Physics and Engineering of Radiation Detection

6.2. Organic Scintillators 343

Wavelength Shifter

(~0.05% by weight)

Scintillator

(~1% by weight)

Energy

Transfer Photons

Ultraiolet Light

Visible

Plastic Base

Radiation

Incident

Plastic Scintillator

Figure 6.2.5: Working principle of a plastic scintillator based on two

scintillation materials, which are homogeneously mixed in a base plas-

tic material.

Pressure

The cause of particle dependence of light yield has already been discussed in
the beginning of the chapter and therefore will not be repeated here. But as a
reminder, this dependence is actually the manifestation of the difference in stopping
powers of the material for different particles. Now, since the stopping power depends
also on the type of material as well as the energy of radiation therefore for most
materials studying the dependence of light output on the stopping power suffices
to characterize it in terms of its suitability as a detection medium. Even though
the light yield for different materials and incident particles can vary considerably
from one another, still for plastic scintillators there is a general trend that can be
identified. for example we see that the light yield decreases with the particle mass
and increases with particle energy. Fig.6.2.6 shows these general trends for plastic
scintillators.
Earlier in the chapter we discussed the temperature dependence of scintillators
and saw two graphs that showed the dependence of the charge pair production and
the light yield on temperature for an inorganic scintillator (CsI). Plastic scintillators
are no exception and show similar behavior.
Now we turn our attention to radiation damage in plastic scintillators. As stated
earlier, most of the plastic scintillators are highly vulnerable to radiation damage.
One of the factors most affected by long term accumulated dose is the decrease in
attenuation length of scintillation light in the bulk of the material. An example of
such deterioration is shown in Fig.6.2.7, which was observed in a plastic scintillator
used in a high energy physics experiment called ZEUS. Here light transmission was
measured using a source at different positions in the detector at two points in time.
The effect of radiation damage is clearly visible with transmission getting poorer
after ten years of irradiation. Since light transmission is directly related to the
attenuation length therefore one can conclude that the attenuation length decreases
with radiation exposure.