Liquid-crystal twisted nematic structures can be made with twist angles other than the conventional 90 degrees . It is shown that theoretically the transmission of such a structure when placed between two polarizers aligned parallel and perpendicular to the respective surface molecular directors is given by T=(1+u2)-1 sin2( theta (1+u2)12/) where u= pi d Delta n/ theta lambda , and theta is the twist angle, d being the cell thickness, Delta n the refractive index anisotropy of the nematic material, and lambda the wavelength of the transmitted radiation. In the idealized case T is taken to be zero. The departures from this, predicted by the above expression, are due to the fact that plane-polarized light becomes elliptically polarized as it passes through the nematic structure. Experiments confirm the validity of the theoretical model.

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A detailed study of alpha interactions on the passivation layer on the end-plate of a true-coaxial high-purity germanium detector is presented. The observation of alpha events on such a surface indicates an unexpectedly thin so-called "effective dead layer" of less than 20 µm thickness. In addition, the influence of the metalisation close to the end-plate on the time evolution of the output pulses is discussed. The results indicate that alpha contamination can result in events which could be mistaken as signals for neutrinoless double beta decay and provide some guidance on how to prevent this.

A four-fold segmented n-type point-contact "Broad Energy" high-purity germanium detector, SegBEGe, has been characterised at the Max-Planck-Institut für Physik in Munich. The main characteristics of the detector are described and first measurements concerning the detector properties are presented. The possibility to use mirror pulses to determine source positions is discussed as well as charge losses observed close to the core contact.