**2. Why is harder to predict radioactive chemical reactants**

To answer the question, the manufacturing route for radioisotopes must be explained. Isotopes can basically be manufactured in 3 different ways (**Figure 1**).

In **Figure 1**, presented in pink, is the nuclear activation form. A thorough study of irradiation must be performed followed by purification essays. An example for promethium-147, used in pacemaker nuclear batteries, by Broderick et al. [3] is shown. Presented in green in the natural decay mode. Radioisotopes are obtained by purification of decay products or concentrated from natural radioactive isotopes. Examples are the Marie and Pierre Curie work in discovering Polonium and Radium and the Uranium-235 enrichment process. In yellow, the spent nuclear fuel reprocessing route is presented. If the isotope is a by-product of fission, then it can be recovered from the nuclear fuel that has already been used. Many isotopes are only found/fabricated through reprocessing. This process is known to be the most dangerous chemical process invented in human history [12]. The chemical process is relatively simple, but the byproducts are sometimes instable, highly radioactive, long lasting, even being able to reach criticality (nuclear chain reaction) [18]. For example, strontium-90 used in nuclear medicine and in nuclear batteries and plutonium-238 used in most RTG (Radioisotope Thermoelectric Generator) are

#### **Figure 1.**

*Modes of radioisotope production: Nuclear activation [3, 4], natural decay [5–11] and spend fuel reprocessing [12–17].*

#### *Start Here When Performing Radiochemical Reactions DOI: http://dx.doi.org/10.5772/intechopen.98766*


**Figure 2.** *Iodine-125 fact sheet.*

produced by reprocessing. Besides that, many stable fission products are high cost and in high demand elements such as rubidium, palladium, and ruthenium.

Performing chemical analysis that would allow to identify contaminant most of the times are impossible. Isotopes with high half-life, may contaminate and permanently disable equipments. For example, Tritium, used in nuclear batteries, have 12.32-year half-life. It means that in 12.32 years the Tritium mass will emits half of its radioactive material. If an equipment such as an EDS would be used, even supposing that one half-life would yield background level radioactivity, the equipment would be contaminated and unusable for 12 years! In large productions centers, each isotope has its own production line containing exclusive equipment for analysis.

Besides that, a radiochemical fact sheet contains very little information. **Figure 2** shows an example of iodine-125, used in radiation therapy.

The issue of not having more information arises when the following comparison is done. For example, in a thyroid cancer treatment with iodine-131, the activity of 5.55–7.40 GBq (150–200 mCi) is administered. Converting:

*Radiopharmaceuticals - Current Research for Better Diagnosis and Therapy*

$$\lambda = \frac{\ln 2}{T\_{\frac{1}{2}}} = \frac{\ln 2}{8.02d \ge 24h \ge 3600} = 10^{-6} \text{s}^{-1} \tag{1}$$

$$A = \lambda N \to N = \frac{A}{\lambda} = \frac{7.40 \times 10^9}{10^{-6}} = 7.40 \times 10^{15} atoms \tag{2}$$

$$\text{mass}\_{sample}(\text{g}) = \frac{\text{MW} \ge N}{N\_a} = \frac{131 \ge 7.40 \ge 10^{15}}{6.02 \ge 10^{23}} = 1.61.10^{-6} \,\text{g} \tag{3}$$

were: *T*<sup>1</sup> <sup>2</sup> ¼ Half-life (s), A = Activity in Bq (decays/second), *λ* ¼ decay constant (s�<sup>1</sup> ), *MW* ¼ atomic mass, *Na* ¼ Avogadro Number ≈ 6.02 x 1023.

The small mass calculated indicate that great chemical purity must exist in the entire course of a methodology/product development. For example, 1% impurity in 1 g of solution results in 0.01 g, an amount that is probably much greater than the total radioactive iodine. If the manufacturer changes significantly, for example, purification steps, new contaminants might be introduced and old expected results might not be achievable.

Ultimately, the best way to achieve the best results in radiation chemistry is to understand how reactions take place, and to recognize the various factors that influence their course.
