Physics and Engineering of Radiation Detection

342 Chapter 6. Scintillation Detectors and Photodetectors

the Birk’s formula. The behavior of the light output with respect to the stopping
power as represented by the Birk’s formula is graphically depicted in Fig.6.2.4.

(dL/dx)sat

dE/dx

dL/d

x Figure 6.2.4: Plot of Birk’s formula in

arbitrary units. The saturation effect of

light output is called Birk’s law.

As stated earlier, the plastic scintillators are manufactured by dissolving one or
more fluors to a base plastic material. Fig.6.2.5 shows the specific purpose of each
of these materials. Also shown are the typical weight concentrations of primary
and secondary fluors in the plastic base. The plastic base absorbs the energy of the
incident radiation and emits ultraviolet photons. If there were no other materials the
photons thus emitted would get quickly absorbed due to their very short attenuation
lengths of the order of a fewmm. Since such prompt absorption of photons is not
desirable for practical photodetectors therefore another material is added to the base
plastic to enhance its scintillation capabilities. This material is generally called the
primary fluor and has the ability to emit ultraviolet photons of larger attenuation
lengths. Since, even with higher attenuation length, the plastic is still not fully
transparent to the photons, another material is added to shift their wavelength.
This secondary fluor is also a scintillator and is generally known as the wavelength
shifter. Its function is to absorb ultraviolet photons and emit light photons. Most
wavelength shifters emit blue light.
As we just learned, the plastic base, though a scintillator itself, can not be effec-
tively used without addition of other scintillation materials. Addition of the primary
fluor in a concentration of about 1% increases not only the photon attenuation length
but also the total light yield. In this configuration the plastic base does not really
act as a scintillator but as an energy transfer medium. It absorbs incident radiation
and then transfers the energy to the primary fluor through resonant dipole-dipole
interactions. This mechanism is generally known as Forster energy transfer.
The light yield of plastic scintillators mainly depends on the following parameters.

Type of material

Type of radiation

Energy of radiation

Temp erature