Neutron radiography

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The neutron radiography (short Neutrographie) is for radiography analogous method of nondestructive testing of materials can be used, particularly in the hydrogen-containing materials (eg. B. many plastics).

Physical basics

In contrast to electrons and protons, neutrons have a high ability to penetrate matter due to their lack of electrical charge. While X-rays mainly interact with the electron shell of the atoms and their absorption in matter therefore increases steadily with the atomic number of the element, the interaction of neutrons with the atomic nuclei takes place and shows no correlation with the atomic number, but the total cross-section is(Absorption and scattering) of thermal neutrons, which is a measure of the strength of the interaction, significantly higher for light elements such as hydrogen, lithium, boron, carbon than for heavy elements. This essential difference makes neutron radiography a method of non-destructive material testing for components made of light elements that complements radiography. However, the use of neutron radiography is limited by the limited availability of powerful neutron sources.

technology

Fig. 1: Simple experimental setup (schematic)

As neutron sources are radioactive sources (neutron generation through nuclear reactions), particle accelerators , research reactors and spallation neutron sources in question, generally only offer the latter two sufficient neutron flux. The thermal neutron beam, collimated as well as possible from a beam tube, shows a spatially different intensity distribution after passing through the sample to be examined, which is registered by suitable detectors. Films and spatially resolving neutron counter tubes and semiconductor detectors in connection with electronic image intensifiers can be used as detectors. In connection with films and other detectors that are permeable to neutrons, a neutron-absorbing converter must be used, which converts the neutrons into radiation that can be recorded through nuclear reactions .

Demonstration examples

Image 2: left neutrography, right X-ray image of a plastic pencil sharpener
Image 3: Neutrography of polyamide gears inside a washing machine program switch made of sheet steel

The figures show two demonstration examples of neutron radiographs that were recorded on the research reactor FRJ-1 at Forschungszentrum Jülich , which has since been dismantled . The simple experimental setup is shown schematically in Figure 1. The thermal neutron beam (neutron flux density approx. 10 6 neutrons / cm², energy approx. 0.04 eV) emerging from a beam tube of the reactor hits the sample a, penetrates the film c without hindrance and becomes in the Converter foil d absorbed or scattered. The flashes of light triggered in the converter film blacken the film with the light-sensitive layer facing the converter film. Film and converter film lie flat on top of one another in a Polaroid film cassette b; they are pulled apart in the illustration for the sake of clarity. The converter foil consists of a mixture of LiF as neutron absorber and scatterer and ZnS sensitized with silver as a scintillator, which is bound in a plastic foil. Figure 2 shows the neutrographic image of a plastic pencil sharpener compared to an X-ray image. While the neutrograph only shows the plastic parts - only the threaded hole can be seen at the point where the fastening screw for the blade is located - the X-ray mainly shows the metal blade. Neutrography and X-ray imaging complement each other. Figure 3 shows three polyamide gears inside a sheet-metal-encapsulated washing machine program control mechanism and demonstrates the possibility of making light, hydrogen-containing plastic parts visible through sheet steel, which is not possible with an X-ray. Note that the gear axles are not visible here either.

Possible applications

The limited accessibility of sufficiently high-intensity neutron sources naturally severely restricts the use of neutron radiography. The following possible applications are conceivable:

  • non-destructive testing of metal bonds (aircraft construction)
  • Testing of plastic-encapsulated components for voids (transformers)
  • Level check on opaque containers (oils)
  • Investigation of radioactive nuclear fuel rods (hydride accumulations)
  • Investigations into the storage behavior of hydrogen in z. B. metal hydrides

With sufficiently strong neutron sources and with sensitive detectors it is also possible to observe dynamic processes.

Web links

literature

  • H. Berger, Neutron Radiography, Elsevier Publishing Company 1965
  • E. Roth et al., Neutrography - a novel detection method for plastics with neutrons, Kunststoffe magazine 1976
  • KV Kasiviswanathan, Introduction to Neutron Radiographie, Neutron Radiographie - Principles and Applications, 1999

supporting documents

  1. ↑ Construction of a dynamic radiography and tomography station with thermal neutrons (PDF; 5.8 MB)
  2. ↑ Look into the running engine. In: World of Physics. Archived from the original on August 17, 2014 .;