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Pharos Fermi Chopper System

The single most important component of Pharos is the fast chopper, which acts like the shutter of a camera. The high-speed chopper is used to create monochromatic neutron pulses from the spectrally broad pulse of epithermal and thermal neutrons emanating from the moderator. This is accomplished by a collimator (slit package) rotating at high frequency and transmitting only short slices of the broad incoming neutron pulse.

The time at which the slit package transmits can be chosen by the experimentalist, and it governs the energy of the transmitted neutrons that subsequently reach the sample. The timing is achieved by setting a phase offset with respect to the master trigger of the LANSCE accelerator. The slit package is levitated by magnetic bearings that, in combination with the motor control, enable active and highly accurate phase control to within 250 ms at a rotation frequency of 600 Hz.