OH2 premonochamber

The x-ray beam from the undulator is premonochromatised by a silicon (1,1,1) channel-cut crystal to a relative bandwidth of DeltaE/E = 2·10-4. Due to the high heat load produced by the intense undulator beam, the crystal has to be cooled down to cryogenic temperatures of about 100K where silicon displays a maximum in the thermal conductivity and a mininum in the linear thermal expansion coefficient. This allows to keep the thermal deformation of the crystal below the limits where photon flux losses occur. The crystal is cooling by a closed-loop liquid nitrogen system.

Energy [keV] U32 gap[mm] Flux [photons/sec]
13.840 13.04 (3.) 6.75.1013
15.847 14.26 (3.) 7.20.1013
17.794 15.51 (3.) 6.00.1013
21.747 18.08 (3.) 3.75.1013
21.747 12.49 (5.) 4.20.1013
23.725 19.50 (3.) 3.00.1013
27.704 14.01 (5.) 1.95.1013

Measured photon flux behind the silicon (1,1,1) premonochromator for the operating points of ID28: Three 1.6 m U32 segments; ring current: 200 mA; primary slits: 1.6 mm (horizontal) x 0.7 mm (vertical); secondary slits: 2.4 mm (hor.) x 1.0 mm (vert.). The number in brackets in the second column indicate the undulator harmonics.