Neutron microscope

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Neutron microscopes use neutrons to create images by nuclear fission of lithium-6 using small-angle neutron scattering. Neutrons also have no electric charge, 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. [1] The system increases the signal rate at least 50-fold. [2]

Contents

Neutrons interact with atomic nuclei 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. [1]

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. [1]

While neutrons have no charge, they do have spin and therefore a magnetic moment that can interact with external magnetic fields. [1]

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. [1]

Neutrons also offer unique capabilities for research in magnetic materials. 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. Polarized neutron beams orient neutron spins in one direction. This allows measurement of the strength and characteristics of a material's magnetism. [1]

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. [3]

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. 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. [1]

Wolter mirror

Glass lenses and conventional mirrors are useless for working with neutrons, because they pass through such materials without refraction or reflection. Instead, the neutron microscope employs a Wolter mirror, similar in principle to grazing incidence mirrors used for x-ray and gamma-ray telescopes. [1]

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. 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. [1]

The microscope uses several reflective cylinders nested inside each other, to increase the surface area available for reflection. [3]

Measurement

The neutron flux at the imaging focal plane is measured by a CCD imaging array with a neutron scintillation screen in front of it. The scintillation screen is made of zinc sulfide, a 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 energy. These fission products cause the ZnS phosphor to light up, producing an optical image for capture by the CCD array. [1]

See also

Related Research Articles

<span class="mw-page-title-main">Microscopy</span> Viewing of objects which are too small to be seen with the naked eye

Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.

<span class="mw-page-title-main">Microscope</span> Scientific instrument

A microscope is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisible to the eye unless aided by a microscope.

<span class="mw-page-title-main">Optics</span> Branch of physics that studies light

Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

<span class="mw-page-title-main">Pinhole camera</span> Type of camera

A pinhole camera is a simple camera without a lens but with a tiny aperture —effectively a light-proof box with a small hole in one side. Light from a scene passes through the aperture and projects an inverted image on the opposite side of the box, which is known as the camera obscura effect. The size of the images depends on the distance between the object and the pinhole.

<span class="mw-page-title-main">Optical microscope</span> Microscope that uses visible light

The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microscope and were possibly invented in their present compound form in the 17th century. Basic optical microscopes can be very simple, although many complex designs aim to improve resolution and sample contrast.

<span class="mw-page-title-main">Camera lens</span> Optical lens or assembly of lenses used with a camera to create images

A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.

<span class="mw-page-title-main">Scintillation counter</span> Instrument for measuring ionizing radiation

A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses.

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

<span class="mw-page-title-main">Reflection (physics)</span> "Bouncing back" of waves at an interface

Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected.

Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).

<span class="mw-page-title-main">Backscatter</span> Reflection which reverses the direction of a wave, particle, or signal

In physics, backscatter is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, although specular backscattering can occur at normal incidence with a surface. Backscattering has important applications in astronomy, photography, and medical ultrasonography. The opposite effect is forward scatter, e.g. when a translucent material like a cloud diffuses sunlight, giving soft light.

<span class="mw-page-title-main">Confocal microscopy</span> Optical imaging technique

Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser scanning confocal microscopy (LSCM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a spatial pinhole to block out-of-focus light in image formation. Capturing multiple two-dimensional images at different depths in a sample enables the reconstruction of three-dimensional structures within an object. This technique is used extensively in the scientific and industrial communities and typical applications are in life sciences, semiconductor inspection and materials science.

X-ray optics is the branch of optics that manipulates X-rays instead of visible light. It deals with focusing and other ways of manipulating the X-ray beams for research techniques such as X-ray crystallography, X-ray fluorescence, small-angle X-ray scattering, X-ray microscopy, X-ray phase-contrast imaging, and X-ray astronomy.

An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.

Diffraction topography is a imaging technique based on Bragg diffraction. Diffraction topographic images ("topographies") record the intensity profile of a beam of X-rays diffracted by a crystal. A topography thus represents a two-dimensional spatial intensity mapping of reflected X-rays, i.e. the spatial fine structure of a Laue reflection. This intensity mapping reflects the distribution of scattering power inside the crystal; topographs therefore reveal the irregularities in a non-ideal crystal lattice. X-ray diffraction topography is one variant of X-ray imaging, making use of diffraction contrast rather than absorption contrast which is usually used in radiography and computed tomography (CT). Topography is exploited to a lesser extends with neutrons, and has similarities to dark field imaging in the electron microscope community.

<span class="mw-page-title-main">Wolter telescope</span> X-ray source magnifier

A Wolter telescope is a telescope for X-rays that only uses grazing incidence optics – mirrors that reflect X-rays at very shallow angles.

<span class="mw-page-title-main">Neutron imaging</span>

Neutron imaging is the process of making an image with neutrons. The resulting image is based on the neutron attenuation properties of the imaged object. The resulting images have much in common with industrial X-ray images, but since the image is based on neutron attenuating properties instead of X-ray attenuation properties, some things easily visible with neutron imaging may be very challenging or impossible to see with X-ray imaging techniques.

The angle of incidence, in geometric optics, is the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal. The ray can be formed by any waves, such as optical, acoustic, microwave, and X-ray. In the figure below, the line representing a ray makes an angle θ with the normal. The angle of incidence at which light is first totally internally reflected is known as the critical angle. The angle of reflection and angle of refraction are other angles related to beams.

<span class="mw-page-title-main">Scanning helium microscopy</span>

The scanning helium microscope (SHeM) is a novel form of microscopy that uses low-energy (5–100 meV) neutral helium atoms to image the surface of a sample without any damage to the sample caused by the imaging process. Since helium is inert and neutral, it can be used to study delicate and insulating surfaces. Images are formed by rastering a sample underneath an atom beam and monitoring the flux of atoms that are scattered into a detector at each point.

References

  1. 1 2 3 4 5 6 7 8 9 10 "What shall we do with a neutron microscope?". Gizmag.com. 21 October 2013. Retrieved 2013-10-21.
  2. Liu, D.; Khaykovich, B.; Gubarev, M. V.; Lee Robertson, J.; Crow, L.; Ramsey, B. D.; Moncton, D. E. (2013). "Demonstration of a novel focusing small-angle neutron scattering instrument equipped with axisymmetric mirrors". Nature Communications. 4: 2556. arXiv: 1310.1347 . Bibcode:2013NatCo...4.2556L. doi:10.1038/ncomms3556. PMID   24077533. S2CID   34185138.
  3. 1 2 "New kind of microscope uses neutrons - MIT News Office". Web.mit.edu. 2013-10-04. Retrieved 2013-10-21.