs shortly after this - before the channel was reset, the hit bits areset to 2 (called OV ROZ). Finally, hit type 3 (OV ALU) indicates an overflow ofthe internal arithmetic logic unit during the drift time determination. For the drifttime measurement, only events with hit type = 0 are of interest.Besides the drift times and hit information, the TDC also measures the pulsewidth. Pulse widths ∆ p are classified into four types:0: NO WIDTH; no pulse width could be determined (e.g. when the stop signalarrives before the actual signal which is a sign for a too small delay ∆)1: SMALL; ∆ p