Neutron radiography renders dense materials like lead and steel virtually transparent, allowing nondestructive tests on a myriad of mechanisms
John T. Lindsay
Senior Research Associate
University of Michigan
Ann Arbor, Mich.
Hallock Hydraulic Inc.
Ann Arbor, Mich.
Automotive transmissions often fail when oil doesn’t reach all areas inside the housings and case. Coke deposits can cause fuel injectors to clog in gas- turbine engines. When problems such as these happen, manufacturers often section the part to examine damaged areas. While this helps determine what’s wrong, it also destroys the sample and prevents further tests on the same part. Neutron radiography and neutron radioscopy offer nondestructive options. The techniques let engineers peer inside engines and transmissions while they are running and examine problems without damaging parts.
Neutron radiography is similar to X-ray radiography in that images are created based on the attenuation of radiation passing through matter. Unlike X-rays, however, the technique is suited for dense materials such as lead and titanium as well as fluids like water and oil. That is because X-rays interact with the electron shell around an atom’s nucleus. X-ray absorption increases with the number of electrons. Therefore, dense materials like lead, titanium, and steel stop X-rays, while materials such as water, oil, and human tissue are essentially transparent. Neutrons, on the other hand, interact with the nucleus of an atom and not the electrons. Neutrons penetrate materials with lots of electrons, and easily detect materials with a few electrons. This allows imaging of phenomena that is impossible with X-rays, such as water flow in soil, oil flow in a transmission, and fuel flow in an operating engine.
In practice a neutron source beams neutrons through an object to a detector. The low-energy neutron beam has a high sensitivity to most materials. Different materials, however, absorb and reflect the beam to different degrees to create an image of the object.
Neutron radiography techniques
Two common types of neutron radiography are film neutron radiography and real-time neutron radioscopy. Neutron radiography uses standard X-ray film developed much like X-ray images. Objects as small as a few micrometers can be imaged with either method.
In film neutron radiography, a screen converts the modulated neutron beam, which is nonionizing, into an ionizing form to expose the film. The type of screen determines the level of resolution. The most common, and highest resolution, screen is vapor-deposited, 25-µm-thick gadolinium metal deposited on an aluminum backing. A 25-nm sapphire layer protects the gadolinium from moisture in the air. The screen absorbs neutrons and, in turn, emits electrons. The electrons are then used to expose the film, giving the highest resolution available. A gadolinium oxysulfide rare-earth phosphor screen suffices for lower resolutions. This screen converts the neutron image into a light image which then exposes photographic film.
Film neutron radiography is used on static images. When objects are moving or changing with time, real-time neutron radiography, or neutron radioscopy, is used. A gadolinium oxysulfide screen converts the neutron image into a light image. It is then intensified and viewed with a TV camera. A computerized processing system enhances the images, which are stored on video tape in real time at a rate of 30 frames/sec. The images are viewed with an extended red camera, a standard camera that also sees light in the infrared spectrum.
Experts typically recommend neutron radiography when X-ray radiography is inadequate, or when parts require nondestructive tests. For instance, one recent application was a spin-loss study to analyze clutch plate spacing under various speeds and loads. The spacing and oil flow at the clutch plate interface contributed to energy losses. Neutron radioscopy determined the presence or absence of lubrication and helped track down obstructions or design faults in the clutch pack.
Neutron radiography has also been used to study two-phase flow. Applications include cavitation in pumps and fuel injectors, boiling and heat transfer in cooling channels and tubes, gas and fluid flow in porous solids, and air entrapment in extrusions and injection molding.
However, using neutron radiography requires a source of neutrons. Reactors that produce neutrons for radiography are different from those that produce power. Two common types of radiography reactors are material test reactors (MTR) and Triga reactors. These reactors have no pressure vessels around them like those surrounding power-producing reactors. They produce neutrons solely for experimental purposes.
Several universities, such as University of Michigan, Ann Arbor, Mich., and Pennsylvania State University, State College, Pa., perform neutron radiography and neutron radioscopy for outside companies. While university facilities are well suited for short-run tests, most are not capable of testing large quantities of parts. As such, these facilities are good for research and development and for locating and solving specific problems. One commercial radiography facility, Aerotest Operations Inc., San Ramon, Calif., performs large-batch neutron radiography.
Edited by Todd Zalud