We describe the science motivation and development of a pair production telescope for medium-energy (~5-200 MeV) gamma-ray polarimetry. Our instrument concept, the Advanced Energetic Pair Telescope (AdEPT), takes advantage of the Three-Dimensional Track Imager, a low-density gaseous time projection chamber, to achieve angular resolution within a factor of two of the pair production kinematics limit (~0.6° at 70 MeV), continuum sensitivity comparable with the Fermi-LAT front detector (<3 × 10-6 MeV cm-2 s-1 at 70 MeV), and minimum detectable polarization less than 10% for a 10 mCrab source in 106 s

SummaryThe bacterial actin homolog MreB, which is critical for rod shape determination, forms filaments that rotate around the cell width on the inner surface of the cytoplasmic membrane. What determines filament association with the membranes or with other cell wall elongation proteins is not known. Using specific chemical and genetic perturbations while following MreB filament motion, we find that MreB membrane association is an actively regulated process that depends on the presence of lipid-linked peptidoglycan precursors. When precursors are depleted, MreB filaments disassemble into the cytoplasm and peptidoglycan synthesis becomes disorganized. In cells that lack wall teichoic acids, but continue to make peptidoglycan, dynamic MreB filaments are observed, although their presence is not sufficient to establish a rod shape. We propose that the cell regulates MreB filament association with the membrane, allowing rapid and reversible inactivation of cell wall enzyme complexes in response to the inhibition of cell wall synthesis.

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a b s t r a c tWe have identified nitromethane ðCH 3 NO 2 Þ as an effective electron capture agent for negative ion TPCs (NITPCs). We present drift velocity and longitudinal diffusion measurements for negative ion gas mixtures using nitromethane as the capture agent. Not only is nitromethane substantially more benign than the only other identified capture agent, CS 2 , but its low atomic number will enable the use of the NITPC as a photoelectric X-ray polarimeter in the 1-10 keV band.

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