After initial tuning of the entire beam channel at 77 MeV / c with a 5 cm thick lead collimator with a horizontal opening of 3 cm placed at F3, the 1.45 mm thick absorber was inserted and the downstream beam momentum (only B2 and B3) was scaled to measure the pion and positron yields at F4. The pion and positron rates at 75 MeV / c were mea- sured with the width of the momentum-defining hori- zontal slit SL1 set at 1.5 cm to be 0.2 M / s and 0.05 M / s, respectively, for a 100 µ A proton beam on the 1 cm thick Be target. The vertical slits at SL1 and SL2 were set at 2.5 cm and 4 cm, respectively, restricting the beam rates to 1 / 4 of the maximum rate. The horizontal and vertical beam spot sizes at F3 were measured to be 4.5 mm × 2.2 mm (rms), respectively, for positrons (10 % wider for pions). In order to suppress positrons based on the displace- ment at F3, the tail in the beam profile needs to be min- imized. E ff ects of slits on the beam profile were stud- ied. Acceptance-limiting jaws SL0 upstream of the B1 magnet caused a broad tail up to 1 / 3 of the total positron intensity when they were closed to narrow (1 cm), while no tail was observed when SL0 was wide open (12 cm). The momentum defining slits SL1 at F1 did not have an impact on the tail. These observations indicated that the jaws SL0 produced an additional source image that al- lowed di ff erent momentum components to pass through SL1. When a 1.45 mm thick Lucite absorber was inserted into the beam near F1, the positron beam position at F3 was displaced by 46 mm with respect to the pion beam as shown in Fig.2. In order to display pions and positrons (solid histograms) in the same plot, the upstream momentum was raised only by 1 %, and the measured position and intensity might slightly be biased due to the geometry of the wire chambers. The horizontal spot sizes of positrons and pions increased to 7.0 and 8.9 mm (rms), respectively, with a tail in the low momentum side. For a thicker absorber (2.0 mm thick Lucite), the separation increased to 50 mm. Fig.3 shows a plot of pion and positron rates at F4 vs. the downstream channel momentum. The rates are normalized to a 100 µ A proton current. After further tuning of beam focusing at F1 and F3, the e + beam contamination with respect to the pion rate was reduced to 1 / 60. The horizontal and vertical beam spot sizes were 12 mm and 9 mm (rms), respectively. The observed pion rate of 40 k / s, being consistent with the prediction, can be increased without significantly a ff ecting the vertical image at F4 to more than 100 k / s by opening the vertical SL1 and SL2 slits, which were set at 2.5 cm and 4 cm, respectively. Due to uncertainties in the fringe fields of the bend- ing magnets, it is usually di ffi cult to obtain an abso- lute beam momentum calibration with accuracy. Two sources commonly used for calibration are the high- momentum edges of positrons from µ + → e $^{+}$νν and surface muons from π + → µ $^{+}$ν . The decay π + → e $^{+}$ν from stopped π + in the production target could provide a high energy source with a definitive peak at 69.8 MeV / c. Unlike the surface muon, the measurement of the decay π + → e $^{+}$ν from the production target does not require special equipment. Figure 3: Pion (closed circles) and positron (open circles) yields at F4 for various downstream momenta. Figure 2: Pion, muon and positron position distributions at F3 (his- tograms). The heavy lines are fitted Gaussian curves for pion s and positrons. 3 3.4. Beam Channel Calibration Source 3.3. Measurements with the Extension 3.2. Measurements without the Extension Momentum (MeV/c) 73 74 75 76 Rate (kHz) 0 10 20 30 40 Pion Positron X position (mm) −60 −40 −20 0 20 40 60 COUNTS 0 200 400 600 800 1000 1200 + + e +