Low energy X-rays now available at C-line

Ken Finkelstein, CHESS, Cornell University

CHESS C1 station has been modified to improve beam delivery for low energy x-ray experiments.

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Figure shows C line optics box (in cave at left), an upstream portion of the beam flight path, and a new optical table. The additional x-ray mirror is visible at the downstream end of optics box. The beryllium window was removed and replaced with a pneumatic gate valve (1st hutch component) interlocked to the optics box vacuum system to prevent damaging downstream flight tube windows. A PLC (Programmable Logic Controller) provides interlock control, and automates box pump-down and helium back-fill. The PLC is controlled using a LCD touch panel interface. The fight tube contains scatter & ion chamber monitors and two independent sets of motorized slits. The newly designed optical table is shown below fight path.

The work was carried out by CHESS operators Chris Whiting, John Conrad, and Engineer Tom Krawczyk with supervision by Operations Manager Chris Conolly & Vacuum Group Leader Bob Seeley, responding to a request by Scientist Ken Finkelstein.

Improving beam quality & flux at low energy involves reducing attenuation along the beam path and increasing the rejection of energy harmonics passed by the monochrometer. Three significant changes were made to improve experiments down to 5KeV: a beryllium window was removed along the flight path between the CESR vacuum window and experiment, a second x-ray mirror system was built and installed downstream of the mono, and a CHESS- design high precision optical table was installed in the hutch. The beamline had a water-cooled, rhodium coated mirror upstream of the mono that suppresses harmonics by up to 10,000, but at 5.3GeV CESR produces lots of high energy photons. In the new configuration beam is reflected upward by the white beam mirror, made monochromatic by a double-bounce silicon or multilayer mono, then reflected downward by the second mirror. This last bounce reduces harmonics for the most demanding low energy experiments, while beam travels through helium all the way to the sample. Our efforts paid off in March when Suzette Pabit, doing cutting edge anomalous-Small-Angle-Scattering (ASAXS), reported no detectable harmonics at the manganese K-edge!