Optical characterization of lithium fluoride x-ray imaging detectors

Abstract

Abstract X-ray microscopy of biological samples is one of the main applications of imaging which plays an important role in Life Sciences [1]. The di usion of this technique needs the development of compact labora- tory x-ray microscopy systems. Important technological issues are the reduction of x-ray source dimensions, the improvement of x-ray optics and the development of new x-ray imaging detectors. Several features, i.e. very high spatial resolution over a large eld of view, large dynamic range, versatility and simplicity of use, make novel solid state detectors based on LiF very attractive as imaging plates for x-ray microscopy applications. Promising results for x-ray contact microscopy using LiF imaging detectors with a resolution close to a few hundred nanometer and large dynamic range have been ob- tained for various soft [2] and hard [3] x-ray sources. These innovative LiF imaging detectors are based on the optical reading of photolumi- nescence (PL) emitted by electronic defects (known as CC) induced by ionizing radiation in LiF crystals or thin lms. In the present thesis a set of samples was irradiated with hard x- rays (6-40 keV) at the TopoTomo beamline of the synchroton source ANKA, other sets with soft x-rays generated by a laser plasma souce. The crystals irradiated at the synchroton source show intense PL signals of the aggregated F 2 and F + 3 centres in the visible spectral range due to a high x-ray attenuation length. The optical response of a series of uniformly irradiated samples has been investigated as iii iv ABSTRACT function of the irradiation dose by varying the irradiation time. Crys- tal and thin lm LiF detectors show a linear PL response covering two decades for both types of aggregated defects in the investigated experimental conditions. Even for x-ray imaging experiments on thin lm exposure times in the order of 1 s are su cient. The z-scan technique of the CLSM has been used to study the spa- tial distribution of the CC in the coloured volume and a comparison between the simulation of the deposited energy density and PL signal pro les obtained along opposing directions has been performed. LiF imaging detectors are ideal for contact microscopy with table- top x-ray sources, in particular for in vivo imaging experiments with laser plasma sources in single shot. These soft x-ray sources have in- tense emission in the water window (280-530 eV). A study of the performance of thin lm detectors irradiated at the TVLPS as function of growth parameters and irradiation conditions has been performed. In contact imaging experiments a resolution down to 630 nm has been obtained. To obtain an even higher resolution it is necessary to optimize the contact between samples and detector de- veloping an appropriate sample holder. Thin lms on silicon substrate exhibit a larger PL response than lms on glass substrate mainly due to the higher re ectivity of the silicon substrate. Finally the high dy- namic range obtained for LiF thin lms allowed to perform an imaging test estimating the thickness of a ying insect wing