Neutron irradiation of the 235U nuclide generates a wide variety of fission products, which are then stabilized through в_-decays or transformed to radionuclides with

Table 8.11 Preparation of 131I-Labeled Nal
Nuclear parameters	Half-life: 8 days. Decay mode and energy: [3_ (keV) 608 and Y (keV) 364.
Utilization	Its gamma radiation is used for diagnostics, and its [3_ radiation is used for therapy of thyroid abnormalities. In addition, several organic molecules (e. g., 131I-MIBG (meta-iodobenzylguanidine), fatty acids, and sodium-o-iodo-hippurate) can be labeled with 131I radionuclide through iodination.
Target material	Tellurium dioxide, 130TeO2 with natural isotope abundance (35%).
Target irradiation	In a research reactor with thermal neutrons, for some days.
Primary nuclear reaction	130Te(n, Y)131Te and 130Te(n, Y)131mTe! 131Te.
Decay of the generated radionuclide	131Te——- ! 131I
Nuclear reactions resulting in	120Te(n, Y)121Te, 124Te(n, Y)125Te, 126Te(n, Y)127Te,
contaminating nuclides	128Te(n, Y)12QTe.
Target processing	The target material is placed into a ceramic vessel and molten in an electric furnace at a temperature of 850°C.
Radiochemical separation	Iodine is separated from the irradiated molten target by dry distillation. Evaporated iodine is collected in diluted NaOH-containing absorbers. As iodine is stable in alkaline solution only; the pH in the absorbers is adjusted to 6—8.
Product purification	Distilled iodine is bubbled through sulfuric acid in a scrubber in order to deposit tellurium isotopes, contaminating isotopes generated from chemical contaminants (e. g., Cu, Fe, Ni, Ag, Se, and Pb), as well as to trap chemical contaminants. This purification is a part of the separation procedure (distillation).
Product finishing	Adjustment of radioactive concentration, dispensing to the ordered number of ampoules.
Other ways of production	a. Technologies of wet distillation, extraction, and chemical separation are also used. b. 131I can be extracted from fission products generated by uranium irradiation.

QQ 131 137 85

longer half-lives, suitable for various applications. The Mo, I, Cs, and Kr fission products have practical importance. Among them, QQMo is very significant because its daughter element, the QQmTc radionuclide, is appropriate for isotope generation (a “cow,” as discussed in Section 8.3), is the most important radioiso­tope of the medical diagnostics IAEA TECDOC-1065 (1QQQ).

Nuclear parameters	Half-life: 60 days. Decay mode and energy: EC (100%), X-ray (keV) 28, and y (keV) 35.
Utilization	General radioactive tracer. Mainly used for labeling RIAs for in vitro investigations, that is, for testing samples extracted from the living body (blood, urine, etc.).
Target material	Xenon gas with natural isotope abundance (0.096%) or enriched xenon gas (99.9%) encapsulated in a pressure — proof aluminum capsule.
Target irradiation	In a research reactor with thermal neutrons, for some
Primary nuclear reaction Decay of the generated radionuclide Nuclear reactions resulting in contaminating nuclides	days. 124Xe(n, Y)125Xe. 125Te——! 125I a. Subsequent activation of the generated 125I isotope through the nuclear reaction 125I(n, Y)126I. Reduction of the rate of 126I nuclide below 0.1% by cooling.
Radiochemical separation	b. Nuclear reactions resulting in contaminating products: 126Xe(n, Y)127Xe, 128Xe(n, Y)129Xe, 130Xe(n, Y)131Xe, 132Xe(n, Y)133Xe, 134Xe(n, Y)135Xe, and 136Xe(n, Y)137Xe After decay, the 125I radionuclide is adsorbed on the inside surface of the aluminum capsule. Xe isotopes are eliminated from the opened capsule by blowing them away. The aluminum capsule is then heated in an electric oven for the dry distillation of the iodine. Iodine vapors are absorbed in diluted sodium hydroxide. Because iodine is stable in alkaline solution only, the pH is adjusted to 8—10.
Product purification	Distilled iodine is bubbled through a scrubber that first has been filled with concentrated sulfuric acid to remove chemical contaminants.
Product finishing	Adjustment of radioactive concentration, dispensing to the ordered number of ampoules.
Other ways of production	a. By periodic tapping, the loop built into the reactor zone in which Xe gas is circulated and irradiated. b. In a cyclotron, through nuclear reactions 125Te(p, n)125I, 124Te(d, n)125I, and 125Te(d,2n)125I with a lower yield.

The common feature of this group is that individual fission products can be extracted from the mixture with multistep separation methods. They are noncarrier added radionuclides, but they contain many potentially contaminating radionu­clides; thus, sophisticated purification is required to obtain sufficiently pure isotopes.

Half-life: 66 h.

Decay mode and energy: [3_ (keV) 1214 and y (keV) 740.

For the preparation of 99Mo/99mTc isotope generator

Enriched uranium containing 45% 235U in aluminum alloy.

In high-flux research reactors, with thermal neutrons, for some days.

A 235U(n, f) mixture of fission products (99Mo content 6%).

Dissolution of the target in NaOH with addition of oxidizing agent (e. g., H2O2), in order to assist dissolution and to provide an oxidized medium.

When dissolving the target in alkaline, beside 99Mo, only a few other elements will be found in the solution. These contaminating elements are separated in two ion — exchange columns and in another column filled with a chelating agent. 99Mo is eluted from the third column; the eluent is evaporated to dry and dissolved again in NaOH. To lower the reduction effect of radiolysis, an oxidizing agent (hypochlorite) is added to the solution.

In alkaline solution, the chemical species is sodium molybdate (Na299MoO4). This solution with very high activity is transported in depleted uranium containers that have a much higher attenuation coefficient against radiation than lead containers.

With a nuclear reaction of 98Mo(n, y), 99Mo can also be obtained, but specific activity is much lower.

The production of the 99Mo radionuclide in fission nuclear reaction is followed by subsequent (3_ — decays:

(8.24)

The activity of 99Mo represents only approximately 6% of the fission mixture. Its extraction from the isotope mixture and its production as sodium molybdate is carried out in steps described in Table 8.13.

Considering the fact that the portion of the 99Mo in the mixture of fission pro­ducts is low (6%), producers supply several users, and users apply several orders of magnitude of TBq 99Mo activity for generator production; the total activity pro­duced is extremely high. Moreover, the 99Mo radionuclide has high gamma energy; thus, the production of 99Mo requires hot cells with very thick (40- 50 cm) lead shielding and results in a huge amount of radioactive waste.

Recently, five countries (Canada, France, Belgium, the Netherlands, and South Africa) produced 99Mo in good quantity, which satisfies these high technical requirements and from which users (99Mo/99mTc generator producers) are supplied.

The extracted 99Mo is used for preparing 99Mo/99mTc radionuclide generators. The principle of isotope generators is described in Section 8.3. Because the half­lives of the parent (99Mo) and daughter (99mTc) nuclides are within the same orders of magnitude, transient equilibrium will develop (see Section 4.1.6). The activity of the system will be directed by the parent nuclide decay. The daughter nuclide is separated from its parent on the place of use. As this separation is made not in the generator production facility but at the user (e. g., in hospitals), it is important that the separation should be simple. From this aspect, chromatographic separation— where the daughter nuclide is eluted from its parent fixed on a chromatographic column—is the most beneficial.

Although many types of isotope generators exist, only some of them have practi­cal importance; e. g., 188W/188Re, 90Sr/90Y, 113Sn/113mIn, 82Sr/82mRb, and 99Mo/99mTc for medical use, and 137Cs/137mBa for industrial use. Among them, the 99Mo/99mTc isotope generator has the highest significance because it provides the most important radionuclide (99mTc) used for isotope diagnostics in nuclear medicine. As much as 80% of the isotope diagnostic investigations in medicine are made with this radionuclide; for this reason, this system is described in detail next.

The dominant role of the 99Mo/99mTc isotope generator can be attributed not only to the ideal nuclear characteristics of the 99mTc daughter nuclide (with gamma energy, which can penetrate through body tissues and with an ideal half-life), but also to its desirable chemical properties. The technetium as transition metal can be bound with several organ-specific complexes, which transfer the labeling isotope to the intended organ. Consequently, the same radionuclide with its beneficial nuclear features is suitable for investigating various organs by varying the linked chemical chelating agent.

In addition to these benefits, the 99mTc radionuclide can be easily extracted (eluted) from the 99Mo/99mTc isotope generator on site if this isotope mixture is adsorbed on a chromatographic column. The chromatographic separation is based on the different retention coefficient of the two radionuclides. Molybdate ions form so-called oligomer aggregates in acidic solutions, so they adsorb stronger in the alumina column than single pertechnetate ions. This binding is strengthened further by higher charges of the molybdate ions. The chemical composition of molybdate and pertechnetate ions present in various pH ranges is summarized in Table 8.14.

The best chromatographic separation is achieved when both the 99Mo parent and the 99mTc daughter nuclides are maintained in the highest oxidation state. This means that the isotope mixture must be maintained under strongly oxidized condi­tions during the whole production.

However, the high activity of the isotope mixture and the resulting dose cause permanent radiolysis (chemical decomposition caused by radiation effect) in the water solution, causing chemical reduction. As a result, the radiation of the isotope mixture has a countereffect against maintaining an oxidizing medium and a good separation because oligomers cannot be separated in lower oxidation states.

220		Nuclear and Radiochemistry
Table 8.14 The Chemical Composition of Molybdate and Pertechnetate Ions in Various pH Ranges
pH Range	Chemical Species	Features
Mo-99 Parent Nuclide pH < 1.5 1.5 < pH < 3 3 < pH < 6 pH > 6	[Mo (VI) OJ2+ [Mo8 (VI) O26]4" [Mo7 (VI) O24]6" [Mo (VI) O4]2"	Single molybdate cations Octamolybdate anions Heptamolybdate anions Single molybdate anions
Tc-99m Daughter Nuclide 1.5 < pH < 3	[Tc (VII) O4]-	Single pertechnetate anions

To maintain a permanent oxidized state with suitable redox potential in the solu­tion, an oxidizing agent, such as hydrogen peroxide and bubbled air, must be added to the isotope mixture.

Based on these considerations, the production of the 99Mo/99mTc isotope genera­tor from fission molybdenum (Na299MoO4) is carried out through steps described in Table 8.15.

The total activity of 99Mo in the generator production plants is in the TBq range at production time. Activities dispensed to individual generators are in the range of 37—370 GBq. Activity to be dispensed to the generator column can be chosen by the user when ordering the generator. As the dose rate constant of the parent nuclide is high, processing the total activity needs hot cells with a lead wall thick­ness of 15 cm, while shielding of the generators requires a lead pot with a lead wall thickness of 5 cm.

The half-life of the daughter nuclide (99mTc) is relatively short (6 h), so it will decompose shortly after the diagnostic investigation. At the same time, the longer half-life (66 h) of the parent nuclide provides comfortable access for the daughter nuclide at least for a week. In practical terms, this means that a hospital laboratory can perform around 100 diagnostic investigations with the daughter nuclide by ordering fresh isotope generator every week and by eluting it once a day.

The principal operation scheme of the 99Mo/99mTc isotope generator is demon­strated in Figure 8.4. The two tubes of the chromatographic column end in injection needles for which the elution agent solution and a vacuumed ampoule are attached by piercing. A vacuum will suck the eluent through the column, which desorbs the generated 99mTc daughter nuclide from the column.

The activity versus time functions of the 99Mo parent nuclide decay and of the 99Tc daughter nuclide generation are shown in Figure 8.11. The process leads to transient equilibrium. It should be noted that the daughter activity reaches its maxi­mum at 24 h, which coincides with the hospital practice, e. g., the elution of the generator once a day guarantees the daily maximum activity for the elution.

Figure 8.11 shows that daughter activity theoretically exceeds parent activity at transient equilibrium (dotted line). However, due to the branching decay of 99Mo (the value of branching factor = 0.96), the activity of the 99mTc daughter nuclide

remains systematically below the parent activity (continuous line). So, approximately 90% of the parent activity can be eluted as 99mTc in equilibrium (Figure 8.12). The activity curve corresponding to the hospital practice—daughter elution once a day—is demonstrated in Figure 8.13. 99Mo/99Tc radionuclide generators can be used economically for a week, with daily activities decreasing according to the parent decay.

Mo-99 Tc-99m calculated ———- Tc-99m experimental Figure 8.11 Activity versus time curves of 99Mo parent and 99mTc daughter nuclides.

99m^c

Half-life 6 h Half-life 2 x 105 years Figure 8.12 The decay scheme of a 99Mo radionuclide.

While 99mTc/99Mo generators are used for diagnostic purposes, another important generator (produced by high-flux reactor irradiation) is the 188W/188Re generator, which also serves a diagnostic purpose:

Irradiation: 186W(n, Y)187W(n, y)188W! 188W/188 Regenerator