Muon beam

TRIUMF is one of the two remaining meson factories in the world, providing intense continuous beams of protons, pions and muons. The TRIUMF accelerator is a six-sector focusing cyclotron, which accelerates H^-ions. By stripping the two electrons with a thin foil, protons are extracted at energies between 183 and 520 MeV with an intensity up to 140 $\mu$A. The primary proton beam usually has an RF time structure of 23 MHz. The cyclotron and the beam lines are shown in Fig. 3.1. Two main beam lines are BL4, primarily for proton experiments, and BL1, mainly for pion and muon experiments. In addition, BL2A will be used for the ISAC radioactive nuclear beam facility now under construction.

  

Figure 3.1: Schematic site drawing of TRIUMF facilities and beam lines.

The muon beam at the M20B channel is obtained from primary protons striking the meson production target 1AT2, typically water cooled beryllium 10 cm thickness and 5 mm $\times$ 15 mm cross sectional area (Fig. 3.2). A cloud of pions produced near the surface of 1AT2 decays into muons and neutrinos.

For our experiment, negative muons of momentum about 27 MeV/c were selected by the momentum analyzing magnet (B1). The electron separator, a crossed electric and magnetic field, selected muons, improving the muon to electron ratio in the beam by more than two orders of magnitude. Final focusing of the beam was done by a set of quadrupole magnets, resulting in a beam size of about 30 mm (FWHM) with a rate of order $5 \times 10^{3}$s^-1.

  

Figure 3.2: Layout of the M20B muon channel.