High Purity Pion Beam at TRIUMF Figure 1: M13 channel with the extension. A.Aguilar-Arevalo$^{a}$, M. Blecher$^{b}$, D.A. Bryman$^{c}$, J. Comfort$^{d}$, J. Doornbos$^{a}$, L. Doria$^{a}$, A. Hussein$^{e}$, N. Ito$^{f}$, S. Kettell$^{g}$, L. Kurchaninov$^{a}$, C. Malbrunot$^{c}$, G.M. Marshall$^{a}$, T. Numao a, $^{∗}$, R. Poutissou $^{a}$, A. Sher$^{a}$, B. Walker$^{c}$, K. Yamada f $^{a}$TRIUMF, 4004 Wesbrook Mall, Vancouver, B.C. V6T 2A3, Canada $^{b}$Physics Department, Virginia Tech., Blacksburg, VA 24061, USA $^{c}$Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada $^{d}$Arizona State University, Tempe, AZ 85287, USA $^{e}$University of Northern British Columbia, Prince George, B.C. V2N 4Z9, Canada $^{f}$Physics Department, Osaka University, Toyonaka, Osaka, 560-0043, Japan $^{g}$Brookhaven National Laboratory, Upton, NY 11973-5000, USA An extension of the TRIUMF M13 low-energy pion channel designed to suppress positrons based on an energy-loss technique is described. A source of beam channel momentum calibration from the decay π + → e $^{+}$ν is also described. Key words: Beam channel, Particle separation, Pion Decay PACS: 41.85.Ja, 13.20.Cz, 29.27.Eg The branching ratio of pion decays [1, 2], R =Γ ( π → e ν + e νγ ) / Γ ( π → µν + µνγ ), has pro- vided the best test of the hypothesis of electron-muon universality in weak interactions. The new TRIUMF PIENU experiment [3] aiming to improve the precision of the branching ratio measurement by a factor of five or more measures positrons from the π + → e $^{+}$ν decay (E e + = 69 . 8 MeV) and the π + → µ + → e + decay chain ( π + → µ $^{+}$ν decay followed by µ + → e $^{+}$νν decay, E e + < 52 . 8 MeV). In order to obtain maximum acceptance with minimum uncertainties arising from positron energy-dependent cross sections, the detector system involving a large NaI crystal is placed on the beam axis. Positrons in the beam (1 / 4 of the rate of pions) severely increase detector rates, trigger rates and background in the π + → e $^{+}$ν spectrum. The TRIUMF M13 channel [4] has therefore been upgraded to suppress the positron contamination in the pion beam. The layout of the existing M13 channel [4] together with the new beam line extension is shown in Fig.1. The beam line takes o ff from the primary proton beam line (BL1A) at an angle of 135 ◦ from a 1-cm thick Be production target (T1). The M13 channel with a maxi- mum angular acceptance of 29 msr is a low-momentum achromatic channel with − 60 ◦ (B1 magnet) and + 60 ◦ 1. Motivation Abstract 2. M13 Extension Design 2.1. Existing M13 channel $^{∗}$Corresponding author: E-mail:toshio@triumf.ca, Tel:1-604-222- 7345 Preprint submitted to Nuclear Instrument and Method May 28, 2018 arXiv:1001.3121v1[physics.acc-ph]18 Jan 2010