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Beam Structure


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 gif 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 gif 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%.

Michael A. Lisa
Tue Apr 1 08:52:10 EST 1997