Optimal neutron Larmor precession magnets

• Published in: IEEE transactions on magnetics Vol. 24; no. 2; pp. 1540 - 1543
• Main Authors: Zeyen, C.M.E., Rem, P.C., Hartmann, R.A., van de Klundert, L.J.M.
• Format: Journal Article
• Language: English
• Published: IEEE 01-03-1988
• Subjects: Coils; Current distribution; Magnetic analysis; Magnetic fields; Magnets; Neutrons; Particle beams; Shape; Solenoids; Spectroscopy
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/20.11539

Spectroscopic techniques based on Larmor precession of particle spins require that for all trajectories of a diverging beam the path integral of the modulus of the magnetic field must be a constant. The amount of precession performed by each spin is then a function of the particle energy only. For cylinder magnets this homogeneity condition can be expressed as a variational problem. An analytical solution is presented for this variation problem. This solution describes the optimal field shape (OFS) to obtain the best possible homogeneity for a given magnet length. In practice the ideal shape can be obtained by superposing a series of solenoids of different lengths but the homogeneity is generally not good enough so that in-beam correction coils are needed that include corrections for the line integral differences caused by the finite-beam divergence. The solution is presented together with a method to implement it in practice using discrete in-beam current distributions. The resulting magnet has a homogeneity of 10/sup -6/, so that the Larmor precession angle is still well defined after 10/sup 4/ turns.< >