Time-Resolved Profiles in Modulated Polarization Spectroscopy
The absorption of a modulated light beam in which the state of the polarization is continuously variable is discussed. It is now possible to measure the average time-dependent profile for the polarization cycle of such a beam, and corresponding theoretical descriptions are needed in order to extract the information contained in it. This work generates such a description. Based on a semiclassical treatment of light/matter interactions, a three-parameter fit function is derived that permits the simultaneous extraction of absorbance, linear dichroism, and circular dischroism from the profile for various types of uniformly oriented, partially oriented, and isotropic materials and combinations of them. The theoretical expressions are used to analyze data for (+)-3-methylcyclopentanone, the results being (1) an increase of the S/N ratio, (2) the simultaneous generation of absorption and dichroism spectra of identical quality, and (3) a description of window adsorbates of the cyclopentanone material. The Jones p...
Selective digital integrator for modulated-polarization spectroscopy: An evaluation using (+)-3-methylcyclopentanone
A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small signals (ac component of interest <10−3×dc component) buried in noise. The polarization of light is altered with fast modulators (modulation frequency ∼50 kHz). A SDI connected to a single detector is used to separately and selectively measure the polarization-dependent intensities of electronic transitions. The SDI is a major improvement over the lock-in approach: it has on-the-fly time-resolved data acquisition and integration capability, and a wide gain-switching-free dynamic range (10 orders of magnitude). This design permits a variety of applications that require detection of modulated or pulsed signals. The advantages of the MPS-SDI method are demonstrated on the first Rydberg electronic transitions of (+)-3-methylcyclopentanone. The selectively measured MPS intensities are provided in 3D form (...
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%