The scintillator materials are classified into two groups: inorganic and organic. The classification is done on the basis of the mechanism of the scintillation process. In the case of inorganic scintillators, the crystal lattice, including its defects, plays an important role, while in the case of organic scintillators, the physical—chemical processes within the molecules of the solid or liquid scintillator material result in light photon emission.

In inorganic scintillators, the radiation or the electrons produced as the result of interactions between the radiation and the scintillator material excite the electrons on the outer shells of the inorganic crystal. During the return of the excited elec­trons to the ground state, photons are emitted. As postulated in the theory of solids, 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. The criterion of the electric conduction process is whether there are electrons in the conduction band or not. In insulators, as the inorganic scintillator materials, the electrons in the valence band are separated from the conduction band by a large gap. Under the effect of radiation, the electrons can pass from the valence band to the conduction band and then return to the valence band, emitting photons whose energy is in the ultraviolet range. These photons, however, are absorbed by the scintillator material. The photons can be detected outside the scintillation material only when the crystal defects are such that they result in the formation of lumines­cence centers in the gap between the valence and the conduction bands. After exci­tation, the electrons return to the valence band through these luminescence centers, emitting blue light. This blue light then provokes the emission of electrons from the cathode of the photomultiplier. In some inorganic scintillators, the crystal defects are created by adding a small percentage of doping material.

The most important inorganic scintillation materials are as follows:

• Zinc sulfide (ZnS): for scintillation purposes, ZnS crystals are dopped with silver or sometimes with copper. ZnS has historical importance (see, for example, the discussion of Rutherford’s scattering experiments in Section 2.1.1). A relatively high portion (about 25%) of the radiation is converted into light photons. The wavelength of the emitted photons is within the sensitive range of the photomultipliers. However, the time of the
light emission is relatively long (2 X 10_7 s), resulting in a longer dead time than other scintillation materials. Another disadvantage of ZnS is that it is produced in a polycrystal­line form consisting of small particles. Both the gamma photons and the emitted light photons are diffracted on the surfaces of the crystal particles, inhibiting the measurements of the radiation energy. For this reason, the ZnS scintillator is usually applied in thin layers for the detection of radiations in a low range (such as alpha particles, protons, deu — terons, and fission products).

• Sodium iodide dopped with tallium [NaI(Tl)]: the luminescence centers formed from the tallium doping. The conversion of radiation to light photons is relatively good (about 8%), and the wavelength of the photons is within the sensitive range of the photocathodes. The main advantage of NaI(Tl) detectors is that large single crystals can be produced that will allow the linear movement of the gamma and light photons without diffraction. Thus, the energy of the radiation can be measured.

Sodium iodide is widely used for spectroscopic purposes. Each of its three main interactions with gamma radiation (the photoelectric effect, Compton scattering, and pair formation, as discussed in Section 5.4) result in the emission of electrons. These electrons are the source of the scintillation that is measured in the detector (Figure 14.4). In the case of the photoelectric effect, the number of emitted elec­trons is proportional to the energy of the gamma radiation, i. e., the intensity of the

Energy of gamma photons entering the scintillation crystal increases ►

The energy transmitted to the scintillation crystal increases; the energy of the gamma photon

simultaneously

decreases

light is also proportional to the energy of the gamma radiation, providing a way to measure gamma energy. The energy of the electrons emitted in the Compton scat­tering depends on the angle of scattering producing a continuous range in the gamma spectrum. In the case of the pair formation, besides the peak associated with the total energy of the gamma photon, other peaks may also be observed. These peaks have 511 keV and 1.02 MeV less energy than the energy of the gamma photon. The 511 keV relates the rest mass of the electron and positron pro­duced in the annihilation process. The occurrence of these peaks with lower energy depends on whether the annihilation photons (one or both) leave the detector or absorb in it.

As seen in Section 5.4, the interactions of the gamma photons with matter, including the interactions with the scintillation material, strongly depend on the energy of gamma photons. At small energies, the photoelectric effect is dominant. When gamma energy increases, so does the cross section of Compton scattering. This scattering leads to a decrease of the energy of the primary gamma photons, and the produced secondary gamma photons that have smaller energy can cause the photoelectric effect. When the energy of gamma photons exceeds 1.02 MeV, pair formation also takes place, again producing secondary gamma photons with smaller energy. As a result, the gamma photons lose their energy in several consecutive processes. Well-defined peaks with characteristic gamma photon energy are obtained by cascade interaction within a short time if there are no scattering and diffraction.

In Figure 14.5, the scintillation gamma spectrum is shown. The peak associated with the gamma energy, as well as the continuous Compton edge, can be observed. A peak associated with the scattering of gamma photons appears at a lower energy.

Figure 14.5 The scintillation gamma spectrum of a gamma emitter. The noisy range of the low energies in a real-life spectrum is not shown.

This peak is produced by the gamma photons scattered from substances around the radioactive nuclide and the detector.

Frequently, the gamma spectra contain other peaks with low energies, such as the X-ray lines resulting from the photoelectric effects of the surrounding material, e. g., the X-ray line of the iodine of the NaI(Tl) scintillation crystal or the lead of the shield­ing. In addition, the X-ray emission of the daughter nuclide may also be present. For example, in the spectrum of the Ba-137 m isotope (the daughter nuclide of Cs-137), the X-ray emission of the stable daughter nuclide, Ba-137, can be seen. This emission is produced when the gamma photons transfer their energy to an electron on the K orbital of the daughter nuclide, 137Ba, and this K electron is emitted. This results in the excited state of the electrons of 137Ba. This excited state returns to the ground state through an electron transfer from an outer (L) orbital to the K orbital emitting the excess energy between the orbitals as a characteristic X-ray photon (see Section 5.4.4).

The energy resolution of spectroscopic scintillators is about 7—10%.

For the measurement of charged particles, NaI scintillators are not used because they are hygroscopic, so they have to be kept sealed and the charged particles with short ranges cannot transfer through the wall of the container.

Besides sodium iodide, other alkali halogenide crystals are applied in scintilla­tion spectroscopy. The most important is cesium iodide, which is not hygroscopic so it can be applied without packaging.

Nowadays, cerium-doped lanthanum bromide [LaBr3(Ce)] scintillation detectors are used. These detectors have some advantages compared to NaI(Tl) scintillation detectors. For example, the energy resolution of [LaBr3(Ce)] scintillation detectors is about 3% for the gamma line of 137mBa (662 keV). Furthermore, they have a higher photoelectron yield; the photon emission is nearly flat.

Other types of the inorganic scintillators are the Li-glass scintillators. Their application in neutron detection will be shown in Section 14.5.5.

The most important organic scintillators are aromatic compounds containing more than one aromatic ring in different combinations. The scintillations are resulted in the easy excitation of the conjugated double bond systems. The mecha­nism of scintillation is associated with the individual scintillator molecules and consists of two steps: (1) the particle transfers its energy to the scintillator mole­cules exciting the electrons and (2) the scintillator molecules go back to the ground state, emitting light photons. The intensity of the light photons is proportional to the excitation energy, i. e., to the energy of the radiation particles. The wavelength of the light depends on the identity of the scintillator.

The organic scintillators are divided into three groups: (1) crystals, (2) liquid solutions, and (3) solid solutions (plastics). From the solid organic scintillators, antracene and stilbene are used. In principle, they can measure all types of radiation (alpha, beta, and gamma); however, they are used only in some cases because other scintillators are more suitable for the measurement of each radiation.

The liquid scintillators are mainly used in the so-called liquid scintillation tech­nique for the effective measurement of beta radiation with low energy. In this technique, both the sample (beta emitter) and the scintillator are dissolved in an organic solvent. In this way, the sample and the scintillator are in direct connection within the molecular dimensions; thus, the absorption can be ignored. This is espe­cially important when measuring weak beta emitters used in medical and biological studies (3H, 14C, 35S, and 45Ca).

The liquid scintillation cocktails is composed of the following substances:

1. Solvent: alkyl-benzenes, aromatic ethers (toluene, xylene, anizole, dioxane).

2. Primary scintillator: diphenyl-oxazols, p-terphenyl, PPO (2,5-diphenyl-oxazol).

3. Secondary scintillator: POPOP (1,4-di-(2,5-phenyl-enyl-oxazolyl)-benzene) and dimethyl — POPOP (1,4-bis-2-(4-methyl-5-phenyloxazolyl)benzene) are the most important.

If the sample is water soluble, alcohol is added to the cocktail or dioxane is applied as a solvent. A suspension, an emulsion, or a gel of the sample can also be prepared and used for measurement. Liquid scintillation cocktails are commercially available.

In multicomponent scintillator systems (solvent, primary, and secondary scintil­lators), the energy flow is not known exactly. In these cocktails, the concentration of the scintillator molecules is as low as l0 3 g/dm3. According to the most accepted theory, the radiation (whose energy is hv) excites the molecules of the solvent (S):

hv 1 S——— ! S* (14.1)

The star means the excited state of the molecule. Then these excited molecules (S*) return to the ground state by exciting the molecules of the primary scintillator (Ps), initiating the scintillation process:

S* 1 Ps—— ! Ps* 1 S (14.2)

The secondary scintillators (Ss), the concentration of which is about 10_4 g/dm3, shift the spectrum of the emitted light toward the higher wavelength:

Ps* 1 Ss——— ! Ss* 1 Ps (14.3)

The excited secondary scintillator emits light photons:

Ss*——- ! Ss 1 light photon (14.4)

The photons with higher wavelength coincide with the sensitive range of the photo­cathode of the photomultipliers. As a result, the quantity of the electrons emitted by the photocathode (discussed in Section 14.2.2) increases. By the application of the secondary scintillator, the efficiency of the beta measurement can be improved. When the sensitivity of the photocathode coincides with the energy of the light photon emitted by the primary scintillator, no secondary scintillator is required.

The liquid scintillators are very sensitive to the chemical impurities present in the sample because of the quenching effects. The intensity of the emitted light dramatically decreases if the sample contains substances with a high quenching
effect (organic substances, sulfur, and halogen compounds). Even the compound containing the beta emitter radioactive isotope itself can have a significant quench­ing effect. If so, the quenching effects have to be corrected by suitable measuring techniques (such as standardization).

Plastic scintillators contain POPOP or terphenyl dissolved in polystyrene. They can measure alpha, beta, and gamma radiations, as well as fast neutrons. They can be applied in any desired geometry, including samples with large sizes.