Neutron Scattering

1 . Neutron Sources
Harald Conrad
1 .1 Introductory remarks
Slow neutrons are a virtually unique probe for the investigation of structure and dynamics of
condensed matter and biomolecules . Neutrons are called slow, if their kinetic energy is below
1 keV. As the first neutrons used as microscopic probes were generated in nuclear reactors,
historic terms like thermal neutrons are also frequently used in the classification of neutrons .
In reactor physics the term thermal is used to distinguish these neutrons, which sustain the
nuclear chain reaction, from the fart fission neutrons with energies of several MeV . Thermal
neutrons, i.e . with an average kinetic energy of - 25 meV, are of particular interest in the
context of this course . They are in thermal equilibrium with an adequate slowing down medium
(moderator) like graphite, light or heavy water at ambient temperature (kBT - 25 meV) .
With the availability of cryogenic moderators, cold neutrons ( = 3 meV) became important
in recent decades, too . Strictly speaking, cold or so called trot neutrons ( = 200 meV) have
to bc considered as thermal, too, because these are neutron gases in thermal equilibrium with
a moderator at a particular temperature . Cold neutrons are in equilibrium with a cryogenic
moderator, e .g . liquid hydrogen at 20 K or solid methane at liquid nitrogen temperature, 77 K .
Hot neutrons are those in equilibrium with e .g . a graphite block heated to 2000 K, say .
These trot neutrons and the even more energetic, so called epithermal neutrons (E > 1 eV)
may in the future gain importance for scattering experiments, in particular with respect to
pulsed accelerator driven neutron sources (sec below). But it is important to realize that there
are no primary sources known, which directly deliver neutrons in the relevant energy range of
typically 10 -3 eV < E < 1 eV . All existing sources emit primary neutrons with energies of
about 10 6 eV or above and we are left with the difficult task to reduce the neutron energy
between 6 and 9 orders of magnitude (moderation) .
1 .2 Free Neutrons
Free neutrons are unstable (half life about 12 minutes) . As a nuclear constituent they are stable,
though, and as bound particles virtually ubiquitous, except in light hydrogen. So, the only
means of generating free neutrons are nuclear reactions . There is a variety of possible reactions,
mostly forced ones, although spontaneous neutron emission is known to exist as well. A