When the nucleus of an atom absorbs a neutron, it immediately releases
energy by emitting one or more gamma rays, referred to as prompt gamma rays.
The energies of the prompt gamma rays are unique for each element, for example,
hydrogen has only a single gamma ray energy while other elements have hundreds
of gamma ray energies. These unique "fingerprints" of the elements allow
identification of the quantity of each element that is present in a material
that is being tested.
Neutrons pass through matter easily and, therefore, penetrate deep into the material being tested. Most prompt gamma rays (induced by neutron absorption) are very energetic and can pass through thick layers of material to arrive at a detector. Therefore, neutron-induced gamma-ray spectroscopy can be used to measure large volumes of material, while many other technologies are limited to the measurement of only the surface layers of the materials under test.
Neutrons travel at velocities similar to the speed of sound and gamma rays travel at the speed of light. Therefore, even fast-moving material on a conveyor belt appears to this measurement technique as if the material were standing still.
Since the absorption of a neutron by an atom is a random event governed by the neutron flux density and the concentration of the element in the material under test, the accuracy of the measurement improves with the population of measured gamma-ray energies. Meaningful estimates of elemental concentrations can be made in a few seconds in well-designed systems and laboratory-type accuracies can be achieved in measurement times on the order of ten minutes. The measurement that is made is a true average of all of the material that has passed in front of the detector during the measurement period.
Neutrons can be generated by isotopic sources or by neutron generators. An isotopic sources is an elemental isotope that gives off a radioactive emission through natural radioactive decay. Gamma-ray emitters or alpha particle emitters can be mixed with an element, such as beryllium, that gives off a neutron when it absorbs a gamma ray or an alpha particle; or some elements give off neutrons directly, such as californium-252, which spontaneously fissions and gives off neutrons in that process.
Neutron generators are electronic devices that generate neutrons in the process of fusing two atoms, either deuterium-deuterium or deuterium-tritium, together by accelerating them through a vacuum. At present the cost of a neutron generator is on the order of $100,000 with an expected life on the order of a couple years.
Californium-252 is the most commonly used neutron source for industrial applications of neutron-induced gamma-ray spectroscopy. The half life of californium is 2.6 years, and, therefore, a californium-252 source is typically replenished every two-and-one-half years by adding one-half of the original loading of californium-252.
The design of an analyzer requires appropriate radiation shielding both for focusing neutrons into the material to be measured and gamma rays to the detector and for preventing radiation from creating a hazard for personnel. Materials are placed near the neutron source for moderating, or slowing down, the neutrons to speeds at which they can be absorbed by the atoms in the material to be tested. Materials that reflect neutrons are strategically placed to focus the neutrons into the material to be tested. And materials that very effectively absorb neutrons are placed near the periphery of the instrument to prevent neutrons from reaching areas where personnel may be. High-density materials are strategically placed to prevent gamma rays from reaching locations where they are not desired.
See also: References and Glossary of Terms.
