Distance From The Source Point

The distance from the source point is measured by projecting a fivefold slit into the detection plane. The distance between the slit and the detection plane d₂ is precisely known from an interferometric measurement. The distance from the projection plane to the radiation source point at the location of the electron beam d₁ can then be calculated from the distance of the projected slits at the detector plane. An accuracy of about 2 mm in the determination of the distance d = d₁ + d₂ to the radiation source point is reached. Typically, a detector to be calibrated is placed about 15 m from the radiation source point at the MLS, which gives a relative uncertainty of about 1.3•10^-4 in the determination of the distance.

The vertical emission angle is normally chosen to be zero (measurement in the orbit plane) by adjusting the detector to maximum signal. A typical uncertainty is 5 μrad for a calibration at 15 m distance.

At BESSY II a typical calibration distance is 30 m. A relative uncertainty of about 0.7•10^-4 in the determination of the distance and a typical uncertainty of 2 μrad for the vertical emission angle alignment is reached. The size a•b of a flux-defining aperture is normally a detector property and not a property of the primary source standard and therefore not included in this discussion.