M. Rzepka

X-ray polarimetry — A novel application of CCDs

Abstract A new method of X-ray polarimetry based on measuring the photoelectron emission direction in a finely segmented MOS CCD has been established using monochromatic synchrotron radiation and planar channeling radiation. For the smallest pixel dimensions (6.8 × 6.8 μm 2 ) available today in commercial optical CCD cameras an analyzing power for linear polarization in the order of 10% has been measured at energies above 10 keV. A strong rise with energy is observed in accord with expectations from Monte Carlo simulations. In addition to events due to the photoeffect in the thin depleted front layer of the CCD, also diffusion spread events resulting from much more abundant conversions deeper inside the chip were found to be very useful for simultaneous measurements of polarization, energy and position.

X-ray detection with submicron impact accuracy

Abstract Utilizing a commercial optical CCD camera with the smallest pixel size presently available (6.8 × 6.8 μ m 2 ) we have measured impact accuracies of 0.9 μm (rms) for 15 keV synchrotron radiation in direct photon counting mode. Of crucial importance was diffusion spreading of the deposited charge such that by centroid reconstruction interpolation between discrete pixel coordinates was possible.

Hard x-ray polarimetry exploiting directional information of the photoeffect in a charge coupled device

A new method of X-ray polarimetry based on the photoeffect and measuring the photoelectron emission direction in a finely segmented MOS CCD has been established with monochromatic synchrotron radiation of known linear polarization. For square pixels of 6.8 micrometer an analyzing power in the order of 10% has been measured at energies above 10 keV. A steep rise with energy is observed in accord with Monte Carlo simulations. In addition to small charge deposits due to the photoeffect in the thin depleted front layer of the CCD, also diffusion spread charge clusters from the much more abundant conversions deeper inside the chip were found to be very useful for simultaneous measurements of the polarization vector, the energy and the position on a photon-by-photon basis. In a first application of spatially and energetically resolved polarimetry the linear polarization of Parametric X- radiation (PXR) produced by 80.5 MeV electrons in a 13 micrometer thick silicon single crystal has been analyzed. The experiment was carried out at the Darmstadt superconducting linear accelerator S-DALINAC providing a low-emittance electron beam. The linear polarization of the (220) reflex observed in 8 narrow angular bins between 20 degrees and 21 degrees with respect to the electron beam direction is consistent with complete local linear polarization. The orientation of the polarization plane, within measurement errors of typically 10 degrees, varies over the diffraction pattern in such a way as to be expected from kinematical theory. The result of this experiment is in contradiction to the only other PXR polarization measurement performed so far.

X-ray polarimetry and position measurement using the photoeffect and charge diffusion in a CCD

A new method of X-ray polarimetry based on the photoeffect in a highly segmented silicon charge coupled device (CCD) has been established using monochromatic synchrotron radiation and planar channeling radiation. For the smallest pixel dimensions (6.8 × 6.8 μm2) available today in commercial optical CCD cameras an analyzing power for linear polarization in the order of 10% has been measured at energies above 10 keV. A strong energy dependence is observed in the energy range from 15 to 40 keV. In addition to events due to the photoeffect in the thin depleted front layer of the CCD, also diffusion spread events resulting from far more abundant conversions deeper inside the chip were found to be very useful for simultaneous measurements of polarization, energy and position with sub-pixel, submicron impact accuracy (0.9 μm rms at 15 keV). The spatial accuracy is in accord with expectations from Monte-Carlo simulations.

A CRYSTAL MONOCHROMATOR FOR A LINEARLY POLARIZED X-RAY SOURCE UTILIZING CHANNELING RADIATION

The performance of a high transmission crystal monochromator to be used as part of a source of linearly polarized X-rays of moderately narrow bandwidth in the energy regime of 10–50 keV has been investigated using channeling radiation produced by 86 MeV electrons from the superconducting linear accelerator S-DALINAC at the Technische Hochschule Darmstadt. Best results on monochromatic photon yields have been obtained with cylindrically bent silicon monocrystals in Laue and with planar mosaic crystals in Bragg geometry, respectively. At energies between 20 and 30 keV monochromatized X-ray intensities of up to 1.5 × 1010photons(s sr μA) have been obtained for axial channeling radiation from a 13 μm thick silicon radiator crystal. The measured yields of narrow band X-rays are such that an intense source of highly polarized planar channeling radiation could well serve to calibrate the linear polarization sensitivity of a CCD polarimeter.

X-ray polarimetry and position measurements using the photoeffect and diffusion in a CCD

A new method of x-ray polarimetry based on the photoeffect in a finely segmented silicon charge coupled device (CCD) first proposed by G. W. Fraser has been confirmed using monochromatic synchrotron radiation and planar channeling radiation. For the smallest pixel dimensions (6.8 multiplied by 6.8 micrometer squared) available today in commercial optical CCD cameras an analyzing power for linear polarization in the order of 10% has been measured at energies above 10 keV. The strong energy dependence of the analyzing power has been measured in the energy range from 9 - 100 keV and is compared to expectations from detailed MC simulations. In addition to events due to photoeffect in the thin depleted front layer of the CCD also diffusion spread events resulting from much more abundant conversions deeper inside the chip were found to be very useful for simultaneous measurements of polarization, energy and position with sub-pixel accuracy (0.9 micrometer rms at 15 keV). For the first time we have now made use of the CCD polarimeter for the measurement of the linear polarization of parametric x-rays (PXR), i.e. radiation which is coherently generated by charged particles traversing a crystal.