Pion Decay-Mode Tagging in a Plastic Scintillator Using COPPER 500-MHz FADC

The PIENU experiment is going to be carried out at TRIUMF to measure the ratio of pion decay rates, R = Gamma(pi⁺ rarr e⁺ nu_e)/Gamma(pi⁺ rarr mu⁺ nu_mu), to an accuracy of 0.1%. In order to achieve this goal, a muon identification with waveform analysis in a target scintillator is necessary. Due to the short lifetime of pions, muon pulses tend to overlap onto the tail of pion pulses. Thus waveform analysis with a high frequency flash-ADC is essential for the identification and separation of muon pulses. We use the 500-MHz flash-ADC of the common pipelined platform for electronics readout system, which was developed at KEK, to readout waveforms from the target scintillator. Muon pulse identification capability of this flash-ADC system was investigated by artificial pulses simulating photomultiplier signals.