Neutron Activation Analysis (NAA)
| Element identification and composition analysis of material is a challenge to scientific community, when it comes down to very low level detection, i.e. when we have to search one unit of some material out of one million unit of another material, which is known as parts per million (ppm). The complexity enhances more when we consider the material even less than ppm level. Similarly, when the question of analyzing very thin layers (nano meter dimension or less) comes, it becomes extremely difficult job. There are several nuclear physics based techniques, such as Rutherford back scattering, nuclear reaction analysis, neutron activation analysis etc. These are extremely sensitive techniques in terms of identification and quantification of materials. One can also analyze very thin l layer of material and with an appropriate analyzing system it is possible to get information of monolayer material deposition. | ||
| Conventionally, major, minor and trace element of materials are defined in the following manner: | ||
| Major: Concentration exceeding 1% by mass | ||
| Minor: Concentration in the range of 0.1% to 1.0 % by mass | ||
| Trace: Concentrations less than 0.1% by mass | ||
| Different trace levels are | ||
| ppm: μg/g (micro gram/gram = 10-6 gm/gm) | ||
| ppb: ng/g (nano gram/gram = 10-9 gm/gm) | ||
| ppt: pg/g (pico gram/gram = 10-12 gm/gm) | ||
| ppf: fg/g (femto gram/gram = 10-15 gm/gm) | ||
| ppa: ag/g (atto gram/gram = 10-18 gm/gm) | ||
| What is NAA? | ||
| In neutron activation analysis (NAA), the sample is exposed to the intense radiation field of a nuclear reactor. The sample is thus bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well- known. Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it. According to the kinetic energies, neutrons can be classified in five groups — thermal, epithermal, resonance, intermediate and fast. When target nuclei are bombarded with neutrons, the four major reactions that take place are neutron capture, transmutation, fission reaction and inelastic scattering. In neutron capture, the target nucleus absorbs (captures) a neutron, resulting in a product isotope, the mass number of which is incremented by one. If the product nucleus is unstable, it usually de excites by emission of gamma rays and/or β-. For example, | ||
/equ5_1.png){kind=small} | ||
| In transmutation reaction, the target nucleus absorbs a neutron, emitting charged/non-charged particles like alpha, proton etc. The unstable product nucleus generally de-excites through β- emission back to the target nucleus. Transmutation neutron reactions are caused by neutrons of high energies (fast or intermediate neutrons). Fission reaction is basically fragmentation of heavier nuclei into relatively lower mass nuclei under neutron bombardment, and is discussed in detail in module 3. The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron captures or (n, gamma) reaction is illustrated in Fig.m5.1. As we know that when a neutron interacts with the target nucleus, the compound nucleus almost instantaneously de-excites into a more stable configuration through emission of one or more characteristic prompt gamma rays. In some cases, there may be emission of delayed gamma rays, but at a much slower rate. This is in accordance with the half-lives of the radioactive nucleus, which could be from a fraction of a second to several years. | ||
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FIGURE m5.1 Neutron capture by a target nucleus followed by the emission of gamma rays | ||
| Let us see the basic NAA processes | ||
| Broadly, there are two types of NAA: prompt-gamma ray NAA or PGNAA and delayed gamma ray NAA or DGNAA. In case of PGNAA, measurements take place during irradiation, and in case of DGNAA, measurements follow radioactive decay. The PGNAA technique is generally performed by using a beam of neutrons extracted through a reactor beam port. It is most applicable to elements (B, Cd, Sm and Gd) with extremely high neutron capture cross-sections. DGNAA is useful for the vast majority of elements that produce radioactive nuclides. The technique is flexible with respect to time, such that the sensitivity for a long-lived radionuclide that suffers from interference by a shorter-lived radionuclide can be improved by waiting for the short-lived radionuclide to decay. | ||
| Let us now see the mathematical formulation required to determine the concentration of any element present in any material. | ||
| The activity equation for NAA can be obtained in the following manner:1,2 | ||
| Let, A = number of decays per second (activity) N = number of atoms of the target isotope = m = mass of the element in the irradiated sample a = isotopic abundance w = atomic weight of the element λ = decay constant φ = neutron flux σ = activation cross section | ||
| If tirr is the time of irradiation then we can write | ||
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| After a delay of time td | ||
/equ5_3.png){kind=small} | ||
| For a counting time of tc | ||
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