What Is Activation Analysis?

Activation analysis is a nuclear analytical technique for determining the elemental composition of a sample by irradiating it with neutrons, charged particles, or high-energy photons and then measuring the characteristic gamma radiation emitted by the radioactive nuclides produced. The technique relies on nuclear rather than chemical reactions, which means that the measurement is independent of the chemical form or bonding state of the target element. When a stable nucleus in the sample captures a projectile, it is converted to a radioactive nuclide that decays with a characteristic half-life and emits gamma rays of discrete, element-specific energies. By measuring the energies and intensities of these gamma rays with a high-resolution germanium detector, analysts can simultaneously identify and quantify dozens of elements in a single irradiation without dissolving or chemically processing the sample. Activation analysis draws on nuclear physics, radiation measurement science, and analytical chemistry, and it is one of the most sensitive multi-element methods available for trace-element determination.

Neutron Activation Analysis

Neutron activation analysis (NAA) is the most widely practiced form of activation analysis. A nuclear reactor or a neutron generator provides the neutron flux. When a thermal neutron is captured by a target nucleus, the resulting compound nucleus is in an excited state and immediately releases energy as prompt gamma rays. The residual nucleus is typically radioactive and continues to decay, emitting delayed gamma rays whose energies identify the element. Two variants are in common use: instrumental neutron activation analysis (INAA), in which samples are irradiated and measured without chemical separation, and radiochemical neutron activation analysis (RNAA), in which chemical separations are performed after irradiation to isolate elements of interest and reduce spectral interferences. As described by the NIST Instrumental Neutron Activation Analysis program, NAA is used by NIST to certify elemental concentrations in Standard Reference Materials across a broad range of matrices, from geological samples to human tissue.

Charged Particle and Photon Activation Analysis

Activation can also be induced by charged particles and by high-energy photons. In charged particle activation analysis (CPAA), protons, deuterons, alpha particles, or other ions accelerated to MeV energies bombard the sample and induce nuclear reactions in a thin surface layer, producing radionuclides that are measured by gamma ray spectroscopy. CPAA is particularly useful for determining light elements such as carbon, nitrogen, and oxygen, which have low neutron capture cross-sections but respond well to charged-particle bombardment. Photon activation analysis (PAA) uses high-energy X-ray or gamma beams from electron accelerators or nuclear sources to initiate photonuclear reactions. The penetrating power of photons allows the technique to be applied to thick solid samples without surface-only sensitivity. The NIST Radiochemical Neutron Activation Analysis page describes how post-irradiation chemical separations complement these methods when the matrix contains elements that would otherwise dominate the gamma spectrum.

Sensitivity, Applications, and Instrumentation

The sensitivity of activation analysis depends on the nuclear reaction cross-section of the target nuclide, the particle flux available from the irradiation facility, the half-life and gamma-ray branching ratio of the product nuclide, and the efficiency of the gamma detector. Detection limits for many elements reach the nanogram per gram range, and for elements with high neutron capture cross-sections such as gold, iridium, or europium, sub-picogram sensitivity is achievable. Gamma ray spectroscopy with high-purity germanium detectors, with energy resolution around 0.1 percent at 1.33 MeV, is the standard measurement approach. A broad survey of charged particle activation analysis techniques and their applications shows that the method is applied in fields from archaeology to semiconductor manufacturing.

Applications

Activation analysis has applications across a range of scientific and industrial domains, including:

• Forensic science, for elemental fingerprinting of trace evidence such as paint chips, glass, or soil
• Nuclear medicine quality control, for verifying trace-element content in radiopharmaceuticals and biological reference materials
• Environmental monitoring, for measuring heavy metals and radionuclides in air particulates, water, and sediments
• Archaeology and art history, for provenance studies of ancient artifacts without destructive sampling
• Semiconductor and materials industry, for measuring dopant and contaminant levels in silicon wafers and advanced materials