Brief description of neutron technologies
Main neutron applications take advantage of few different technologies listed below.
- Neutron activation (NA)
- Neutron transmission (NT)
- Neutron activation analysis (NAA)
- Neutron imaging (NI)
In most systems the measurement setup consists of neutron source, object under study and detector which can be counter, spectrometer or camera system.
Development of the technologies for practical applications concentrate in developing the appropriate neutron source and detector system. The neutrons interact mater in with few different ways.
These material interactions are the basis of the technologies listed above. When neutrons travel through matter, the atoms inside can be activated to excited state. This excited state can immediately relax or it can relax after a period of time.
Another main interaction is scattering where neutron traveling through the matter collides with the atoms inside the material and changes direction due to these collisions.
Most practical neutron application take advantage of these two mechanisms. We will go through these technologies in more detail below.
Neutron activation (NA)
In neutron activation the goal is usually just to create radioactive isotope. This can be useful for example if we try to create a type of radiation by irradiating the sample material with neutrons and when the sample material becomes radioactive it will decay through the desired radiation. Most commonly this technology is applied in production of radioisotopes for various purposes. Mainly for use as marker in industry. For example medical industry uses extensively various radioisotopes for imaging purposes. Radioisotopes are also used for liquid- and gas- flow measurements and leak detection in many fields of industry.
Neutron transmission relies on the neutron scattering that is different for different materials and different neutron energies. In its simplest form we can just measure the transmittance of neutrons and gammas through the object under study and draw conclusions of what it is made from. In many elements there is resonances for certain neutron energies. If we can measure the energy spectrum of the transmitted neutrons we can identify these resonances. The big challenge for these technologies are the lack of good methods to measure the energy spectrum of neutron beam.
Neutron activation analysis (NAA)
NAA uses the neutron activation to perform analysis for the object under study. Principle is that object is irradiated with neutrons. Once it becomes radioactive through neutron capture or inelastic scattering we can detect the resulting decay radiation and analyze it in order to make deductions about elemental composition and structure of the object. In most applications we measure the energy spectrum of the characteristic gamma decay which gives us the elemental composition of the object under study. There can be large number of applications for this technology. Most prominently it is being used in oil prospecting, analysis of cement mixtures and various mining applications.
NAA consists of various different methods most common ones listed below. Using combination of these nearly all elements can be made to give some sort of response.
- DGNAA delayed gamma neutron activation, measuring gamma response after neutrons have dissipated. Works for radioisotopes that have long half life >1ms
- PGNAA prompt gamma neutron activation. Measuring induced gamma response created immediately after neutron hits the atom under study.
- NS neutron spectroscopy. Measuring the change to the neutron beam caused by the object.
- NM neutron multiplication. Detection of fissile materials such as uranium, thorium and plutonium is very effective via neutron based technologies. This is especially interesting for security and border control applications in detection of special nuclear material (SNM).
Neutron imaging (NI)
There are few variations of NI. The neutrons scatter differently in different materials. Figure below illustrates the probability of neutron and X-ray scattering for different elements. Fairly different scattering probabilities between X-ray and neutrons make them good complementary techniques.
The object under study is irradiated with neutrons. If the object is made from parts that scatter neutrons differently, it will result on shadow image similar to the traditional x-ray image. And can be recorded with film or camera system. Neutron imaging can be used in materials that are practically invisible to the x-rays and thus it is often used in situations where x-ray is not suitable. Also neutrons penetrate materials that x-ray does not. For that reason neutron imaging is used for example in imaging of large structures like bridges in order to inspect quality of the structure. On the other hand, use of contrast agents make it possible to image microscopic features such as hairline fractures. In short, neutron imaging can be used to image objects in very large scales and very small microscopic scales.
In some more advanced variations NAA and NI techniques are combined. The image pixel (or in case of 3d imaging voxel) contains elemental data from neutron activation. This allows us to create image that also tells us the elemental composition of the object or pixel/voxel under study. This is especially useful in applications where the object does not have uniform composition and we want to know what it is made from. In recent years these methods have been under intense study for use in security and border inspection applications and for medical industry.