2.6. Interaction of Neutral Particles with Matter 139
A.3 Transmutation
It is a reaction in which an element changes into another one. Neutrons of all energies
are capable of producing transmutations. For example, when a Boron-10 nucleus
captures a slow neutron it transforms into Lithium-7 and emits anα-particle,
n+B 510 →Li^73 +α.A.4 RadiativeCapture
Radiative capture is a very common reaction involving neutrons. In such a reaction,
a nucleus absorbs the neutron and goes into an excited state. To return to stable
state, the nucleus emitsγ-rays. In this case no transmutation occurs, however the
isotopic form of the element changes due to increase in the number of neutrons. The
reaction is represented byA(n, γ)A+1 or
n+Xnp+p→Xpn+p+1+γ. (2.6.3)Radiative capture is generally used to produce radioisotopes, such as Cobalt-60
n+Co^59 →Co^60 +γ.A.5 Spallation............................
Spallation refers to the fragmentation of a nucleus into several parts when a high
energy neutron collides with it. This process is important only with neutrons having
energy greater than about 100MeV.
A.6 Fission..............................
This is perhaps one of the most important reactions a neutron can initiate. In this
process a slow neutron is captured by a heavy nucleus, such as uranium-235, taking
it into an excited state. The nucleus then splits up in fragments after a brief delay.
Several neutrons andγ-ray photons are also emitted during this process. Fission of
uranium-235 can be written as
n+U 92235 →I 39139 +Y 5395 +2n+γ. (2.6.4)It should be pointed out that although iodine and yttrium are the most probable
elements produced during this fission process, however in a sample of large number
of fission processes, fragments of varying atomic numbers can be found.
The fission process is the source of thermal energy produced in nuclear reactors.
A nuclear reactor core is a controlled environment where neutrons are allowed to
produce the so calledchain fission reaction. In this process the neutrons emitted
from the fissioning nucleus produce more fissions, which produce even more neutrons.
As a result the fission initiated by a few neutrons spreads quickly to the whole
fissioning material. The large number of fission fragments thus produced quickly
loose their energy in the material due to their heavy masses. This energy is released
in the form of heat, which is the main source of thermal energy in a nuclear reactor.
The thermal energy is then converted into electrical energy through other processes.