Neutrons provide unique penetrating radiation

In the short-wavelength world of matter waves -- past the UV portion of the spectrum and beyond x rays -- materials look very different and the rules of imaging are different than with photons. "Neutrons show you things that x rays will never be able to," said Wade Richards of the Univ. of California/Davis research nuclear reactor in Sacramento, CA (Figure 1).

Figure 1. Neutron radiograph of a flower corsage shows the method's ability to image thin biological samples. Photo courtesy of Nray Services.

The ways that neutrons interact with matter are very different from the way x rays interact with matter. X rays interact with the electron cloud surrounding the nucleus of an atom. Neutrons interact with the nucleus itself. In general, x-ray attenuation increases as the atomic number of the target material increases; usually, the attenuation is greater for lower energy x rays. Some light elements (such as hydrogen, boron, and carbon) have high thermal neutron attenuation coefficients, while some heavier elements (such as lead) have relatively small attenuation coefficients. The methods can be used in a complementary fashion. And while the imaging materials are different, neutron radiography is similar to x-ray radiography: a beam of particles penetrates the target, and the shadow of the device is captured.

Patrick Doty, Senior Scientist at Sandia National Labs. (Albuquerque, NM) said that because the intensity of neutron scattering varies irregularly with atomic number and neutron energies vary over a large range (and thus a wide range of useful wavelengths), neutrons can probe the structure of matter over many scales of length.

Neutrons can be divided into several energy groups: cold, thermal, epi-thermal, fast, and relativistic. This article concentrates on thermal and fast neutrons. Thermal neutrons have energies of roughly 0.03 eV or less, whereas fast neutrons are 10 to 15 MeV. Fission reactors produce neutrons with a wide range of energies, but because most reactors also have large volumes of moderator (to slow the neutrons down so they will be more efficient at initiating new fissions), reactors are excellent sources of thermal neutrons.

Beams of fast neutrons can be generated indirectly from particle accelerators (which accelerate charged particles into a target that gives off neutrons). "Fast neutrons are very highly penetrating," Doty said. "You can tailor the energies to look at a material, or you can look at shielded things -- you can look through lead shielding." Radioisotopes such as californium and amerecium-beryllium can generate fast neutrons as well, although this is not generally used for radiography.

At the McClellan Nuclear Radiation Center (MNRC) located just north of Sacramento, CA, Wade Richards, Tom Majchrowski and others use thermal neutrons from the small nuclear reactor for a variety of applications, including inspecting aircraft parts for corrosion.

The MNRC is a small nuclear reactor: whereas a power-generating reactor produces 2000 to 3000 MW, the McClellan reactor produces 2 MW. In nuclear reactors, neutrons are generated by fission in uranium. One neutron hits a 235U and two neutrons emerge. The neutrons emerge isotropically, heading in all directions. To use the neutrons for imaging, part of the shielding (10-ft.-thick concrete walls outside the tank of water in which the uranium sits) is removed. Pipes lined with neutron-absorbing and -scattering materials remove the particles that are too far out of line, providing a collimated beam of neutrons to the imaging bays. The reactor provides a flux of about 4 X 10⁶ neutrons/cm²/s at the radiography plane.

Richards said that in the 2D and 3D computer tomography neutron radiography systems in his facility, a converter screen uses gadolinium (which has a high cross-section for thermal neutrons), which emits beta particles when bombarded with neutrons, and the beta particles are caught by a fluorescing zinc sulfide material. Once the image has been converted to visible light, it can be captured on film or by a video camera sitting outside the neutron-beam axis. (Other converters can also be used, including lithium carbonate and plastic scintillators, depending on the application and neutron energies used.) A major application of neutron radiography at the reactor is for real-time imaging of military aircraft wings as a standard maintenance method to detect corrosion.

The McClellan site is the only facility in the world equipped with a robot stage and a video-camera radiography system that allows real-time imaging of objects 34 ft. long by 12 ft. high and as heavy as 5000 pounds.

There is always some concern about how much one can bombard the object before the nucleus becomes radioactive. For aluminum, Richards said there is a 2.5-minute decay time. "Within 10 minutes, it's all gone."

Figure 2. Neutron radiograph of a handgun shows a wide range of contrasts within a complex mechanical structure.

Glen MacGillivray at Nray Services (Petawawa, ON, Canada) described the complementary relationship between x-ray and neutron radiography: "When one is looking for a flaw, the contrast between the flaw and the unflawed material is paramount. If the attenuation in the material is too large, then insufficient beam penetrates to allow inspection." For applications that require distinguishing materials that attenuate differently, usually one wants to find the higher attenuator within a bulk material of lower attenuation. "So," MacGillivray said, "finding lead in a paraffin block (or a needle in a haystack) would work for x rays while looking for paraffin in a lead block (or a straw in a needlestack) would work for neutrons."

In addition to imaging corrosion in aircraft wings, neutron radiography's ability to detect hydrogen and carbon yield other applications. Neutron computer tomography has been used to analyze flaws and misalignments in o-rings, which are used for sealing joints in rockets and other heavy machinery. The method allows researchers to look through the outer (usually metallic) materials to see how artifacts from the fabrication process sometimes put crimps into the rings when they are in place (work of this sort for NASA has been done at McClellan).

In a similar way, neutron radiography can be used for viewing lubricants or other details of interest within a metal structure (Figure 2), or for looking at explosives contained within metals.

"Thermal neutron radiography is limited to reactor-based sources for high-resolution production-rate work," MacGillivray said. Accelerator and isotopic sources of neutrons can be used for imaging, but with sacrifices in speed and/or resolution. Images are obtained using a variety of techniques, including direct to film from gadolinium foil, indirect to film from activated foils of dysprosium (Dy, atomic number 66) or indium, to a CCD device from a scintillator, or to an imaging plate from a scintillator. "Imaging speeds of several thousand frames per second have been obtained," MacGillivray said.

MacGillivray said the neutron flux required for imaging depends on the application. "We have successfully used image-plane neutron fluxes ranging from 5 X 10⁴  to 4 X 10⁷ neutrons/cm²/s for film imaging," he said. MacGillivray will be presenting his invited paper, "Imaging with neutrons: the other penetrating radiation," at the Penetrating Radiation Systems and Applications conference at SPIE's Annual Meeting in July.

In addition to the applications mentioned above, neutron radiography can be used with a contrast agent to look for residual ceramic core material inside jet engine turbine blades. Another application is an indirect method of inspecting nuclear fuel -- a very radioactive material that would, if inspected directly, fog the film.

Figure 3. In addition to being gathered from nuclear reactors, neutrons can be generated by linear accelerators. This machine generates neutrons by accelerating protons into a beryllium target. The systems can generate from 10⁸ to 10¹³ neutrons/second. Photo courtesy of AccSys Technology.

Beams of fast neutrons, with energies from 10 to 15 MeV, offer different imaging capabilities than thermal neutrons. These energetic particles can be produced by linear accelerators.

Robert Hamm of AccSys Technology (Pleasanton, CA) said his company generates neutrons from linear accelerators by knocking an electron off a proton or deuteron (usually from hydrogen gas), accelerating the positively charged particle, then bombarding a target (usually beryllium). The target then emits secondary radiation (neutrons), mostly moving in the same direction as the ion beam. (Figure 3). "Fast neutrons penetrate matter very easily," Hamm said.

Fast neutron radiography can be used in prospecting for diamonds or other minerals, Hamm said. It also shows mineral inclusions in rocks. For this application, users shine two beams at the target, one after the other. One neutron beam is at a resonant energy for the mineral, the other is off-frequency. The difference between the images provides information about the interior of the rock.

Meanwhile, James Hall and others at Lawrence Livermore National Labs (Livermore, CA) produce fast neutrons using a "DD source." This is a system in which a deuterium beam is accelerated to the desired energies and hits deuterium with a pressurized gas cell (at 2 to 3 atm). The interaction creates neutrons mostly going in the same direction as the incoming beam.

Hall uses fast neutrons to penetrate steel, lead, or uranium. "We can look at voids of a few cubic millimeters in size behind up to four inches of uranium," he said, "and we can see it well."

Hall wants to create a system that will fit into a small laboratory and can image cubic-millimeter-scale voids or other structural defects in heavily shielded thick materials. The system should also be able to acquire tomographic image data sets, allowing users to gain a 3D image of the object.

To detect fast neutrons, Hall's group is using a rigid 4-cm-thick plastic scintillator indirectly viewed by a single commercial CCD camera. A thin mirror made of front-surfaced-aluminized Pyrex glass reflects light from the scintillator to the camera, which is well out of the neutron beam path. The camera's CCD is a cryogenically cooled thinned, back-illuminated 1024- X 1024-pixel CCD with an antireflective coating on its active area. The detector can be used for image integration times as long as an hour.

Applications include looking at thick objects not well penetrated by thermal neutrons. Hall's group has imaged a 6-in.-thick uranium and polyethylene slab assembly (with features machined into the polyethylene). The group also imaged a set of nine conventional step wedges. The step wedges were fabricated from lead, Lucite^®, mock high explosive, aluminum, beryllium, graphite, brass, polyethylene, and stainless steel. All were 0.5-in.-thick pieces with uniform steps ranging from 0.5 in. to 5 in. in width. All of steps in each of the materials within the detector's field of view could be discerned in the final processed image. A series of other imaging experiments, including tomographic imaging, have also been performed.