Measurements of Acetone Fluorescence and Phosphorescence at High Pressures and Temperatures
Laser-induced fluorescence and phosphorescence signals of liquid acetone at pressures from 1 to 15 atm were measured and analyzed. Liquid acetone, which was preprocessed to minimize any dissolved oxygen, was injected as a low velocity jet into a pressurized chamber with either an air or nitrogen co-flow. The actone was excited at 266 nm. The laser-induced fluorescence intensity from the liquid was found to be invariant with pressure and co-flow conditions. The measured phosphorescence lifetimes were also found to independent of pressure, for both nitrogen and air environments. The phosphorescence lifetime for the liquid was measured to be ~1 μs, which is significantly smaller than the lifetime for acetone vapor (200 μs). This is likely due to self-quenching by acetone. The effect is further seen in the phosphorescence to fluorescence (P/F) ratio comparison between the liquid and vapor. The P/F ratio for liquid is 0.012, which is much smaller than the vapor value of 9.0. The overall phosphorescence intensities, however, were seen to decrease in the air co-flow cases compared to the nitrogen co-flow. Diffusion of oxygen into the liquid jet was observed, and the estimated concentration was found to be a few orders of magnitude less than the saturated concentration.
Principles of Photochemistry of Metal Complexes
Many of the principles that govern the photochemistry of metal complexes are the same as those that have been developed for understanding the photochemistry of organic compounds. Photochemical reactions differ from thermal reactions in that they are initiated by the absorption of light rather than by the application of heat. Photochemical reactions can therefore only occur if the compounds being activated have a chromophore in the electromagnetic spectrum that corresponds in wavelength with that of the exciting radiation. The number of thermodynamically favorable products that are accessible from photochemical reactions are greater than those that are obtainable from thermal reactions. This situation is a result of the excess energy that is possessed by the excited state after the absorption of a photon. The addition of this photon energy to the ground state energy of the molecule results in the excited state having considerably more stored energy; as a result, the excited state molecule can be considered to be quite different from its ground state precursor. This difference usually leads to the excited state having different bond distances and angles than does the ground state. These molecular distortions that the molecule undergoes after absorption of the photon can be either isotropic over the entire molecule, or anisotropic where the differences from the ground state are primarily located in one region of the molecule.
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%