140 Chapter 2. Interaction of Radiation with Matter
A.7 TotalCrossSection
The total neutron interaction cross-section is the sum of the cross-sections of all the
processes described above.
σt=σelastic+σinelastic+...... (2.6.5)The total cross section for neutrons of energy up to 200MeVis shown in Fig.2.6.2
(21). It is obvious that at very low neutron energies the variation in cross section
with respect to energy is larger as compared to higher energies.
Figure 2.6.2: Total neutron cross section for uranium-238 (21).A.8 PassageofNeutronsthroughMatter
A neutron interacts with the nuclear particles predominantly through the strong
nuclear force. The strong force is extremely short range and therefore the particle
must be very close to the nucleus to be affected by it. Neutrons, owing to their
effective electrical neutrality, can get extremely close to the nucleus. In contrast,
positively charged particles, such as protons andα-particles, experience Coulomb
repulsion as they try to approach the nucleus. Unless their energy is fairly high,
these charged particles can not penetrate deep enough to experience the strong nu-
clear force. Another major difference between neutrons (or for that matter, any
other neutral particle) and charged particles is that the neutrons do not loose their
energy through electromagnetic interactions with the material atoms. Hence they
can penetrate deeper into the material as compared to the charged particles. This
higher penetration capability is quite problematic in terms of developing effective