As mentioned in the previous chapter, the half-life of the radioindicator and the duration of the studies have to be comparable. Both too short and too long half­lives have disadvantages. If the half-life is too short, the radioisotope can disinte­grate before finishing the investigations. If the half-life is too long, the measurable
activity demands a greater quantity of the radioactive nuclides and the impact of the radiation on the environment is higher than needed. So, the most suitable isotope has to be chosen within these options. The available radioactive isotopes depend on the elements in question. Most light elements have no radioac­tive isotopes, which could be useful in practical applications; the half-lives are too short. For example, the half-lives of the 6He, 8Li, and 8B isotopes are less than a second. Tracer studies with these elements can be performed by altering the natural isotope ratio and subsequent activation. Hydrogen is an exception because the half­life of 3H, tritium, is fairly long (12.28 years). Tritium emits weak beta radiation.

As the atomic numbers become higher, the choice of radioactive isotopes increases, which can be used well for radioactive indication. If more than one radioactive isotope is available, we can choose the most suitable one for the given study. For example, carbon has two radioactive isotopes: 11C with a half-life of 20.48 min and 14C with a half-life of 5730 years. 11C can be applied for medical applications, while 14C is suitable for the synthesis of organic substances or radio­carbon dating (as discussed in Section 4.3.6).

The shortest half-life for tracer studies is about 2—3 min. For example, adsorp­tion studies have been done with the 208Tl isotope, whose half-life is 3.1 min. As another example, the application of 15O (half-life is 122 s) in medical research is mentioned. Of course, these studies with such short-lived isotopes need careful pre­liminary training. In addition, the radioactive isotopes have to be produced in very high activities, which require radiation protection and automation. The studies can only be performed in a location where isotope production can take place.

If a radioactive isotope with a short half-life has a parent nuclide with a longer half-life, the daughter element can be separated repeatedly from the parent element. Such a system is called an isotope generator or “cow”; the separation operation is called “milking.” Generators can be produced from the parent—daughter nuclides of decay series, such as:

About 50 parent—daughter pairs can potentially be applied in isotope generators. The most important is the 99Mo—99mTc generator, which is widely used in medical applications (see Sections 8.7.1.4 and 12.2.6):

99 66 hours 99m 6 hours 99

99Mo 99mTc 99Tc

The suitable compound of 99Mo is adsorbed on a chromatographic column (e. g., aluminum oxide). Since the half-life of 99Mo is 66 h, the generator can be

Parent and daughter nuclides on absorbent

transported and used for several weeks. The short half-life of 99mTc is desirable for medical applications; this isotope can be eluted (milked) by using physiological sodium chloride solution from the chromatographic column, and after some chemi­cal preparations, it can be used in many ways in medical diagnostics. The scheme of the isotope generators is shown in Figure 8.4.

The radioactive indicators can be used in the most effective way when their specific activity (activity per mass) is fairly high. The specific activity of the carrier — free isotopes is the highest because the radioactive isotope is not diluted with the inactive isotope of the same element (no-carrier-added). Production of the carrier — free isotopes, however, is not always simple, and it is usually more expensive than the production of carrier-added isotopes. As seen in Section 6.2.1, the simplest and cheapest nuclear reactions, namely (n, Y) reactions, do not give carrier-free isotopes because only the number of neutrons changes in the nuclear reaction. The high spe­cific activity is also important in these reactions; however, the production of the iso­topes with long half-lives would require a long irradiation time, which is too expensive or even impossible to do. For example, the half-life of 36Cl is about 301,000 years; the irradiation time is obviously much shorter, so most of the target chlorine isotope remains inactive and is present as a carrier. If the presence of the car­rier is not allowed (because the specific activity is too low), carrier-free isotopes can be produced through nuclear reactions with charged particles in accelerators. These isotopes are usually more expensive. Nevertheless, the price can differ depending on

the half-lives of the isotopes produced. For example, the [2]Na isotope (whose half­life is 24 h) produced in nuclear reactors is much cheaper than the 22Na isotope (whose half-life is 2.6 years). The 24Na isotope can be used for several days, while 22Na can be applied for years. In conclusion, the overall quantities needed for a given period should be considered when deciding which isotope should be purchased.

As usual in chemistry, the radioindicators have to be sufficiently pure. In the case of radioactive isotopes, purity includes different terms: chemical, radioactive, and radiochemical purities can be defined.

Chemical purity is the same as in chemistry: the ratio of the chemical quantities in the number of particles, moles, or masses expressed in the usual concentration units (percent, ppm, etc.). Radioactive purity is measured by the amount of radia­tion. It represents the fraction of the radioactivity that comes from a given radionu­clide. Since the radioactivity depends on the number of radionuclides and the decay constants (Eq. (3.1)), the chemical and radioactive purities are usually differ­ent because of the different values of the decay constants. The radioactive purity can be different even for a given isotope mixture if the isotopes emit different parti­cles or electromagnetic radiation. In addition, the probability of the transitions can also be different, which has to be taken into account. This is illustrated by the example of a Pu— Am isotope mixture shown in Table 8.1.

Radiochemical purity shows what fraction of the radioactive isotope is in the compound defined by a certain chemical formula. For instance, sodium carbonate labeled with the 24Na isotope (24Na2CO3) can contain sodium hydroxide (24NaOH) as an impurity. The radiochemical purity is determined by the ratio of the two com­pounds. The term of the radiochemical purity is especially important in organic chemistry and in radiopharmacons, where impurities can be formed in the synthesis and by the radiolysis of the product. So, the radiochemical purity is determined mostly by thin-layer chromatography.

The potential use of radioactive indicators is strongly affected by the range of the radiation. Because of their short range, alpha emitters are applicable only for some special cases. An example is the use of the dominantly alpha particles emit­ting transuranium elements as radioactive indicators. The measurement of alpha activity needs special preparation of the samples, so only static investigations are possible. In addition, alpha emitters are also used for therapy in nuclear medicine.

Table 8.1 Chemical and Radioactive Purities of the Pu— Am Isotope Mixture. The
bold fonts show the purity values

239Pu 241Am

Chemical purity (m/m%)

Half-life (years)

Alpha particle/100 g

Radioactive purity for alpha particles (%)

Probability of gamma radiation with ca. 60 keV Radioactive purity for gamma radiation with ^60 keV (%)

In this case, the short range is desirable; the isotope loses its high radiation energy within a short distance (e. g., within a tumor) and does not damage the healthy tissues significantly.

The so-called weak beta emitters (namely, isotopes with low beta energy (e. g., 3H, 14C, 35S, 45Ca isotopes)) are essential in the biological and medical applica­tions. Because of the self-absorption of the weak beta radiation (see Section 5.3.5), special techniques are needed for the measurements (e. g., liquid scintillation spec­trometry, discussed in Section 14.2.1). Similar to alpha radiation, only static inves­tigations can be done in biological systems. In chemical systems, however, dynamic or in situ investigations are also possible under special conditions (see Section 9.3.6).

As radioactive indicators, the hard beta emitters (isotopes with high beta energy) and gamma radiators can be used easily. The application of the gamma radiation is especially advantageous because of the discrete energy of the gamma photons. The long range of these isotopes is also required for dynamic and in situ studies.

Dual or multiple radioactive indicator methods describe the use of two types of isotopes or isotopes with differing half-lives and/or differing energies of the radia­tion. When two radioactive isotopes of the same element is used, the isotopes with shorter half-lives results in higher activity, allowing the study of the fast process. The isotope with the longer half-life gives information on the same process on a longer timescale. An additional advantage of the application of the two isotopes of the same element is that less of it is needed; thus, the radiation dose is smaller, which is an important factor in some instances, such as in medical applications.

Exploiting the differences in the energy of the radiation is mostly feasible in the case of the gamma emitters, where the spectra are composed of characteristic peaks with discrete energy, which can be separated. The separation of the beta energies is problematic because of the continuous spectra (see Figure 4.10); thus, the activity measurements frequently have to be complemented by chemical separation.