Ames Laboratory Neutron and X-ray Scattering Group

Ames Laboratory Neutron Scattering Group:

The group had its origins at the Ames Laboratory Research Reactor (ALRR) in the late 60's. When the ALRR was shut down in 1977, the group relocated to the Oak Ridge Research Reactor (ORR) at ORNL, moving three instruments, a triple axis spectrometer, TRIAX, and two diffractometers (one a polarized instrument and one fixed Ei machine), to that facility in 1978. Around 1986-7 the group became involved in plans to renovate the HB1A spectrometer at the HFIR. The instrument was built as a collaboration of the Ames Laboratory and ORNL neutron scattering groups with respect to funding and design effort. It was on line in 1990 when the HFIR came back up after an extended outage. The ORR had been shut down permanently by this time and the HFIR became the group's base of operations at ORNL. The TRIAX spectrometer was moved and installed at the Missouri University Research Reactor (MURR) in 1993-1995 and continues to be a productive neutron instrument.

HB1A is a fixed-initial-energy triple-axis spectrometer operating with Ei = 14.6 meV. The monochromator is a double crystal system with a vertically focusing pyrolitic graphite monochromator (M1) in the main beam of the HB1 beam tube. At the second monochromator position (M2), two PG monochromator options are available: a vertically focusing unit and a double focusing unit, the choice depending on experiment. The double crystal monochromator configuration provides one of the most intense beams of this energy at the HFIR and the cleanest beam in terms of higher order contamination by ?/2 (I?/2 ˜ 10-4 x I?) and very low gamma and fast neutron background. The analyzer can be selected from various analyzer crystals available: pyrolitic graphite, beryllium, silicon and germanium.

The recent upgrade of the HFIR at ORNL provided larger beam tubes for extracting much taller beams. This together with a larger monochromator at the M1 position as well as an evacuated flight path for 2 meters of the beam line, combined to improve the flux on sample by a factor of 2-3. The M2 shield was also replaced with a much larger and more effective version. The evacuated flight path, taller opening beam plug and M2 shield were designed and fabricated at Ames Laboratory. One of the major changes was eliminating the airpad module and tanzboden for support and motion of the analyzer/detector system and replacing it with a precision circular rail system. This has proven to be a very successful modification in terms of instrument accuracy and reliability.

The electronics and computer system controlling the spectrometer were also completely replaced. The SPICE triple-axis control software was implemented on HB1A by the HFIR neutron scattering staff. This is a state-of-the-art instrument operation and data acquisition application using PC's and LabView software. The transition to this system was very smooth, one of the design goals of the SPICE software design, and the spectrometer was brought into a functioning condition the same day the software was installed. This change required the complete replacement of almost all of the electronics on the machine. The beam intensity at the sample is comparable to ILL spectrometers, and the upgrade which is planned will result in about a factor of two more intensity.

The spectrometer became operational in April of 2003 and is now actively in use in the HFIR Center for Neutron Scattering (CNS) user program. Since Apr-03, 61 experiments involving 27 different investigators have been performed on the instrument. The HB1A spectrometer has proven to be an outstanding spectrometer for inelastic studies of excitations below ~9 meV at and below room temperature and for energies well above 20 meV above room temperature. It is also an excellent instrument for elastic studies of structural and magnetic transitions because of very low higher-order contaminations. The installation of the upgraded analyzer/detector shield and evacuated flight path and motorized slit system, will be carried out during a reactor outage and will not interrupt the operating schedule of the machine. It will make HB1A a truly world-class instrument.