What shall we do with a neutron microscope?

Neutrons have a set of unique properties that make them better suited than light, electrons, or x-rays for looking at the physics and chemistry going on inside an object. Scientists working out of MIT's Nuclear Reactor Laboratory have now invented and built a high-resolution neutron microscope, a feat that required developing new approaches to neutron optics.

Why would anyone want to use neutron imaging to study materials? Optical microscopes tell you what the reflectivity of the surface of a material is, but little else. X-ray microscopes tell you what the mass density of the insides of an object is, but again, little of any structure that isn't mirrored in the density of the material.

In contrast, neutrons are heavy compared to the other particles (photons and electrons) used in forming images, and have no electric charge, properties that make it possible to look deeply inside an object while gaining information about the structure that is not accessible through the other forms of microscopy. Unfortunately, these same properties make it difficult to focus a beam of neutrons – a prerequisite for forming an image.

Neutrons do interact with atomic nuclei via the strong force. This interaction can cause the neutrons to scatter from their original path, and can also remove neutrons through absorption. Either way, a neutron beam that is penetrating a material becomes progressively less intense. In this way, neutrons are analogous to x-rays for studying the invisible interiors of objects.

However, while the darker regions of an x-ray image indicates how much matter the x-rays have passed through, the density of a neutron image provides information on the neutron absorption of the material. This absorption can vary by many orders of magnitude among the chemical elements.

As a result, a neutron image provides different information about the composition and structure of the interior of an object than do x-ray images. In particular, neutron imaging has great potential for studying so-called soft materials, as small changes in the location of hydrogen within a material can produce highly visible changes in a neutron image.

Neutrons also offer unique capabilities for research in magnetic materials. Neutrons may be uncharged, but they do have spin, and hence also a magnetic moment. It can help to think of a tiny bar magnet within the neutron that can interact with other magnetic fields. The neutron's lack of electric charge means there is no need to correct magnetic measurements for errors caused by stray electric fields and charges, another argument for using neutrons to study magnetism.

The most informative approach to using neutrons to study magnetic materials is likely the use of polarized neutron beams, beams in which the neutron spins are oriented in the same direction. This allows measurement of the strength and characteristics of magnetism within a material. Such information is extraordinarily difficult to determine in any other way, and cuts to the essence of the magnetic properties of a material.

Neutron images, such as those used in nondestructive testing, have been based mainly on shadowgraphs – images produced by casting a shadow on a surface, usually taken with a pinhole camera. Such methods, however, always involve an awkward balance between low illumination levels (and hence long exposure times) and poor spatial resolution – both being the natural result of using only pinhole optics.

Similar problems are associated with the pinhole optics of the camera obscura, a camera that forms an image of a scene by projecting light from the scene through a pinhole. A rule of thumb states that a good balance between illumination and resolution is obtained when the diameter of the pinhole is about 100 times smaller than the distance between the pinhole and the image screen, effectively making the pinhole an f/100 lens.

Optimum, however, is not necessarily good. The level of illumination on the image screen projected from an f/100 pinhole would be more than 1,000 times dimmer than that from a standard f/2.8 camera lens. Perhaps worse, the resolution of the pinhole lens cannot be smaller than the diameter of the hole. The resolution of an f/100 pinhole is about half a degree, making the camera obscura barely able to notice that the Moon looks like a disk rather than a point of light. However, an f/100 glass lens with a diameter of an inch can see lunar craters smaller than 10 miles (16 km) across.

The potential for dramatically improving the performance of pinhole-based neutron optics led the MIT Nuclear Reactor Laboratory group to develop an imaging neutron microscope. Their goals were to increase both the resolution of the image and the level of illumination, so that the neutron microscope can quickly produce higher-quality images. Unlike the case of an optical microscope, however, there is no equivalent of optical glass from which lenses for neutrons can be made. Conventional mirrors also tend not to work, as the neutrons simply go through them.

The key to the design of the neutron microscope is the Wolter mirror, similar in principle to grazing incidence mirrors used for x-ray and gamma-ray telescopes.

When a neutron grazes the surface of a metal at a sufficiently small angle, it is reflected away from the metal surface at the same angle. When this occurs with light, the effect is called total internal reflection. However, owing to the way neutrons interact with the electrons in a metal, it would be better to call this total external reflection – the neutrons refuse to enter the material. Fortunately, the critical angle for grazing reflection is large enough (a few tenths of a degree for thermal neutrons) that a curved mirror can be constructed. Given curved mirrors, an optical system that creates an image can be made. The figure below shows a cartoon of a four power neutron microscope after the MIT design.

Having formed a neutron image, it is necessary to find a way to visualize it. In the MIT microscope, the neutron flux at the imaging focal plane was measured by a CCD imaging array with a neutron scintillation screen placed in front of it. The scintillation screen is made of zinc sulfide (a traditional fluorescent compound) laced with lithium. When a thermal neutron is absorbed by a lithium-6 nucleus, it causes a fission reaction that produces helium, tritium, and a lot of excess energy. These fission products cause the ZnS phosphor to light up like a Christmas tree, producing an image in light that can be captured with the CCD array.

MIT’s new neutron microscope is a proof-of-principle, attaining only a four-fold magnification and 10-20 times better illumination than earlier pinhole neutron cameras. However, it points the way toward new approaches to study properties of whole classes of fascinating and potentially useful materials.

Source: MIT Nuclear Reactor Laboratory