Prompt gamma neutron activation analysis (PGNAA) and pulsed fast thermal neutron activation (PFTNA) are non-contact, non-destructive analytical techniques used in online analysis systems to determine the elemental composition of bulk raw materials. Both of these techniques are known collectively as neutron activation analysis and function by bombarding materials with neutrons.
The neutrons interact with elements in the materials, which then emit secondary, prompt gamma rays that can be measured. Similar to X-ray fluorescence (XRF), each element emits a characteristic energy signature as it returns to a stable state.
PGNAA and PFTNA overview
Prompt gamma neutron activation analysis and pulsed fast thermal neutron activation are based on a subatomic reaction between a low energy neutron and the nucleus of an atom. When a thermal, or rather low energy neutron (<0.025 eV) approaches near enough to, or collides with, a nucleus of an atom, an interaction between the neutron and the nucleus takes place. Energy from the neutron is transferred to the nucleus and temporarily elevates it to an excited energy state. The energy is then released, nearly instantaneously, in the form of a gamma ray. The gamma-ray given off has a distinct energy associated with the atom from which it was released. In essence the gamma-ray emitted is like a “fingerprint” of the element. The emitted gamma-rays are detected and an energy spectrum generated which can then be analyzed for elemental composition.
PGNAA and PFTNA online analyzers detect the gamma rays using scintillation detectors. These detectors are composed of a high purity crystalline structure which, when exposed to a gamma-rays, produces photons proportional in energy to the energy of the gamma-rays that enter the crystal. A photo-multiplier tube coupled to the crystal converts the pulses of light into electrical signals proportional in energy to the photon. Sophisticated high-speed electronic circuits then amplify and process the electrical pulses, yielding a composite energy spectrum. The spectrum is analyzed to determine information about specific elements.
Each element has a different tendency to interact with neutrons; those with a high tendency are measurable. There must also be a sufficient amount of the element to interact with the neutrons. What is noted as the “threshold of detection” for an element is a function of the amount of material being analyzed, the percentage of the element in that sample and the tendency for that element to interact with neutrons.
Neutrons used in the analysis technique are supplied by either a radioisotope, Californium 252 (252Cf), or from a neutron generator system.
The radioisotope Californium 252 (252Cf) undergoes spontaneous fission and produces neutrons that are used in the analysis process. Neutrons from 252Cf have a distribution of energy with an average energy of 2.6 MeV and a most probable energy of 0.7 MeV.
Neutrons from a neutron generator are produced electrically in a special type of compact linear accelerator. Isotopes of hydrogen, deuterium (2H) and tritium (3H,) are utilized in a fusion reaction to produce the neutrons. Ions, created in the accelerator’s ion source, are accelerated and focused onto a target where high energy collisions enable fusion and the creation of neutrons. The neutrons produced from this type of reaction are of a fairly high energy, 14 MeV, and are considered “fast” neutrons. As well, instead of continuously running, the small linear accelerator is continuously pulsed off and then on creating periods where the neutrons are not being produced.
Since only low energy, thermal neutrons (less than 0.025 eV) are required to enable the analysis technique; the energy of the neutrons from both the radioisotope and the neutron generator must be reduced. This is accomplished with special materials and by analyzer design so that a majority of the neutrons are converted to thermal neutrons.
When neutrons are supplied using a neutron generator, the term PFTNA is used as it describes what is taking place to allow PGNAA:
- A neutron generator is “Pulsed” on and off when it is operational and is not continually on.
- The 14 MeV neutrons are considered Fast neutrons.
- The Fast neutrons must be reduced to an energy range termed “Thermal” neutrons so that the PGNAA process can take place.
- NA stands for neutron activation.
PGNAA analyzers are situated directly on the conveyor belt and penetrate the entire raw material cross-section, providing minute-by-minute, uniform measurement of the entire material stream, not just a sample. Surface analysis technologies such as XRF, X-ray diffraction (XRD), and other spectral analysis technologies measure limited depths and surface areas that may not be representative of the entire amount of material on the belt. With PGNAA, sample errors are reduced, and the high-frequency of analysis helps reduce variation in material quality.