Physics and Engineering of Radiation Detection

6.1. Scintillation Mechanism and Scintillator Properties 321

When radiation traverses a scintillation material it departs some of its energy
along its track to the particles of the medium. If the energy is greater than the band
gap of the material, electrons in the valence band jump up to the conduction band.
The vacancy left behind in the valence band by the electron produces an effective
positive charge calledhole. Both the electron in the conduction band and the hole
in the valence band are then free to move around in the material. Eventually the
electron in the conduction band falls to an energy level below the lower level of the
conduction band. If this energy level is the luminescence center of the material,
the electron jumps further down to the lower luminescence center and either emits
a scintillation photon or dissipates its energy though a non-radiative means. From
there the electron jumps to the valence band and combines with the hole.
The electron in the conduction band can also jump to a so calledelectron trap.
These traps are metastable energy states that are formed by impurities and defects
(in case of crystalline scintillators). An electron trapped there can remain there for
an extended period of time that can be as long as more than an hour and as short
as a few nanoseconds. Eventually the electron jumps back to the conduction band
after receiving enough energy by thermal agitation or some other means. From there
it can jump to the luminescence center and cause scintillation light to be emitted.
These delayed photons constitute the so calleddelayed lightorphosphorescence.

Eg

Radiation

Incident

Electron Hole

Scintillation

Light

Luminescence

Center

Conduction Band

Valence Band

Trap

E

Thermal Agitation

or Radiation

Figure 6.1.1: Principle of production of prompt and delayed scintillation light

by incident radiation.

Now that we know how scintillation light is produced, we will proceed to the
discussion of the important parameters related to use of scintillation materials as
radiation detection media.