next up previous contents
Next: Magnets Up: 2.2 Experimental setup for Previous: 2.2.2 SR spectrometer

Particle counters

The particle counters used in conventional $\mu$SR measurements are usually plastic scintillator (dark gray in Fig.11) attached to a photo-tube (black) through a light-guide; the photo-tube gives an electrical pulse when a charged particle ($\mu^+$ or e+) passes through the scintillator. The muon counter, which is placed between the beam window and the sample, provides the muon arrival signal (t=0). The scintillator of the muon counter is thin (thickness $\sim$0.25 mm) so that it doesn't prevent muons from reaching the sample. The positron counters (Backward, Forward, Left and Right counters) are placed symmetrically with respect to the sample position, so that the muon spin polarization can be reconstructed as described in section 2.1. The scintillators for the positron counters are relatively thick ($\sim$1 cm) to have a good detection efficiency for the relativistic decay positrons (momentum $\sim$50 MeV/c).