introduction-weak-interaction-volume-one

to be aligned in a direction opposite to that of its spin vector 6 . I n
the rest frame of the approaching electron, the nucleus appears as a
positive current, and hence it produces a magnetic field H parallel t o
its orbital angular momentum r . Thus, when 6 and r are parallel, ther e
is a repulsive force between the electron and the nucleus, and when the y
are antiparallel, an attractive one . Obviously, electrons will be scattered
to both sides of nuclei because of ordinary electrical forces, but mor e
tend to be scattered on the side corresponding to r and antiparallel .
However, in order to utilize Mott scattering, we must first translat e
the longitudinal polarization of the decay electrons into transvers e
polarization . There exist a number of possible methods for effectin g
this transformation . One is to apply a transverse static electric field (56 )
to the electron beam . This leaves the spins of nonrelativistic particle s
unchanged, while their direction of motion is varied continuously .
However, if the electrons are relativistic, then their spins tend t o
precess in the same direction as their momentum vectors . This problem
may be overcome by bending the electron beam through an angle greate r
than the necessary 900 . A second method of changing the sense of electro n
polarization is to produce fields, E and H, so that they are mutually
perpendicular, an d
IEI/IHI = ve (3.7 .20 )
An electron beam is then introduced at a normal to both fields, and
electrons of a particular energy, corresponding to v , remain undeflected ,
e
since the effects of the electric and magnetic fields cancell each othe r
out . However, the spin axes of the electrons are rotated through th e
desired angle, since they are affected only by the magnetic field .
A further method (57) is to introduce the electron beam into a semicircular
arc of, for example, aluminium foil, in which multiple scatterin g
occurs . This does not affect the spin axes of the electrons, but, i n
a few cases, will result in a 90 0 change in their direction of motion ,
thus translating any longitudinal polarization into transverse polarization .
Experimentally, it is necessary to have a very thin foil i n
which Mott scattering may occur, in order to reduce the probability of