Neutron microscopes use

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

to create images by

nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...

lithium-6 Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant on Earth. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon ( for lit ...

using small-angle neutron scattering. Neutrons also have no

electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...

, enabling them to penetrate substances to gain information about structure that is not accessible through other forms of microscopy. As of 2013, neutron microscopes offered four-fold magnification and 10-20 times better illumination than pinhole neutron cameras. The system increases the signal rate at least 50-fold. Neutrons interact with

atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...

via the strong force. This interaction can scatter neutrons from their original path and can also absorb them. Thus, a neutron beam becomes progressively less intense as it moves deeper within a substance. In this way, neutrons are analogous to x-rays for studying object interiors. Darkness in an x-ray image corresponds to the amount of matter the x-rays pass through. The density of a neutron image provides information on neutron absorption. Absorption rates vary by many orders of magnitude among the

chemical elements A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

. While neutrons have no charge, they do have

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

and therefore a

magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...

that can interact with external

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

Applications

Neutron imaging has 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 material A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel ...

s. The neutron's lack of

means there is no need to correct magnetic measurements for errors caused by stray

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

s and charges. Polarized neutron beams orient neutron spins in one direction. This allows measurement of the strength and characteristics of a material's magnetism. Neutron-based instruments have the ability to probe inside metal objects — such as fuel cells, batteries and engines to study their internal structure. Neutron instruments are also uniquely sensitive to lighter elements that are important in biological materials.

Shadowgraphs

Shadowgraphs are images produced by casting a shadow on a surface, usually taken with a pinhole camera and are widely used for

nondestructive testing Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage. The terms nondestructive examination (NDE), n ...

. Such cameras provide low illumination levels that require long exposure times. They also provide poor spatial resolution. The resolution of such a lens cannot be smaller than the hole diameter. A good balance between illumination and resolution is obtained when the pinhole diameter is about 100 times smaller than the distance between the pinhole and the image screen, effectively making the pinhole an f/100 lens. The resolution of an f/100 pinhole is about half a degree.

Wolter mirror

Glass lenses and conventional mirrors are useless for working with neutrons, because they pass through such materials without

refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...

or reflection. Instead, the neutron microscope employs a Wolter mirror, similar in principle to grazing incidence mirrors used for

x-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

and

gamma-ray telescope Gamma-ray astronomy is the astronomical observation of gamma rays,Astronomical literature generally hyphenates "gamma-ray" when used as an adjective, but uses "gamma ray" without a hyphen for the noun. the most energetic form of electromagneti ...

s. 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 Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...

. The critical angle for grazing reflection is large enough (a few tenths of a degree for thermal neutrons) that a curved mirror can be used. Curved mirrors then allow an imaging system to be made. The microscope uses several reflective cylinders nested inside each other, to increase the surface area available for reflection.

Measurement

The neutron flux at the imaging

focal plane In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the '' focal points'', the principal points, and the nodal points. For ''ideal'' ...

is measured by a CCD imaging array with a neutron scintillation screen in front of it. The scintillation screen is made of

zinc sulfide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...

, a

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

compound, laced with

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

. When a thermal neutron is absorbed by a

nucleus, it causes a

fission reaction Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioac ...

that produces

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

, tritium and energy. These fission products cause the ZnS

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or vi ...

to light up, producing an optical image for capture by the CCD array.

References

{{reflist Neutron instrumentation Microscopes

Applications

Shadowgraphs

Wolter mirror

Measurement

See also

References