1. The neutron
The neutron is an uncharged particle which forms part of the nucleus of any atom (with the exception of H1). Being uncharged, it is not electronically repelled from a nucleus. The interaction of a neutron with the nucleus forms an intermediate known as a compound nucleus. This is also the short range nuclear interaction. These compound nuclei may react in a number of ways, with one of the most important occurring when the neutron is released with the subsequent reformation of the original nucleus. This release is called scattering.
2. Theory
When considering the interaction of a nucleus and a neutron, consideration has to be given in terms of the effective nuclear cross section, σ. Take a beam of neutrons, in which the nuclear flux is n sec-1 cm-2. If the beam passes through matter in which there are c nuclei of a given kind per cm3, the number of neutrons intercepted per second in a thickness dx is denoted by
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If there is an initial flux of n0, this will be reduced after a distance y to
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The scattering cross section, σS, is distinguished from the adsorption cross section, σ A, and so σ = σS + σ A. These cross sections are generally of the order of 10 -24 cm2 (10-28 m2) and have the unit name, barn.
The cross section depends on the kinetic energy of the neutrons which will differ greatly depending on the velocity of the neutron (in accordance with E = ½mV2 where m = mO for E << 94MeV. The effective nuclear cross section shows a dependancy on the neutron energy which gives important information about the structure of the nucleus. Should the energy of the neutron be close to that of the nucleus, a resonance occurs which helps in the capture of the neutron and also will increase the value of σA.
In considering the speed of the available neutrons, it is important to define a thermal neutron.
A fast neutron will typically have an energy of somewhere greater than 100eV, with slow neutrons having an energy range of between 0.01 and 10eV. If the energy of the neutrons have the same order of magnitude as the ordinary gas molecules (kT), the neutrons are called thermal neutrons with a typical kT at 300K of 0.026eV[4].
For thermal neutrons, σA (H1) = 0.31 barn and σA (H2) = 6.5 x 10-4 barn (H1 is hydrogen and H2 deuterium). While both have high scattering cross section (and so effective in slowing fast neutrons), many of the thermal neutrons produced would be lost by capture to H1 (H1+ 0n1 -> H2). For this reason, D2O is a more efficient neutron moderator than water.
In water, a thermal neutron would have on average 150 collisions before capture, whereas D2O requires 104.