Backscattering Spectrometer (BASIS)

The Backscattering Spectrometer on beamline 2 received operational approval on May 19, 2006. We have measured the first spallation neutrons from the target coming down the guide! Using a Kapton sample, the neutron count rate went up 3 orders of magnitude during beam production. A screenshot was taken of a diffraction measurement with an 8-pack of ³He filled Linear Position Sensitive Detector tubes (LPSDs) set up around a Mica sample.

Backscattering Spectrometer at SNS. Click image for a larger view.

The SNS Backscattering Spectrometer is a near-backscattering, crystal-analyzer spectrometer designed to provide extremely high energy resolution, as low as 2.2 - 2.6 microeV at the elastic peak (depending on sample size). The design requires a long initial guide section of 84 m from moderator to sample in order to achieve the timing resolution necessary to achieve the desired energy resolution. The design is optimized for quasielastic scattering, but will provide 0.1% resolution in energy transfer up to energy transfers of 18 meV. This spectrometer will provide an unprecedented dynamic range near the elastic peak of plus and minus 258 microeV, approximately seven times that of comparable reactor based instruments. For experiments that require the full dynamic range available at reactor based instruments (or greater), we expect this spectrometer to have a count rate approximately 50 to 100 times greater than the current best spectrometers.

Backscattering Spectrometer at SNS. Click image for a larger view.

Eugene Mamontov is the SNS instrument scientist responsible for the design and construction of this instrument. It was selected for early consideration by the Instrument Oversight Committee (currently the Experimental Facilities Advisory Committee) in November 1998. It was enthusiastically received by members of the Instrument Advisory Team, and the conceptual design was well received by the IOC at its July 1999 meeting. The instrument completed final design review in October 2004. After a year and a half of procurement and installation activities, it was approved through Instrument Readiness Review to operate for commissioning.

The following documents contain more detailed information about the expected performance and design of the spectrometer:
EFAC presentation, October 2002 (PDF 1.72M)
Conceptual Design Report (PDF 470K)
Resolution Function (PDF 485K)
Recent Presentation (PDF 1.99M)
Instrument data sheets (PDF)

Internal Team Site

Instrument Readiness Review Site