Figure 4-6. Thomson effect.
(1) Only when a high-energy photon passes very close to the nucleus of a
heavy atom can it interact with the electric and magnetic fields in such a way that the
photon or part of its energy is converted at the same instant into two particles--an
electron and a positron of equal mass and equal but opposite electrical charge. This
conversion requires photon energy of 1.02 MeV.
The electron can cause ionization in its path and may end up as a free
(3) The positron can also cause ionization in its path. However, the
positron, once formed, has a very short life. After losing its speed (energy) by colliding
with and ionizing atoms of matter, the positron undergoes a unique and final interaction
with an electron. They unite and annihilate each other, giving off two (gamma ray)
0.511-MeV photons. These new photons may produce further ionization by
photoelectric effect or by Compton effect.
Section III. RADIATION--DETECTION AND MEASUREMENT
Since none of the five senses can detect the presence of x-radiation indirect
methods must be employed. Although the ionizing capability is harmful and in some
cases deadly to living tissue, it is this same ability which is used to provide detection
through two means: (1) chemical changes. and (2) electrical changes.