Annual Review 2024-2025

Estimation of Compton and Rayleigh scattering from white-beam optics Diamond’s beamlines I12, I15 and I20 and the future SWIFT rely on wigglers to produce X-rays with a smooth spectrum up to energies of 80 keV. The increase in electron energy from 3.0 GeV today to 3.5 GeV in Diamond-II will not only increase the power load on the first optic but also increase the portion of the power emitted at higher photon energies (Figure 4a). First beamline optics are normally made from silicon because of its favourable thermal properties. For X-rays of energy over 57 keV, the cross section for Compton scattering in silicon exceeds that for photoelectric absorption, while the cross section for Rayleigh scattering is not much less. These processes scatter X-rays into the components surrounding the first optic. Since the scattered power is estimated to be 3 times greater in Diamond-II than currently, proper Compton shielding must be planned based on the spatial distribution of scattering estimated on various planes around the first optic (Figure 4b & c). A cooled copper “roof” should be within 10 mm of the first optic’s active surface. A cooled copper mask should protect the upstream end of the second optic. No mechanical or electrical components should be placed within 200 mm of the reflected beam. This guidance has been included in the procurement specifications for the first beamline optics. O P T I C S A N D M E T R O L O G Y A N N U A L R E V I E W 2 0 2 4 / 2 5 36 Figure 4: (a) Current (“Diamond”) and Diamond-II (“D-II”) spectrum of I15 wiggler through the primary slit aperture. (b) Diamond I20, 20 mm wiggler gap: Total power density (Compton + Rayleigh) scattered onto the plane perpendicular to the reflected beam and centred 1 m downstream of the centre of M1. (c) Diamond-II I20, 11 mm wiggler gap: Scattered power density on the same plane as in (b). Figure 3: (a) Model of cryo-cooled Si tangential deformation. Threshold power levels are defined as values at which deformation starts to increase steeply. (b) Measured flux on I04 versus incident power. A A B C B Power [W] Energy (eV) Width accros M1 (mm) Width accros M1 (mm) Height above M1 (mm) Height above M1 (mm) Flux on DCM (ph/s.0.1% BW) Power [W]