Triple-Axis Spectrometer (HB-3)

The triple-axis spectrometer is one of the most versatile instruments for measuring excitations in solids via neutron scattering. HB-3 is a colossal flux thermal neutron three-axis spectrometer designed for inelastic measurements on single crystals over a wide range of energy and momentum transfers. While the energy and momentum range for measurements is quite large at HB-3, the instrument is the ideal location to perform experiments at high energy transfers. This is due to a combination of its location directly at the end of the beam tube and the availability of a beryllium monochromator. Examples of experiments which would be performed on the HB-3 triple-axis spectrometer (in preference to other HFIR triple-axis spectrometers) are excitations at high energies such as phonons in high-temperature cuprates and magnons in iron and nickel.A wide range of sample environments, including closed-cycle helium (Displex) refrigerators, 4He cryostats (with dilution option), cryomagnets, and furnaces can be accommodated on HB-3.

In the current installation, the HB-3 triple-axis spectrometer has a fixed-focus PG002 monochromator optimized at 20 meV. In the near future, the monchromator will be upgraded to provide three crystal choices (PG 002, Be 002, and Si 111) with continuously variable focusing. The variable focusing will be calibrated to maintain the smallest beam size at the sample position, thus optimizing the incident neutron flux at each energy.

Pyrolitic graphite provides the highest neutron intensity because it has a high neutron reflectivity. The high-quality beryllium monochromator will allow measurements with good energy resolution while relaxing the q-resolution. Silicon 111 crystals have the advantage that there is no second-order reflection, thus providing a higher order contamination-free beam. The availability of these three monochromator crystal choices will make the spectrometer extremely versatile for a wide range of studies of the properties of materials.

The in-pile collimation can be reduced from its intrinsic value of 48' to a user-selectable value of 30' or 15' at the push of a button via the shutter control panel. This spectrometer also provides an in-pile sapphire filter for the white beam before the monochromator. Söller collimators from the monochromator-sample, the sample-analyzer, and the analyzer-detector can be changed as required by the user. With the upgraded HFIR beam tubes, the beam diameter of the effective source that illuminates the monochromator is 4.625 in.