We investigate the phase structure of pure compact U(1) lattice gauge theory in 4 dimensions with the Wilson action supplemented by a monopole term. To overcome the suppression of transitions between the phases in the simulations we make the monopole coupling a dynamical variable. We determine the phase diagram and find that the strength of the first order transition decreases with increasing weight of the monopole term, the transition thus ultimately getting of second order. After outlining the appropriate topological characterization of networks of currents lines, we present an analysis of the occurring monopole currents which shows that the phases are related to topological properties.Comment: 22 pages (latex), 14 figures (available upon request), BU-HEP 94-

A multileaf collimator (MLC) optimized for SBRT delivery with the CyberKnife ® Robotic Radiosurgery System (Accuray Incorporated, Sunnyvale, CA, USA) is described. The MLC is exchangeable with the alternate fixed and variable circular aperture collimator systems. The non-coplanar workspace is effectively equivalent for all three collimation types. The same range of tracking options, including real-time respiratory motion tracking, and the same tolerance on beam pointing accuracy (0.95 mm) is maintained with all three collimation types. The MLC includes 52 flat-sided leaves, each of which is 90 mm tall and projects 3.85 mm width at the nominal treatment distance of 800 mm SAD. The design allows 100% overtravel and unrestricted interdigitation. Leaf position is determined by primary motor encoders and is checked with a secondary optical camera system. Maximum leakage, including inter-leaf and under the closed position leaf-tip gap was measured on five units to be 0.44%, while mean leakage and transmission ranged from 0.22%-0.25%. Leaf positioning accuracy measured over the full range of leaf positions, all robot and MLC orientations, and including variation with leaf motion direction and accumulated leaf motion after initialization had a mean error <0.2 mm, with 2%-98% range of ±0.5 mm (projected at 800 mm SAD) on three units tested. The only factor found to effect leaf positioning accuracy was sag under gravity, which systematically altered leaf positions by 0.1 mm. Tilting the leaves to reduce inter-leaf leakage results in 0.5 mm asymmetry in leaf-side penumbra at 100 mm depth, and a partial leaf-edge transmission pattern analogous to the tongue and groove effect observed with interlocking leaves.

An integrated dosimetry and cell irradiation system (IDOCIS) with laser-accelerated proton beams was developed, characterized, calibrated and successfully used for systematic in vitro experiments. Due to the broad exponentially shaped energy spectrum, the low-energy range of the protons (<20 MeV) and the high pulse dose, the absolute dosimetry for this beam quality is challenging. Therefore, a dedicated Faraday cup is used as an energy and dose rate independent absolute dosimeter that has been calibrated consistently with three independent methods. A transmission ionization chamber providing online relative dose information is cross-calibrated against the Faraday cup. Providing both online and absolute dose information, the IDOCIS allows for quantitative dosimetric and radiobiological studies at current low-energy laser-accelerated proton beams. Finally, first dosimetric characterizations of a laser-accelerated proton beam with the IDOCIS are presented.

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