Carrier-free 54Mn can be produced from the natural isotopes of the iron 56Fe (d, a)54Mn and 54Fe(n, p)54Mn nuclear reactions. In carrier-added form, it can be produced from natural manganese by the 55Mn(n,2n)54Mn nuclear reaction. Manganese is one of the rare elements that consists of only one isotope—in this case 55Mn. The half-life of 54Mn is 312 days, and it disintegrates by electron cap­ture and gamma radiation.

8.6.7 Iron Isotopes

Fe-52 is produced by the spallation of nickel-58 with protons (58Ni(p, spallation)52Fe); its half-life is 8.3 h, it is a positron emitter nuclide, but its daugh­ter nuclide, the metastable Mn-52m, has a shorter half-life, so an Fe-52-Mn-52m generator can be prepared.

Fe-55 is produced from iron by the 54Fe(n, Y)55Fe nuclear reaction. Carrier-free Fe-55 isotopes is produced from manganese by the 55Mn(d, p)55Fe reaction. half­life is 2.7 years, and it decays by electron capture.

Fe-59 forms from the subsequent (n, Y) nuclear reaction of stable iron isotopes, half-life is 44.5 days, and it emits (3_ and gamma radiation.

8.6.8 Cobalt-60

It can be produced from the stable isotope of cobalt in the 59Co(n, Y)60Co nuclear reaction, and its half-life is 5.5 years. It has two gamma lines at 1178 and 1333 keV. Co-60 is applied in sterilizing and therapeutic irradiations. For additional information, see Section 8.8.

Under the effect of oxidative agents, oxidized layers form on the metal surfaces. The mechanism of the formation of the oxidized layer can be studied easily by labeling one of the reaction partners (e. g., the oxidative agent) with its radioac­tive isotope. The distribution of the radioactive isotopes resulting in the different oxide formation mechanisms is schematically shown in Figure 9.10. The study of the activity profile, such as by autoradiography as described in Section 14.5.2, gives information on the mechanism and rate-determining step of oxidation reactions.

In TEM, the sample is bombarded by an electron beam. The resolution is deter­mined by the wavelength of the electron and the numerical aperture of the electron optical lens. Similar to the wavelength of neutrons, the wavelength of the electrons is calculated by Eq. (4.93). The energy of the electrons in TEM microscopes is typ­ically 100—300 keV. At 100 keV electron energy, and taking into consideration the mass of the electron, the wavelength of the electron is about 4 X 1012 m. The res­olution (d) is quantitatively expressed as follows:

d = _A_

2n sin a 2NA

where A is the wavelength of the electron, n is the refractive index of the medium of the lens, a is the half-angle of the maximum cone of radiation that can enter or exit the lens, and NA is the numerical aperture. This means that theoretically the maximum resolution of the electron microscope is about 2 pm. Practically, the best resolution is in the order of tenths of a nanometer.

Since the electron is a charged particle with a relatively short range, the sample must be thin to transmit the electrons. Therefore, the method requires special prepa­ration of the samples. The thin samples are placed on gold sample holders, as illus­trated in Figure 10.21.

As mentioned previously, the imaging is possible using the elastically scattered electrons. Inelastic electron scattering has a positive and a negative effect. Although it disturbs the imaging because the change of the wavelength changes the focal length (chromatic error), it allows chemical analysis via the emission of the characteristic photons (electron microprobe).

Figure 10.21 A gold sample holder for TEM. The diameter is about 3 mm.

The irradiating electrons are scattered on the electrons of the irradiated atoms. Thus, the light elements have a smaller degree of scattering than the heavier ones. For this reason, the compounds consisting of light elements, such as organic sub­stances, are covered by contrast material with a high atomic number (e. g., osmium, lead, gold, silver, etc.). These metals are evaporated or adsorbed onto the surface of the sample.

The demands of in vitro isotope diagnostic procedures are different from those of imaging as follows:

• A longer half-life is desirable, so labeled products can be stored for a longer time.

• A wider range of radiation energy is acceptable. A lower gamma energy is even prefera­ble so that the staff dose can be decreased. The reason is that we can ensure the same measurement geometry of the radioactive specimen (see Chapter 14), i. e., both the standards and the samples are in the same type of (plastic) vial, volume, and medium. Therefore, the attenuated fraction of the radiation is the same, and the accuracy of the measurement is not reduced. Iodine-125 is the radionuclide most frequently used for in vitro diagnostic procedures.

• For concentration measurements, beta emitters can also be used, but for this procedure, a special measuring technique, the so-called liquid scintillation counting, is required, as beta radiation is absorbed in the sample and its vial, so we cannot use an external radia­tion detector to measure it (see also Sections 5.3.4, 5.3.5, and 14.2.1).

Before discussing the biological effects of radiation, the dose of the natural back­ground radiation is revealed. This is the radiation dose that has been present during the evolution of living organisms; they obviously somehow adapted to this radia­tion dose. Every other effect of radiation has to be compared to this effect of back­ground radiation.

Both natural and artificial radioactive isotopes were presented in Sections 13.1 and 13.2; their abundance in the different spheres of Earth was discussed in Section 13.3. These radioactive isotopes and the cosmic ray irradiate living organ­isms as external radiation sources. The cloud chamber photograph (see Section 14.5.1) of the background radiation is shown in Figure 13.5. The tracks of the alpha (thick tracks) and beta (thin tracks) are shown well. A solid-state detector picture (Section 14.5.3) of the cosmic ray exposed on the Cosmos 2044 satellite is shown in Figure 13.6.

In addition, living organisms can incorporate radioisotopes, which cause internal radiation exposure. The types and distribution of the radiation doses, as well as the mean effective doses, are quantitatively given in Table 13.5.

Figure 13.5 A cloud chamber photograph of background radiation. (Thanks to Dr. Peter Raics, Department of Experimental Physics, University of Debrecen, Hungary, for the photograph.)

Figure 13.6 A solid-state detector picture of the cosmic ray exposed on the Cosmos 2044 satellite. The thick track is probably the track of a heavy oxygen ion. (Thanks to Dr. Istvan Csige, Department of Environmental Physics, Institute of Nuclear Research, University of Debrecen, Hungary, for the photograph.)

Table 13.5 shows the part of natural background radiation that lacks any anthro­pogenic activity, which has been present during the whole history of the Earth. Since the nucleogenesis or primordial isotopes (see Section 13.1) cannot be formed under natural conditions on the Earth, their quantity continuously decreases. The decrease is obviously very slow because of the very long half-lives. Nowadays, nuclegenesis, cosmogenic isotopes, and the cosmic ray mean an effective dose rate of about 2—2.5 mSv/years, depending on the geographical location.

In addition, there is natural radiation that is present as a result of anthropogenic activity. These are also listed in Table 13.5. For example, the radioactivity of rocks in the deep layer of the Earth’s crust does not irradiate the living organisms if they are in their original place. When they are brought to the surface of the Earth (e. g., by mining of coal, phosphates, and natural gas), however, radioactive isotopes get onto the surface, increasing the natural radioactivity and the effective dose. Similarly, the 222Rn isotope is a natural radioactive isotope. It has a high atomic number, so it accumulates in closed places such as in caves. The building materials
always contain uranium and, of course, its daughter nuclides, including 222Rn. As a result, living in houses increases the effective dose rate. As seen in the decay series of U (see Figure 4.4), the daughter nuclides of Rn are solid, so they accumu­late in lungs, increasing the internal radiation dose rate. When flying by aircraft, the cosmic ray increases the external radiation dose rate. The total effective dose rate of these types of radiation (which is natural, but a consequence of anthropo­genic activity) is about 2 mSv/years.

The effective dose rate of artificial radioactivity is less than 0.05 mSv/years. This consists of the fallout of nuclear explosions and the emission of nuclear reac­tors. This accounts for about 1% of natural background radiation, and it continu­ously decreases. Medical irradiation represents about 0.5 mSv/years, but this value strongly depends on the level of medical intervention. This value can also decrease with the technical improvement of the instruments used in nuclear medicine but can increase by a larger segment of the population taking part in preventive medi­cal examinations.

192Ir is produced by the 191Ir(n, Y)192Ir nuclear reaction. Its half-life is 74 days, and it disintegrates by electron capture and (3_ and gamma radiation. This nuclide has historical importance, as the isotope was used in the discovery of the Mossbauer effect. For more information, see Section 8.8.

8.6.19 Gold-198

198Au is produced by the 197Au(n, Y)198Au nuclear reaction. The half-life is 2.7 days, and it emits (3_ and gamma radiation. 198Au is an ancient radiopharmaceuti­cal; gold colloids have been produced by the reduction of gold salt with ascorbic acid, and they are used in cancer therapy.

8.6.20 Mercury-203

203Hg is produced by the 202Hg(n, Y)203Hg nuclear reaction. Its half-life is 46.6 days, and it emits (3_ and gamma radiation. As a widespread industrial tracer, 203Hg is used to determine the volume of mercury in the cells of natrium chloride electrolysis with mercury cathode (see Section 11.2.4).

The combination of electrochemical studies with radiotracer methods can help solve many electrochemical and corrosion problems, such as the corrosion of the structural material of nuclear power plants and the contamination and decontamina­tion of the corrosion products.

Important factors of contamination and decontamination are the sorption pro­cesses which, as discussed previously, can be well studied by radiotracer methods. In such studies, the radioactivity of the solution and/or the solid phase, including the electrodes in the electrochemical studies, can be measured. The radioactivity of the surface is proportional to the surface excess concentration of the sorbed spe­cies. The measurements frequently can be obtained by stopping the process and
measuring the activities after sampling. In addition, the processes can also be stud­ied in situ. The in situ methods are based on the thin-layer principle. This means that the self-absorption of beta particles with low and medium energy, as well as gamma- or X-ray radiation below 20 keV energy, is so high that the detectors can observe the radiation of only a thin liquid layer. If the solution phase is eliminated from the surface of the solid (electrode), the radioactivity of the sorbed species can be determined. It can be achieved in three ways:

1. In the foil method, the detector and the solution of the labeled adsorbate is separated by the adsorbent deposited on a thin foil (or the adsorbent itself is the foil).

2. In the thin-layer method, the solution is continuously circulated in a thin layer (about 0.5 mm) between the detector and the electrode.

3. In the so-called electrode sinking method, which is the combination of the two previous methods, radioactivity is measured alternatively in the two positions of the electrode. Sinking to the bottom where the background is very low, the surface excess concentration is measured, while in a position higher than the range of the beta radiation, the radioactiv­ity of the solution is measured.

Further Reading

Atkins, P. W. (1998). Physical Chemistry. 6th edition. Oxford University Press, Oxford.

Bartha, L. (1967). Observation of recrystallization of tin by autoradiography. J. Appl. Radiat. Isot. 18:789-790.

Crank, J. (1979). The Mathematics of Diffusion. Clarendon Press, Oxford.

Elektrochemie, Z. Berichte der Bunsengesellschaft fur physikalische Chemie. 56:380-386. http://onlinelibrary. wiley. com/doi/dx. doi. org/10.1002/bbpc.19520560427/abstract. May 3, 2010.

Gerischer, H. Vielstich, W. (1952). Untersuchungen mit radioaktiven Indikatoren uber Austausch — und Diffusionsvorgange an Silberelektroden.

Haissinsky, M. (1964). Nuclear Chemistry and its Applications. Addison-Wesley, Reading, MA.

Hoffman, R. E. and Turnbull, D. (1951). Lattice and grain boundary self-diffusion in silver. J. Appl. Phys. 22:634-639.

Imre, L. (1933). Grenzflachengleichgewichte und innere Gleichgewichte in heterogenen Systemen. Teil I. Z. Phys. Chem. Abt. A 164:343-363.

Imre, L. (1933). Zur Kinetik der Oberflachenvorgange an Kristallgittern. II. Die Elementar — prozesse bei der Ausbildung einer aus mehreren Komponenten bestehenden Grenzschicht. Z. Phys. Chem. Abt. A 164:327-342.

Imre, L. (1942). Uber die Anwendbarkeit der radioaktiven Indikatormethode zur Bestimmung Der Oberflache fester Korper I. Kolloid Z. 99:147-157.

Imre, L. (1944). Uber die Anwendbarkeit der radioaktiven Indikatormethode zur Bestimmung der Oberflache fester Korper II. Kolloid Z. 106:39-46.

Kazarinov, V. E. and Andreev, V. N. (1984). Tracer methods in electrochemical studies. In: Comprehensive Treatise of Electrochemistry (eds. Yeager, E., Bockris, J. O’M., Conway, B. E., Sarangapani, S.). Plenum Press, New York, NY, London, pp. 393-443.

Konya, J. (1977). Study of surface reactions of the Fe/Fe heterogeneous isotope-exchange system with a radioactive indicator (59Fe), part I. Determination of iron exchange cur­rent. J. Electroanal. Chem. 84:83-91.

Konya, J. and Baba, A. (1980). Study of surface reactions of the Fe/Fe heterogeneous iso­tope-exchange system with a radioactive indicator (59Fe), part II. Rates of anodic and cathodic part-processes at corrosion potential. J. Electroanal. Chem. 109:125—139.

Nagy, N. M. and Konya, J. (2005). The relations between the origin and some basic physical and chemical properties of bentonite rocks illustrating on the example of Sarmatian ben­tonite site at Sajobabony (HU). Appl. Clay Sci. 28:257—267.

Nagy, N. M. and Konya, J. (2009). Interfacial chemistry of rocks and soils. Taylor & Francis, Boca Raton, FL.

Philibert, J. (1991). Atom Movements. Diffusion and Mass Transport in Solids. Les Editions de Physique, Paris.

Sheppard, C. W. (1948). The theory of the study of transfers within a multi-compartment sys­tem using isotopic tracers. J. Appl. Phys. 19:70—76.

Solomon, A. K. (1949). Equations for tracer experiments. J. Clin. Invest. 28:1297—1307. < http://www. ncbi. nlm. nih. gov/pmc/articles/PMC439688/pdf/jcinvest00400-0051.pdf > (accessed 25.03.12.)

Varga, K., Hirschberg, G., Baradlai, P. and Nagy, M. (2001). Combined application of radio­chemical and electrochemical methods for the investigation of solid/liquid interfaces. In: Surface and Colloid Science (ed. Matijevic, E.), vol. 16. Plenum Press, New York, NY, pp. 341—393.

Varga, K. (2004). The role of interfacial phenomena in the contamination and decontamina­tion of nuclear reactors. In: Radiotracer Studies of Interfaces, Interface Science and Technology (ed. Horanyi, G.), vol. 3. Elsevier B. V., Amsterdam, pp. 313—358.

From both the technology management and economic points of view, knowing the quantity of substances processed in large and closed industrial equipment is essen­tial. In the same time, in most cases, removal of large quantities of substances for volume or mass measurement outside the equipment is not feasible, or else it would cause considerable contamination and substance loss.

For substances that are or can be stirred to a homogeneous mixture, the radio­tracer technique offers a simple method for determining their quantity. The known dosage (activity and volume) of a radiotracer should be added to the substance of unknown quantity, and after thorough mixing, a sample should be taken for
determining the radioactive concentration of the homogeneous mixture. The decrease in radioactivity compared to that of the tracer reflects the dilution rate of the tracer in the total quantity of the substance. Thus, the simple isotope dilution principle (discussed in Section 10.1.6.1) can be applied to the measurement:

Va0 1 j = (V 1 Vj)ah (11-14)

where V is the volume of the substance to be determined, Vj is the volume of the tracer, a0 is the radioactive concentration of the substance to be determined, aj is the radioactive concentration of the tracer, and ah is the radioactive concentration of the mixture developed after homogeneous stirring.

Arranged to the volume of the substance to be measured:

V = Vj (11.15)

ah — a0

As a0 = 0 and aj» ah:

V = Vj a (11.16)

ah

Consequently, the volume (V) of the substance in the equipment can be deter­mined by measuring the volume (Vj) and radioactive concentration (aj) of the tracer as well as the radioactive concentration of the pool (ah) afrer dilution. Equations

(11.11) —(11.13) are valid not only for volumes but also for masses; therefore, sub­stance masses can also be determined based on the same principle.

A well-known example of the substance quantity (mass or volume) determina­tion with the radiotracer technique is the regular measurement of metal mercury mass (used as a cathode metal) circulating in chloro-alkali-electrolysis cells. For this measurement, the Hg-203 radioisotope is used in metal mercury form with activity of 37 MBq per electrolysis cell. In one cell, approximately 1 —4 tons of mercury is circulated, and generally, 40 electrolysis cells are operated in one plant.

Some additional examples of substance quantity measurement with radiotracer technique are mentioned here:

• Determining the slag quantity in metallurgical shaft and cupola furnaces.

• Determining the metal melt quantity in electric furnaces.

• Determining the raw meal residue in fluidization homogenizers in a cement factory.

To distinguish from the medical imaging modalities that measure the attenuation of radiation originating from an external source, called transmission imaging (e. g., con­ventional X-ray radiography and CT), the imaging of radiation emitted by radio­pharmaceuticals inside a patient’s body is generally referred to as emission imaging.

• Static scintigraphy: images from one or several views are acquired after the distribution of the radiopharmaceutical reaches equilibrium.

Static imaging was possible even before the gamma camera was invented, using a radiation detector that scanned the target organ, having a print head yoked to it.

• For dynamic imaging, a series of images from the same view is taken, following the pro­cess of accumulation, secretion, or excretion of the radiopharmaceutical. Besides obtain­ing the distribution at different points in time, we can investigate how the radioactivity concentration of any region changes with time.

• Whole-body imaging means linking several static images by the computer to show an area bigger than the field of view. A large-field-of-view gamma camera generally covers the width of a patient’s body, and we need three to five steps to scan its length. See details of this process later in this chapter.

• The last and most powerful way of imaging is tomography, when the distribution of the radiopharmaceutical in various sections of the body is calculated from measurements of many projections. Consecutive slices together represent a three-dimensional distribution:

• SPECT (single photon emission computed tomography) applies gamma emitters, while

• PET utilizes positron emitters (see Section 12.6).

When passing through some substances, heavy ionizing particles break the chemi­cal bonds and produce damaged channels. The diameter of these channels is about 5 nm; however, their diameter can be augmented to as large as 10 qm using chemi­cal development. In amorphous substances, the tracks are round, assuming that the ionizing particles enter the substance perpendicularly. In crystalline substances, the shape of the tracks fit the crystal lattice. The density of the tracks is propor­tional to the intensity of the radiation. Solid-state detectors can be used to measure the energy of the particles because the diameter of the tracks is proportional to the energy of the particles, providing that the procedures are done strictly under the same conditions.

In Figure 14.11, the tracks of the alpha particles emitted by 252Cf (6.1 MeV) are shown. Moreover, tracks of the heavy particles are shown in Figures 13.4 and 13.6.

Figure 14.11 Tracks of the alpha particles emitted by 252Cf (6.1 MeV). (Thanks to Dr. Istvan Csige, Department of Environmental Physics, University of Debrecen— Institute of Nuclear Research, Hungary, for the photograph.)

The basic characteristics of the most important solid-state detectors are summa­rized in Table 14.1. The plastic detectors are developed by 6 mol/dm3 of NaOH or KOH. Glass and silicate (such as mica) detectors are developed by hydrogen fluoride.

An application of solid-state detectors was presented in Section 4.3.3. The minerals also record nuclear processes that occur spontaneously in nature.