GAPS (Get Access to a Polarized Spectrometer) : A Cold 3-Axis CRG at the ILL

The LLB has proposed the construction of a new fully polarized triple-axis spectrometer instrument at the ILL in the framework of the major upgrade program Endurance2. The CRG will be operated by LLB and it is planned that 50% of the beam time will be available through 2FDN proposal calls. The start of installation of the GAPS CRG is scheduled for 2022.

Short description : The CRG GAPS aims to build at the ILL a word-class compact cold-neutron triple axis spectrometer (cold-TAS), with a high neutron flux, and primarily dedicated to polarization analysis. Fully equipped for spherical polarization analysis (µPAD), GAPS will noticeably increase the capacities of polarized neutron for inelastic studies at ILL. Its compactness is ensured thanks to a double-monochromator geometry for the primary spectrometer, while the secondary spectrometer will be adapted from the already existing and recently upgraded 4F1 instrument at LLB.


  • Frustrated and/or quantum magnets
  • f-electron systems with multipolar, Kondo insulating or exotic superconducting states
  • Unconventional superconductors
  • Multiferroic materials
  • Materials Sciences

Strengths : The great advantage of the double-monochromator solution is that the sample table is immobile when changing the incident neutron energy, demanding much less space on the ground for movement. The background is also lower. Cooper-Nathans based simulations show a similar flux of a double-monochromator instrument compared to a conventional TAS with a single monochromator. For the design proposed in GAPS, the incident neutron flux is found to be 80% of the flux of a conventional TAS (same total distance source-sample as for a conventional TAS and including the pyrolithic graphite transmission).