In nuclear reactors, radioactive tracers can be produced by nuclear reactions with neutrons or by the reprocessing of spent fuel elements (fission products and trans­uranium elements). Fission products can also be obtained by the radiation of ura­nium as the target. In accelerators (cyclotrons or linear accelerators), the targets are irradiated with positively charged particles. The general characteristics of nuclear
reactions are discussed in Chapter 6. The most important radioactive tracers are shown separately in Section 8.6.

The preparation of radioactive tracers consists of two steps: the preparation of the isotope by nuclear reaction and the preparation of the desired compound by chemical processes. In most irradiation processes, the chemical species of the target and the product are different, that is, the chemical species needed for the applica­tion cannot always be produced directly. Two main processes are responsible for this. First, the radiolysis of the target can take place during irradiation, resulting in the change of the chemical species. Second, the other constituents of the target can also be transformed in nuclear reactions, and the product can contain other radioac­tive isotopes too. In addition, further reactions that may lead to the formation of undesired side products are the nuclear reactions of the other isotopes and chemical impurities of the target, and secondary nuclear reactions with the already produced radioactive isotopes.

The target has to be selected so that the quantity of the polluting product is kept at a minimum. For this reason, the target has to be a very pure substance and, if possi­ble, in elementary form. Oxides and carbonates are also suitable because the nuclear reactions of oxygen and carbon can be ignored, and the products are stable isotopes.

When the other nuclides of the irradiated element enter in nuclear reactions producing undesirable radioactive isotopes, the target has to be enriched after irradi­ation, that is, the concentration of the isotope has to be increased. For example, nat­ural silver consists of two isotopes: 107Ag and 109Ag. By irradiation of silver with neutrons, 108Ag and 110Ag isotopes are produced in a (n, Y) reaction. When only one of these isotopes is needed, silver isotopes can be separated by mass spectrometry.

Enriched targets are used when the concentration of the target nuclide is very low in the substance with a natural isotopic ratio. For example, 18F isotope is pro­duced from 18O by proton irradiation. In the case of enriched targets, the specific activity of the product nuclide also increases.

In some nuclear reactions, secondary nuclear reactions can also take place. For example, in the production of 125I, the subsequent nuclear reaction of 125I occurs: 125I(n, Y)126I. The effect of the secondary nuclear reactions can be limited by con­trolling the irradiation time or cooling the undesirable isotope if its half-life is shorter than that of the main product.

The desired radioactive isotopes can be separated by radiochemical methods (such as chromatography, ion exchange, distillation, sublimation, precipitation, and thermochromatography). The simpler the method is, the better.

As mentioned previously, the radioactive isotopes have to be manipulated fur­ther to obtain the chemical compounds needed for the specific application, which includes the production conditions (pH, redox potential, etc.), chemical reactions, and purification procedures.

It is important to remember during the production of the radioactive isotope that a carrier-free or just minimally carrier-containing isotope has a high level of spe­cific activity. Carrier-free isotopes can be produced in nuclear reactions in which the atomic number changes or the daughter nuclide of the product is also radioac­tive, and they can be separated from the parent nuclide produced by the nuclear

reaction. For example, the Szilard—Chalmers reaction can be used to produce cer­tain carrier-free radioactive isotopes. This method is based on the recoiling of the produced radioactive isotope, which leads to breaking its chemical bond. In this way, a new chemical compound is formed, and the target and the product, contain­ing different isotopes of the same elements, can be separated by chemical proce­dures because the radioactive and the inactive isotopes are in different chemical compounds. For example, in I(n, Y) I nuclear reactions, the iodine in the target

can be an organic compound or iodate, and the radioactive iodine is present as iodide ion. Bromine and chlorine isotopes have similar nuclear and Szilard—Chalmers reactions. In addition, the same reactions can be used for the inactive chromium, manganese, phosphorus, and arsenic isotopes in chromate, manganate, phosphate, and arsenate ions. No-carrier-added radioactive isotopes with high specific activity can be produced by nuclear reactions with high cross sections, especially when the half-life of the product is too short to allow irradia­tion for a suitably long time, that is, the maximum activity of the radioactive prod­uct can be approached (see Section 6.1 and Eqs. (6.9) and (6.11)).

As discussed in Section 6.2.1, the nuclear reactions with neutrons can easily be created in nuclear reactors. Radioactive isotopes can be produced by the irradiation of a target substance located on the irradiation channels of the nuclear reactors. The other possibility for the production of radionuclides in nuclear reactors is the reprocessing of spent fuel elements. Fission products and isotopes transuranium elements can be obtained in this way. The two methods can be combined: a target containing 235U isotope can be irradiated in the irradiation channels of the reactor, and then the radioactive isotopes can be separated from the target. This procedure is significant in the production of fission products with short half-lives.

As discussed in Section 7.3.2, the first step of reprocessing of spent fuel ele­ments (or the irradiated 235U) is the separation of transuranium elements, in most cases by extraction with tributyl phosphate, followed by subsequent chemical pro­cedures. The number of fission products is about 300, including the isotopes with longer half-lives. These fission products are the isotopes of many chemical ele­ments; therefore, the chemical procedure is usually complicated. At first, the chem­ically similar fission products are separated by such methods as extraction, ion exchange, and precipitation, and then the individual isotopes are separated from the groups of the chemically similar elements.

As an example of the separation of fission products, the separation of 140Ba is shown here. Lead nitrate solution is added to the solution of fission products, and then lead sulfate containing 140Ba(II) ions is precipitated with sulfuric acid (coprecipitation):

140Ba21 1 Pb(NO3)2 + H2SO4 ! (140BaPb)(SO4) 1 H2O (8.17)

The precipitate, polluted with 90Sr, is digested with KNaCO3 and dissolved in nitric acid. Then barium— lead carbonate is precipitated with ammonium carbonate and dissolved in nitric acid again. The precipitation with carbonate and dissolution with nitric acid is repeated until the radioactive purity of the precipitate becomes

high. When the desired purity is attained, concentrated hydrochloric acid is added to the solution at 0°C. Lead ions are precipitated as lead chloride, and barium ions remain in the solution. The residual lead ions are eliminated by electrolysis. By this method, carrier-free 140Ba isotopes are obtained.

Radioactive isotopes can be produced by irradiation with charged particles (as discussed in Section 6.2.3) in cyclotron (see Figure 8.7), or in linear accelerators (see Figure 8.8). This method is older than the nuclear reaction with neutrons in nuclear reactors. As discussed in Section 6.2.6, the heavier transuranium elements have been produced by irradiation with charged particles. During the isotope pro­duction in accelerators, the target becomes very hot; therefore, the cooling is very important, and even cryogens are applied, if required (see Figure 8.9). The require­ments to the target are the same as in the nuclear reactors.

Some radioactive isotopes are produced by spallation reactions too (see Section 7.3.2).

In Figure 6.7, the different possibilities that lead to the production of a nuclide with a Z atomic number and an A mass number are summarized, including the for­mation of the nuclide by radioactive decays. When selecting a method for isotope production, the general nuclear reactions, the requirements of the isotope in terms of purity and use, and the available techniques are to be taken into account.

9.1 The Thermodynamic Concept of Classification (Distribution of Radioactive and Stable Isotopes)

For radioactive indications, the most important factor that has to be considered is the distribution of the radioactive tracer (microcomponent) and the inactive carrier (macrocomponent). As mentioned in Section 8.2, the radioactive indicator has to be homogenously distributed in the studied system. In this chapter, the condition of the homogeneous mixing of the radioactive indicator and the inactive (stable) car­rier will be investigated via examining the change of the mixing entropy.

Let us assume two solutions with the same concentration C, each containing the macro — and microcomponent, respectively, as the same chemical species and have the same temperature. When the concentration C is the same, the dilution-free energy does not have to be taken into account; thus, the entropies of the two solu­tions are expressed as:

N and n are the atoms of the macro — and microcomponents, respectively; T is the temperature; R is the gas constant; and S0 is the absolute entropy. When the solu­tions are mixed, the entropy changes as follows:

Sn = n R ln T — R ln — C 1 sn

n n 1N n

Sn = N R ln T — R ln N C 1 SN n1 N

Nuclear and Radiochemistry. DOI: http://dx. doi. org/10.1016/B978-0-12-391430-9.00009-3

© 2012 Elsevier Inc. All rights reserved.

When mixing the solution, it will be diluted for both components. The dilution can qualitatively be expressed by the molar fractions of the micro — and macrocomponents:

where Xn and XN are the molar fractions. The partial molar concentration of the components is:

The change of the entropy as a result of mixing can be expressed in Eqs. (4.1)—(4.4) as follows:

ASelegy — (Sn + SN) — (Sn + Sn) — — nR ln Xn — NR ln Xn

By dividing both sides of Eq. (9.9) by the value of (n + N), the molar mixing entropy is obtained for mixing:

when the system consists of i components (besides two), the molar mixing entropy for i components is:

ASMegy —-R£Xi ln Xi

The end of the mixing process is mathematically reached when the primary differ­ential quotient of the entropy is equal to zero:

^(ASMegy) — 0 (9.12)

This extremum of mixing entropy can be calculated as follows. Let us divide the whole mixture to elementary volumes containing (An 1 AN) atoms and choose an arbitrary kth element, in which the molar ratio of the ith component is:

From Eqs. (9.11)—(9.13), we obtain the following:

Equation (9.12) can be solved for any ith component, assuming that the total quan­tity of this component remains the same:

E S(ni)k = 0 (9.17)

k

From Eq. (9.16), we obtain the following:

Equation (9.18) is solved by using the method of the Lagrange multiplier, i. e., each member of Eq. (9.16) is multiplied by an undetermined constant (a) and added to Eq. (9.18):

In Eq. (9.19), the coefficients of S(Ani)k are equal to zero, i. e.:

From here:

— e—(1+a) — constant

An + AN

Equation (9.21) expresses the well-known fact that when mixing equilibrium, the radioactive indicator is homogeneously distributed in the whole system.

Previously, the system has been divided into elementary volumes containing (An + AN) atoms. When the total number of the elementary volumes is r, the moles of the ith components (any of these components can be the radioactive indicator) in the whole system are:

r(Ani)k = щ (9.22)

The total number of atoms of all components in the whole system is: r( An 1 AN) = п 1N (9.23)

Therefore, the molar fractions are as follows:

(Ani)k = ni = X

An 1 AN n 1N i

Equation (9.24) expresses that the ratio of the micro — and macrocomponents is the same in any elementary volume as it is in the whole system. It also means that in this case, the mixing entropy is maximal.

Equation (9.12) and its solution, Eq. (9.24), are valid even if the radioactive indica­tor is distributed among different chemical species of the macrocomponents. However, the value of a has not been restricted, so the different chemical species may be characterized by different constants. Therefore, Xi can be different for chemical species. As an example, the iron ions and hemoglobin are mentioned (see Section 8.2). Since there are no exchanges between iron ions dissolved in water and the iron(II) ions within the hemoglobin, when a radioactive iron isotope is added, it will be mixed with the iron ions dissolved in water, but it will not exchange with the iron ions of hemoglobin. Therefore, the specific activity of the two species of iron will be different. Depending on the chemical bonds, the distribution of a radioactive iso­tope can be different within the same molecules. For example, a 14C-labeled side- chain can be bonded to a 14C-labeled benzene; the specific activities of the two carbon atoms may be different, depending on the specific activities of the reactants. In these cases, mixing entropy has local maxima for the different species and bonds. When the radioactive indicator is homogeneously distributed among the different chemical spe­cies and bonds (e. g., isotope exchange can take place), the mixing entropy has reached its absolute maximum. In this case, the subsequent effects cannot change the homoge­nous distribution, as shown by Hevesy’s experiments for the separation of 210Pb (RaD) from lead chloride (PbCl2) (discussed in Section 8.1).

The molar fraction, Xi, can decrease because of the decay of the radioactive iso­tope; this effect, however, can be taken into consideration easily using decay kinetics.

NAA, discovered by G. Hevesy and H. Levi in 1936, is an activation analytical method (see Section 10.2.1).

In NAA, the sample is irradiated with neutrons, initiating nuclear reactions. Having no charge, neutrons can be captured easily by the different atomic nuclei. All elements, except for helium, have isotope(s) reacting with neutrons (as detailed in Section 6.2.1) in an (n, Y) nuclear reaction. As a result of the nuclear reaction, radioactive isotopes are produced. This process is called “activation.” These iso­topes have one more neutron than the inactive target, so they will typically decom­pose by emitting negative beta particles. Because of the continuous spectra, beta particles are measured with difficulty, especially when the sample contains many components. The beta decay, however, is frequently followed by the emission of gamma photons with discrete energies. The energy of these gamma photons is char­acteristic of the target elements and suitable for qualitative analysis. The activity/ intensity of gamma photons provides quantitative analytical information. The gamma photons are the result of radioactive decay, and they are present after the irradiation.

Besides neutrons, the samples can be activated by charged particles too. Table 10.2 describes this method as CPAA. The basic concepts (the detection of the emitted photons) are similar to NAA.

As discussed in Section 5.5.2, neutrons are produced in neutron sources, genera­tors, nuclear reactors, or spallation neutron sources. Since there are only a few spallation neutron sources all over the world, the irradiation is generally achieved in nuclear reactors (or sometimes neutron generators).

The way of neutron production determines the flux (neutron sources < neutron generators < nuclear reactors) and energy of the neutrons. In neutron generators, fast neutrons with 14 MeV energy are formed, which induce (n, Y), (n, p), (n, a), (n,2n) nuclear reactions. As seen in Figure 6.4, the general tendency is that the cross section of nuclear reactions with neutrons is inversely proportional to the neutron energy. For this reason, thermal neutrons produced in nuclear reactors are the most important in the (n, Y) nuclear reactions of NAA. In addition, the flux of the neutrons is highest in the nuclear reactors. As a result, neutron activation stud­ies are usually performed in the research nuclear reactors.

The high range of gamma radiation provides the possibility of nondestructive NAA. In this method, the samples are analyzed directly by an instrumental technique called instrumental neutron activation analysis (INAA). Of course, the instrumental measurements can be supplemented by radiochemical separation, if required. This method is called radiochemical neutron activation analysis (RNAA).

During activation, the inactive nuclei transform to radioactive ones via nuclear reactions. This means that the specific activity of the sample increases, usually from zero to a certain value. The sensitivity of the method is determined by the number of the radioactive nuclei formed. The number of the radioactive nuclei is deter­mined by the kinetic law of activation, as discussed in Section 6.1. Equation (6.11) gives the number of radioactive nuclei formed from any stable nuclide at a certain nuclear reaction characterized by cross section, flux of the irradiating particles, irra­diation, and cooling time. If all these factors are known, and the activity can be measured with high accuracy, absolute measurements can also be done.

These absolute measurements, however, are usually limited by the lack of accu­rate knowledge of the cross sections, the flux at the position of the sample, and activity after irradiation and cooling. In practice, mostly relative measurements are made; the samples are compared to a standard with known concentration of the ele­ments expected in the sample. The standard is simultaneously irradiated with the sample under the same conditions (flux, time, and irradiation position). The gamma radiation of the sample and the standard are measured under the same conditions as well. Standard samples may be homemade or purchased: commercial multielement Standard Reference Materials (SRMs) are available.

The advantages of NAA are that small sample sizes (1—200 mg) are suitable for the simultaneous analysis of many elements with low detection limits. The detec­tion limits of the elements are shown in Table 10.4. The accuracy of NAA is about 5%, and the relative precision may be better than 0.1%. INAA does not destroy the samples; thus, it can be useful when the sample has to remain intact (archeological, artistic, criminal, and other samples).

Because of the very small size of the sample, the sampling and preparation of the sample is critical. The sample must characterize the average composition of the object to be analyzed. Any contamination must be avoided.

The method’s major limitation is that, although almost all elements have iso­topes that can participate in nuclear reactions with neutrons, the produced radioac­tive isotopes of some light elements have very short half-lives, so they cannot be analyzed in neutron activation. Similarly, some elements have small neutron cap­ture cross sections, which cause difficulties in the analysis, and there are other ana­lytical methods, which often provide better sensitivities.

The nuclear reactions of the different nuclides can interfere with each other, for example, because the nuclear reactions produce the same radioactive nuclide. The

activation of bronze or brass results in the following nuclear reactions: 64Zn (n, Y)65Zn, 64Zn(n, p)64Cu, and 63Cu(n, Y)64Cu. As seen, MCu is produced from both zinc and copper. These interferences, namely, all possible nuclear reactions in the sample, always must be taken into consideration.

Energy (keV)

The neutron activation spectrum of sodium is shown in Figure 10.7. As a result of the irradiation, a 23Na(n, Y)24Na nuclear reaction takes place, and the spectrum shows gamma lines characteristic of 24Na.

The term in vivo refers to measuring or imaging the distribution of a radiopharma­ceutical in a living organism. Such procedures have been known for a long time, but the most commonly used imaging device, the so-called gamma camera (see Section 12.4.1), was developed by Hal Anger in Berkeley in 1957. While the gamma camera detects single photons, the pair of photons emitted when a positron meets an electron (annihilation radiation, as discussed in Section 5.3.3) can also be imaged using a so-called PET (discussed further in Section 12.6).

12.1.3 Therapy with Unsealed Radioactive Preparations

If it is possible to deposit beta — or alpha-emitting radiopharmaceuticals into or close to the organ or tissue to be deactivated or destroyed, then this short-range radiation will affect only a few layers of cells, or, when evenly distributed in an organ, it will irradiate the targeted organ selectively. This procedure requires radiopharmaceuticals accumulating specifically in the target organ, and preferably nowhere else in the body.

Besides natural radioactive isotopes, artificial radioactive isotopes are present in

the environment. They originate from different anthropogenic activities:

1. Radioactive wastes of isotope laboratories, including the research, medical, and industrial laboratories.

2. Radioactive wastes of nuclear energy production and reprocessing technologies (see Section 7.3). From nuclear plants, radioactive isotopes can be introduced into the environment in accidents (Section 7.2) and by regular emissions (Section 7.1.1.1), of which the emission of gaseous radioactive isotopes (T, 14C, 85Kr, 133Xe, 135Xe, and I isotopes) is the most important. A nuclear reactor with 440 MW electric power produces 2.7 X 1019 Bq/year radioactivity. Some important fission products are shown in Figure 13.1. The fission products are present in nuclear waste, as discussed in Section 7.3 and shown in Figures 7.6 and 7.7.

In addition, there are some isotopes which emit beta particles with low energy (79Se 95Zr, 109Pd, and 135Cs). Their radioactivity is not very high; however, they have long half-lives, so they will be present in nuclear waste disposal and may remain in the environment for a long time.

3. The radioactive isotopes of nuclear bombs and the experimental nuclear explosions. As discussed in Section 7.5, the first nuclear explosions that affected the atmosphere were two US explosions in Japan (Hiroshima and Nagasaki in August 1945), a Soviet explo­sion (1949), a British explosion (1952), and a Chinese explosion (1964). When exploding a nuclear bomb equivalent to a 1000 ton traditional trinitro-toluol (TNT) bomb, 48.5 g of fission products is emitted into the atmosphere. This mass seems to be low; however, the radioactivity is extremely high (3.7 X 1021 Bq). A significant portion of the fission products has a short half-life, so the radioactivity decreases rapidly. After 24 h, it is 5.9 X 1016 Bq. This is still a very high level of radioactivity. The radioactive isotopes of the nuclear explosions have the following half-lives (see Figure 7.7): t1/2 < 1 day for 131 isotopes; 1 < t1/2 < 10 days for 117 isotopes; 10 < t1/2 < 30 days for 9 isotopes; 30 days < t1/2 < 1 year for 12 isotopes; 1 year < t1/2 < 10 years for 7 isotopes; 10 years < t1/2 < 100 years for 3 isotopes; t1/2 > 100 years for 10 isotopes.

The longtime pollution obviously originates from the isotopes with long half­lives. The most important polluting radioactive isotopes are 14C, 90Sr, 137Cs, 95Nb, 106Ru, 106Rh, 140Ba, 140La, 144Ce, 144Pr, and Pu. Before 1963, 1.2 X 1016 Bq (about 400 kg) of 239Pu were emitted into the atmosphere.

The nuclear wastes of isotope laboratories and nuclear energy production are treated and stored under very strictly checked conditions (as discussed in Sections

7.3 and 8.9). The radioactive products of the nuclear explosions, however, freely got into the environment. In 1963, the United States, the Soviet Union, and the United Kingdom signed the Limited Test Ban Treaty, pledging to refrain from test­ing nuclear weapons in the atmosphere, underwater, or in outer space. The treaty permitted underground tests. Many other nonnuclear nations have acceded to the Treaty; however, some countries, which possess nuclear weapons, have not. As a result of the Limited Test Ban Treaty, the radioactivity of the atmosphere
originating from the nuclear explosions has decreased, and that from nuclear energy production has increased. The radioactive pollution reached its maximum between 1961 and 1965. Additional significant radioactive pollution entered the environ­ment during the Chernobyl and Fukushima accidents (described in Section 7.2).

These isotopes can be obtained as fission products from reprocessing of the spent fuel elements. They are applied for medical purposes only in special cases because the products of reprocessing contain long-life fission products of the same ele­ments. The isotopes can be separated by mass spectrometry.

Sm-153: the production will be discussed in detail in Section 8.7.1.1.

The chemical reactions in which the reactants and products are chemically identical but have different isotopic composition are called “isotope exchange reactions” (see Section 3.1.5). This also means that there are no chemical changes and the

enthalpy of the reaction is zero, and therefore the reaction is directed by the change of entropy (AS) (AG = —TAS). The isotope exchange studies are classified on the basis of the phases present as homogeneous and heterogeneous isotope exchange studies.

For PIXE, typically protons are produced in small energy accelerators. Quadruple magnets focus the protons, and the sample to be analyzed is hit by this proton beam. The protons eject electrons from the K or L orbital of the atoms in the sam­ple. From here, the processes are the same as in XRF: the characteristic X-ray photons emitted by the sample are detected by semiconductor detectors (e. g., SiLi).

Using traditional SiLi PIXE detectors, the concentration of the elements that have an atomic number greater than 13 can be measured. The sensitivity of the K and L lines is the highest in the range of atomic number 20 < Z < 35 and 75 < Z < 85, respectively. The most important trace elements of the biological and geological systems are mostly in just these ranges. This gives the significance of PIXE studies. There are special SiLi detectors with an ultrathin window, which are applied for the measurement of elements from carbon to iron. In Figures 10.25 and 10.26, the spectrum of an aerosol is shown using two detectors: a traditional SiLi PIXE detector and a SiLi detector with an ultrathin window.

The method has 10_6—10_7g/g relative, and 10_9—10_12g absolute detection limits with 5—10% error. The sensitivity can be increased using a very thin proton beam (with a diameter measured in micrometers). The method is called micro — PIXE, and its absolute detection limit is 1015—1016 g. This very high sensitivity is the main advantage of PIXE compared to XRF.

The main field of PIXE applications is the study of atmospheric aerosols. Filtering through different pore-sized membranes, fractions of air with different particle sizes are collected. The quantity of each fraction is small; thus, the analysis requires very sensitive techniques. Using PIXE, the elementary composition of the different fractions is measured directly, positioning the membrane filter to the win­dow in front of the proton beam (Figure 10.27). The sample remains unchanged after the PIXE analysis, so it can be subjected to another analytical technique (gravimetry, microscopy, particle distribution analysis, or other spectroscopic

techniques), which is essential since main light elements (H, C, N, O) composing aerosol particles (soil, humus, etc.) cannot be analyzed by PIXE. Therefore, PIXE and chemical analysis of the light elements (e. g., PGAA and electron microprobe) can be combined.

If we encounter a photon with, for example, 80 keV energy, there is no way to tell whether it is an X-ray or a gamma photon because the names refer to the origin of the photon: X-rays always come from the electron shells, while gamma rays originate from the nucleus.

Since there is a wide energy range (up to ~ 100 keV) where both gamma — and characteristic X-rays can occur, the devices for detecting gamma rays are also capable of measuring X-rays of the same energy. So the gamma camera might also be called “electromagnetic camera”; for instance, when using thallium-201 for imaging, which decays with electron capture, actually we mainly detect its charac­teristic X-rays between 68 and 85 keV.

Note that the number of photons detected by a gamma camera and an X-ray device is different by several orders of magnitude. So a gamma camera would be blinded by a regular X-ray generator, while the radiation of a patient injected with 600 MBq of a Tc-99m-labeled radiopharmaceutical would be under the detection limit of a standard medical X-ray detector.

A common type of radiation detectors are the gas-filled tubes. These are cylindrical gas-filled capacitors where one electrode is the cylinder itself and the other elec­trode is a wire that is electrically insulated from the cylinder. Under the effect of ionizing radiation, the atoms or molecules of the gas are ionized, producing posi­tive ions and electrons. When direct potential is applied across the tube, the positive ions will migrate toward the negative electrode (anode), while the elec­trons toward the positive electrode (cathode) induce an electric current. Thus, the migration of ions to the electrodes results in an electric impulse that can be mea­sured, for example, by a simple electric circuit (see Section 14.4). The time needed for the ionization and migration to the electrodes determines the dead time, which is about 10_4 s. The formation and shape of the electric impulses and the regenera­tion of the detector are illustrated in Figure 14.2.

The ionization detectors can measure alpha and beta radiation. Since gamma radiation does not form ions directly, only the secondary electrons emitted in the scattering processes and in the photoelectric effect lead to ionization (Section 5.4), and the efficiency of the measurement of gamma radiation is low.

In Figure 14.3, the quantity of the ions that reaches the electrodes of the gas — filled tube is shown as a function of direct voltage. The function has characteristic ranges, of which some can be used for radiation detection. Range I is the range of

recombination. In this range, the voltage is too low to collect all the ions and elec­trons initiated by the radiation. Thus, this range is not used for the detection and measurement of radiation.

Voltage then increases to a threshold value where all ions and electrons can reach the electrodes. By continuing to increase the voltage, a plateau is reached where no additional ions and electrons are formed (Range II). This is the voltage range, where ionization chambers work. As seen in Figure 14.3, there is a big dif­ference in the numbers of ions produced by alpha and beta radiations. Thus, the ionization chambers can differentiate between alpha and beta particles. In the volt­age range of the ionization chambers, the number of the primary ions is propor­tional to the energy of the radiation; therefore, the energy of the radiation can be determined. However, the number of the primary ions is usually low, so the appli­cation of the ionization chambers is limited. Therefore, ionization chambers are used to determine the total activity; for example, in dosimeters and signals where different amplitudes are not discriminated, all impulses are counted together. In other words, the measurements are made in an integrated way.

By further continuing to increase the voltage in these detectors, the primary ions ionize additional gas atoms or molecules when flying toward the electrodes, which results in the formation of secondary ions. As a result, the number of the collected ions increases and the current increases by several orders of magnitude. This range (Range III) is the so-called proportional range. The detectors working in this range are called “proportional counters.” Since the number of the secondary ions is pro­portional to the number of the primary ions, and consequently to the energy of radi­ation, the proportional counters give some information on the energy of the radiation. By increasing the voltage, the proportionality becomes less clear. Similarly, the number of ions produced by the alpha and beta particles gets closer. This range is the semi-proportional range (Range IV) and is not appropriate for radiation detection.

Beyond the semi-proportional range, a new plateau is formed (Range V). This is the so-called Geiger—Muller range, the range of Geiger—Muller counters or tubes. As seen in Figure 14.3, the electric impulses are independent of both the type and the energy of the radiation. Therefore, the Geiger—Muller counters can measure the total activity or intensity of the alpha and beta radiation. Since the range of the alpha particles is short (as discussed in Section 5.2), the measurement of alpha particles needs very thin windows (<1.5 mg/cm2). If the window of the detector is thicker, only the beta particles are detected.

By an additional increase of voltage, continuous discharge is formed (Range VI), which can damage the detector.