Neutron Sciences Directorate

Using Bragg’s Law, a triple-axis spectrometer (TAX) selects the wavelengths of neutrons before and after they hit the sample, which directly probes the energy and momentum response of various materials. TAX provides a very high neutron flux thanks to its location directly at the end of the beam tube that is situated slightly away from the tangential path toward the reactor core. The beam diameter of the effective source that illuminates the monochromator is 4.625 inches. A sapphire filter is placed before the monochromator to reduce epithermal neutrons. Inside the drum, there are 3 monochromators mounted on a unit. They are pyrolytic graphite (PG 002), beryllium (Be 002), and silicon (Si 111), each of which is equipped with vertical focusing device that keeps the beam height at 1 inch over a wide range of incident energies at the sample position.

PG (002) provides the highest neutron intensity because it has a high neutron reflectivity. The unique high-quality Be (002) monochromator extends the energy range and provides good energy resolution at high energy transfers. Si (111) crystals have the advantage of an absent second-order reflection, thus providing a clean beam. The availability of these three monochromator crystal choices makes this spectrometer extremely versatile for a wide range of studies.

The momentum and energy resolution can be further defined by changing the horizontal divergence of the beam through Söller collimators. The in-pile collimation can be reduced from its intrinsic value of 48' to a user-selectable value of 30' at the push of a button via the shutter control panel. Collimators from the monochromator-sample, the sample-analyzer, and the analyzer-detector can be changed manually. The PG analyzer has a fixed vertical focusing optimized for the final energy of 14.7 meV. The single 3He detector is tall enough for the choice of various other final energies too.