Neutron generators, target materials

Even so, demand for them is increasing year by year and in some cases quite spectactularly. As an example, both thulium and erbium are used as target materials in some sealed-tube neutron generators and erbium is also used in other generators designed specifically for use in cancer therapy. 

Spallation occurs when a high-energy cosmic ray breaks a target nucleus into two or more pieces. These interactions commonly eject neutrons. The secondary neutrons slow down to thermal energies and eventually react with other nuclei in the target material to generate heavier species. Production of cosmogenic nuclides by secondary neutrons increases with depth to a peak at between 0.5 and 1 m below the surface. Therefore, in order to get an... 

Three basic approaches may be used to generate man-made radionuclides, two of which involve neutron irradiation of suitable target materials in a nuclear reactor. In one case this is to... 

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. " ... 

Concerning the generation of the radionuclides, the amount produced is determined by the amount of activable substances present in the neutron field, the abundance of the parent nuclide in the target material, the cross section for the respective nuclear reaction, the neutron flux at the position of the substance and, finally, by its residence time in the neutron field. Short-lived radionuclides will reach their... 

The extremely large hydrogen density in combination with the high thermal stability of several hydrides makes these materials suitable for applications as moderators in nuclear fission reactors (Keinert, 1971 Mueller et al., 1968). A small-scale application of metal hydrides is found in neutron generators. Here a thin layer of the metal hydride saturated with deuterium or tritium is used as a target for accelerated D and T ions, where the DT reaction leads to a high neutron flux (Reifenschweiler, 1972). 

Fig. 7.4. The proton beam strikes the lead target generating neutrons which are moderated in the surrounding heavy water blanket. Molten salt carrying fissile material for heat generation and electric power production circulates in the heavy water blanket through double-walled pipes. Some of this power drives the accelerator. Nuclear waste including that produced in the molten salt is also circulated through the blanket in a separate loop and transmuted to stable and short-lived nuclides which are extracted and...

Plutonium is made in nuclear reactors by the above reaction where is the target nucleus. Although there are other reactions using different combinations of particles in Equation 2, in most cases these require energetic bombarding particles generated in accelerators. Also, since there are no common radioisotopes that generate neutrons, there is essentially no probability that other materials in laboratories will be made radioactive by exposure to radiation from byproduct materials. 

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