PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

study themselves due to their unique properties like large mass (~ 2000 times the e - mass), short
range strong interaction, essentially no electric charge, long life time, anomalous magnetic moment
( n =–1.913 N , N being the nuclear magneton) etc. Responding to all known fundamental
interactions viz. strong, electromagnetic, weak and gravitational, they are a powerful tool for
addressing questions from the domains of particle physics, nuclear physics and astronomy [3, 23].
Neutrons especially in the thermal energy range ~ 25meV (in thermal equilibrium with a moderator
at ~ 300 K), stand out as a unique probe for condensed matter. The significant advantages of
thermal neutrons can briefly be summarised as [24-27]:
1. The thermal neutron energy (meV) is ~ energies of atomic motions. A wide range of energy
scales may be probed, from the neV energies associated with polymer reptation, through
molecular vibrations and lattice modes in meV to eV transitions within the electronic structure
of materials.
2. The wavelengths of thermal neutrons (~ Å) are of the same order as atomic spacing in
condensed matter. Through various neutron scattering techniques from diffraction to ultra small
angle scattering, structural information over many orders of magnitude (~ Å to 10 4 Å) in scale
can be obtained.
3. Neutrons primarily undergo the strong interaction with nuclei, rather than the diffuse e - clouds
of atoms, in contrast to X-rays. Therefore n’s can discern light atoms (e.g. hydrogen) in the
presence of heavier ones, and distinguish neighbouring elements.
4. The neutron’s magnetic moment is ideally suited to study microscopic magnetic
structures and magnetic fluctuations.
5. Because of its charge neutrality and spin ½, the two component neutron spinor evolves in a
magnetic field B through the interaction Hamiltonian –μ n σ.B, enabling observation of the
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