Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 49

scientist, James Chadwick, discovered these particles in 1932. Like protons, they are
also composite particles made up of three quarks. However they are not as stable
outside of the nucleus as protons and decay within about 15 minutes.
Because neutrons do not carry electrical charge, they can penetrate most mate-
rials deeper than other charged particles. Their main interaction mechanism with
other particles is through collisions and absorption, releasing other detectable par-
ticles. This is how they are generally detected.
Apart from other applications, neutrons are also being extensively used in ra-
diation therapy to destroy cancerous tumor cells in the body. One astonishing
application of their use in this area is in destroying metastasis cancer in the body.
Scientists are using the high neutron absorption cross section of an element called
Boron-10 to do radiation therapy of patients whose cancer cells have spread in the
body or who have tumors in inoperable locations. Boron-10 is administered to the
cancerous cells and then the body is bombarded by slow neutrons. The neutrons
are almost exclusively absorbed by Boron-10 atoms, which as a result release other
heavy subatomic particles. Due to their short range, these subatomic particles de-
stroy the tumor cells only in their vicinity thus causing minimal damage to healthy
tissues.

Basic Properties of Neutrons

Rest mass = 1. 675 × 10 −^27 kg= 939. 55 MeV/c^2

Electrical charge = Zero

Mean life 14.76 minutes

As discussed earlier, another important application of neutrons is in nuclear power
plants where they are used to initiate and sustain chain fission reactions necessary
to create heat and thus generate electricity.

E.1 SourcesofNeutrons

Neutrons are very valuable particles because of their ability to penetrate deeper in
matter as compared to charged particles. Production of free neutrons is therefore of
high research significance. In this section we will have a look at the most important
of the neutron sources available today.

Spallation Sources

Spallation is a violent reaction in which a target is bombarded by very high
energy particles. The incident particle, such as a proton, disintegrates the nucleus
through inelastic nuclear reactions. The result is the emission of protons, neutrons,
α-particles, and other light and heavy particles. The neutrons produced in such a
reaction can be extracted and used in experiments.
A general spallation reaction with a proton as the incident particle can be written
as
p+ST→SF 1 +SF 2 +....+SFm+(k)n,