Regional Basic Professional Training Course in Korea

❙ 87 ❙
2. Radiation Protection
a positive and negative electrode attached to each side. Under ordinary circumstances, all
the electrons in the crystal structure are in the lowest allowable energy states. However,
when energy is deposited in the crystal, the electrons are excited to energy states near the
valence band, as in the case of the scintillator, but in a semiconductor the electrons get
trapped in this higher energy state. As usual, all the missing electron vacancies or holes
propagate to the valence band.
When a potential difference is applied to the crystal, the electrons travel toward the
positive electrode and the holes toward the negative electrode, thus generation a current.
It is important that current not flow when there is no applied potential. And it is also
important that all the charge carriers be detected when there is an applied potential.
Semiconductor detectors are similar to ionization chambers in that the energy deposited
by the incoming ionizing radiation is exactly equivalent to the number of electron‐hole
pairs produced and it is thus important that all the charge be collected. As with an
ionization chamber detector, semiconductor detectors suffer from the problem of
re‐combination of the produced charge. The ability of these detectors to furnish accurate
energy data is degraded by the trapping of electrons which creates within the crystal
structure a space charge that reduces the electric potential between the electrodes, further
facilitating re‐combination of electron hole pairs.
Principle of Operation
An electron‐hole pair is produced when incoming ionizing radiation deposits energy
which allows an electron to move to an excited or higher energy state. In certain crystal
materials, the electrons get trapped in an energy state, known as the conduction band, and
cannot decay back to a lower energy state. Semiconductor materials have the property
that, under the influence of an electric potential difference, the electrons in the conduction
band can move and at the same time the crystal structure can propagate, between
neighboring lattice points, a net positive charge because of the missing electron. This