As mentioned previously in Section 8.1, de Hevesy used the Pb-210 isotope in the first tracer experiments to determine the solubility of lead salts. Pb-210 has been used for self-diffusion studies of metallic lead, and the determination of exchange current density of lead amalgam.

As seen in Figure 4.4, Pb-210 is an intermediate member of the decay series of U-238, and it has a longer half-life (21.6 years) than the previous and the subsequent members of the U-238 decay series. Therefore, it has the tendency to accumulate and is considered as a polluting radioactive isotope in the environment. In some
places (e. g., Matraderecske, a village in Hungary), the atmospheric concentration of Ra-222, and simultaneously the concentration of Pb-210, is higher as usual. Since Pb-210 is formed as a result of alpha decays (see Figure 4.4) due to the recoil of the nucleus, Pb-210 can introduce into the glass plates of the pictures, photos on the walls of the houses in these places. If the age of the glass plates is known (e. g., photos were taken on special occasions such as a wedding), the change of the radon concentration in the house can be determined from the Pb-210 activity.

Pb-210 emits weak beta particles; recently, it usually has been measured by the liquid scintillation technique (see Section 14.2.1). Before the construction of liquid scintillation spectrometers, the weak beta radiation of Pb-210 was determined by measuring the activity of Bi-210, the daughter nuclide of Pb-210. Before the mea­surements, Pb-210 and Bi-210 were separated. The sample containing Pb-210 was covered by aluminum foil to absorb the weak beta particles. The daughter nuclide of Pb-210, Bi-210, however, emitted beta particles with higher energy, which can transmit through the aluminum foil. The accumulation of Bi-210 gives infor­mation on the activity of Pb-210. After reaching the secular equilibrium between Pb-210 and Bi-210 (which takes approximately 50 days), the activities become the same.

The production process that consists of neutron irradiation of the target and the subsequent encapsulation of the irradiated pellets is demonstrated through the example of 192Ir-sealed source production (see Table 8.18).

60Co-sealed sources are produced in the nuclear reaction of 60Co(n, Y)60Co with similar processing as that of 192Ir, but applying much longer irradiation time (i. e., several months).

8.8.1 Quality Control of Sealed Radioactive Sources

At the production of sealed (encapsulated) radioactive sources, quality control includes activity testing, surface contamination testing, and leakage testing (see Table 8.19).

10.2.1 Basic Concepts

As discussed in detail in Chapters 5 and 6, radioactive (and other) radiations may have different impacts on the substances. The radiation can be absorbed or scat­tered, and, as a result of the interactions with the radiation, the irradiated substances themselves can emit different radiations, including particles or electromagnetic photons. All these phenomena provide analytical information on the different
structural levels of matter. The changes of intensity of the entering radiation, as well as the type, energy, or energy distribution, and the activity or intensity of the emitted radiation can be used in qualitative, quantitative, structural, and species analysis of the bulk phases, interfaces, and species bound to the surfaces of the substances.

Different types of radiations (namely, photons in the whole range of the electro­magnetic spectrum, electrons and beta particles, neutrons, and positively charged particles) are used for the irradiation. The emitted radiations can also be photons and particles (electrons and positively charged particles). The emission is the result of the interactions between the entering radiation and the nuclei, nuclear field, and orbital electrons.

As seen from the list of the entering and emitted radiations, there are many ana­lytical methods using the interaction of radiation with matter. They can be classi­fied on the basis of the entering and the emitted particles, as summarized in Table 10.2. As usual, the interactions are significantly influenced by the mass and charge of the particles (as discussed in Section 5.1). The classification is made on the basis of the mass of the particles.

For the sake of completeness, Table 10.2 includes the analytical method in which the substance is irradiated with photons with lower energy than the nuclear radiation (e. g., nuclear magnetic resonance, electron spin resonance, infrared, near­infrared, visible, ultraviolet spectroscopy, or dynamic light scattering). Of course, these methods traditionally belong to other disciplines of chemistry, so they are not discussed in detail here. It is important to note, however, that they also utilize the interactions of radiation with matter. In addition, nuclear magnetic resonance can be considered to be a nuclear analytical method in which the magnetic field of spe­cial nuclei is excited by electromagnetic radiation with low energy. At the same time, the highest-energy electromagnetic radiation (gamma photons) also excites the nuclei: the two terminal ranges of the electromagnetic spectrum have an impact on the same part—namely, the nucleus of the atoms.

In each row of Table 10.2, the emitted particles are the same; only their energy is different. The photons or particles within the same energy range are detected with the same methods, independent of the irradiation. As an example, the emission of X-ray photons of X-ray fluorescence analysis (XRF), electron microprobe, and ion (including proton)-induced XRF is mentioned. As will be discussed later in this chapter (Section 10.2.3.1), the X-ray photons (shown in row 4 of Table 10.2) are emitted as a consequence of the electron emission from the K or L electron orbital of the samples to be analyzed (the photoelectric effect, discussed in Section 5.4.4). High-energy gamma photons (shown in row 4 of Table 10.2) are emitted when irra­diation with neutrons or charged particles induces nuclear reactions (such as NAA, PGAA (see Sections 10.2.2.1 and 10.2.2.2), and charged particle activation analysis (CPAA; Section 10.2.5.2)).

As seen in row 5 of Table 10.2, electrons can be emitted after irradiation of the matter with photons, electrons, or ions. When Auger electron emission (see Sections

5.3 and 5.4.4) results from electron emission from the K or L electron orbital of the samples to be analyzed, they are detected and measured by the same techniques,

a, alpha particle; AES, Auger electron spectroscopy; CPINRA, charged particle-induced nuclear reaction analysis; CPAA, charged particle activation analysis; EIID, electron-induced ion desorption; EELS, electron-energy-loss spectroscopy; EMP, electron microprobe; ESCA, electron spectroscopy for chemical analysis; ESR, electron spin resonance; EXAFS, extended X-ray absorption fine structure; IEX, ion-excited X-ray fluorescence spectroscopy; IMMA, ion microprobe mass analyzer; IMXA, ion microprobe X-ray analysis; INS, ion neutralization spectroscopy; IR, infrared spectroscopy; ISS, ion scattering spectrometry; LEED, low-energy electron diffraction; LAMMA, laser microprobe mass analysis; n, neutron; NAA, neutron activation analysis; NEXAFS, near-edge X-ray absorption fine structure; NIR, near-infrared spectroscopy; NMR, nuclear magnetic resonance; p, proton; PGAA, prompt gamma activation analysis; PIXE, particle-induced X-ray emission; RHEED, reflection high-energy electron diffraction; RIBS, Rutherford backscattering spectroscopy; SAM, scanning Auger microanalysis; SANS, small-angle neutron scattering; SEM, scanning electron microscopy; SIMS, secondary ion mass spectroscopy; UPS, ultraviolet photoelectron spectroscopy; UV, ultraviolet spectroscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction analysis; XRF, X-ray fluorescence analysis; XANES, X-ray absorption near-edge structure.

Source: Adapted from Nagy and Konya, with permission from Taylor & Francis.

independent of the irradiation method. On the basis of the emitted radiation, gamma, X-ray, electron, and charge particle spectroscopic methods are classified.

In classical radioanalysis, those methods are used when the emitted radiation originates from the nucleus or the internal electron orbitals. Accordingly, classical analytical methods are the activation analytical methods, especially NAA and XRF. In fact, XRF does not require nuclear processes: the irradiating X-ray photons can be produced by an X-ray tube using the electron transition between the internal electron orbitals of the cathode of the X-ray tubes. Furthermore, the emitted X-ray photons also originate from electron transition between electron orbitals. As dis­cussed previously in this chapter, the emission of the same particles with similar energy requires similar detection and measuring techniques, independent of the irradiation method. For this reason, the methods providing X-ray photons and elec­trons (e. g., electron microprobe, AES, and X-ray photoelectron spectroscopy (XPS)) are also considered to be nuclear analytical methods. At the same time, the photoelectric effect may or may not be accompanied by a nuclear process. Thus, the term “radioanalysis” is used in a very broad sense: all methods may be consid­ered to be radioanalysis in which emitted particles and photons are analyzed.

To summarize, nuclear analysis refers to all types of detection and measurement techniques of the emitted radiation. As mentioned several times previously, radiation can be emitted both from the nucleus and from the orbital electrons. The radiation that originates from the nucleus can be the consequence of nuclear reactions or excita­tion of the nucleus. The radiation that originates from the electron orbitals relates to the excitation and de-excitation of the electrons or ionization. Common characteristics of these methods are that they are selective, sensitive, and frequently indestructible.

The methods listed in Table 10.2 differ in the depth of the introduction of the radiation, the interacting part of the substance, and the number of the interactions of the radiation with matter; and these characteristics determine which properties of the substance can be investigated using a particular method. The depth of the intro­duction of the radiation determines how thick the studied layer is, i. e., whether the properties of bulk phases, the interface, or the species adsorbed on the surfaces may be studied. The mass, charge, and energy of the radiation influences the thick­ness of the studied layer. The layer of absorption of the radiation is usually deeper in the case of light and neutral radiations. The energy of the radiation, however, strongly modifies this general tendency. For example, high-energy X-rays or elec­trons are introduced deeply, so the properties of the bulk can be studied. At small X-ray or electron energies, the structure of the surface layer can be studied.

In Table 10.3, the thickness of the studied layer, the primary information, the typical sensitivity, and the detectable elements and species of the methods included in Table 10.2 are summarized.

Jozsef Varga

Department of Nuclear Medicine Institute, University of Debrecen, Debrecen, Hungary

Nuclear medicine applies unsealed radioactive preparations for medical purposes. A preparation is called “unsealed” if it can mix with its environment and may participate in both chemical reactions and biological processes.

Note that radioactive materials are applied in medicine as external and sealed radiation sources as well (Figure 12.1). The cobalt gun has been used extensively as an external source for radiation therapy of tumors (teletherapy). Brachytherapy involves treating a disease by exposure to a radioactive sub­stance. Doctors place a small radioactive source (pellet or seed) in or a short distance from a malignant (e. g., cervical) tumor. Thus, high doses of radiation can be used while reducing the risk of damage to nearby healthy tissue and increasing the likelihood that the tumor is destroyed. The sealed source is then removed so that no radioactive substance is left in the patient’s body. Both teletherapy and brachytherapy belong to radiation therapy rather than nuclear medicine.

The basis of nuclear medicine is the radiotracer technique developed by Gydrgy Hevesy (1885—1966) in 1924; he was awarded the Nobel Prize for chem­istry for this achievement in 1943. The principle is that changing an atom in a molecule for its radioisotope will not significantly change its chemical and bio­logical behavior, while the movement, distribution, and concentration of the mol­ecule (and its derivatives) can be followed by measuring its radiation—even processes in a live human or animal can be studied using external radiation detec­tors. Modern devices allow the detection of such small amounts of the tracer that the function of the organ that we want to study is not affected (in contrast to some X-ray contrast materials). However, biological processes are so complicated that human applications require special considerations; see further details in Sections 12.4 and 12.6.

Nuclear and Radiochemistry. DOI: http://dx. doi. org/10.1016/B978-0-12-391430-9.00012-3

© 2012 Elsevier Inc. All rights reserved.

As mentioned in Chapter 3, there is a much higher number of radioactive isotopes than stable isotopes. Radioactive isotopes, including natural and artificial ones, are present in the environment, causing part of the background radiation. Radiation of the natural radioactive isotopes has affected living organisms during their evolu­tion. Obviously, living organisms could adapt to natural background radiation. The increase in radiation that results from the anthropogenic-generated radioactive iso­tope effects must always be compared to natural radioactivity when its effects on living organisms are discussed.

Nickel-63 can be produced from the stable isotope of nickel in the 62Ni(n, Y)63Ni nuclear reaction. Its half-life is 100 years, and it emits (3_-radiation. It is used as a radiation source of electron capture detectors in gas chromatographs.

8.6.9 Copper Isotopes

Cu-64 can be obtained from natural copper by the 63Cu(n,^)64Cu nuclear reaction, half-life is 12.7 h. Copper-64 has branching decay emitting both positive and nega­tive beta particles. The carrier-free Cu-64 isotope can be obtained from zinc in the 64Zn(n, p)64Cu nuclear reaction. Simultaneously, the 64Zn(n,^)65Zn reaction also takes place. The two product nuclides, Cu-64 and Zn-65, can be separated by electrolysis.

Cu-66 is produced from copper by the 65Cu(n, Y)66Cu nuclear reaction. The half-life of Cu-66 is 5 min, and it emits negative beta particles.

Natural zinc contains 66Zn isotope, too. By the (n, Y) and (n, p) nuclear reactions of 66Zn, inactive 67Zn and 65Cu, respectively, are formed. However, the half-life of 65Cu is short enough (5 min), so its decomposition can be waited, it does not pol­lute 64Cu.

8.6.10 Zinc-65

See at copper isotopes. The half-life is 244 days, and decays are electron capture and negative and positive beta decays.

In pure substances, the movement of the own particles, the so-called self-diffusion, can easily be studied by a labeled species of the substance. As mentioned in Section 8.1, both stable and radioactive isotopes can be used for labeling. In the case of stable labeling isotopes, mass spectrometry or subsequent activation is used to measure self-diffusion. Other techniques that can differentiate the isotopes with different mass numbers, such as nuclear magnetic resonance, are also used for the study of self-diffusion.

In this chapter, the application of radioactive isotopes in self-diffusion studies is discussed. As an example, the self-diffusion of metals is studied so that the bottom of

(C)

Metallic bulk phase

Figure 9.10 The distribution of the radioactive isotopes resulting in the different oxidized layer formation mechanisms. The oxidizing agent is labeled by the radioactive isotope.

(A) The oxidation process is determined by the diffusion of metal toward the solution.

(B) Both the metal and the oxidizing agent diffuse toward each other. (C) The oxidizing agent moves faster than metal atoms. (D) The oxidizing agent moves faster than metal atoms; the difference of the transport rate, however, is smaller than in C.

the metal piece is evenly covered (e. g., electrolysis) by a layer of the carrier-free radioactive isotope of the metal, and then the local distribution of the isotope is mea­sured after a given length of time. In the meantime, the temperature is kept constant. Under these conditions, the self-diffusion can be described by Eq. (9.34); thus, the self-diffusion coefficient is determined from the slope of the ln I versus x2 function. The transport of the radioactive tracer increases the mixing entropy. The change of enthalpy can be disregarded because of the very low concentration of the radiotracer. The first self-diffusion studies were done by George Hevesy and Gyula Grah.

In Table 9.1, the self-diffusion coefficients of different metals are listed. As seen, the diffusion coefficients give information on the crystal lattice, and consequently on the properties of metals. In noncubic lattices, the diffusion coefficients depend on the direction, the diffusion is anisotropic. In Table 9.1, || and _L denote the direc­tions parallel and perpendicular to the c-axis of the crystal lattice, respectively. As seen, different authors sometimes give different values, but within the same order of magnitude. This shows the uncertainty of self-diffusion measurements.

In solid crystalline substances, diffusion has two mechanisms:

• Diffusion in a crystal grain or volume diffusion. This takes place inside a crystal grain or in a single crystal.

• Grain boundary diffusion, which is characteristic in polycrystals. An example of this is shown in Figure 9.11.

The self-diffusion coefficients are suitable to study the structural changes of sub­stances under different physical effects. An example is shown in Figure 9.12, where

the effect of heat treatment and deformation by compression is seen. Primary recrystallization takes place as a result of compression, while secondary recrystalli­zation occurs by 1 h anneal at an elevated temperature.

Table 9.1 also shows the self-diffusion coefficients for some liquids. As seen, the self-diffusion coefficients of liquids are several orders of magnitude higher than those of solid substances. In addition, the activation energy of self-diffusion in liquids is less by about an order of magnitude.

SEMs image the sample to be analyzed using the electrons elastically scattering from the surface. Thus, the surface of thick layers can be studied. The surface of the sample has to be covered by a conductive layer, such as metal or graphite. The surface is scanned by an electron beam. The diameter of this electron beam (<1 pm) determines the horizontal resolution of the SEM.

A portion of the electrons scatters inelastically, ejecting electrons from the K or L electron orbitals. Similar to XRF, the following characteristic X-ray emission provides qualitative and quantitative analytical possibilities. These instruments are called “electron microprobes.”

In electron microprobes, the characteristic X-ray photons are detected in two ways: in an energy-dispersive or a wavelength-dispersive manner. Similar to X-ray fluorescence spectrometers, energy-dispersive systems are equipped with semicon­ductor detectors, the resolution of which is about 130 eV. In wavelength-dispersive systems, the detector is a single crystal at a precise angle. The structure, including the characteristic spacing between the planes of the crystal lattice, is known. The wavelength of the X-ray photons is measured using the Bragg formula (Eq. (10.14)). The resolution of the wavelength-dispersive detectors is about (10 eV). All elements, except hydrogen, helium, and lithium, can be measured.

Electron microprobes have 0.01% relative and 1014g absolute detection limits with 3% relative accuracy.

In Figure 10.22, the SEM picture of montmorillonite clay treated with lead ions is shown. The left picture shows the image (morphology) obtained by the elastically scattered electrons. The right picture is the lead map obtained by the energy — dispersive spectrum of the characteristic X-ray photons.

In Figure 10.23, the wavelength-dispersive X-ray spectrum obtained at a certain area of a clay sample treated by manganese ions is shown.

The scanning of a sample surface provides the possibility of analyzing the ele­mentary composition along a straight line. The concentration profiles of different elements of montmorillonite clay treated with lead ions are shown in Figure 10.24.

The sample is mixed with a liquid scintillator that converts beta radiation into light, which can be detected using photomultiplier tubes (PMTs). Unfortunately, there are various factors that may interfere with the conversion of decay energy emitted from the sample into light photons reaching the PMTs, which usually reduces counting efficiency. This process is called quenching and should be corrected. We usually encounter three major types of quenching:

• Photon quenching occurs with the incomplete transfer of beta particle energy to solvent molecules.

• Chemical (sometimes called “impurity”) quenching causes energy losses in the transfer from solvent to solute.

• Optical or color quenching causes the attenuation of photons produced in the solute. For example, plasma samples contain many different substances in variable amounts, so each sample may absorb light to a different extent.

We can estimate the quenching effect by applying external or internal standards or by using multiple energy channels.

As mentioned in Section 13.4.1, the organs have different sensitivities to radiation. The sensitivity of an organ to radiation is determined by the proliferation and dif­ferentiation of its cells and tissues. This means that cells with faster proliferation are more sensitive to radiation. Moreover, living organisms, in which the biological functions of the cells and tissues are strongly differentiated, are also more sensitive to radiation. These facts have important consequences, both in medical applications and in radiation protection. For example, the cells of tumors can be damaged by irradiation because their proliferation is faster than that of healthy cells and tissues. At the same time, children and pregnant women have to be protected against radia­tion very carefully.

The radiation exposure can be acute or present for a long period of time (i. e., chronic). Acute radiation exposure refers to the delivery of (usually high) doses of radiation in a short period of time (within days). Acute radiation exposure of human beings can occur through accidents, wars, criminal activity, or medical impacts. Some radiation exposures can be present constantly in the environment, such as background radiation and the continuous elevated doses of radiation in radioactive workplaces.

Radioactive irradiation can cause somatic and genetic effects. The somatic effects manifest themselves in the individual, while the genetic effects (mutations) are observed in their descendants. The most serious genetic effect is when the indivi­dual’s reproductive capacity is affected, and there are consequently no descendants.

The low and high doses have different biological impacts, which are called “sto­chastic” and “deterministic” effects, respectively. The term “stochastic” means a random effect that is only the probability of damage (e. g., the induction of cancer and genetic defects) that can be caused by a certain radiation exposure. Stochastic effects are usually related to exposures to low levels of radiation exposure over a long period of time. Stochastic effects have no threshold level of radiation exposure below which we can say with certainty that cancer or genetic effects will not occur.

Deterministic effects are related to much higher levels of radiation exposure, usually over a much shorter period of time than is the case for stochastic effects. The deterministic effects have a threshold radiation dose, below which the deter­ministic effects are not observed. However, above the threshold dose, the severity of the deterministic effect is proportional to the radiation dose.

The effect of high doses is illustrated in Figure 13.7. Some curves of Figure 13.7 have been obtained in animal tests. The curve for humans has been constructed from the data collected after accidents. As seen, the tumor frequency and absorbed dose are in well-determined correlations. Moreover, there are thresh­old doses below which no excess cancer cases have been reported.

In Table 13.6, the biological impacts of high radiation doses are listed in the case of acute and cronic radiation exposures. The data are based on the radiation effects observed in human populations following the nuclear explosions in Hiroshima and Nagasaki in 1945.

As seen in Table 13.6, both acute and chronic radiation exposures can cause radiation sickness. This sickness has a variety of symptoms, including the following:

• General symptoms: fainting, fatigue, weakness, nausea and vomiting, diarrhea, dehydra­tion, and hair loss.

• Cutaneous symptoms: inflammation of exposed areas (redness, tenderness, swelling, bleed­ing), bruising, skin burns (redness, blistering), open sores on the skin, and sloughing of skin.

• Mucosal symptoms: mouth ulcers, ulcers in the esophagus, stomach, or intestines, bleed­ing from the nose, mouth, gums, and rectum, vomiting blood, and bloody stool.

Low radiation exposure has stochastic effects. This means that only the proba­bility of effects can be provided. The effect—dose functions may show linear, sub­linear (with threshold dose), and supralinear curves (with effects increasing with the increase in the radiation dose). In addition, there are views that very small doses stimulate the repair activity of DNA, so the radiation may have a desirable impact. This process is called “hormesis,” and there are many disputes whether this effect even exists at all. In conclusion, we do not have exact information on the radiation effects, which, in fact, is the most important in life for most of us.

As mentioned in Section 13.4.2, the impact of the radiation is due to radical for­mation. The effects of low doses can hardly be estimated because there are many other factors that also produce radicals in living organisms. This means that the effects of low-level radiation cannot be separated from the effects of other environ­mental risks such as stress, carcinogens (tobacco smoke, nonradioactive species, etc.), aging, and individual physical conditions. As mentioned previously, living organisms have different ways to protect against radicals by natural radical scaven­gers, and they have various repair mechanisms for restoring the damages to the bio­logical molecules. If, however, this repair mechanism fails, different diseases appear. Since any radiation exposure has some risk of producing radicals, it should be avoided if possible. For this reason, the limits of radiation are determined by the “linear-no-threshold” hypothesis. This hypothesis is questioned from time to time; however, it provides a pragmatic means of estimating radiation risks and is consis­tent with the (limited) data that are available.

The standards of radiation protection control the receipt, possession, use, trans­fer, and disposal of radioactive material in such a manner that the total dose to an individual (including doses resulting from licensed and unlicensed radioactive material and from radiation sources other than background radiation) does not exceed the standards for protection against radiation prescribed in the regulations. However, nothing shall be construed as limiting actions that may be necessary to protect health and safety. The standards have three basic aspects:

1. It must be proved that the application of radioactive material results in more improve­ments for the community than the risk to health.

2. The risk has to be decreased as low as reasonably achievable (ALARA).

Table 13.6 The Biological Impact of High Radiation Doses

Acute Radiation Exposure

1000 Sv Death immediately

100 Sv Damage of the central nervous system; death within hours

10 Sv Damage of blood-forming tissues; death within days

1 Sv Radiation thickness

Chronic Radiation Exposure

0.01 Sv/nap Weakness after 3—6 months; death after 3—6 years

0.001 Sv/nap Radiation sickness, symptoms are observed after several years

3. There are dose limits that are strictly prohibited under any conditions. There are dose lim­its for individual members of the public and occupational dose limits.

The standards for individual members of the public say that the total effective dose equivalent to individual members of the public from the licensed operation does not exceed 1 mSv/year, exclusive of the dose contributions from background radiation, medical investigations, and occupational doses. The annual limit of the effective occupational dose is 100 mSv/5 years, but the 50 mSv/year is permitted only in a 5-year period. Therefore, the mean occupation dose limit is 20 mSv/year. Other dose limits are defined for any tissues including occupational and public dose limits. For example, the occupational dose limit for the lens of the eye is 150 mSv/year; for skin, hands, and feet, it is 500 mSv/year. These limits for the members of the public are the tenth part of the occupational dose limits; namely, 15 mSv/year for the lens of the eye, and 50 mSv/year for skin, hands, and feet. The occupational dose limits have been determined such that they should be similar to the risk of other occupational limits (e. g., the risk of death of bus drivers).

Medical applications of radiation have no dose limits. The basic principles of protection for medical exposures can be summarized as follows: medical exposures should be justified by weighing the diagnostic or therapeutic benefits they procure against the radiation detriment they might cause, taking into account the benefits and risks of available alternative techniques that do not involve radiation exposure. The doses from medical exposure should be the minimum necessary to achieve the

required diagnostic objective or the minimum required to the normal tissue for the required therapeutic objective. This principle is in accordance with the ALARA principle. In Figure 13.8, the mean effective dose of the patients is shown in several nuclear medical diagnostic methods.

In conclusion, we can say that background radiation has always been present during the history of humanity. Protection from excess radiation exposure is legally controlled. The standards are very strict for protecting the members of the public. The occupational dose limits, or course, must be higher, so that people working with radioactive material and radiation face a higher level of risk. However, safety regulations are to be followed strictly in order to minimize this risk.

Further Reading

Feher, I. and Deme, S. (2010). Sugarve’delem (Radiation Protection). ELTE Eotvos Kiado, Somos Kornyzetvedelmi Kft, Budapest.

United States Nuclear Regulatory Comissions, 2007. < http://www. nrc. gov/reading-rm/doc- collections/cfr/part020/ > (accessed 28.03.12.)

Szabo, S. A. (1993). Radioecology and Environmental Protection. Akademiai Kiado/Ellis Horwood, Budapest/New York, NY.

Valentin, J. (2006). The Scope of the Radiological Protection Regulation. Elsevier, Amsterdam, http://www. icrp. org/docs/Scope_of_rad_prot_draft_02_258_05v06.pdf (accessed 28.03.12.)