Irradiation by neutrons

Principles and Characteristics In neutron activation analysis (NAA) the sample is irradiated by neutrons. The principal reaction in NAA is ... 

The following procedure can be used for the separation of " Tc from molybdate irradiated by neutrons ... 

For the extraction of Tc from molybdemun irradiated by neutrons or separated from uranium fission products, inorganic sorbents, especially aliuninum oxide have widely been applied. In preparing a Tc generator from irradiated molybdenum , MoOj is dissolved in cone, nitric acid, the solution is diluted and passed through an aluminum oxide column. The column is then eluted by 0.2 N H2SO4 to extract Tc. If molybdenum is adsorbed by AljOj as molybdatophos-phate instead of molybdate, the exchange capacity of molybdenum increases from... 

Finally, concerning irradiations by neutrons and X-rays, the two cases must be distinguished. The neutrons create geometrical defects, displacing nuclei as you have suggested in the case of MgO, eventually even changing the chemical nature (i.e., realizing the impurity situation I mentioned earlier). 

An important reaction used quite widely for this purpose is irradiation by neutrons and measurement of die energies of radiations emitted. The source of the neutrons may be a nuclear reactor, a particle accelerator, or an isotopic source, that is, a sealed container in which neutrons are produced by alpha rays emitted by a source such as radium, sodium-24(24Na), yttrium-88f8sY), etc., and arranged so that the alpha rays react-with a substance such as beryllium which in turn emits neutrons. The neutrons react with stable nuclides in the sample to produce radioactive ones. Thus... 

Scientists in the United States and elsewhere quickly confirmed the idea of uranium fission, using other experimental procedures. For example, a cloud chamber is a device in which vapor trails of moving nuclear particles can be seen and photographed. In one experiment, a thin sheet of uranium was placed inside a cloud chamber. When it was irradiated by neutrons, photographs showed a pair of tracks going in opposite directions from a common starting point in the uranium. Clearly, a nucleus had been photographed in the act of fission. 

Curium, berkelium, californium and einsteinium were separated from the americium samples irradiated by neutrons. For preliminary separation the anion exchange in hydrochloric acid and lithium chloride solutions was used as well as the HDEHP extraction. Mutual separation of the transamericium elements was made by using DIAION CK08Y cation exchange resin. Nuclides prepared and separation methods adopted are summarized in Table 1 (1-15). 

When uranium is irradiated by neutrons, in addition to neutron capture followed by fission (n,f), several diffeimit processes occur scattering, (n,y), (n,2n) reactions, etc. All these reactions are of inqxirtance for the reactor designer as well as for the chemists who have to manufacture new fuel pellets and take care of the spent fuel elemmts. The probability for the various reactions dqi ds on the neutron energy. As is seoi in Figure 19.3, three regions are clearly distinguishable ... 

Figure 17.9 Gamma ray spectrum of a kombu sample irradiated by neutrons.

The are a number of different procedures, the most popular of which is termed the Ko method (e.g., Negi et al. 1997). Small samples of sediment (e. 0.15 g) are irradiated by neutrons from a nuclear reactor for a few tens of hours. This aetivates the nuelei of atoms in the sample, leading to gamma emission which can be measured using a Ge detector. The method can measure a wide range of elements simultaneously, with excellent sensitivity and detection limits for some elements (e.g., Smodis et al., 1993 Negi et al., 1997). 

Thus, the retention of helium atoms produced by the nuclear reaction is the main cause of swelling of the °B-containing materials irradiated by neutrons. The approximate equation between swelling and He-retention in a boron carbide sample can be derived as... 

Fig. 1. Specific pinning force Fp/p (defect density) vs. elementary interaction force fp for defects in Nb. Data are for pinning by dislocation loops ( ), voids due to irradiation by neutrons (O) and Ni ), and Nb2N precipitates (A). (From Kramer [ ].)...

When PH3/3He mixtures are irradiated by neutrons, the additional nuclear reaction 3He(n, p)T has to be taken into account. The tritium atoms formed react with PH3 to give HT and PH2T [11]. 

When solid orthophosphates are irradiated by neutrons at low temperature, the decomposition is very small, but some atomic hydrogen is produced and trapped in the crystal lattice. Solutions of phosphites and hypophosphites are oxidised to orthophosphates by y-rays whether external oxygen is present or not, and with solid phosphites and hypophosphites radicals can first be obtained (13.210) and (13.211). Neutron bombardment of anhydrous phosphites or hypophosphites gives a mixture of phosphites, hypophosphites, hypophosphates and orthophosphates. 

While neutrons, charged particles, and gamma photons can be equally used for irradiation, the most frequent choice is the irradiation by neutrons. Since the neutron lacks an electric charge, it is more penetrating and makes the analysis of bulk samples possible. Additionally, the high cross-sections of neutron-induced reactions offer extremely high analytical sensitivities for many elements. 

In isotope power sources (actually radionuclide sources), energy of radioactive decay of nuclei is used. The radionuclides themselves are products of fission or activation due to irradiation by neutrons. 

Determination of silver can be carried out by polarography in 0.1 M KNO3 electrolyte. A dropping mercury electrode is placed into the solution to be analyzed and by changing the charge on the electrode the Ag in solution will form an amalgam with the mercury at different potentials. Polarographic waves are produced from which the concentration of the silver can be obtained. Neutron activation analysis (NAA) is based on the formation of radioactive isotopes when a sample is irradiated by neutrons. NAA is not extensively used in routine analysis because costs are prohibitive, but there is a limited use in the field of biomedical research [37],... 

Fission track (FT) is among the most sensitive active methods and is based on the fact that U nuclides may undergo fission when bombarded by neutrons. The sample is placed between two thin films and is irradiated by neutrons (see Figure 5.11). The recoil of the fission producfs creafes small deficiencies in the film that can be developed by alkaline treatment (or HF) to form craters that can be seen by an optical microscope (see Figure 5.13a). The FT method is useful for detection and location of the particles, and may also provide some semiquantitative information (the density of the tracks around the craters) but the particles then need to be transferred (relocated) for further characterization. 

TRANSMUTATION The conversion of one element to another. This happens all the time in nuclear decay. In the context of nuclear waste treatment it is the process by which radionuclides are irradiated by neutrons or other particles to convert them into nuclides that are either stable or have shorter half-lives. 

The critical temperature of A15 (cP8) compounds like Nb3Ge or V3Si is very sensitive to lattice disorder and decreases under irradiation by neutrons, a particles, heavy ions or electrons. In the above A15 compounds the Si (or Ge) atoms constitute a b.c.c. lattice and the V (or Nb) atoms are arranged in linear chains along the <1 0 0> directions. Labbd and Friedel (1966) have developed a model of A15 compounds in which the one-dimensionality of transition-metal atoms gives rise to... 

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