Neutron Sciences Directorate

The fully instrumented HFIR will eventually include 15 state-of-the-art neutron scattering instruments, seven of which will be designed exclusively for cold neutron experiments, located in a guide hall south of the reactor building. The currently available instruments and the status of new instruments can be found on the instruments page. Particularly prominent in the cold neutron guide hall are the two small-angle neutron scattering (SANS) instruments, each terminating in a 70-ft-long evacuated cylinder containing a large moveable neutron detector. In addition to the instruments, laboratories are equipped for users to prepare samples.

Perhaps the most exciting development at HFIR is the successfully commissioned cold source in 2007. The HFIR source has a brightness measured to be the best in the world. The cold source increases the available neutron flux from 4 to 12 Å. For neutron scattering experiments, it’s ideal to match the wavelength and energy of the neutron to the length and energy scales, respectively, of the materials under investigation. Therefore, for studying large-scale structures (e.g., molecular organization, nanopore-size distributions, and aggregate size and shape) and low-energy excitations (e.g., excitations in frustrated systems and various problems in magnetism, superconductivity, and correlated electron systems), the best neutrons are those with long wavelengths and low energies—cold neutrons.

The thermal neutron spectrum of a reactor produces neutrons with wavelengths on the order of a few tenths of a nanometer, well matched to the study of atomic lengths scales and lattice vibrational energies. By passing the thermal neutrons through a vessel of low-temperature liquid hydrogen, the neutrons are slowed down by inelastic collisions with the hydrogen. This produces slower, long-wavelength neutrons better suited for studies of soft matter and low-energy excitations.

Cold neutrons reflect well from some surfaces and thus can be transported over long distances with little loss. Neutron guides can, therefore, be constructed to transport the cold neutron beam to the guide hall outside the reactor building where there is more room to work and where the sensitive instruments can be located in a very low radiation background. The HFIR source illuminates four such neutron guides, bringing beams to seven new instrument positions in the cold neutron guide hall. Two of these guides currently serve the two operational SANS instruments. Instruments on the other guides are being installed or are in development.