Introduction

Gamma-ray diffraction is a non-destructive method used to extract crystallographic data from a sample, which can be a large bulk object, a small test sample or a single crystal. The technique is able to give great resolution even for thick samples and good three-dimensional spatial measurements and real-time density measurements inside a small specified volume.^[1]The method works in a way similar to X-ray diffraction (XRD), with the difference of using much more energetic photons. A typical photon used for XRD has a wavelength of about 1 Å and energy of 1keV, whereas a typical gamma ray used in diffractometry has a wavelength of about 0.03 Å and energy of 400 keV^[2]. Due to several different mechanisms in which gamma-rays interact with matter several approaches, both inelastic and elastic, can be taken. Due to the weak interactions of gamma-radiation with matter, the mean free path of the particles is typically from 1 – 4 cm and thus large samples and little to none cutting or surface preparation of the sample needs to be done.

Gamma-ray source

Unlike X-rays which are produced by interactions between electrons from an external source and electrons on an atom’s shells, the gamma-rays are produced by changes within the nucleus of the atom^[3]. The ways to generate gamma-rays have generally been too low for successful application and the intrinsic short lived nature of the highly radioactive sources restricts the locations in which these instruments can be constructed; for example a great gamma-ray source ¹⁹⁸Au has a half-life of 2.7 days as compared to a much less radioactive ¹⁹²Ir which has a half-life of 74.2 days. If the intensity of the gamma-ray beam is low the measurements will take an unreasonable amount of time. Because of this, many of the analysis instruments are located near nuclear reactor stations, in which these source isotopes can be created. The usage of commercially available ⁵¹Cr source allows usage of up to 5 half-lives and hence only need to be replaced once every 100 days, allowing the instrument to be used further away from aforementioned reactors.

Comparison to X-ray diffraction

The functional instruments operate in a manner comparable to a conventional X-ray or neutron diffractometer. However, the advantage of gamma over X-ray radiation is based on the fact that X-ray sources typically generate a much wider and complicated line shapes, while gamma-rays sources generate highly monochromatic energies, enabling imaging if higher resolution. The Kα1 and Kα2 peaks generated by a typical X-ray source vary by about 10^-3 while the respective variance for gamma-ray sources is about 10^-6.

Instruments

The two inelastic gamma-ray scattering applications include quasi-elastic gamma spectrometer (QUEGS) and Compton Gamma-ray Spectrometer (COGS).

The first of these, QUEGS is derived from Mössbauer spectroscopy, while COGS is based on Compton scattering. 

Several different instruments that use gamma ray diffraction have been devised:^[4]

• MUGS, a general purpose, crystal physics-oriented instrument for elastic scattering, ab solute structure factors, crystal perfection and thermal motion studies.
• SMUGS, a dedicated crystallography instrument for single crystal diffraction, free from absorption and extinction.
• GAPS, a powder diffractometer with multicounter which can be used with the Rietveld method of structure solving.
• QUEGS, a quasi-elastic instrument for studies of diffusion and low energy excitations, and possibly, for phonon measurements.
• COGS, a Compton scattering spectrometer for studying the electron momentum distributions in solids.

Use cases

As an example, voids in aluminium castings have been analyzed in the automobile industry and defects inside jet engines turbine blades in the aircraft industry.^[1] Especially metal castings can be analyzed without further sample preparation and has been used to detect cracks, voids and bubbles inside the homogenous material and inclusions in composite and ordnance materials. Compared to typical transmission methods, gamma-ray spectroscopy has been shown to increase the contrast by a factor of four in these use cases.

References