Page 236 - Radiochemistry and nuclear chemistry

220 Radiochemistry and Nuclear Chemistry

If impurities are present they can create energy levels between the valence and conduction
bands, as describexl in w Following excitation to the conduction band through absorption
of energy an electron may move through the conduction band until it reaches an impurity
site. At this point it can "decay" to one of the impurity electron levels. The de-excitation
from this level back to the valence band may occur through phosphorescent photon
emission. Again, since this photon would have an energy smaller than the difference
between the valence and conduction bands, these crystals are transparent to their own
radiation.
To be useful as a scintillator a substance must possess certain properties. First, there must
be a reasonable probability of absorption of the incident energy. The high density in solid
and liquid scintillators meets this condition. Following absorption, emission of luminescence
radiation must occur with a high efficiency and - as mentioned - the scintillator must be
transparent to its own radiations. Finally, these radiations must have a wavelength that falls
within the spectral region to which the PMT is sensitive. Since this is not always the case,
particularly with liquid scintillators, "wave-length shifters" are added (e.g. diphenyl-
oxazolbenzene (POPOP) to solutions of p-terphenyl in xylene). Further, "quenching"
substances which absorb the light emitted from the scintillator should be absent. This is a
particular problem in liquid scintillation counting.
Table 8.3 lists the properties of some common scintillators. The data indicate that the
greater density of inorganic crystals makes them preferable for ~,-ray counting. The
resolving time is shorter for the organic systems whether liquid or solid. When large
detector volumes are necessary a liquid solution system is the simplest and most
economical.
The scintillator must be coupled optically to the PMT so that there is a high efficiency of
transfer of the light photons to the PMT photo cathode. Since PMTs are sensitive to light
in the visible wavelength region, both scintillator and PMT must be protected from visible
light. Figure 8.16 shows a typical combination of a "well-type" crystal phosphor and PMT.
The light sensitive photo cathode of the PMT is a semitransparent layer of a material such
as Cs3Sb which emits electrons when struck by visible light. The emitted photoelectrons are
accelerated through a series of 10-14 electrodes (dynodes) between which a constant voltage
difference is maintained. When the photoelectrons strike the nearest dynode, secondary
electrons are emitted as the dynodes are also covered with Cs3Sb. Consequently, there is
a multiplication of electrons at each dynode stage and at the last dynode the number of
original electrons have been increased by about a factor of 106 over a total voltage drop in
the photo tube of 1000 - 2000 V. The electrical signal is normally generated from a voltage
change between ground and the anode caused by a resistor between anode and bias supply.

8.5.1. Gas scintillator detectors

Several high purity gases are useful scintillators, notably N 2, He, Ar, Kr and Xe. Except
for N 2, much of the emitted light lies in the UV range. Hence, PMTs sensitive to UV must
be used or a wave-length shifting gas like N 2 added. The scintillations produced are of very
short duration, a few ns or less, which puts them among the fastest of radiation detectors.
Gas scintillators have easily variable size, shape and stopping power. The latter by changing
the gas pressure. They are often unusually linear over a wide range of particle energy and