Typically the IU Cyclotron facility can provide a packet of protons
approximately once every 30 ns, which corresponds to one beam bunch per
cycle for the 36 MHz Main-Stage Cyclotron. For the current experiment it
was necessary to suppress most of the beam so that there was a 1:12
pulse selection. This meant that the protons were incident on the target
approximately once every 360 ns. The spacing was required to prevent the
overlap of high energy-loss neutrons from one pulse with low energy-loss
neutrons from the subsequent pulse in the time-of-flight detector. In
general,
the pulse selection was about 93%-95% efficient, meaning that about 95%
of the total protons incident on the target were in the primary pulse.
Figure 
shows the output of the proton monitor which is
situated
near the target. The proton monitor measures elastically scattered protons
from the target. Therefore the number of counts in the proton monitor is
proportional to the amount of beam. Figure shows the primary
pulse with 95% of the beam and the rest scattered amongst other pulses
spaced at integral multiples of 30 ns.
The other notable feature of the beam is that the polarization of the
protons is flipped 
at the source every 
seconds.
The spin state (+ or -) is saved as part of the experimental data and used
to help cancel out possible systematic asymmetries.
Figure: Log plot of the proton monitor signal versus time showing the
pulse selection of the proton beam. Pulse selection efficiency is
approximately 95%.