Page 62 - Neutrons for Sciences and Society

Neutrons for Science
3.1 The interaction of neutrons with matter
All samples comprise of atoms, and it is with the nuclei of these atoms that the neutron interacts. There are two possible results : either the neutrons are absorbed (captured) or they are scattered. The probabilities of these two events are expressed by what is called their cross-section. Following this convenient notation the probability of an interaction with a neutron is Nσ/A. σ is
the cross-section, N the number of nuclei in a surface area A. The cross-section is expressed in units of the barn, an area of 10-24 cm2. Often capture cross-sections are proportional to the neutron wavelength, and vary considerably from one atom (more specifically one nucleus or isotope) to another. The value is close to zero for helium-4, but very large for helium-3. Boron-10, cadmium, gadolinium and lithium-6 have all very large capture cross-sections. This enables them to be used to create neutron detectors, or be used in devices to control a reactor. In contrast deuterium, beryllium, carbon, oxygen and magnesium capture very few neutrons. Aluminium absorbs a little, but not too much. This makes these elements the materials of choice with which to construct a reactor, in particular heavy water and graphite as a reflector.
With such contrasting behaviour of the two isotopes of helium it is worth adding that firstly helium-3 is only present as 1 in 104 in natural helium, which can thus be used in the neutron beam path (rather than a vacuum). Secondly helium-3 is a by-product of the nuclear industry, and is very interesting for scientists.
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