Abschlussarbeiten

The present thesis details the properties of position-sensitive detectors suited for the Compton polarimetry of high-energy X-ray radiation, investigating double-sided Ge(i) and Si(Li) strip detectors as well as a CdTe-based sensor equipped with the novel Timepix detection chip. In the case of the strip detectors, special concern was dedicated to the so-called charge sharing effect, which denotes an incident photon's charge cloud being distributed across several segments. This obviously hampers the determination of the actual interaction position, a crucial parameter for the Compton-polarimetric analysis. A set of sophisticated software routines to recover such events was developed. Using this modified analysis, the number of multiplicity-1 events reported by the Si(Li) detector was increased by about 30%, while the Ge(i) system, having a much smaller strip width, saw an increase by up to a factor of 30 at energies above 200 keV. Consequently, the number of events usable for the Compton analysis, which has to be restricted to exposures with exactly two discernible interactions, grew in a similar fashion, such that, for the Ge(i) sensor, the computed degree of polarization deviated by as much as 9% from previously obtained values. The wide-stripped Si(Li) detector, on the other hand, proved to be much less susceptible to distortions induced through charge splitting. In addition, Timepix acquisitions made during an electron acceleration experiment conducted at the JETI laser system were evaluated. To avoid saturation of the sensor, extensive shielding and indirect exposure, utilizing a plastic body for scattering, were necessary. While the employed Time-over-Threshold mode returns a pixelwise measure for the energy deposition, neither an absolute nor inter-pixel calibration was possible. However, a clear correlation between the total output signal and the intensity reported by an electron spectrometer could be observed, confirming the Timepix sensor's general capability of energy-resolved measurements. Furthermore, electron tracks that were visible in the obtained data were used to calculate a rough estimate of the primary photon energy by comparing their extent to the predictions of the Continuous Slowing Down Approximation. This yielded initial electron energies in the MeV range, which are in good agreement with the achieved energies reported for the JETI experiment in question.

The present thesis reports on the study of the linear polarization properties of bremsstrahlung produced in polarized electron atom collisions. The experimental investigation of bremsstrahlung photons has been performed using the polarized electron source (SPIN) at the TU Darmstadt. Gold and carbon targets were bombarded with 100 keV electrons whose spin was oriented parallel, anti-parallel and transverse with respect of the beam axis. In addition, an unpolarized electron beam was used for a reference measurement. For the detection of the bremsstrahlung photons, a novel Si(Li) Compton polarimeter has been employed at two different observation angles. This detector enabled the determination of the degree of linear polarization as well as the orientation of the polarization vector for various energies of the bremsstrahlung photons. The emphasis of the work was on the so-called polarization transfer where the polarization of the incoming electron spin influences the polarization of the emitted x-rays. This gives rise to an enhanced degree of linear polarization and to a rotation of the photon polarization vector with respect to the unpolarized case. Both effects were observed by comparing data for the unpolarized and the transversely polarized electron beam. The experimental results are in qualitative agreement with fully relativistic calculations.