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Very Small Angle Neutron Scattering 


The problem: the 30 m SANS instruments at NIST are capable of making measurements over angles corresponding to scattering vectors from q = 0.0008 Å-1 to 0.7 Å-1. The UltraSANS (USANS) instrument covers the range from q = 0.00003 Å-1 to 0.01 Å-1. So it is possible for researchers to cover a wide range of q values by using two quite different instruments having differing sample requirements and needing corresponding setting up as well as coordinated scheduling. In the recent Expansion Workshop there was interest expressed by the community for a single instrument having a q range extending to somewhat lower values than SANS with adequate intensity to make measurements practical in the time-frame typically available to users.


The vSANS solution: The planned vSANS instrument which will cover the usual SANS range will also allow configuration to cover the range between q  10-4 Å-1 to 10-3 Å-1 with a sample beam current of (104 neutrons/s). The key requirements are a second position-sensitive detector system having a 1 mm pixel size and a longer sample-detector flight path of 20 m (i.e., a 40 m instrument). To increase beam intensity at the sample, several options are being considered. One is a multiple confocal beam arrangement which is accomplished by a set of baffles containing pinholes to aim the beams at the central detector pixel. Intermediate baffles are used to remove cross-talk. If the pinholes at the sample baffle are very small, the wings of the Fraunhofer diffraction pattern arising from them spreads an undesirable background "halo" of counts over a wide area of the detector. To reduce this halo intensity, focusing lenses after the sample can be used to image the source pinholes on the detector. Then the relevant stop is set by the rim diameter of the usable lens portion, which can be much larger than a sample pinhole that would have been required in the absence of the lens, and the halo is correspondingly reduced. Gravitational fall of the neutrons becomes a problem with the long flight paths involved, and this can be compensated through the use of prisms just after the baffles.

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Last modified 29-August-2012 by website owner: NCNR (attn: Przemek Klosowski)