Biomedical Engineering Reference
In-Depth Information
decay by emitting photons, which can be observed and related quantitatively to the
action of the radiation. If the decay of the excited state is rapid (
10
-8
or
10
-9
s), the
process is called fluorescence; if it is slower, the process is called phosphorescence.
Scintillators employed for radiation detection are usually surrounded by reflect-
ing surfaces to trap as much light as possible. The light is fed into a photomulti-
plier tube for generation of an electrical signal. There a photosensitive cathode con-
verts a fraction of the photons into photoelectrons, which are accelerated through
an electric field toward another electrode, called a dynode. In striking the dynode,
each electron ejects a number of secondary electrons, giving rise to electron mul-
tiplication. These secondary electrons are then accelerated through a number of
additional dynode stages (e.g., 10), achieving electron multiplication in the range
10
7
-
10
10
. The magnitude of the final signal is proportional to the scintillator light
output, which, under the right conditions, is proportional to the energy loss that
produced the scintillation.
Since materials emit and absorb photons of the same wavelength, impurities are
usually added to scintillators to trap energy at levels such that the wavelength of
the emitted light will not fall into a self-absorption region. Furthermore, because
many substances, especially organic compounds, emit fluorescent radiation in the
ultraviolet range, impurities are also added as wavelength shifters. These lead to the
emission of photons of visible light, for which glass is transparent and for which
the most sensitive photomultiplier tubes are available.
Good scintillator materials should have a number of characteristics. They should
efficiently convert the energy deposited by a charged particle or photon into de-
tectable light. The efficiency of a scintillator is defined as the fraction of the energy
deposited that is converted into visible light. The highest efficiency, about 13%, is
obtained with sodium iodide. A good scintillator should also have a linear energy
response; that is, the constant of proportionality between the light yield and the
energy deposited should be independent of the particle or photon energy. The lu-
minescence should be rapid, so that pulses are generated quickly and high count
rates can be resolved. The scintillator should also be transparent to its own emit-
ted light. Finally, it should have good optical quality for coupling to a light pipe or
photomultiplier tube. The choice of a particular scintillation detector represents a
balancing of these factors for a given application.
Two types of scintillators, organic and inorganic, are used in radiation detection.
The luminescence mechanism is different in the two.
Organic Scintillators
Fluorescence in organic materials results from transitions in individual molecules.
Incident radiation causes electronic excitations of molecules into discrete states,
from which they decay by photon emission. Since the process is molecular, the
same fluorescence can occur with the organic scintillator in the solid, liquid, or
vapor state. Fluorescence in an inorganic scintillator, on the other hand, depends
on the existence of a regular crystalline lattice, as described in the next section.
