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Spin Precession Magnets


Downstream of the dump magnet in figure gif on both the and flight paths are a set of dipoles with horizontal fields and a superconducting solenoid. These magnets are used to precess the polarization of the outgoing neutrons. The primary concern is that the neutron polarimeter can only measure a polarization if it is transverse to the neutron direction of motion ( or ). If one wishes to measure the longitudinal component of the neutron polarization it is necessary to precess that polarization to a measurable state. The magnets, and in particular the solenoids, are also used to occasionally flip the neutron polarization by . This, combined with the fact that the beam polarization is flipped every seconds is enough to cancel out any systematic asymmetries which may exist in the detector. The dipoles also serve the purpose of sweeping away any charged particles produced in the target or the surrounding shielding so they will not interact with the detector downstream. The first dipole magnet downstream from the dump magnet on the line is a permanent magnet placed there for safety. Should the dump magnet fail the entire beam would follow the path of the neutrons and get swept skyward by the permanent magnet.

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