initial pion momentum of 76.8 MeV / c, which was degraded to 73.6 MeV / c by a 2.0 mm thick Lucite absorber. Separation of 53 mm between the pion and positron images was expected at F3. The positron image size was expected to be 16 mm (FWHM) with a small tail at the low momentum side, while the pion size was 18 mm (FWHM) with a 10 % tail. Two thirds of the pions are lost due to decay-in-flight, scattering, and collimation, resulting in a pion to positron ratio of 100 expected at F4. Third order aberrations might cause some broadening of the beam spot but were not expected to impact the suppression factor. (B2 magnet) bends, a quadrupole doublet (Q1–Q2) between the production target and B1 for collecting pions, a quadrupole triplet (Q3–Q5) between the two bends, and a quadrupole doublet (Q6–Q7) downstream of B2 for the final focusing. There are three foci: F1 between B1 and Q3, F2 between Q5 and B2, and F3 after Q7. Beam acceptance-defining slits SL0 are located just upstream of the first bending magnet B1, and there are momentum-defining slits SL1 and SL2 at F1 and F2, respectively. About 10 cm downstream of SL1, there are two absorber / slit wheels each with four mounting positions that allow 4 × 4 combinations of slit and / or absorber settings. The maximum 75-MeV / c pion yield at F3 is 0.8 M / s for 500 MeV, 100 µ A TRIUMF cyclotron operation. The new extension starts at F3 (0.9 m downstream of Q7), and consists of a –70 ◦ bending magnet (B3) at 1.5 m downstream of Q7, and a 30-cm diameter aperture quadrupole triplet (Q8-Q10) after B3. A 5 cm thick lead collimator with a 3 cm square hole placed at F3 stops the displaced positrons and redefines the pion image. The new focus F4 is 1.5 m downstream of Q10. The total length of the extension between F3 and F4 is 4.5 m. To cause a momentum spread between positrons and pions, 1.45 mm and 2.0 mm thick Lucite degraders are mounted on the absorber wheel near F1. Here the momentum width of the incoming beam is restricted to 1.5 % (FWHM) by closing the SL1 horizontal slit to 1.5 cm. The beam after F1 is tuned for the pion momentum corresponding to the pion energy after the energy loss. Energy-loss-based particle separators have been used since the early days of particle physics experiments [5]. The di ff erential energy loss at 75 MeV / c for pions and positrons is large enough for clean particle separation, and this concept can be applied simply to the existing M13 structure. When a thin foil is inserted at the ab- sorber wheel near F1, a momentum spread between pi- ons and positrons results due to the energy loss di ff er- ence. Following subsequent momentum analysis by B2, pions and positrons separate into two distinct horizon- tal distributions at F3. A collimator can be placed such that it stops only positrons. A small momentum tail of the positron beam due to bremsstrahlung results in some positrons at the pion spot, giving < 1 % positron con- tamination. Due to the energy-loss variation in the ab- sorber, the pion beam has a significant low-momentum tail, which results in degradation of the image at F3. Showers from stopped positrons may be a source of background with a 23 MHz radio frequency (RF) struc- ture of the proton beam from the TRIUMF cyclotron if the detector system were located near F3. The extension of the M13 channel provides con- trolled pion beam quality, although there is a pion inten- sity loss due to decay-in-flight. The blurred pion beam image due to a combination of momentum spread by the absorber and dispersion by the B2 magnet can be rede- fined with a collimator at F3 for an improved image at a new downstream focus F4. The extension isolates the shower source from the detector, allowing better shield- ing for γ -rays from the stopped positrons. Simulation was done using a Monte Carlo beam transport program, REVMOC [6], which calculates up to second-order optics including the e ff ects of multiple Coulomb scattering. The calculation was done at the Tests were carried out in two stages; the first stage was done at F3 before the installation of the extension, and the second at F4 after installation. The incoming beam was measured at F3 or F4 with a telescope consisting of two plastic scintillator beam counters (0.6 cm × 15.2 cm × 20.3 cm and 0.3 cm × 3 cm × 4 cm), 6 layers of 10-cm diameter wire chambers arranged in the wire orientation of X-U-V-X-U-V (0 ◦ and ± 60 ◦ with respect to the vertical axis), and a 48 cm diameter 48 cm long NaI(T ℓ ) crystal [7] surrounded by two cylindrical layers of 8.5 cm thick, 2 × 25 cm long pure CsI crystals [8]. The time of arrival of particles at the beam counter with respect to the cyclotron RF time provided particle identification based on the time-of-flight (TOF) together with the energy losses in the telescope counters. 3. Measurements 3.1. Detector and measurements 2.3. The Extension 2.2. Design Principle 2