Neutron Scattering

Fie. 16.4: Scattering geometry for measuring the interference terni between nuclear- and
magnetic scattering tivith polarised neutrons, but withoutpolarisation analysis .
The relevant cross sections to measure the interference tenu in this geometry are :
z
z
d6_ - - Ynro bLQ) M QQ1 =b 2 _2Y ro bM+ 2 ,nro~ Mz
2,UB (dS2)++
2U B ~ 2,UB
T
b,M real °
z
z
= b(Q ) +Ynr MQ1 =b2+2Y bM+~Y"°j M 2
2,UB
B 2,UB
(16.8)
(16.9)
Besides the magnitude square of the amplitude for nuclear- and magnetic- scattering,
respectively, these cross sections contain one tenu, in which a product of the magnetic- and
nuclear- amplitudes appears . This interference tenu is especially useful, if the amplitude of
magnetic scattering is much smaller than the amplitude ofnuclear scattering :
YA MI « Ibl (16.10)
2,PB
This is for example the case, if an external magnetic field induces a weak magnetisation in the
paramagnetic state, when the ration between magnetic- and nuclear- amplitude is often below
10 -3 . This implies that the contribution from magnetic scattering to the total signal is in the
order of 10-6 or less, and thus no longer measurable . However, if we take data in two
measurements, once with the neutron polarisation parallel and once anti-parallel to the
magnetic field, we can determine the so-calledfipping ratio :
16- 10