Neutron imaging

Generally the neutron source is a research reactor,[1] [2] where a large number of neutrons per unit area (flux) is available. Some work with isotope sources of neutrons has been completed (largely spontaneous fission of Californium-252,[3] but also Am-Be isotope sources, and others). These offer decreased capital costs and increased mobility, but at the expense of much lower neutron intensities and significantly lower image quality. Additionally, accelerator sources of neutrons have increased in availability, including large accelerators with spallation targets[4] and these can be suitable sources for neutron imaging. Portable accelerator based neutron generators utilizing the neutron yielding fusion reactions of deuterium-deuterium or deuterium-tritium.[5]

After neutrons are produced, they need to be slowed down (decrease in kinetic energy), to the speed desired for imaging. This can take the form of some length of water, polyethylene, or graphite at room temperature to produce thermal neutrons. In the moderator the neutrons will collide with the nucleus of atoms and so slow down. Eventually the speed of these neutrons will achieve some distribution based on the temperature (amount of kinetic energy) of the moderator. If higher energy neutrons are desired, a graphite moderator can be heated to produce neutrons of higher energy (termed epithermal neutrons). For lower energy neutrons, a cold moderator such as liquid deuterium (an isotope of Hydrogen), can be used to produce low energy neutrons (cold neutron). If no or less moderator is present, high energy neutrons (termed fast neutrons), can be produced. The higher the temperature of the moderator, the higher the resulting kinetic energy of the neutrons is and the faster the neutrons will travel. Generally, faster neutrons will be more penetrating, but some interesting deviations from this trend exist and can sometimes be utilized in neutron imaging. Generally an imaging system is designed and set up to produce only a single energy of neutrons, with most imaging systems producing thermal or cold neutrons.

In some situations, selection of only a specific energy of neutrons may be desired. To isolate a specific energy of neutrons, scattering of neutrons from a crystal or chopping the neutron beam to separate neutrons based on their speed are options, but this generally produces very low neutron intensities and leads to very long exposures. Generally this is only carried out for research applications.

This discussion focuses on thermal neutron imaging, though much of this information applies to cold and epithermal imaging as well. Fast neutron imaging is an area of interest for homeland security applications, but is not commercially available currently and generally not described here.

In the moderator, neutrons will be traveling in many different directions. To produce a good image, neutrons need to be traveling in a fairly uniform direction (generally slightly divergent). To accomplish this, an aperture (an opening that will allow neutrons to pass through it surrounded by neutron absorbing materials), limits the neutrons entering the collimator. Some length of collimator with neutron absorption materials (E.g. boron) then absorbs neutrons that are not traveling the length of the collimator in the desired direction. A tradeoff exists between image quality, and exposure time. A shorter collimation system or larger aperture will produce a more intense neutron beam but the neutrons will be traveling at a wider variety of angles, while a longer collimator or a smaller aperture will produce more uniformity in the direction of travel of the neutrons, but significantly fewer neutrons will be present and a longer exposure time will result.

The object is placed in the neutron beam. Given increased geometric unsharpness from those found with x-ray systems, the object generally needs to be positioned as close to the image recording device as possible.

Though numerous different image recording methods exist, neutrons are not generally easily measured and need to be converted into some other form of radiation that is more easily detected. Some form of conversion screen generally is employed to perform this task, though some image capture methods incorporate conversion materials directly into the image recorder. Often this takes the form of a thin layer of Gadolinium, a very strong absorber for thermal neutrons. A 25 micrometer layer of gadolinium is sufficient to absorb 90% of the thermal neutrons incident on it. In some situations, other elements such as boron, indium, gold, or dysprosium may be used or materials such as LiF scintillation screens where the conversion screen absorbs neutrons and emits visible light.

A variety of methods are commonly employed to produce images with neutrons. Until recently, neutron imaging was generally recorded on x-ray film, but a variety of digital methods are now available.

Track etch is a largely obsolete method. A conversion screen converts neutrons to alpha particles that produce damage tracks in a piece of cellulose. An acid bath is then used to etch the cellulose, to produce a piece of cellulose whose thickness varies with neutron exposure.

Several processes for taking digital neutron images with thermal neutrons exists that have different advantages and disadvantages. These imaging methods are widely used in academic circles, in part because they avoid the need for film processors and dark rooms as well as offering a variety of advantages. Additionally film images can be digitized through the use of transmission scanners.

A neutron camera is an imaging system based on a digital camera or similar detector array. Neutrons pass through the object to be imaged, then a scintillation screen converts the neutrons into visible light. This light then pass through some optics (intended to minimize the camera's exposure to ionizing radiation), then the image is captured by the CCD camera (several other camera types also exist, including CMOS and CID, producing similar results).

Neutron cameras allow real time images (generally with low resolution), which has proved useful for studying two phase fluid flow in opaque pipes, hydrogen bubble formation in fuel cells, and lubricant movement in engines. This imaging system in conjunction with a rotary table, can take a large number of images at different angles that can be reconstructed into a three-dimensional image (neutron tomography).

When coupled with a thin scintillation screen and good optics these systems can produce high resolution images with similar exposure times to film imaging, though the imaging plane typically must be small given the number of pixels on the available CCD camera chips.

Though these systems offer some significant advantages (the ability to perform real time imaging, simplicity and relative low cost for research application, potentially reasonably high resolution, prompt image viewing), significant disadvantages exist including dead pixels on the camera (which result from radiation exposure), gamma sensitivity of the scintillation screens (creating imaging artifacts that typically require median filtering to remove), limited field of view, and the limited lifetime of the cameras in the high radiation environments.

X-ray image plates can be used in conjunction with a plate scanner to produce neutron images much as x-ray images are produced with the system. The neutron still need to be converted into some other form of radiation to be captured by the image plate. For a short time period, Fuji produced neutron sensitive image plates that contained a converter material in the plate and offered better resolution than is possible with an external conversion material. Image plates offer a process that is very similar to film imaging, but the image is recorded on a reusable image plate that is read and cleared after imaging. These systems only produce still images (static). Using a conversion screen and an x-ray image plate, comparable exposure times are required to produce an image with lower resolution than film imaging. Image plates with imbedded conversion material produce better images than external conversion, but currently do not produce as good of images as film.

A digital technique similar to CCD imaging. Neutron exposure leads to short lifetimes of the detectors that has resulted in other digital techniques becoming preferred approaches.

An emerging method that produces a digital detector array with very small pixel sizes. The device has small (micrometer) channels through it, with the source side coated with a neutron absorbing material (generally gadolinium or boron). The neutron absorbing material absorbs neutrons and converts them into ionizing radiation that frees electrons. A large voltage is applied across the device, causing the freed electrons to be amplified as they are accelerated through the small channels then detected by a digital detector array.