SLAC National Accelerator Laboratory
BL4-2 Biological Small Angle Scattering/Diffraction

BL4-2 Optics for B-SAXS/D Experiments

Note: We currently run the new 2T wiggler at a reduced field to manage thermal load on the existing optic components. Once optics upgrade is completed (planned in 2007), we will be running it at its full field. This is expected to increase beam flux by an order of magnitude.

Wiggler
                • 2 Tesla (20.0 kGauss)
                • 10 periods (20 poles)

Wiggler

New 2T 20-pole wiggler

Focusing mirror
                •  Cylindrically bent toroidal mirror
                •  Pt-coated silicon
                •  Focusing both horizontally and vertically in a 1:1 ratio
                •  Radii: 1500 m (vertical, adjustable) x 10 cm (horizontal)
                •  Energy cutoff: 11.5 keV

flux

                •  Mean angle of incidence: ~7 mrad
                •  Normally takes a 0.25-2mrad horizontal fan for B-SAXS/D experiments

Monochromator
                • Parallel crystal geometry (1st crystal water-cooled)
                • Upward reflecting
                • White light mode available

        1. Si(111) double crystal
                •  ~1011 photons/s at specimen
                •  high energy resolution (Dl/l ~5x10-4)
                •  better harmonic rejection

        2. A pair of W/B4C multilayers, which recently replaced older Mo/B4C multilayers
               •  ~1012 photons/s at specimen
               •  modest energy resolution ~2%
               •  best for time-resolved studies
               •  multilayers are composed of 125 W/B4C layer pairs with 25Å periodicity deposited on a Si substrate  

Energy range:
                • ~6 keV to 11.3 keV focused

Absorption in beamline (does not include BSAXS/D experimental apparatus):
                • 546 micron Be
                • 1.6 m He
                • 15.2 micron C

Beam spot size (FWHM):

• 0.2 mm x 1.0 mm focused, typical; beam size can be reduced per user requirements, as small as 0.1 mm x 0.1 mm with some loss of flux