Neutron Activation Analysis- A Radioactive Analytical Technique
Neutron Activation Analysis(NAA) is a radioactive analytical technique useful for qualitative and quantitative analysis of a substance. The peculiar feature of the technique is, it can analyze the minor components or any constituent which have a low concentration in a sample. NAA uses the energies of gamma rays for analysis purposes.
NAA was first developed by G. Hevesy and H.Levi in 1936, where they used (Ra₂₂₆ + Be) as the neutron source and an ionization chamber as the radiation detector for the method. They promptly recognized that the element Dysprosium(Dy) was highly radioactive after being irradiated by the neutron source. This led to the foundation for the use of nuclear reactions in analytical fields.
When an analyte is irradiated with neutrons, certain of its elements become radioactive and emit gamma radiation of various energies. These radiations are the fingerprint of the elements and the amount of radiations given off at a certain energy is indicative of the quantity of the element present in the analyte.
Principle of Neutron Activation Analysis
This can be understood by the following:
Irradiation of sample with neutrons
The neutrons are captured by the elements of the sample to produce unstable radioisotopes or radionuclides
The radionuclides immediately start decaying to gain stability
The decay produces the emission of gamma rays and in some cases beta particles
The energies of these gamma rays are deterred by the semiconductor detectors
One of the important factors considered in this process is the half-life of the decaying isotopes. Half-life (t ½ ) of any radioactive material is the time required for half of the initial number of atoms to disintegrate. It is dependent on the concentration of the element. The number of radioactive atoms remains constant throughout the decay process. 50 percent of radioactive atoms disintegrate in one half-life, 75 percent in the second half-life, and 87.5 percent in the third half-life.
Radioactivity follows the first-order kinetics for the decay of isotopes. It is given by:
A = dN/dt = 𝛌N,
A = 𝛌N₀e⁻𝛌t
A = A₀e⁻𝛌t
t ½ = 0.693/𝛌
A = activity
A₀ = activity at an initial
N = number of radioactive atoms present in the sample at time t
N₀ = number of radioactive atoms present
𝛌 = radioisotopes’ constant
t ½ = half-life of the radioactive atom
Instrumentation of Neutron Activation Analysis
The NAA instrument has the following three components:
- Neutron source
- Sample cell
1. Neutron Source
The neutron source is the core component of the instrument. The sources which are commonly used are:
- Nuclear reactors
- Gas discharge tubes
- Isotope producers
2. Sample Cell
The sample cell is usually a heat-sealed vial made up of either quartz or polyethylene. 50 milligrams of sample is encapsulated in the vials. Before analysis, the sample is carefully prepared by a suitable preparation method that removes all the impurities from the sample.
The detectors used in NAA are designed to detect gamma radiation. The most commonly used detectors are:
- Gas-ionizing detectors
- Scintillation detectors
- Semiconductor detector
This detector consists of a tube, two electrodes, and a counter. The tube is filled with argon gas. The cathode used is a grounded metallic casing and the anode is a rod in the middle of the detector.
When the x-ray particles enter the tube, they ionize the argon gas, producing a large number of Ar⁺/e⁻ ion pairs. Ar⁺ moves towards the cathode while e⁻ move to the anode and this movement of charges produces an electric potential which is measured by the counter.
This detector consists of a scintillator made up of sodium iodide and a photomultiplier tube. The radiations fall on the scintillator which converts the rays into visible radiations.
When these visible radiations fall on the photocathode, the photons present in visible radiations get multiplied due to reflection on the dynodes present in the photomultiplier tube. The anode collects these photons that show signals on the readout.
This detector is divided into three regions namely- a p-type region made up of silicon, an n-type region up of silicon doped with lithium and an intrinsic region made up of silicon doped with lithium ions. X-rays enter the intrinsic region and excite electrons to a conduction band. This results in a resistance decrease in the region which allows the flow of current through the device. The detection is based on the increase in conductivity when struck by radiation.
The radioactivity measured by the detectors is recorded in the form of gamma spectra which shows photo-peaks. Sophisticated computer programs are used to process the data, that smooth the spectral data, and determine the net areas of gamma-ray photopeaks.
The program then translates the area into count rates as counts per minute or cpm. These programs are capable of resolving overlapping and complex photopeak energy regions. Additional data for decay time differences, electronic dead time losses, and unresolved interferences and compares the sample data (cpm/weight) to the standard data (cpm/μg) to calculate elemental abundance in the sample.
Types of Neutron Activation Analysis
Depending on the types of parameters which are considered for the analysis the NAA can be classified into various categories.
If the parameter is the time of decay, then NAA is classified as:
- Prompt Gamma Neutron Activation Analysis or PGNAA is used for the analysis of elements having rapid decay rates or weak gamma emission intensities. The analysis is carried out during the irradiation process.
- Delayed Gamma Neutron Activation Analysis or DGNAA, measures the gamma radiations after the irradiation procedure. Elements having longer decaying irradiation and decaying periods are analyzed by DGNAA.
If the parameter is the kinetic energy of the neutrons, then the analysis is done by:
- Epithermal neutron activation analysis or ENAA, analyses the neutrons having a low kinetic energy of 0.5 eV to 0.5 MeV.
- Fast neutron activation analysis or FNAA, analyses the neutrons of high kinetic energy that is more than 0.5 MeV.
The next parameter is the behavior of the elements,
- Instrumental neutron activation analysis or INAA is the most widely used NAA, where the sample is directly injected into the instrument without any chemical pretreatment or separation. But is only applicable for the elements where no interference of the impurities takes place.
- Radiochemical neutron activation analysis or RNAA is the process in which the sample is first chemically separated and then directed toward the analysis.
Advantages of Neutron Activation Analysis
- It is a non-destructive technique
- It pisses high accuracy and precision
- Requires a small amount of sample
- Negligible or low zero blank contribution
- Highly sensitive
Disadvantages of Neutron Activation Analysis
- The instrument is expensive
- Requires high maintenance
- Limited availability of nuclear reactors
- Requires special and strict disposable procedures
Applications of Neutron Activation Analysis
- Archaeology– NAA is used to elucidate the elements of the artifacts discovered
- Geology– NAA is helpful in detecting the trace elements of rare earth elements in the rocks. Also, it locates the ore deposits.
- Semiconductor Industry– NAA helps to determine the standards of the purity of the semiconductors.
- Agriculture Industry– NAA helps in tracing the elements of the soil that may determine the quality of the soil. It also tracks the origin of the traces of fertilizers or pesticides that are washed away into water bodies.
- Forensic Science – In forensic science laboratories, NAA is used to analyze metallic poisons, glass, soil, fiber, inks, dyes, paints, gunshot residues, explosives, etc. It can be used to determine the elemental concentrations of blood.
NAA is a non-destructive analytical technique using radioactivity for analysis purposes. It is a highly sensitive technique with a detection limit in parts per billion(ppb). It is a very useful technique but nowadays its popularity is decreasing because of the demerit of radioactivity which has a negative impact on the environment as well as human beings.
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