Neutron Activation Analysis

Neutron activation analysis (NAA) is a nuclear process used to determine the concentration of elements in a given material. A non-radioactive sample can sometimes become radioactive after bombardment with gamma radiation. It is the analytical technique in which radiation is induced by bombardment. It utilizes neutron for bombardment, lack of charges and mass of neutron allows efficient penetration and energetic transfer to the nucleus.

Neutron activation analysis (NAA) is a nuclear process used for detecting the presence and determining the concentration with great sensitivity of most chemical elements without destroying the sample to be analyzed. It allows discrete sampling of elements as it does not consider the chemical form of a sample, but focuses completely on its nucleus.

Activation analysis has, in a few decades, become one of the most important methods for determination of minor, trace, and ultra-trace elements in solid samples. The main advantages of activation analysis are its accuracy and sensitivity. Moreover, it is an independent method, i.e., not subject to the same systematic errors as other, more commonly used analytical methods. The method is applied in the semiconductor industry, medicine, biology, criminology, archaeology, geochemistry, and environmental studies or quality control.

What is Neutron Activation Analysis?

Analytical technology in which radioactive emissions are monitored from a sample that has been bombarded with the neutron. NAA is qualitative and quantitative analysis for trace elements. It measures specific radiation from radioactive nuclei.

History

Neutron Activation Analysis was discovered in 1936 by George Charles DE Hevesy and Hilde Levi.

Each radioactive element has a specific and well-known decay rate. By measuring the decay rate element and it is concentration can be found.

Types of Neutron Activation Analysis

There are two types of NAA:

1Prompt Gamma Neutron Activation Analysis (PGNAA)

1. Elements that produce stable isotopes.
2. Measurements take place during the irradiation.
3. Elements with weak decay gamma-ray intensities.
4. Applicable to elements with short half-lives.
5. Planer detector is used (a flat, large collection surface area and can be placed close to the sample).

2Delayed Gamma Neutron Activation Analysis (DGNAA)

1. Elements that produce unstable isotopes.
2. Measurements take place after irradiation.
3. A flexible method of analysis.
4. Applicable to elements with sort, medium, and long half-lives.
5. The detector is used (surrounds the sample with a large collection surface area).

Prompt and delayed gamma radiations are unique for each element just like fingerprints.

Principle

The principle of activation analysis is that a particle such as a neutron, proton, Alpha (α) – particle, etc., or photon like gamma (γ) – rays, bremsstrahlung (radiations from accelerated particles) induces a nuclear reaction in an atom of a target element. Then the product is detected and quantified by photon or particle emission or, if radioactive then by its decay properties.

Mechanism

The sample is bombarded with neutrons. They make radioactive isotopes that emits particles that are beta particles and gamma rays. Beta particle’s emission is energy-wise more continuous than gamma rays. It is emission is discrete. Gamma rays emission measured preferably. Beta rays emission is more sensitive. The energy is imparted by the neutron. That gives energy to radioactive molecules nucleus that excites the nucleus into high energy level. This excited state is unfavorable so the compound nucleus emits beta rays and gamma rays to relax and be in a stable configuration. The most stable configuration leads to a radioactive nucleus.

Now, the newly formed radioactive nucleus decay by emitting, alpha, beta, gamma photons.

The decay process depends upon the particle and their half-life it is slower.

Detector

The most common types of detectors which are used in Neutron Activation Analysis are as follows:

Scintillation Detector

It uses a radiation-sensitive crystal most commonly thallium-doped sodium iodide which emits light when struck by gamma photons. These detectors have excellent sensitivity and stability and a reasonable resolution.

Semiconductor Detector

It utilizes the semiconducting elemental germanium. The germanium is processed to form a positive intrinsic negative diode and when cooled to seventy seven degrees kelvin by liquid nitrogen to reduce dark current and detector, noise produces a signal which is proportional to the photon energy of the incoming radiation.

There are two types of germanium detector:

• Lithium drifted germanium
• High purity germanium.

Forensic Application

• It helps in trace element examination.
• It is used for examination the poisoning by metals.
• It is used in hair and nail examination.
• The revelation of Gunshot Residues (GSR)

NAA technique is used for revealing mineral gunshot residues. These indicial traces after being picked up from the crime scene are used as material proofs for judicial investigations. Samples of metallic powder residue can be realized after several shots by different kinds of weapons with local and foreign ammunition.

Advantages

The major advantages of Neutron Activation Analysis are:

• The relative freedom from matrix effects and interferences
• High accuracy and sensitivity
• Very low or zero blank contributions
• Multi-elemental analysis
• It is the Non-destructive technique.
• Low amount of sample required
• Minimal sample pre-treatment required
• Time-efficient for analyzing many samples.

Disadvantages

• Full analysis of long-lived nuclides is time-consuming.
• Irradiated samples remain radioactive for some time.
• It is costly in comparison to other techniques.