Neutron Scattering - JUWEL - Forschungszentrum Jülich

RESEDA 3
1 Introduction
Neutrons are well suited as probes for investigating matter non-destructively, due to
their large penetration depth caused by their vanishing electric charge. The energy E of
a neutron with mass m corresponds to a temperature T and a velocity v:
E = 3
2 kBT = 1
2 mv2 , (1)
where kB is the Boltzmann constant. Then the wavelength of the neutron is given by:
λ = h
mv
with Planck`s constant h.
In neutron scattering mainly cold (E ∼ 0.1 − 10meV, T ∼ 1 − 120K , λ ∼ 30 − 3Å),
thermal (E ∼ 5−100meV, T ∼ 60−1000K, λ ∼ 4−1Å) and hot (E ∼ 100−500meV, T ∼
1000−6000K, λ ∼ 1 − 0.4 Å) neutrons are used. The energy range as well as the
wavelength of neutrons allows investigations of dynamics and structures of a broad
range of samples. The so-called elastic neutron scattering (no energy transfer) provides
the possibility to observe the structures of the studied samples. Inelastic neutron
scattering (with energy transfer) and quasi-elastic scattering (with a small energy
transfer) experiments provide information on the dynamics in the sample.
The fact that a neutron is a spin 1/2 particle bears another great advantage of neutron
scattering: The corresponding magnetic moment of μ = −0.966 · 10 -26 J/T makes them
sensitive to electro-magnetic interactions and allows to polarize neutrons by using
magnetic fields in polarization devices.
In neutron spin echo (NSE), the spin of the neutron is used, in order to analyze the
energy transfer in quasi-elastic scattering experiments. NSE provides high energy
resolution being typically in the μeV to neV range when using cold neutrons. The NSE
energy resolution does not depend on the wavelength spread of the incident beam, as
opposed to other types of spectrometers.
2 Neutron Spin Echo
2.1 Elastic scattering
The NSE technique is based on the Larmor precession of polarized neutrons. To
understand the basic principle, we first assume two uniform DC magnetic fields B1 and
B2 located before and after the sample position, respectively (figure 1). We also assume
elastic scattering at the sample (no energy change). An initially polarized neutron beam
passes through the magnetic fields before and after being scattered at the sample. The
spin of the neutrons must be aligned perpendicular to the magnetic fields (in