The production of S-35 is similar to the production of carrier-free P-32; it is pro­duced in the 35Cl(n, p)35S nuclear reaction by irradiating the KCl target. The irradi­ated KCl is dissolved in water, while sulfur is dissolved as sulfate. Chloride and sulfate are separated by anion exchange. The half-life of 35S isotope is 87.9 days, and it emits weak beta particles, similar to C-14. As a by-product, a weak beta emitter 36Cl isotope is formed by the 35Cl(n, Y)36Cl nuclear reaction, the half-life of which is 301,000 years. In addition, the 41K isotope of KCl is activated in the 41K (n, Y)42K nuclear reaction. The half-life of the beta and gamma emitter 42K, how­ever, is short enough (12.6 hours) so its disintegration can be waited. For additional information, see Section 8.7.1.2.

The diffusion of mercury vapor in plastics can be studied by a radiotracer method using the Hg isotope. Hg has a half-life of 46.9 days and emits beta particles of 208 keV and gamma radiation of 279 keV. The gamma activity of 203Hg can be measured easily by a NaI(Tl) scintillation detector.

The diffusion studies are as follows. Discs are cut from the plastic samples and they are placed onto the plane top of glass vessels containing a drop of mercury labeled with 203Hg. The diameters of the plastic discs and the top of the glass ves­sels have to be the same and they have to be fitted tightly to avoid the escape of mercury from the vessel. Mercury evaporates in the glass vessel and introduces into the plastic disc. After a given experimental time, the plastic disc is sliced by a microtome into layers that are about 10 micrometers thick. The gamma activities of the plastic slices are plotted as a function of the square of the distance measured
from the surface connecting to the mercury vapor. The virtual diffusion coefficient can be determined using Eq. (9.34).

In Figure 9.5, the logarithm of gamma activities (ln I) is plotted as a function of the square of the distance measured from the surface connecting to the mercury vapor (t = constant). From the slope of the straight line, the virtual diffusion coeffi­cient is determined if the time of diffusion is known. The virtual diffusion coeffi­cients of different plastic samples are very similar, around 10_10 cm2 s_1.

During the migration of mercury in plastic samples containing sulfur, a chemical reaction takes place between mercury and sulfur, influencing the profile of the ln I versus x2 plot. As a result of this chemical reaction between mercury and sulfur, a maximum is observed at a given distance (Figure 9.6). The place of the maximum

depends on the sulfur concentration in the plastic and the time of migration. This phenomenon is analogous to the principle of chromatography.

The principle of the recoil-less nuclear resonance absorption, or Mossbauer spec­troscopy, was discussed in detail in Section 5.4.7. Here, the emphasis is on its chemical applications: Mossbauer spectroscopy allows the analysis of chemical compounds that contain elements which have a Mossbauer nuclide. From a practi­cal point of view, the most important is iron, the Mossbauer nuclide of which is 57Fe. Its abundance in natural iron is about 2.2%. The gamma radiation source is 57Co, and its gamma radiation of 0.0144 MeV can excite the nucleus of the 57Fe isotope. The chemical state (valency) and the chemical environment modify the energy of the nuclear levels, which can be measured, thus allowing the study of the chemical species.

Besides the physical requirements listed previously, we can utilize a radionuclide only if a suitable molecule can be labeled with it. There are very important classes of mole­cules in the body that are so small that we cannot label them with any gamma-emitting radioisotope without altering their structure, i. e., they do not contain any atom with a suitable gamma-emitting radioisotope, so we could label them only by attaching a group of atoms that would significantly change their biological behavior.

Fortunately, there is another possibility for labeling many biologically important molecules: applying positron emitters (see Section 4.4.2). There are positron-emitting radioisotopes of the most important constituents of organic molecules: carbon, oxygen, nitrogen, and fluorine (see Table 12.6).

13.4.1 Dose Units

Previously in this book, the radioactive radiation has been characterized by the half-life, the type, the energy or energy distribution of the emitted particles or elec­tromagnetic radiation, and the activity. These properties are not sufficient to char­acterize the biological effects of radiation because the effects are due to the absorbed energy and the subsequent material changes induced. These effects are characterized by different terms, the radioactive doses.

The most common radiation effect is ionization. The different types of the radio­active radiation ionize the substances directly or via the electrons formed in the scattering processes. Thus, as a first approximation, the biological (or other radio­logical) effects are characterized by the number of the ions produced in the air under the effect of the radiation. The number of the ions related to the irradiation dose (or ion dose) and expressed as coulomb per kilogram (C/kg) in dry air. It should be pointed out that ionization produces electrons and positive ions in equal quantity, so the total number of the charged particles (ions + electrons) is double the ion dose. The previous unit of irradiation dose was 1 rantgen (R), which expresses the number of ions in 1 cm3 of air. The relation between the two is the following: 1 R = 2.58 X 10"4 C/kg.

The biological effects are due to the absorbed radiation energy. This is defined as an absorbed dose in 1 kg of material and expressed in joules per kilogram (J/kg). This unit has its own name, namely gray (Gy). This means that 1 J/kg = 1 Gy (gray). The previous unit of absorbed dose was rad: 1 Gy = 100 rad.

The irradiation and absorption doses can be related by taking into consideration that the formation of one ion and one electron demands 53.9 X 1019 J, assuming the average composition of the air. Since the charge of an electron is 1.6 X 1019 C, the formation of 1 C requires 33.7 J of energy. Therefore, a 1 C/kg irradiation dose is equivalent to a 33.7 Gy absorbed dose in air, assuming total absorption.

As discussed in Chapter 5, different radioactive radiations have different interac­tions with matter. This also applies to the biological effects. The biological effects of the different radiation types are taken into account by the radiation weighting factors, such that the absorbed dose is multiplied by the radiation weighting factors. In this way, the so-called equivalent dose is obtained. The unit of the equivalent dose is the sievert (Sv): Sv = radiation weighting factor X Gy. The value of the fac­tors is very different for each type of radiation. The radiation weighting factor for X-ray and gamma radiation has been chosen to be 1. However, the radiation weighting factor is 5—20 for neutrons, depending on the neutron energy, 5 for protons, and 20 for alpha particles and fission products. The previous unit of equiv­alent dose was rem: 1 Sv = 100 rem.

The effect of the radiation depends not only on the type and energy of the radia­tion but also the sensitivity of the organs and tissues to the radiation. This different sensitivity to stochastic radiation damage (see Section 13.4.4) is considered in the Publication 60 published by the International Commission of Radiological

Protection (CRP), in the Euratom basic standards for radiation protection dated May 1996 by the tissue weighting factor: 0.20 for gonads; 0.12 for colon, bone marrow (red), lung, and stomach; 0.05 for bladder, chest, liver, thyroid gland, and esophagus; 0.01 for skin, bone surface, and others. The sum of the tissue weighting factors is 1 for the whole body. The dose of a whole human body is the effective dose. To calculate the effective dose, the individual organ dose values are multi­plied by the respective tissue weighting factor and the products added. The unit of the effective dose is sieverts.

If the radiation is present for a long time, the dose rate is used, which is defined as the ratio of the dose and the time of irradiation. The background radiation, for example, is expressed in mSv/year (see Table 13.5).

I-123 is produced from Xe-123 in cyclotron (see Eq. (8.28)). Its half-life is 13 h, and it disintegrates with electron capture and gamma radiation. Because of its short half-life, the I-125 isotope is used in nuclear medicine for examinations of pregnant women and children.

I-125 is applied as irradiation source in X-ray fluorescence studies. Its half-life is 60 days, and it disintegrates with electron capture and gamma radiation. For more information, see Section 8.7.1.3.

I-131 is formed by the irradiation of tellurium and by beta decay of the product: 130Te(n, Y)131Te!131I. The half-life of 131I is 8 days, and it has в_ and gamma radiation. During the production of Mo-99 from the spent fuel elements of nuclear reactors, I-131 is separated by acidic treatment, that is, I-131 is a by-product of Mo-99 production. I-131 obtained in this way has a higher specific activity than I-131 produced by irradiation of tellurium. I-131 has important medical applica­tions. For more information, see Section 8.7.1.3.

8.6.17 Xenon Isotopes

Similar to Kr isotopes, xenon isotopes are fission products and emitted into the air from nuclear reactors and reprocessing plants.

Because of their fairly high sensitivity, radiotracer methods are widely applied in all fields where the interface plays an important role in the reactions (interfacial chemistry, colloid chemistry, heterogeneous catalysis, etc.). The surface quantity of the substances is about 10-9—10-8 mol/cm2; thus, the study of the interfacial reac­tions requires analytical methods that are able to detect and precisely measure the change of these small quantities. The radioactive isotopes fulfill this requirement. In addition, radiotracer studies are applicable in a very broad concentration range of the solution or gas interacting with the interface: from carrier-free to saturation concentrations or to critical pressure. The high concentrations are reached using inactive carriers (isotopic effects can usually be disregarded).

Another advantage is the possibility of multiple indications if the radioactive isotopes can be separated using their radiochemical properties (type and/or energy of radiation and half-life, as discussed in Section 8.3). By multiple indications, interfacial processes can be studied from the direction of both bulk phases. For example, in ion exchange processes, both ions can be labeled. In this way, the equivalency of the process can be checked or the effect of other interfacial processes (e. g., adsorption) or the influence of the exchange of additional ions (e. g., hydrogen and hydroxide ions of the water) can be studied. These experiments

give significant information on the interfacial processes of ion exchangers, includ­ing natural ion exchangers such as clay minerals, rocks, and soils.

The study of the cation exchange of calcium montmorillonite clay mineral and manganese(II) ions illustrates the difficulties of multiple radioactive indication. Calcium ions can be labeled as weak beta emitter 45Ca isotopes. For labeling man — ganese(II) ions, the 54Mn isotope can be used, which disintegrates via electron cap­ture and emits X-ray and gamma radiation. The gamma radiation of the 54Mn isotope can be measured by scintillation (see Section 14.2) as well as semiconduc­tor (see Section 14.3) detectors, and the beta radiation of the calcium isotope does not disturb the measurement of gamma radiation. However, the radiation of the 54Mn isotope disturbs the measurement of beta emitter isotopes. The weak beta particles of 45Ca can be measured by the liquid scintillation technique (discussed in Section 14.2). The liquid scintillation beta spectrum of 45Ca is shown in Figure 9.16. The spectrum of 54Mn obtained by a liquid scintillation spectrometer can be seen in Figure 9.17. The sharp peak originates from the electrons emitted after the electron capture from the K orbital. The energy of these electrons is 4.7 keV. When both isotopes, 45Ca and 54Mn, are present simultaneously, the shape of the spectrum is determined by their ratio (Figures 9.18 and 9.19). In Figures 9.16—9.19, the horizontal axis is the channel number proportional to the logarithm of the energy; the vertical axis shows the intensity.

As seen in Figures 9.18 and 9.19, the sharp peak of 54Mn can be eliminated when its activity is relatively large. Thus, the activity of 45Ca can be determined. When the radioactivity of 54Mn is small, however, the peaks corresponding to 45Ca and 54Mn cannot be separated, and the activity of 45Ca cannot be measured. This is a rare and interesting case in chemical analysis when the interfering effect of the “impurity” (54Mn) is higher if it is present in small quantities than if it is present in large quantities. The degree of interference has been experimentally determined as follows: the gamma and “beta” activities of solutions containing 54Mn isotopes

Figure 9.16 Liquid scintillation beta spectrum of the 45Ca isotope.

Figure 9.17 Liquid scintillation beta spectrum of the 54Mn isotope. The sharp peak originates from the electrons emitting after the electron capture from K orbital.

Figure 9.18 Liquid scintillation spectrum of 45Ca and 54Mn in small quantities.

Figure 9.19 Liquid scintillation spectrum of 45Ca and 54Mn in large quantities.

with different activities are determined; the beta activity is plotted as a function of gamma activity. When the solutions contain both 45Ca and 54Mn isotopes, the beta activity of the 54Mn isotope is calculated on the basis of gamma activity using a predetermined plot, and then this value is subtracted from the total beta activity. The difference is treated as the beta activity of 45Ca isotope.

Utilizing sensitivity of the radioactive tracers, it is possible to detect leakages in technological equipment or pipes far earlier than appearance of the leaked materials would be observed in the contaminated component or would endanger product quality or cause any technical risk. For instance, if there is a leakage in a heat exchanger, where migration of the cooling agent into the cooled material (e. g., product) causes obvious deterioration in the product quality, this contamination is detected using traditionally applied analytical methods only if the rate of leakage exceeds detection limits of the given method. With radioactive tracers, however, leakages even in the very early phase can be detected and eliminated readily (Figure 11.1).

Among the radiotracer leakage test methods, leak detection and leak localization on oil pipelines have the utmost importance. Pipelines generally run underground.

For a radiotracer test, NH^Br solution is injected into the streaming oil at a pres­sure upgrading station. This radioactive tracer is miscible with the oil, so the streaming oil will incorporate and transport the radioactive tracer along the pipe­line. At those spots where the pipeline leaks, oil seeps out to the surrounding soil layers. When the radioactive cloud reaches these spots, together with the oil, a small amount of radioactive tracer will also get into the soil. The NH482Br tracer is well adsorbed in the soil. The nonradioactive natural oil stream fills up the pipeline gradually, washing the radioactive cloud into a huge storage tank. But the small radioactive contamination remains in the soil at the location of the leak. With suitable detectors, its location and intensity can be determined.

In the past, the location of the radioactive spot was identified with hand detec­tors by walking along the pipeline on the surface. For this technique, high radio­tracer activities (10—100 GBq) were necessary because soil layers covering the pipeline absorbed a considerable portion of the radiation (Figure 11.2). Today, new detectors have been developed that are built into so-called pigs passing together with the oil stream inside the pipeline, and they both detect radioactivity and mea­sure the distance. Based on this modern detection technique, count versus distance plots provide information on the location of leakage, and the detected counts give

the rate of leakage. For this detection technique, 1 — 10 GBq of radiotracer is suffi­cient for a leakage test (Figure 11.3).

Further examples of leakage tests that were carried out using the radioisotope tracer technique include the following:

• Localization of leaking spots on high-voltage electric cables under gas pressure.

• Localization of leaks in gas-filled telecommunication cables.

• Localization of damaged spots on bitumen-lined concrete tanks.

In this section, we describe the main components of a “traditional” Anger camera: a gamma camera with analog signal processing (Figure 12.4).

1. The collimator is a sheet or disc made of lead, containing (mostly parallel) holes. Radiation arriving from the patient’s body can get across it only along the holes (i. e., in a perpendicular direction); otherwise, the septa of the holes will absorb it. As a result, the “image” of a point source will be a small spot on the crystal. The spatial resolution of the camera is primarily determined by the collimator.

2. The special scintillation detector is generally a thallium-activated sodium iodide [NaI(Tl)] monocrystal, which is rectangular or circular in most cases. For imaging the 141 keV gamma radiation of Tc-99m, the crystal thickness is around 9 mm. Gamma photons hitting the crystal will produce light in a process called “scintillation.”

3. There are many (19—100) photomultiplier tubes (PMTs) attached to the crystal. Each of them responds to the light, and those closer to its source produce larger electric signals than the distant ones.

4. The output signals of all PMTs are forwarded to the matrix (or Anger) circuit.

5. It calculates the X and Y coordinates, and a Z signal proportional to the energy of the original gamma particle.

6. The three signals (X, Y, and Z) are interfaced to a differential discriminator.

7. The discriminator selects only the signals in a specified narrow (15—20%) energy window; in this way, Compton-scattered radiation can be partly removed from the image.

8. In older cameras, a persistent scope helps to position the patient.

9. Analog—digital converters (ADCs) will form numbers from the X and Y signals.

10. A computer will form a digital image from the signals. Each picture element (pixel) contains the number of photons detected in the corresponding small (square) area of the detector. We then display, process, and store these digital images.

Track detectors (cloud chambers, bubble chambers, autoradiography, and solid — state detectors) can visualize the track of the radiation particles. They are mainly used in nuclear physical studies.

14.5.1 Cloud Chambers and Bubble Chambers

The Wilson’s cloud chamber is filled with supersaturated vapor (e. g., very pure alcohol vapor) in which liquid drops are formed under the effect of radiation. In this way, the trace of the radiation can be seen. The photos taken in cloud chambers have been shown previously in this book (e. g., Figures 4.11, 5.7, 5.14, 5.31, 6.1, 13.3, and 13.5).

Bubble chambers operate similarly, but the phases are opposite. Bubbles are formed in a superheated transparent liquid under the effect of radiation and visual­ize the track of the radiation particles.