Nuclidic purity

Tc is available through the /l -decay of Mo (Fig. 2.1.B), which can be obtained by irradiation of natural molybdenum or enriched Mo with thermal neutrons in a nuclear reactor. The cross section of the reaction Mo(nih,v) Mo is 0.13 barn [1.5], Molybdenum trioxide, ammonium molybdate or molybdenum metal are used as targets. This so-called (n,7)-molybdenum-99 is obtained in high nuclidic purity. However, its specific activity amounts to only a few Ci per gram. In contrast, Mo with a specific activity of more than in Ci (3.7 10 MBq) per gram is obtainable by fission of with thermal neutrons in a fission yield of 6.1 atom % [16]. Natural or -enriched uranium, in the form of metal, uranium-aluminum alloys or uranium dioxide, is used for the fission. The isolation of Mo requires many separation steps, particularly for the separation of other fission products and transuranium elements that arc also produced. 

Nuclidic purity. The nuclidic purity is the fraction of the total activity due to the stated radioactive nuclide. Even another radioactive nuclide of the same element decreases the nuclidic purity and can disturb activity measurement. However, nuclidic purity turns out not to be essential if different nuclides can be distinguished in measurement as in y-ray spectrometry with... 

Commercial radioactive nuclides are usually available with nuclidic purities exceeding 98%. However, one should remember that the purity decreases gradually when the contaminating nuclide has a longer half-life than that of the stated one. For example, Hg (64.14 h) used for scan of kidney and spleen often contains " Hg (46.61 days) as an impurity. Even if the amount of the latter is 1 % at the time of assay, it will exceed that of the former in about 20 days. 

Isotope concentration While deuterium model reactions are often conducted with pure deuterium oxide as the isotope source, tritium oxide is rarely used at anything close to nuclidic purity (note that tritiated water at 50 Ci/mL, the highest specific activity normally available commercially, has a tritium/hydrogen ratio of only about 1.6/98.4). Therefore, the concentration of tritium in HHO is usually much lower than that of deuterium in HjO, and this difference will be important if the source concentration is a factor in the rate equation. Analogously, model exchange reactions with deuterium gas are often done at one atmosphere of pressure, whereas in most cases tritium gas is used at lower pressures. This can result in substantially slower tritium exchange rates. 

It will be recalled that is 100% abundant and is the heaviest stable nuclide of any element (p. 550), but it is essential to use very high purity Bi to prevent unwanted nuclear side-reactions which would contaminate the product Po in particular Sc, Ag, As, Sb and Te must be <0.1 ppm and Fe <10ppm. Polonium can be obtained directly in milligram amounts by fractional vacuum distillation from the metallic bismuth. Alternatively, it can be deposited spontaneously by electrochemical replacement onto the surface of a less electropositive metal... 

Measurement of specific activity. The half-life of a nuclide can be readily calculated if both the number of atoms and their rate of decay can be measured, i.e., if the activity A and the number of atoms of P can be measured, then X is known from A = XP. As instrumentation for both atom counting and decay counting has improved in recent decades, this approach has become the dominant method of assessing half-lives. Potential problems with this technique include the accurate and precise calibration of decay-counter efficiency and ensuring sufficient purity of the nuclide of interest. This technique provides the presently used half-lives for many nuclides, including those for the parents of the three decay chains, U, U (Jaffey et al. 1971), and Th. 

Calorimetry. Radioactive decay produces heat and the rate of heat production can be used to calculate half-life. If the heat production from a known quantity of a pure parent, P, is measured by calorimetry, and the energy released by each decay is also known, the half-life can be calculated in a manner similar to that of the specific activity approach. Calorimetry has been widely used to assess half-lives and works particularly well for pure a-emitters (Attree et al. 1962). As with the specific activity approach, calibration of the measurement technique and purity of the nuclide are the two biggest problems to overcome. Calorimetry provides the best estimates of the half lives of several U-series nuclides including Pa, Ra, Ac, and °Po (Holden 1990). 

Studies of short-lived radionuclide generators (4-6) do not adequately treat the quantitative problems of the daughter nuclide elution or those specific to their optimal clinical use. Two essential physical characteristics of a generator are the yield of the daughter nuclide and its radiochemical and radionuclidic purity. To realize the full potential of a short-lived radionuclide generator for medical studies requires that these two characteristics are optimized and are compatible with parameters important to clinical use such as total perfused volume and duration of the scintigraphic examination. 

An alternative means of radionuclide production employs neutron capture reactions in nonfissile nuclides. Again a high-purity target is used and a mixture of the unconverted target material, the... 

Examples of the Partitioning Goals (Recovery Yield, Product Purity) for Long-lived Nuclides... 

In order to determine the half-life, the decay scheme, and other nuclear characteristics of a radioactive nuclide, it is important to use a sample of very high radiochemical purity. In addition in the measurement of nuclear reaction cross sections, fission yields and in activation analysis, the amounts of the radioactive nuclide produced must be determined. Thus It Is also necessary to determine the yield of... 

All quality control procedures that are applied to nonradioactive pharmaceuticals are in principle applicable to radiopharmaceuticals. In addition, tests for radio-nuclidic and radiochemical purity must be carried out. Furthermore, since radiopharmaceuticals are short-lived products, methods used for quality control should... 

Radionuclidic Purity Radionuclidic purity is defined as the fraction of the total radioactivity in the form of the desired radionuclide present in a radiopharmaceutical. Radionuclide impurities may arise from impurities in the target material or from fission of heavy elements in the reactor [2], In radionuclide generator systems, the appearance of the parent nuclide in the daughter nuclide product is a radionuclidic impurity. In a "Mo/"mTc generator, "Mo may be found in the "mTc eluate due to breakthrough of "Mo on the aluminum column. The presence of these extraneous radionuclides increases the radiation dose to the patient and may also obscure the scintigraphic image. 

For production of uranium compounds suitable for use in nuclear reactors or for isotope separation, further chemical procedures are applied, as indicated in Fig. 11.9. Nuclear purity means that the compounds are free of nuclides with high neutron absorption cross section, i.e. free of boron, cadmium and rare-earth elements. Selective extraction procedures are most suitable for this purpose. Uranyl nitrate hexa-hydrate (U02(N03)2 6H2O UNH) is obtained by concentration of solutions of U02(N03)2, and ammonium diuranate ((NH4)2U207 ADU) by precipitation with ammonia. 

It will be recalled that Bi is 100% abundant and is the heaviest stable nuclide of any element (p. 550), but it is essential to use very high purity Bi to prevent unwanted nuclear side-reactions which would contaminate the product... 

Radiochemical purity. The radiochemical purity is the fraction of the stated radioactive nuclide present in the stated chemical form. For tracers of elements stabilized in two or more oxidation states, it is necessary to check their oxidation state by their chemical behavior, ion exchange for example, preferably just before the experiment. In organic compounds labeled with a radioactive nuclide, it is desirable that the number and position of labeling of the radioactive nuclide are unique. However, when the number and position of the nuclide in a compound do not essentially affect its chemical behavior as is often the case in tritium-labeled ones, use of a mixture of a compound labeled with different number of the stated nuclide or labeled at different positions is acceptable. The purity of some labeled compounds decreases gradually due to oxidation, self- or radiolytic decomposition during long storage. Such a labeled compound should be assayed and purified, if necessary, before use. 

If the purity of a radioactive tracer is guaranteed in ordinary single tracer work, no energy discrimination is necessary. However, when the y-ray spectrum consists of a main intense peak and other minor ones, use of single channel analyzer set at the main peak will considerably decrease the background of counting and also possible interference by impiuity nuclides. 

In selection of a radioactive nuclide as a tracer, attention must be paid to the following factors half-life, radiation emitted, specific activity, chemical form, and purity along with license of use and availability. In the case of a non-iso topic tracer, the element of the tracer must be chosen after due consideration as stated above. In any case, radiation safety and legal regulations have to be carefully followed throughout acquisition, storage, use, and disposal of a radioactive tracer. 

Another situation occurs as a result of an (n,y) reaction, in which an intermediate radionuclide decays to the product of interest. This route is followed to make for example, with the Xe(n, y) Xe process. The neutron capture product Xe beta decays to with a 16.9 h half-life. Because the final product can be chemically separated from the target, specific activity may approach the theoretical value for the pure radionuclide. Obviously, the use of high chemical purity targets and processing reagents is necessary to avoid introducing stable nuclides of the same element as the product. In the example, this means that both the... 

Solvent extraction and ion exchange are widely used to produce various radionuclides from irradiated targets. The common routine is that the target is dissolved in an aqueous phase, often a mineral acid, and then treated with a suitable extractant or ion exchanger to isolate the nuclide of interest. Several steps are often applied to obtain the desired radiochemical purity. 

The aim of the present improvement work on the PUREX process is to make the separations more selective and to create effluent streams of high purity. Thus, modifications are performed to make neptunium end up in a fraction for later transmutation in a reactor or accelerator-driven system. This can be achieved by a better control of redox conditions in the process. Today neptunium is partially co-exlracted with plutonium and uranium. There are also suggestions to withdraw product streams with Tc and respectively, i.e., long-lived nuclides that might be of interest for transmutation. 

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