Instrumental Neutron Activation Analysis
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- Introduction
- Principle
- Neutron Activation
- Neutrons
- Nuclear reactions by neutrons
- Thermal neutron capture cross section and Neutron Flux
- Instrumental Neutron Activation Analysis notation
INTRODUCTION
Trace element studies have become increasingly important in several fields of the earth and life sciences. Nuclear analytical techniques were developed to provide precise and accurate results, which were better than the classical chemical methods. Neutron activation analysis was first studied by von Hevesy and Levi in 1936. With the advent of nuclear reactors generating neutron fluxes in the range 1012 n/cm 2*sec-1neutron activation analysis came into prominence. Commercially available high resolution germanium detectors in the 1960s revolutionized the satisfactory analysis of complex gamma spectra obtained from the irradiated samples. Better computing hardware and software of the 1970s and 1980s made neutron activation analysis well established and versatile for the trace element analysis of biological, geological and environmental materials.
Extensive bibliography of the use of neutron activation analysis for the period 1950-1970, categorized alphabetically not only by the names of the elements but also by the name of the authors was provided by De Soete, Gibels and Hoste.
References to the work done during the period 1970-1980 may be found
(Muecke, Potts)
PRINCIPLE
Neutron Activation Analysis is a nuclear analytical technique.The procedure consists of irradiating a sample with neutrons generated in the MIT nuclear reactor and measuring the activities from the resulting radioactive isotopes; using a standard (known abundances), the concentrations in the sample (unknown) are determined precisely and accurately.
NEUTRON ACTIVATION
Neutrons
Neutrons are generated in a nuclear reactor like MIT Research Reactor. Neutrons may also be generated by 3-5 MeV fast neutron generators. Neutrons can be classified according to their kinetic energy.
Typically neutrons are classified as:
- Slow neutrons
- Thermal neutrons of energy about 0.025 eV
- Epithermal neutrons of energy about 0.2 eV
- Resonance neutrons of energy in the range 1 to 1000 eV
- Intermediate neutrons of energy range 1 to 500 keV
- Fast neutrons of energy range above 0.5 MeV
Nuclear reactions by neutrons
Nuclear reactions occur when the nuclei of atoms react when bombarded by particles like neutrons, charged particles, etc.. For example when a sample containing sodium is bombarded by thermal neutrons in a reactor, the nuclear reaction occuring is represented by the equation below.
The reaction is called (n,gamma) reaction. This is an example of thermal neutron activation of sodium.
The interesting thing in this kind of interaction is - stable isotopes of the element are activated by the thermal neutrons. Referring to the Periodic Table of Elements , we see that 23Na is the stable isotope of the sodium element with an isotopic abundance of 100%.
There are several different types of neutron reactions that occur caused by bombarding neutrons. The most common modes of reactions are neutron capture, transmutation, fission and inelastic scattering. Neutron capture is the most important nuclear reaction for activation analysis, especially for thermal neutron activation analysis.
Thermal neutron activation cross section and neutron flux
Different neutron reactions occur with different probabilities. The measure of the probability with which a certain reaction occurs is called the cross section given in the units of cm2 and is usally denoted by the symbol sigma.
Instrumental neutron activation analysis (INAA)
The notation of instrumental neutron activation analysis (INAA) came into use to differentiate the technique from the radiochemical neutron activation analysis (RNAA). RNAA involves radichemistry and radiochemical separations of the activated samples, where as INAA means absolutely no chemistry is involved.
The details of theory and procedure of activation analysis are given in many text books.
REFERENCES
1. D. De Soete, R. Gijbels and J. Hoste,
Chapter 12: Survey of Neutron Activation Analysis: pp565-736,
Neutron Activation Analysis,
Wiley Interscience 1972.
2. G. K. Muecke,
Short course in Neutron Activation Analysis in the Geosciences, Minerological Society of Canada (1980).
3. P. J. Potts,
Chapter 12: Neutron Activation Analysis, Handbook of silicate rock analysis,
Blackie New York (1987).