Reactor-produced radionuclides

Reactors are sources of neutrons, and thus most reactor-produced radionuclides are neutron-rich ft emitters. Reactor-produced radionuclides are of relatively low specific activity if the target nucleus is the same element as the product radionuclide, because the target and the product cannot then be chemically separated. 

Rh is a ft emitting radionuclide suitable for therapeutic applications. It has a 35.4-h half-life and emits 0.566 MeV and 0.248 MeV ft particles and a 319 keV gamma photon. It is a reactor-produced radionuclide that is also potentially available from the separation of fission products in... 

Radionuclides produced in nuclear reactors are of minor interest, because they exhibit fi decay. Furthermore, reactor-produced radionuclides with half-lives <10h are hardly applicable, because of the time needed for transport from a nuclear reactor to the site of application. 

An example of a reactor-produced radionuclide is l, which has been used many years for diagnosis of thyroid function. The y rays of this radionuclide are easily measurable from outside, but the radiation dose transmitted to the patients by the particles is not negligible, and the half-life (8.02 d) is relatively long. Besides, excretion of l may cause contamination problems. 

In June, 1946, the American Atomic Energy Commission announced in the journal Science that they would provide cyclotron- and nudear reactor-produced radionuclides for scientific research to qualified persons throughout the world. The first shipment of a C-14 tracer was to the Bernard Skin and Cancer Hospital in St Louis. 

Table 38.1 lists some examples of reactor-produced radionuclides important to nuclear medicine. There are three general reaction types used neutron radiative capture (n,y) neutron capture followed by particle emission, e.g., (n,n ), (n,p), and (n,a) and fission (n, f). The radionuclides in Table 38.1 are arranged in order of mass numbers. 

Examples of reactor-produced radionuclides of current interest to nuclear medicine... 

A list of reactor-produced radionuclides of current interest to nuclear medicine is given in O Table 38.1 along with modes of production and corresponding nuclear cross sections. From this list, the production and a brief description of chemical processing of the following radionuclides will be discussed ... 

Radionuclides find applications in many fields. Their major use, however, is in medicine, in both diagnosis and therapy. The production of radionuclides is carried out using nuclear reactors as well as cyclotrons. The reactor produced radionuclides are generally neutron excess nuclides. They mostly decay by P emission. The cyclotron produced radionuclides, on the other hand, are often neutron deficient and decay mainly by EC or emission. They are especially suitable for diagnostic studies. The reactor production of radionuclides is described in Chap. 38 of this Volume this chapter treats radionuclide production with cyclotrons. It is worth pointing out that today more than 300 cyclotrons exist worldwide (cf. Directory of Cyclotrons, lAEA-DCRP/CD, 2004), many of them in hospitals they produce short-lived radionuclides for medical use. Thus, radionuclide production science and technology at cyclotrons has become a very important feature of modern nuclear medicine. 

The Mo/ Tc generator consists of an alumina column in which Mo (a fission- or reactor-produced radionuclide) is adsorbed as Mo-molybdate. Mo decays 87% to " Tc, and a Tc-pertechnetate solution is easily obtained by eluting the column with sterile sahne solution. Tc-radiopharmaceuticals are then synthesized by adding the generator eluate to one of the many commercially available radiopharmaceutical kits. Because the parent radionuclide decays to " Tc with a half-life of 66 h, the Mo/ Tc generator can be used daily for up to 1 week. 

The fissioning of U and Pu in a nuclear reactor produces a large number of radioactive fission products. Most of these decay to stable isotopes within a few minutes to a few years after the fuel has been discharged from the reactor and therefore pose no problem in the management of nuclear fuel wastes. There are, however, a number of longer lived radionuclides that must be considered in assessing the environmental impact of any nuclear fuel waste disposal vault in the geosphere. 

The spectrum of radionuclides available for application in the life sciences broadened appreciably with the invention of the cyclotron by Lawrence in 1930 and the possibility of producing radionuclides on a large scale in nuclear reactors in the late 1940s. By application of T and C, important biochemical processes, such as photosynthesis in plants, could be elucidated. 

The third means of radionuclide production involves target irradiation by ions accelerated in a cyclotron. One example of this approach is provided by the production of Ge, which decays with a 280 day half-life to the positron emitter Ga. Proton irradiation of Ga produces Ge in a (p,2n) reaction. After dissolution of the target material a solution of the Ge product in concentrated HCl is prepared and adsorbed on an alumina column which has been pre-equilibrated with 0.005 M EDTA (ethylenediaminetetraacetate) solution. The Ga daughter may then be eluted using an EDTA solution in a system which provides the basis of a Ga generator. Cyclotron production of radionuclides is expensive compared with reactor irradiations, but higher specific activities are possible than with the neutron capture process. Also, radionuclides with particularly useful properties, and which cannot be obtained from a reactor, may be prepared by cyclotron irradiation. In one example, cyclotron produced Fe, a positron emitter, may be used for bone marrow imaging while reactor produced Fe, a /3-emitter, is unsuitable. " ... 

Reactor produced beta emitting radionuclides such as Lu, and Am... 

Radionuclides of high specific activity are produced either through accelerator irradiation or through secondary reactions in the target ( 15.6) in a reactor. Maximum specific activity is obtained when the radioactive nuclide is the only isotope of the element. This is not possible to achieve in regular reactor irradiation through (n,y) capture processes. For example, reactor-produced Na may be obtained in specific activities of 2 X 10 Bq g while the specific activity of accelerator-produced Na may exceed 10 Bq g however, the total activities available are usually the inverse. 

The expanded use of the cyclotron in the late 1930s and of the nuclear reactor in the early 1940s made available a variety of radionuclides for potential applications in medicine. The field of nuclear medicine was founded with reactor-produced radioiodine for the diagnosis of thyroid dysfunction. Soon, other radioactive tracers. 

The longer half-lives of these reactor-produced tracers facilitated their availability and decreased their cost compared to cyclotron-produced radionuclides. Carbon-14 and tritium emitted oifly beta particles with a very short range in tissue, so they could not be used to examine regional biochemistry with radiation detectors pointed at the human body. Three decades were to elapse before physicians and scientists returned to the cyclotron as an important, indeed essential, tool in molecular imaging. 

Thereafter, cosmic rays were observed and explained, and many cosmic-ray-produced radionuclides were identified. The number of known and characterized radionuclides increased dramatically with the development and application of nuclear-particle accelerators in the 1930s and nuclear-fission reactors in the 1940s. 

For a general and detailed discussion of activation kinetics see O Chap. 38, Vol. 4, on the Reactor-Produced Medical Radionuclides. ... 

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