Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

INTERACTION OF RADIATION WITH M ATTER 195
from the atom and the rest of the photon’s energy is converted into kinetic energy
of the ejected photoelectron. If the energy of the photon exceeds 1.02 MeV, another
interaction mechanism, called pair production, can occur. In pair production,
the photon interacts with the nucleus of the absorbing atom and all of its energy
is converted into mass through the production of a positron and a negative electron.
The 1.02-MeV threshold energy required for pair production represents the
energy equivalent of the mass of the two newly created particles. Any energy that
the photon may have in excess of 1.02 MeV is transferred to the pair of particles as
kinetic energy. The Compton electrons and photoelectrons as well as the electron
and positron pair become primary ionizing particles and proceed to lose their energy
by ionization and excitation in the media in which they were produced. They
are the agents through which energy is transferred from the X-ray or gamma field
to the absorbing medium. When the positron loses all its kinetic energy, it combines
with a negative electron. The two particles are annihilated and two photons, called
annihilation radiation, of 0.51 MeV are created.
The interactions of alpha and beta radiation are governed by deterministic processes
and therefore alphas and betas have a finite range and can be completely
stopped. Gamma (photon) interactions, on the other hand, are stochastic events.
Since the interactions are governed by laws of probability, a gamma-ray (photon)
beam does not have a finite range; it can only be reduced in intensity by increasingly
thicker absorbers. The fractional reduction in intensity per unit thickness of
absorber is called the attenuation coefficient, while the fractional absorption of energy
from the beam per unit thickness of absorber is called the absorption coefficient of the
absorbing material. Both these coefficients are functions of the photon energy and the
absorber material.
In the context of interaction with matter, neutrons are classified according to their
kinetic energy as thermal and fast. Neutrons are produced through nuclear reactions
and by nuclear fission. All neutrons have kinetic energy when they are produced
and hence may be considered to be fast. These fast neutrons lose energy by colliding
elastically with atoms in their path, and then, after being slowed to thermal energy,
they are captured by nuclei in the absorbing medium. Many nonradioactive isotopes
become radioactive after capturing a neutron. When a hydrogen nucleus is struck by
a fast neutron, the nucleus is knocked out of the atom and becomes a proton, which
is a positively charged, high specific ionization primary ionizing particle. It loses its
kinetic energy by ionization and excitation interactions with the absorber atoms.
m Problems
5.1. The density of Hg is 13.6 g/cm3 and its atomic weight is 200.6. Calculate the
number of Hg atoms/cm3 .
5.2. The density of quartz (SiO2) crystals is 2.65 g/cm3 . What is the atomic density
(atoms/cm3 ) of silicon and oxygen in quartz?
5.3. Compare the electronic densities of a 5-mm-thick piece of aluminum and a piece
of iron of the same density thickness.