8
Scintillation and Semiconductor
Detectors
81Scintillation Detectors
As stated in Chapter 7, the detection efficiency of g- and x-rays in gas
detectors is very low, because these penetrating radiations travel through
the low-density gas with little interaction. To improve counting efficiency
for these radiations, solid and liquid scintillation detectors with high
density are used. These detectors have the unique property of emitting
scintillations or flashes of light after absorbing g- or x-radiations. The g- or
x-rays interact with scintillation detectors via photoelectric, Compton,
and/or pair production mechanisms, whereby the detector molecules are
raised to higher energy states through ionization or excitation. These high-
energy states return to ground states by emitting light photons. The time to
reach the ground state is called the scintillation decay time. The light
photons produced are converted to an electrical pulse by means of a pho-
tomultiplier (PM) tube (described later). The pulse is then amplified by a
linear amplifier, sorted by a pulse-height analyzer (PHA), and then regis-
tered as a count. Different solid or liquid detectors are used for different
types of radiation. For example, sodium iodide detectors containing a trace
of thallium (NaI[Tl]) are used for g- and x-ray detection, whereas organic
detectors such as anthracene and plastic fluors are used for b−particle
detection.
In liquid scintillation counting, a b−emitting radioactive sample and an
organic scintillator are dissolved in a solvent. The b−particle interacts with
solvent molecules emitting electrons. The latter interact with the organic
scintillator, whereby light photons are produced, which are then directed to
two PM tubes coupled in coincidence. A pulse is generated by the PM tube,
which is registered as a count, as in the solid scintillation counting.
Organic scintillators usually have a lower density and, hence, a lower
counting efficiency than inorganic scintillators. The decay time also limits
the efficiency of a detector at high-count rates. The faster decay time allows
high-count rate capability. The decay time for organic scintillators is much
shorter than that for inorganic scintillators. For example, the decay time for