Neutron imaging
Circa 1960, Harold Berger (US) and John P. Barton (UK) began evaluating neutrons for investigating irradiated reactor fuel.
The first commercial facilities came on-line in the late 1960s, mostly in the United States and France, and eventually in other countries including Canada, Japan, South Africa, Germany, and Switzerland.
This can take the form of some length of water, polyethylene, or graphite at room temperature to produce thermal neutrons.
Eventually the speed of these neutrons will achieve some distribution based on the temperature (amount of kinetic energy) of the moderator.
Fast neutron imaging is an area of interest for homeland security applications, but is not commercially available currently and generally not described here.
To produce a good image, neutrons need to be traveling in a fairly uniform direction (generally slightly divergent).
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.
Until recently, neutron imaging was generally recorded on X-ray film, but a variety of digital methods are now available.
Film is generally the highest resolution form of neutron imaging, though digital methods with ideal setups are recently achieving comparable results.
The most frequently used approach uses a gadolinium conversion screen to convert neutrons into high energy electrons, that expose a single emulsion X-ray film.
Neutron radiography is a commercially available service, widely used in the aerospace industry for the testing of turbine blades for airplane engines, components for space programs, high reliability explosives, and to a lesser extent in other industry to identify problems during product development cycles.
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.
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.
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.
Neutron exposure leads to short lifetimes of the detectors that has resulted in other digital techniques becoming preferred approaches.
The device has small (micrometer) channels through it, with the source side coated with a neutron absorbing material (generally gadolinium or boron).
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.
A system to scan cargo containers using fast neutron and gamma-ray radiography was developed by CSIRO and trialled in Brisbane International Airport in 2005–2006.
[9] Unlike X-ray scanning, which can detect metallic items such as firearms but has problems with other substances, fast neutron and gamma-ray radiography is sensitive to a wide range of materials.
