A non-conventional neutron polariser concept

Abstract

Even in an idealised case conventional methods of neutron polarisation inevitably loose at least 50% of the intensity by removing one spin state from the incident beam. We demonstrate, however, that it should be possible to polarise a pulsed beam of slow neutrons without loosing any particle. This so-called “Dynamical Neutron Polarisation” (DNP) method is based upon acceleration and deceleration, respectively, of the two neutron spin states during passage through an NMR-like arrangement of crossed static and radio-frequency magnetic fields, followed by spin-dependent spin rotation in a homogeneous precession field. Precise ramping of the strength of this precession field synchronously with a time modulated π / 2 -spin turn device at the exit position allows to stop precession for all neutron wavelengths exactly at the moment where both spin states are aligned parallel, albeit on the cost of a tiny energy difference between them. We present a theoretical description of DNP which is essentially based upon a semi-classical spin-rotation formalism and present a typical result of a thorough investigation of the performance of such a non-conventional polariser with respect to a manifold of instrumental parameters.