Irradiation with Neutrons

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

The concentration of Mn in steel can be determined by a neutron activation analysis using the method of external standards. A 1.000-g sample of an unknown steel sample and a 0.950-g sample of a standard steel known to contain 0.463% w/w Mn, are irradiated with neutrons in a nuclear reactor for 10 h. After a 40-min cooling period, the activities for gamma-ray emission were found to be 2542 cpm (counts per minute) for the unknown and 1984 cpm for the standard. What is the %w/w Mn in the unknown steel sample ... 

The experiments are conceptually very straightforward although often more complicated in practice. A chosen target material is irradiated with neutrons (or other projectiles). Following the irradiation the target may, if desired, be thermally or otherwise treated (annealed) to effect solid-state reactions, after which the sample is dissolved and chemically processed in order to separate the various expected products and to measure their yields. 

Suppose a solid compound A is transformed into B when it is heated at 200°C. An untreated sample of A shows no induction period, but a sample of A that was irradiated with neutrons does show an induction period. After the induction period, the irradiated sample gave similar kinetic behavior to that of the untreated sample. Explain these observations. 

NAA at its simplest is a technique whereby some of the elements in the sample are converted into artificial radioactive elements by irradiation with neutrons. Figure 2.13 shows a schematic diagram of this process. These artificial nuclei decay by one or more of the standard pathways for radioactive... 

In essence, NAA involves converting some atoms of the elements within a sample into artificial radioactive isotopes by irradiation with neutrons. The radioactive isotopes so formed then decay to form stable isotopes at a rate which depends on their half-life. Measurement of the decay allows the identification of the nature and concentration of the original elements in the sample. If analysis is to be quantitative, a series of standard specimens which resemble the composition of the archaeological artifact as closely as possible are required. NAA differs from other spectroscopic methods considered in earlier chapters because it involves reorganization of the nucleus, and subsequent changes between energy levels within the nucleus, rather than between the electronic energy levels. 

The number of protons is unique to the element but most elements can exist with two or more different numbers of neutrons in their nucleus, giving rise to different isotopes of the same element. Some isotopes are stable, but some (numerically the majority) have nuclei which change spontaneously - that is, they are radioactive. Following the discovery of naturally radioactive isotopes around 1900 (see Section 10.3) it was soon found that many elements could be artificially induced to become radioactive by irradiating with neutrons (activation analysis). This observation led to the development of a precise and sensitive method for chemical analysis. 

To synthesize Pa-233, thorium nitrate is irradiated with neutron. Pa-233 formed, as shown above, is dissolved in 3M nitric acid. The solution is heated. A manganous salt and permanganate are added to this solution. Manganese dioxide, Mn02, is precipitated. Pa-233 co-deposits onto this precipitate. The precipitate is washed with water. It is then dissolved in 6M hydrochloric acid. 

Uranium-233, like uranium-235 and plutonium-239, forms a fissionable isotope used as nuclear fuel. This isotope can be made from natural thorium by irradiation with neutrons, as follows ... 

Figure 9 Relation between the RBE of neutrons for regression of lung metastases in patients and their doubling time. The closed circles correspond to measured values of RBE. The open circles correspond to values estimated from irradiation with neutrons only. For the 15-MeV neutrons used in this study [produced by a (d,T) generator], the RBE for the tolerance of the most important normal tissues is about 3. As a consequence, neutrons are a good indication (RBE >3) for tumors having doubling times greater that 100 days. In contrast, they should not be used for rapidly growing tumors. (From Ref. 32.)...

Fermi realized this meant that, if uranium, the heaviest known element, was irradiated with neutrons, it might decay to form a previously unknown transuranic element. Uranium has an atomic number of 92 beta decay would convert it to element 93 , anew member of the Periodic Table. 

There are certain unusual types of defects in metal systems that are noteworthy. It has been found (Taylor Doyle, 1972) that in NiAl alloys A1 atoms on the Al-rich side do not substitute on the Ni sublattice instead there are vacancies in the Ni sites. For example, at 55 at.% Al, 18% of Ni sites are vacant while the A1 sites are filled. Such vacancies determined by composition are referred to as constitutional vacancies. Other alloys have since been found to exhibit such vacancies, typical of these being NiGa and CoGA. Another rather curious aspect of defects is the formation of void lattices when metals such as Mo are irradiated with neutrons or more massive projectiles (Gleiter, 1983). Void lattices arise from agglomeration of vacancies and are akin to superlattices. Typically, neighbouring voids in Mo are separated by 200 A. An explanation for the stability of void lattices on the basis of the continuum theory of elasticity has been proposed (Stoneham, 1971 Tewary Bullough, 1972). 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

In a second series of experiments, the crysts of LA were mixed with uranium oxide and irradiated with neutrons so that fission of U-235 took place. Again no expln was produced. Bombardment with slow neutrons of Nla did produce an expln, but this was not due to neutrons, but simply to the well known fact that Nig usually explodes at ordinary temp as soon as ammonia (which is usualLy mixed with. Nig) becomes evaporated... 

When performing irradiations with neutrons or high-energy protons, it is common to measure the beam intensity using a monitor reaction. A thin foil of a... 

The sample being analyzed in this experiment contains fissionable material that had been sealed in a quartz tube and irradiated with neutrons. The material was dissolved in strong mineral acid to prepare it for radiochemical analysis of fission products. You will be informed of the date and time of the end of bombardment (EOB), the type of material, the amount of fissionable material in your sample, and the acid type and strength. You can assume that the person who prepared the sample based the amount of fissile material to be irradiated and the period of irradiation on the available neutron flux and the desired amounts of fission product radionuclides. You should estimate the activities of the major fission products that remain after the time interval between formation and analysis. 

A 10 go sample of uranium oxide Irradiated with neutrons 1... 

All of these initial experiments had been based on the assumption that the reaction of deuterons with would be the most likely reaction in forming To eliminate the possibility of a favored reaction of neutrons with an experiment was devised in which a solution of saturated ammonium nitrate was irradiated with neutrons. It was expected that no detectable amount of radiocarbon would be produced. Instead, a relatively small amount of the precipitate paralyzed the screen-wall counter Within a few month s time, despite strong theoretical arguments to the contrary, it was demonstrated experimentally that the thermal neutron mode of production was heavily favored and that the half-life was on the order of years or millenia (15). 

The elements with the atomic numbers 97 and 98 (berkelium and californium) at first could not be produced by irradiation with neutrons, because isotopes of Cm exhibiting jfi" transmutation were not known. After milligram amounts of " Am had been produced by reaction (14.14), was obtained in 1949 by Thompson, Ghiorso and others by irradiation with a particles ... 

There are some special methods of chemical synthesis recoiling phenomenon (recoil synthesis). For example anthracene labelled with " C is synthesised by irradiating acridine with neutrons through N(n,p) C reaction. Similarly labelling is realised with the use of He( ,p) H or Li(n,a) H. For this purpose the compound to be labelled is mixed with He gas or Li,COj and then irradiated with neutrons. 

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