Page 140 - Radiochemistry and nuclear chemistry

Absorption of Nuclear Radiation 125

comparison to ionization energies (usually < 15 eV) and to the energies involved in
chemical bonds (normally 1 - 5 eV). Therefore, nuclear radiation can cause ionization in
its passage through matter; this is reflected in the common name ionizing radiation.
Neutrons of energies < 100 eV are included because their absorption (capture) by nuclei
results in emission of nuclear radiation with energies ~, 100 eV.
The passage of such high energy radiation through matter results in the transfer of energy
to the atoms and molecules of the absorber material. This transfer of energy continues until
the impinging particle of the radiation has reached the same average kinetic energy as the
atoms comprising the material; i.e. until thermal equilibrium is obtained.
In considering the absorption of nuclear radiation it is appropriate to view the overall
process from two aspects: (1) processes occurring to the nuclear particles themselves as
their energies are reduced to the thermal equilibrium value; such absorption processes are
the principal consideration of this chapter; (2) processes in the absorbing material due to
the effect of the transfer of energy. This transfer results initially in excitation and ionization
which cause physical and chemical changes. The study of these effects is the domain of
radiation chemistry and is considered in Chapter 7.

6.1. Survey of absorption processes

The reduction in the intensity of a beam of ionizing particles can be caused either by
reaction with the nuclei of the absorbing material (nuclear reactions) or with the atomic
electrons (electron collision). In Table 6.1 the most important processes involved in the
absorption of nuclear radiation in matter are listed along with the probability for each
process. Comparison shows that the probability of interactions with electrons is
considerably greater than that of a nuclear reaction; the only exception to this is the case
of neutron absorption. In fact the principal mode of interaction between the particle and the
atoms of the absorbing material involves the electromagnetic fields of the particle and the
atomic electrons. Since neutrons are neutral particles, in order for them to transfer energy
it is necessary that they experience a collision with a nucleus. Consequently for all particles
except neutrons, nuclear reactions can be neglected in considering the processes involved
in the reduction of the intensity of the particle beam.
As nuclear radiation passes the atoms of an absorber, it can transfer some of its energy
to the atoms. If the amount of energy transferred is sufficient, ionization of the atom
results. The positive ion and the electron thus formed are known as an ion pair. Frequently
the electrons from this primary ionization have sufficiently high kinetic energy to cause
secondary ionization in other atoms. The number of electrons produced in secondary
ionization is often larger than that of the primary ionization but the average kinetic energies
of the secondary electrons are lower than those of the primary electrons. In many
interactions the initial radiation transfers insufficient energy for ionizations; instead an
electron is raised to a higher, excited energy level of the atom. These excited atoms rapidly
return to lower energy states by emission of electromagnetic radiation such as X-rays,
visible light, etc. For neutrons the absorption process involving the capture of the neutron
(of. w167 and 10.6) imparts sufficient recoil energy to cause ionization and excitation.