16 Oct 2008



The General Materials Diffractometer (GEM) is used for high intensity, high resolution neutron diffraction and pair distribution function experiments to study the structure of crystalline powders and amorphous materials.

​​​​​​​​​​​​Gem instrument at target station 1

Principle Measurements

  • Pair distribution function (PDF) / total scattering measurements of disordered (poly)crystalline, nanocrystalline and amorphous materials.
  • Powder diffraction measurements for Rietveld refinements of atomic and magnetic structure.
  • Diffraction measurements of textured samples

Key Facts

  • The GEM detector array was completed in December 2003.
  • The completed GEM detector array has 7270 elements (not including monitors) in 86 modules.
  • The GEM shutter was opened for the first time on 12th October 1999.
  • The size of a typical datafile for one run on GEM is nearly 37 megabytes.
  • The GEM detectors cover a scattering angle range from 1.1° to 169.3°.
  • GEM's incident flight path is L1=17.0 metres.
  • GEM's scattered flight path ranges from 1.0 metres to 2.9 metres.
  • Over 100 kilometres of electrical cable were installed for the GEM detectors.
  • When complete, the GEM scintillator detector system will contain 660,000 individual optic fibres, whose total length will be about 350 kilometres.
  • The GEM vacuum vessel is made of 25mm grade 304 stainless steel, and is designed to attain pressures as low as 1x10-6 mbar.
  • The windows in front of the detectors are 0.7mm aero grade aluminium - when the vacuum vessel is under vacuum, the windows deform by about 45mm at the centre.
  • The GEM vacuum vessel has a volume of 1.3517m3 (i.e. 1351.7 litres).
  • The GEM beam-stop weighs about 80 tonnes, with a canister of boron carbide at the centre to absorb neutrons.
  • GEM has about 200 tonnes of bulk steel shielding, and about 43 tonnes of wax in shielding tanks.

Instrument links: 

Contact information for the Gem instrument​

Location and contact information for the Gem beamline

​ContactsAlex Hannon, Ivan da Silva and David Keen
Telephone01235 445198