340 Chapter 6. Scintillation Detectors and Photodetectors
Although we can not use absolute numbers of scintillation and optical proper-
ties of plastic scintillators to represent most of the materials but there are certain
trends that these materials follow. Unfortunately these trends point towards their
disadvantages than advantages as is obvious from the following list.
Nonlinearity of Light Output: The major problem associated with plastic
scintillators is their nonlinear behavior. That is, their light output per unit
length has a nonlinear dependence on the energy loss per unit length of the
particle’s track. This behavior is characterized by the so calledBirk’s formula
(6)
dL
dx=
AdE/dx
1+kdE/dx, (6.2.2)
wheredL/dxrepresents the light output per unit length,dE/dxis the energy
lost by the particle per unit length of its path,Ais the absolute scintillation
efficiency, andkis a parameter that relates the density of ionization centers
to the energy loss. dE/dxin the above equation is usually determined by
the Bethe-Bloch formula, that we studied in Chapter 2. We will discuss this
behavior a little bit more shortly.Delayed Fluoroscence:Some of the fluorescence modes of plastic scintillators
have long lived components that do not decay exponentially. This may result
in emission of delayed light or afterglow. Such an afterglow is undesirable in
radiation detectors since it not only introduces uncertainty in the measurements
but also makes the material unsuitable for use in high radiation fields.Dependence of Light Yield on Gas Pressure:The light yield of plastic
scintillators has a significant dependence on pressure. It has been observed
that the light yield of typical plastic scintillators can decrease by up to 10%
with increase in partial pressure of oxygen and other gases in the environment.Dependence of Light Yield on Magnetic Field:The light yield of plastic
scintillators has a nonlinear dependence on magnetic field. However the effect
is not significant in moderate magnetic fields and one could essentially ignore
it in typical applications.Decrease in Light Yield with Time:The degradation of scintillation prop-
erties of plastics is well known and has been thoroughly studied. It has been
found that the light yield of typical plastic scintillators decreases with the pas-
sage of time.
Mechanical Instability:Although plastic scintillators are chemically stable
but their mechanical structure is somewhat fragile. If they are not handled
carefully, they can develop small cracks. The most dramatic effect of these
cracks is the degradation in optical properties of the material.
Vulnerability to Foreign Elements:Any foreign material, such as oil and
dust, may degrade its optical and chemical properties.
Vulnerability to Radiation Damage:Plastic scintillators are highly vul-
nerable to radiation induced damage. We will discuss this in detail later in the
Chapter.