Neutron Activation Analysis Explained: Principles, Types, & Instrumentation

Neutron Activation Analysis(NAA) is a radioactive analytical technique useful for qualitative and quantitative analysis of a substance. The peculiar feature of NAA is that it can analyze the minor components of any component that has a low concentration in a sample. NAA uses the gamma-ray energy for analysis purposes.

Neutron Activation Analysis 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.

Principle of Neutron Activation Analysis

When an analyte is irradiated with neutrons, its certain 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.

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. The half-life (t½) 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 Where,

A = activity
A₀ = activity at an initial rate
N = number of radioactive atoms present in a sample at the time
N₀ = number of radioactive atoms present at A₀
𝛌 = radioisotopes’ constant
t½ = half-life of the radioactive atom

Instrumentation of Neutron Activation Analysis

Neutron Activation Analysis only has three components:

  • Neutron source
  • Sample cell
  • Detector 

I. Neutron Source

The neutron source is the core component of the instrument. The commonly used sources are:

  • Nuclear reactors
  • Gas discharge tubes
  • Isotope producers
  • Fusors 

II. 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 which removes all the impurities from the sample.

III. Detectors

The detectors used in NAA are designed to detect the gamma radiations. The most commonly used detectors are:

  • Gas-Ionizing Detectors: 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 an 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⁻ moves to the anode and this movement of charges produces an electric potential which is measured by the counter.
  • Scintillation Detectors: 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.
  • Semiconductor Detector: This detector is divided into three regions namely a p-type region made up of silicon, an n-type region made 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, 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 compare 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 that are considered for the analysis, NAA can be classified into various categories.

If the parameter is a 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 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.  However, it 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 towards the analysis.

Conclusion

Neutron Activation Analysis 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 that harms the environment as well as human beings.

Suksham Gupta

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