Beam Composition

The incoming pion rate (650 - 750 k/$s$) was adjusted by a set of horizontal and vertical slits situated downstream of the bending magnets. The beam spot had a horizontal full width half maximum (FWHM) of $\sim$ 3.4 cm and vertical FWHM of $\sim$ 4.4 cm and the momentum spread was $\sim$ 9% ($\delta p/p$ at FWHM) (26). The final composition of the beam at the target was determined from both the time of flight (TOF) of particles in the beam as well as energy deposition in four beam telescopes. The final beam incident on the target consisted of 71% $\pi ^-$, 18% $e^-$ and 9% $\mu^-$ (Figure 3.2).

Figure 3.2: Time of flight (TOF) structure of the unseparated beam particles as measured by the beam telescopes showing the $\pi ^-$, $e^-$, and $\mu^-$.