We had the long-standing problem of huge intensity differences between X18B and X19A, some users claimed to see a factor of 50 more intensity at X19A compared to X18B.
The white beam slits at X18B are a distance of 17m away from the source, and the sample is about 20m away from the source. The white beam slits reduce the vertical beamsize to 1mm, and an additional pair of slits reduces the horizontal beamsize to 10mm for reference measurements. Therefore, the acceptance of the beamline is 0.05x0.5mrad2.
At X19A, the white beam slits are 7.5m away from the source. The vertical size is 1mm, but there are no adjustable horizontal slits. The mask upstream of the collimating mirror limits the vertical size to 20mm. Therefore the acceptance of the beamline is 0.133x2.66mrad2. The intensity at X19A should be a factor 14 higher than the intensity at X18B.
We measured the intensity at both beamlines 100eV above the Cr K-edge at beam-currents of 270-280mA. At X18B, we observe a current of 4.97·10-7A in the 30cm long Oxford ion-chamber, and a current of 1.6·10-8A in the PIPS detector, which is 66mm away from the center of a Cr foil. At X19A, we observe a current of 1.44·10-6A in the 60mm long ion chamber, and a current of 1.15·10-7A in the PIPS detector, which was also 66mm away from the same Cr foil. Based on these results, the intensity in the ion-chambers is a factor of 12 higher at X19A compared to X18B, and a factor of 7 higher with the PIPS-detectors.
When we moved the PIPS-detector closer to the sample, we gained a factor of 2, and the intensity of the PIPS-detector mounted in the sample chamber was even a factor of 7 higher. This explains the factor of 50 observed by some users: At X19A, it is possible to mount the PIPS-detector closer to the sample, since the beam is focused to a small spot, compared to the line at X18B. When we moved the PIPS-detector closer to the sample at X18B, we doubled the signal.
We also compared the effect of windows on the intensity: The aluminized mylar window and a 25μm thick Al-window yield an intensity of 1.15·10-6A and 1.09·10-6A, respectively, and no window increases the intensity to 2.42·10-6A.
We finally compared all three PIPS-detectors: All three give a signal of about 2.8·10-6A under similar conditions. Small differences are attributed to slightly different distances from the sample.
The conclusion is that the intensity ratio between X19A and X18B is about 10, and that the signal observed in the PIPS-detector depends crucially on the distance to the sample.
X18b-x19a-intensity.doc