A trigger for the identification of pions stopped in an active target

The total cross sections of the π+p → π+π+n and π−p → π+π−n reactions near threshold can be used to obtain scattering lengths which are directly comparable to predictions of chiral perturbation theory. Data for these reactions were taken at TRIUMF using a segmented active scintillator target. The signature of a π+ in an active target segment was a prompt pulse caused by the particle stopping, followed by a second pulse due to the π+ to μ+ decay. TRIUMF 500 Mhz transient digitisers were used to record the scintillator outputs in 2 ns steps so that the double pulses could be identified with high efficiency in off-line data analysis. A second level trigger able to reject events in less than 10 μs was necessary to avoid a prohibitively high deadtime due to the long read-out time of the digitisers. It was implemented with fast ECLine electronics and increased the useful event acquisition rate by a factor of more than forty.