Figure 10.12
Typical primary and secondary scintillators.
(a) Primary; 2,5-diphenyloxazole (PPO).
(b) Secondary; 1,4-bis- 2 - ( 5 - phenyloxazolyl)-benzene (POPOP).by combustion to^3 H 2 O and^14 CO 2 which are subsequently absorbed and measured in a constant matrix.
Semiconductor Detectors
These detectors depend on the interaction of high-energy radiation with crystals of semiconductors such
as germanium or silicon. The former generally finds use for medium or high-energy γ-radiation and the
latter for low-energy γ- and X-radiation. When the energy of the radiation is absorbed in the crystal,
free electrons and positive holes are produced in numbers proportional to the energies of the incident
photons. A thin gold layer is plated on to each end of the crystal and electrodes are attached to this so
that a potential of 3–5 kV may be applied across the crystal. Thus the electrons and positive holes will
migrate to the electrodes and produce electrical pulses with sizes proportional to the energy of the
incident radiation. Detectors based upon these principles provide the basis for modern γ-ray
spectrometry and have become of great importance in X-ray spectrometry also.
Although simple in concept, semiconductor detectors are rather complex in construction, because of
practical difficulties which have to be overcome. These problems derive from the small numbers of
electrons and positive holes produced in the initial interactions. To enable the electrical pulses to be
distinguished, it is necessary to reduce thermal randomization of the electrons to a minimum and to
reduce any background currents to a very low level indeed. Refrigeration with liquid nitrogen reduces
thermal randomization to a satisfactory level. Background currents can originate from leakage around
the outside of the crystal via impurities adsorbed onto the surface. Operation of the detector inside a
high-vacuum jacket overcomes