New Equipment at W1

W. Caliebe, U. Brüggmann, M. Lohmann, and P. Machek1

1Institute of Physics, Academy of Science of the Czech Republic, Cukrovarnicka 10, 16253 Prague, Czech Republic

The end-station at W1 houses two different instruments: A heavyweight high-resolution 6-circle diffractometer, which is mainly used for surface scattering, and an in-vacuum high-resolution spectrometer for resonant inelastic x-ray scattering.

The spectrometer is designed to work in dispersive geometry and that requires a position sensitive detector. For the past 3 years, the same direct illumination CCD-camera had been used and during that time, some radiation damage had occurred in the chip, which manifested itself in a non-uniform background over the area of the detector. The background subtraction resulted in some very sharp features in the spectra, which could not be avoided. In 2007, the CCD was replaced with a new direct illumination CCD, which is slightly larger in the vertical direction than the old one. The new CCD has 1380x400 pixels with a pitch of 20µm in both directions. In order to increase its lifetime, a fast computer-controlled shutter was installed in front of the sample. The shutter closes when the detector does not collect the data which reduces the overall exposure of the CCD to x-rays. The difference in the spectra is quite dramatic as can be seen in figure 1: Emission spectra of the same sample (V2O5) were collected at the same energy below the V K-absorption edge where the intensity is rather low. The spectra measured with the old CCD show some sharp artifacts, which clearly limit any attempts to analyze the data quantitatively. The spectra measured with the new CCD do not show these artifacts, and the data are significantly smoother – especially the background on the high-energy side of the emission lines.

Figure 1: V Kα emission spectra of V2O5 at an incident energy of 6467eV measured with the old and the new CCD detectors. Two artifacts of the old CCD at 4938 and 4954eV are clearly visible.

At the same time, new spherically bent analyzer crystals with three different orientations were purchased. Now, the spectrometer is equipped with large spherically bent Si(311), Si(400), Si(220), and Si(111) analyzers, which cover most emission lines of the 3d transition metals and 4f rare earth elements between 4 and 11keV with a Bragg angle between 60o and 86o. However, a few lines require analyzer crystals with different orientations, which are not easily available. It is planned to start a new program to manufacture the spherically bent analyzer crystals in the near future, and

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then, these missing analyzer crystals will be manufactured. One significant difference between the new spherically bent analyzer crystals and the old cylindrically bent ones is the difference in effective area by a factor of approximately six, which results in an increased signal and better data quality.

By the end of the year, a Mythen-Detector [1], a 1D Si-strip detector, arrived for tests with the diffractometer. Each individual strip has its own amplifier and acts as a separate detector, and works as individual counter each. The thickness of 300µm gives an upper limit of about 16keV with sufficient efficiency, and the electronic noise limits its range down to 5keV without signal independent counts, which is perfectly matched to most applications at W1. The maximum counting rate for an individual channel is 106 without any requirements for dead time correction, which is a factor of 30 more than that of the existing NaI-detector.

Figure 2: Experimental set-up with the Mythen-detector at W1. The standard flight tube is replaced with a 1m-long X95 profile, and the detector is mounted at its end.

The cross-talk between neighboring elements and photon-sharing is taken care of. The detector consists of 1280 strips with a width of 50µm and a height of 4mm. This makes it perfectly suited for grazing incidence reflection (GID) experiments, since it covers an angle of 3.4o, if it is mounted at a distance of 1m from the sample. The angular resolution corresponds to 0.0028o, which is better than what can be obtained with the existing slit-system. The divergence of the incident beam is still the limiting factor. The detector measures in parallel what has to be measured in serial with a point detector without any impact on the data-quality, which reduces the measuring time by a factor of about 20-100, depending on the measured angle-range. It is also possible to rotate the detector by 90o: In that geometry, it is possible to measure reflectivity and diffuse scattering with one scan, which reduces the measuring time by a factor of 2, however, since both are measured with the same attenuators, the quality of the diffuse scattering scan is slightly worse. The detector is mounted at the center of an X95-profile, which makes it rather easy to rotate the orientation of the detector by 90o.

First tests with the detector required that it was started to count by hand at each individual point of a scan, but the time required for a GID-scan or an Ω-2Θ map was a factor of 60 faster than that with a point-detector. Figure 3 shows a GID Si(220)-reflection of a Si(111)-wafer which was measured in 5 minutes with the Mythen-detector. Right now, the detector is integrated within the data-collection program online as a virtual counter, and in order to retrieve additional information (maximum counting rate, integrated counting rate, regions of interest) from each frame for scans.

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It is planned to use the detector also with the spectrometer since the spectrometer requires also just a 1D-detector with rather low spatial resolution in the vertical direction. The noise freedom of the 1D-detector compared to a CCD-detector might significantly improve the data-quality, but first tests have to be made. Since the detector cannot be mounted in vacuum, a different set-up has to be designed first.

Figure 2: Si(220) GID-Reflection from a Si(111) surface.

Finally, a new chamber was designed for the spectrometer, in which a closed-cycle cryostat can be mounted. The chamber makes use of the existing sample chamber with the two valves that separate the small sample chamber from the large spectrometer vessel. This reduces the time to obtain low pressure (10-7Pa) to a few minutes instead of several hours. The cryostat allows cooling down samples to temperatures of about 10K. First experiments are planned for the beginning of December 2007.

References

[1] B. Schmitt, http://pilatus.web.psi.ch/mythen.htm

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