Polychromatic Beam Monitoring
This detector is currently developed in a collaboration with PSI (CH) through cantonal fundings.
Many synchrotrons around the world are been upgraded or in preparation to be upgraded towards diffraction limited storage rings (DLSRs) which will deliver X-ray beams with increased brilliance and reduced emittance to the beamlines, offsetting current limits and thus opening up new challenges for the machine teams, beam line (BL) designers, scientists, and BL users. One of these will be to keep the smaller beam stable, in position, intensity and energy distribution, on the smaller target samples. To cope with this, it will be more and more important to measure, realtime, the beam’s position, intensity and, ideally, shape, along the whole optical path, starting from the insertion devices (IDs). Diamond, either polycrystalline [[i],[ii]] or single crystal [[iii]], and more, recently, Silicon Carbide (SiC) [[iv]], have all demonstrated excellent non-destructive X-ray Beam Position Monitors (XBPMs) for monochromatic beams, whereas there is currently no commercial solution for polychromatic, pink- and white-, beams, i.e., for monitoring locations before monochromators. To be able to predict expected current signals and heat loads for different types of XBPMs and thus predict the feasibility of polychromatic beam monitoring in different facilities and beamline locations we have implemented a combined multi-software algorithm, linking together: (i) SPECTRA[[v]], to numerically evaluate the characteristics of radiation emitted from various synchrotron radiation (SR) sources, (ii) GEANT4, to calculate generated carriers for the different sensors, (iii) MATLAB, to convert the emitted radiation in generated current maps and (iv) COMSOL [[vi]], a finite element modelling (FEM) tool to calculate heat loads based on both direct and Joule heat. As a test case, we applied the algorithm to analyse the response of a 2.3µm SiC XBPM currently installed and under tests at the microXAS beamline of the Swiss Light Source at PSI. The 2.3µm SiC XBPM is located at 22 m from the U19 undulator, before the monochromator and after a 250µm window (pinkbeam location). Fig.1A. presents the flux spectral distribution at this position. As an example, the complex spatial flux distribution of the 8th harmonic at the XBPM position is shown in Fig.1B. The spectral transmittance of the beam after th 250µm thick diamond (CVD) window calculated using MATLAB is shown in Fig.1C. it can be readily noticed that the 1st harmonic is completely removed on the spectral flux. The spectral integrated spatial distribution of the filtered beam is shown in Fig.1D. The figure shows that features present at single harmonics (see Fig.1B) are largely smeared out. The number of electron-hole pairs generated on SiC and Blade XBPMs with different thicknesses is presented in Fig.1E. As shown in the figure the XBPM is mainly sensitive to low photon energies due to the high absorption at low energies [[vii]]. The spatial current density on a 2.3µm thick SiC XBPM, Fig.1F, is calculated using the photon-current conversion table of Fig1E. The analysis shows the very large (>2mm) FWHM of the current density distribution, especially along the X-direction. Fig.1G and Fig.1H show the integrated current on a 5x2.5mm2 SiC XBPM for a knife edge scan of the (filtered) beam. The analysis allows calculating expected currents (up to 500mA) and resolutions. In Fig.1G and Fig.1H we also consider the effect of 0.7x0.4mm2 slits, located in front of the XBPM, which results in lower current (around 15mA) and inferior resolutions. At the conference the results of current signals and heat loads on different devices: (i) SiC, (ii) novel patented [[viii]] SiC XBPMs, (iii) diamond and (iv) blade monitors (based on Auger emission) as well as the extension towards other beamlines and facilities (Diamond, SOLEIL and SPRNG8) will be presented at next Synchrotron and Radiation Instrumentation (SRI2021) conference in Hamburg.
Fig.1 A) Whitebeam flux spectral distribution. B) Spatial distribution of 8th harmonic. C) Pinkbeam flux spectral distribution after 250um diamond window. D) Spectral integrated spatial distribution. E) number of electron-hole pairs generated on SiC XBPMs. F) spatial current density. G) and H) total current signals from SiC XBPM for different beam positions.
[i] D.Shu, Journal of Synchrotron Radiation 5, 636-638 (1997)
[ii] H. Sakae, Journal of Synchrotron Radiation 4, 204-209 (1997)
[iii] E. Griesmayer, Proceedings of the 13th International Conference on Synchrotron Radiation Instrumentation (2019)
[iv] S.Nida, Journal of Synchrotron Radiation 26, 28-35 (2019)
[v] T. Tanaka and H. Kitamura, “SPECTRA - a synchrotron radiation calculation code,” J. Synchro-tron Radiation 8, 1221 (2001)
[viii]Granted patent number 2019P10004EP