Semiconductor detectors operate similarly to ionization chambers (discussed in Section 13.1). This means that the charged particles produce negative and positive charge carriers, depending on the type of the semiconductor material, which move toward the oppositely charged electrodes. As a result, an electric impulse is formed. The main advantage of the semiconductor detector is that the energy producing the charge carriers (about 3.6 eV) is much less than that in the gas-filled tubes (^30 eV) of detectors or scintillation detectors (2—300 eV). Thus, the same radiation particles are able to produce more electric charges in the semiconductor detectors (by several orders of magnitude) than either in gas-filled tubes or in scintillation detectors.

The basic material of the semiconductor detectors is germanium or silicon. Their operation is usually explained by the theory of solids. This theory postulates that instead of having discrete energies as in the case of free atoms, the available energy states form bands. In the ground state, the electrons are in the valence band. Under the effect of radiation, the electrons can move from the valence band to the conduction band, increasing the conductivity. When an electron moves from the valence band into the conduction band, the produced vacancy, “the hole,” also takes part in the electric conduction because filling the hole with an electron of the adjacent atom requires a small amount of energy. The electrons and the holes move in opposite directions; the hole can be considered to be a positive charge.

When the basic material of the semiconductor (Si or Ge) is doped with an elec­tron donor (such as P, As, and Sb), or electron acceptor (e. g., Al, B, Ga, and In), an excess quantity of the electrons or holes, respectively, are produced. These types of semiconductors are called “n-type” (negative) or “p-type” (positive) semiconduc­tors, respectively. In these semiconductors, the excitation energy is even lower than in the pure germanium and silicon semiconductors.

Ge, Ge(Li), and Si(Li) semiconductor detectors are used for the measurement of gamma and X-ray radiations, respectively. The energy resolution of the Ge(Li) detectors widely applied in gamma spectroscopy is much better than that of scintil­lation detectors. For example, the half-width of the photoelectric peak of the 137mBa isotope (the daughter nuclide of Cs-137) (662 keV) is 2—3 keV (<0.5%) when measured with a semiconductor detector, while this value is 7—10% when measured with a scintillation detector. However, the efficiency of the semiconductor

Gamma energy (keV)

detectors is about one order of magnitude worse than that of the scintillation detec­tors. In addition, the semiconductor detector requires high-quality signal-processing units (e. g., charge and spectroscopic amplifiers). GeLi and SiLi semiconductor detec­tors have to be kept continuously at the temperature of liquid nitrogen because the thermal energy is enough to transfer the electrons from the valence band to the con­duction band, increasing the noise. Recently, high-purity germanium (HPGe) detec­tors also have been used, which can be allowed to warm up to room temperature when not in use.

The photoelectric peaks of the Ra-226 and its daughter nuclides measured by semi­conductor and scintillation detectors are shown in Figure 14.7. This figure illustrates the differences in the gamma spectra obtained by scintillation and semiconductor detectors. As seen, semiconductor detectors have a higher resolution but a lower efficiency than the scintillation detectors.