monochromatic soft-Xray beam position monitor


Ultra-thin (<1um) Silicon Carbide free-standing membranes for beam intensity and position monitors of soft x-ray beamlines




X-ray absorption and dichroism measurements are widely used to characterize the electronic and magnetic properties of matter. Synchrotron beamlines are well suited for this task because they allow to continuously scan the energy of the incoming photons over a wide energy range at high resolution. A particular interesting photon energy range is the soft x-ray range between 400 and 1600 eV because it contains the important L2, 3 and M4, 5 edges of the transition metals and rare earths, respectively. Most synchrotrons run one or more beamlines for x-ray absorption experiments in this energy range. The quantitative analysis of such X-ray absorption data requires a good in situ characterization of the photon flux hitting the sample. The flux typically varies significantly with the storage ring current or with the changing polarization of the incoming light. Therefore, respective beamlines are usually equipped with a dedicated beam intensity monitor (BIM) as the last optical element in the beam before the sample. The main criteria for a BIM are linearity of the signal with respect to the transmitted beam, a strong monitor signal in the order of magnitude of the sample signal and at the same time, a high beam transmission ratio while introducing little or no distortion in the beam shape. Current available BIM technologies are “gold-meshes” and “Diamond conductive thin films”. Recently Silicon Carbide (SiC) free-standing membranes have been validated for hard-Xray Beam Position Monitoring (XBPM), as alternative to Diamond CVD ones [[1]]. In this work we show that ultra-thin (<250nm) Silicon Carbide (SiC) free-standing membranes can be an alternative to the widely used gold meshes or diamond thin films for the Beam Intensity Monitoring (BIM) of soft X-ray beamlines.  Main benefits of the new SiC sensors, as compared to both technologies are:

a) elimination of diffraction effects. This is known problem of mesh-type BIMs such as gold meshes. This effect becoming stronger when beam sizes reduced [[2]].

b) increase of the electric signal as compared to both diamond thin films or gold mesh. This thanks to the high yield of the internal photoemission effect, characterized by a low electron-hole generation energy (7.8eV)

c) possibility of monitor not only the intensity, but also the beam lateral position, thus achieving full X-ray Beam Position Monitoring (XBPM) functionality.


Fig.1 240nm SiC device assembled on a 4cmx3cm PCB. Electrical connections for the 2x2 sensors array can be seen from front view (left) whereas the 2mm free-standing thin membrane can be seen from back view (right).

Fig.2 shows theoretical X-ray transmissions for different Silicon Carbide and diamond sensors (100nm [2], 3um [[3]] and 20um[[4]]). As can be seen, SiC XBPM become applicable beginning from about 500 eV photon energy, where the important transition metal edges and the rare-earth edges are found. Preliminary results of current signals and transmission uniformity on different devices will be presented at next Synchrotron and Radiation Instrumentation (SRI2021) conference in Hamburg.

[1] S. Nida et al J. Synchrotron Rad. 26, 28 (2019)

[2] K. Kummer et al. Rev. Sci. Instrum. 84, 035105 (2013)

[3] K. Desjardins et al. J. Synchrotron Rad. 21, 1217 (2014)

[4] E. Griesmayer et al. AIP Conference Proceedings 2054, 060052 (2019)