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Neutron Activation Analysis
S.P. Murarka, in Encyclopedia of Materials: Science and Technology, 2001
1 Neutron Activation Analysis
Neutron activation analysis refers to a technique of analyzing materials for their elemental composition by use of neutrons. Neutrons, absorbed by the matter, activate elements into a state leading to new materials and radioactive decay of some of these newly formed materials. By monitoring the radioactive decay and neutron dose of materials one can identify the elements and determine their amounts in the sample irradiated with neutrons. In its simplest form the neutron activation process can be defined as follows:
The activated species undergo decay according to the following reaction:
where B is relatively stable product and in the simplest case of these decays the radiation could be α, β, or γ radiation. In more complex decay schemes B could still be a rather unstable nucleus and radiation could be proton, neutron, ionized deuterium, or some other species. An example of this neutron-activation process is:
(3)Ni62+n1âNi63âCu63+β
where the square brackets indicate the activated nickel atom, which is unstable and decays to copper and β radiation. Not all neutron absorptions lead to such activated atoms. In some cases neutron irradiation of atoms leads to stable isotopes of the irradiated atom, for example
A reaction cross-section for each of these interactions determines the probability of such interactions (Eqns. (3) and (4)). The reaction cross-section is called an absorption cross-section, generally denoted by Ï given in the units of barn (= 10â24cm2).
Both the neutronâatom reaction and the absorption cross-section depend on the neutron energy. In general thermalized neutrons (energy in the range of about 10â2eV) are used, although fast neutrons or other activating particles are also used to produce special reaction products to aid in identifying or determining the concentration in cases where thermal neutron activation is not possible. For example, the analysis of light elements such as hydrogen, boron, carbon, nitrogen, and oxygen, is carried out using the so-called nuclear reaction technique where ionized nitrogen, deuterium, proton, and helium are used as the activating particles (Lanford et al. 1983). To produce nuclear reactions by these particles, accelerators are required to accelerate ionized gas atoms or molecules to very high energies (in the range of several to tens of MeV). This discussion is confined to thermal neutron activation analysis.
Thermal neutron activation analysis can be employed for most of the elements in the periodic table, with sensitivity in the range of a few parts per million (ppm) to a few parts per billion (ppb). The sensitivity could be improved to a few parts per trillion (ppt) for some elements, with the use of large neutron fluxes. Since neutrons are neutral, most materials are transparent to neutrons and reactions of the type (1) are very simple to follow and standard tables of the reaction products, emitted radiation, and reaction cross-sections are available (Handbook of Chemistry and Physics 1991). It should be emphasized that in most circumstances several elements present in the same sample can be analyzed by one activation process, especially with the use of the sophisticated computerized multichannel analyzers, with a minimum to no sacrifice of the sensitivity.
1.1 Analysis
The number (Ni0) of radioactive nuclide present at the end of an irradiation with a thermal neutron flux of Ï is given as
where subscript i refers to the nuclide that is irradiated, ni is the number of the target nuclei present in the irradiated sample, and Ïi is the reaction cross-section for a given nuclear reaction converting nuclide i to a radioactive, monitorable product nuclide. λi is the decay constant of the radioactive nuclei thus produced and is equal to 0.693/t1/2 where the t1/2 refers to the so-called half-life of the radioactive material. Half-life is defined as the time required to decay half of the original amount of the radioactive material. For example, the half-life of Ni63 decaying to Cu63 is determined to be approximately 92 years.
In Eqn. (5)ni can be obtained using the known mass of the irradiated element (me), the isotopic abundance of the target nuclei given as a fractional abundance fi, the atomic weight of the target elemental material Ae, and Avogadroâs number NA such that
Values of Ïi are now available for all known nuclei, especially for thermal neutrons. Similarly values of t1/2 are available in tabular form.
![Neutron activation analysis pdf Neutron activation analysis pdf](https://bradscholars.brad.ac.uk/bitstream/handle/10454/6680/Anatolian%20Studies%202005%2055%2025-59.pdf.jpg?sequence=4&isAllowed=y)
The quantitative determination of Ni0 is carried out by measuring the number of disintegrations per unit time. The measurement is done by use of so-called counters that detect the α, β, or γ radioactive decay products and by knowing the efficiency of detection for the given counter. Generally standard radioactive sources with a variety of α, β, or γ decay modes and different half-lives are commercially available to obtain the efficiency of each counting system. It must be pointed out that in determining the detection efficiency, sample and standard size effects should be considered. One can make use of multichannel analyzers to determine the energies spectroscopically and intensities of γ rays so that identification and quantity estimates can be made for each radioactive nuclide present in the irradiated sample. Details of counting methods are available in reference books (Kruger 1971, DeSorte et al. 1972) and from equipment manufacturers. Different counters are recommended for counting α, β, and γ particles.
Finally in this section, it should be pointed out that the Ni0 in Eqn. (5) refers to number of radioactive nuclei produced just after the irradiation. If the analysis (counting) is carried out at a later time, a correction for the decay in the elapsed time must be made. This is done by use of the equation
![Neutron Activation Analysis Vs Petrography Neutron Activation Analysis Vs Petrography](http://www.pnas.org/content/pnas/102/32/11213/F1.large.jpg)
where De is the measured activity after the elapsed time te, D0 is related to Ni0 through the detection efficiency, and λ is the decay constant.
1.2 Optimization
The neutron activation analysis consists of a series of well-planned procedures to optimize the sensitivity and accuracy (Kruger 1971). Safety, contamination, and cost must also be carefully considered and maximized or minimized. Besides determining the desired accuracy, optimum radiation monitoring system, and irradiation time, the following considerations are necessary.
(a) The choice of optimum nuclear reaction
In this case the target nucleus, its isotopic abundance, reaction cross-section, reaction product, radioactivity (type, half-life and level), interference with other target nuclei present in the sample, sample preparation, need for separation and specialized analysis tools, and storage of the irradiated samples are considered.
(b) Availability of desired neutron flux
To optimize the analysis, it is essential to have available a neutron flux that yields the desired product nuclei and radioactive decay, measurable in a given time frame. When more than one element is analyzed in the same sample, the choice of total neutron flux must be made carefully so that desired sensitivity and accuracy are achieved. The cost of the irradiation and its handling from the analytical laboratory to irradiation facilities needs to be evaluated.
(c) Sample preparation and irradiation conditions
This may be essential if certain target nuclei present in the sample may interfere in the analysis of the other nuclei, especially when the interfering nuclei lead to a very high dosage of radioactivity. In this case a chemical separation of interfering nuclei will be necessary. Also, sample volume and homogeneity need to be addressed. Liquids and gases are generally not desirable. Materials that may liquefy, evaporate or produce gases, or that may explode during or after irradiation, should not be used.
1.3 Limitations
Kruger (1971) has defined and discussed four limitations to neutron activation analyses. (i) The chemical limitation is related to the inability of this technique to distinguish the chemical nature (bonding, ionization state) of the element under consideration. (ii) The nuclear limitation has two components: (a) the interfering nuclear reactions (e.g., S1430i(n,γ)âS1431iandS1531i(n,p)âS1431i yield same product nuclei), and (b) the changes in the irradiation conditions resulting when the sample is inserted in the neutron flux. (iii) The γ-ray spectroscopy limitations are associated with so-called Compton scattering, gain and threshold shift of the spectrometers, and quality control of the useful spectrometers. It may be pointed out that with improved detector crystal preparations and powerful electronics and computers these limitations are practically eliminated. (iv) Sensitivity and accuracy limitations are associated with sample conditions, irradiation conditions, postirradiation processing, and radiation measurement. Nuclear constants, e.g., half-life, reaction cross-section, decay scheme, interfering and competing reactions, and accurate flux determination are important parameters that affect the accuracy.
1.4 Sensitivity of NAA
The minimum concentration (maximum sensitivity) in ppm of the detectable element can be calculated using the following equation (Kruger 1971).
Where A is atomic number, Am(t) is minimum detectable count rate at the time of counting, E is overall counting efficiency, Y is chemical yield, and F is self absorption losses. Using the selected values of the above parameters and compilations of Ï, t1/2, f, and A, Meinke (1955) has determined the maximum sensitivity of various elements listed in Table 1.
Table 1. Calculated sensitivity by neutron activation analysis.a
Element | Sensitivity (ppb) | Element | Sensitivity (ppb) | Element | Sensitivity (ppb) |
---|---|---|---|---|---|
Na | 0.35 | Ga | 2.35 | Sb | 0.2 |
Mg | 30 | Ge | 0.2 | Te | 5 |
Al | 0.05 | As | 0.1 | I | 0.1 |
Si | 50 | Si | 2.5 | Cs | 1.5 |
P | 1 | Br | 0.15 | Ba | 2.5 |
S | 200 | Rb | 1.5 | Hf | 1 |
Cl | 1.5 | Sr | 30 | Ta | 0.35 |
K | 4 | Y | 0.5 | W | 0.15 |
Ca | 190 | Zr | 15 | Re | 0.03 |
Sc | 0.1 | Nb | 500 | Os | 1 |
V | 0.05 | Mo | 5 | Ir | 0.015 |
Cr | 10 | Ru | 5 | Pt | 5 |
Mn | 0.03 | Pd | 0.25 | Au | 0.15 |
Fe | 450 | Ag | 5.5 | Hg | 6.5 |
Co | 1 | Cd | 2.5 | Tl | 30 |
Ni | 1.5 | In | 0.005 | La | 0.1 |
Cu | 0.35 | Sn | 10 |
- a
- Calculated for a flux of 1013 n cmâ2 sâ1 by Meinke (1955).
Neutron activation analysis provides a means to analyze quantitatively very small amounts of matter. The necessary information on reaction cross-sections, decay schemes, percentage abundance, etc. are compiled in many reference books and handbooks including the Handbook of Chemistry and Physics (1991). It must, however, be noted that analysis is destructive and irradiated samples are radioactive and hazardous and therefore should be handled and stored by properly trained people.