Showing papers by "Charles L. Melcher published in 2012"

Spectroscopic refractive indices of monoclinic single crystal and ceramic lutetium oxyorthosilicate from 200 to 850 nm

Abstract: The four real values of the dielectric function tensor of the monoclinic crystal Lu2SiO5 or lutetium oxyorthosilicate (LSO) have been determined using generalized ellipsometry from 200 to 850 nm. The three principal values are fit to the Sellmeier model and they indicate that the band gap of LSO is less than ∼9 eV. The off-diagonal element e12 is non-zero over the entire spectrum, but it is very close to zero for wavelengths longer than ∼400 nm, indicating that structurally monoclinic LSO is nearly optically orthorhombic in this wavelength region. The spectroscopic dielectric functions of three isotropic ceramic LSO samples are presented, which are consistent with the dielectric functions of single-crystal LSO when the effects of optical density are included. As a comparison, the dielectric functions are also determined using relativistic electronic structure and optical calculations based on the recently developed potential functional of Tran and Blaha [Phys. Rev. Lett. 102, 226401 (2009)].

Praseodymium valence determination in Lu2SiO5, Y2SiO5, and Lu3Al5O12 scintillators by x-ray absorption spectroscopy

Abstract: Until now, determination of both Pr3+ and Pr4+ at the low concentration levels commonly used in single crystal scintillators has proven to be difficult. We have found that it is possible to use synchrotron radiation and superconducting tunnel junction detectors to measure the X-ray absorption on the M4 and M5 edges of Pr to directly determine Pr3+ and Pr4+ in Lu2SiO5, Y2SiO5, and Lu3Al5O12. The spectra were measured at room temperature and compared to model samples of trivalent and tetravalent praseodymium, which provided clear signatures of the two charge states. The results show predominant Pr(III) in most samples.

Measuring the non-proportional response of scintillators using a Positron Emission Tomography scanner

Abstract: A novel way of measuring the non-proportional response of scintillation materials, using a Positron Emission Tomography (PET) scanner, has been developed and tested Using a Siemens Biograph mCT, a modified Compton coincidence technique is performed where the Compton scatter angular information data is collected by taking advantage of the fine angular sampling that is inherent to the PET scanner Using the scatter angle information, the energy deposited in the sampled scintillator can be calculated Comparing the calculated energy deposited versus the measured scintillator response yields the Compton electron non-proportional response

Comparison of count rate sensitivity performance for a LSO-TOF system with a Cherenkov radiation based PbF 2 -TOF system

Abstract: The use of Cherenkov light production and timing for TOF PET has been suggested, and there are papers indicating the possibility to improve timing in this way The present paper, however, focuses only on count rate sensitivity issues by comparing a LSO-LSO system against a potential PbF2 system This has been accomplished by using a simple two detector set up first looking at LSO-LSO coincidences and then exchanging the LSO crystal on one of the channels with PbF2 and then extracting the coincidence sensitivity for a pure PbF2 system A simple comparison between the resulting LSO-LSO coincidence count rate and the PbF2-PbF2 coincidence count rate shows more than a 10 to 1 difference In addition we have evaluated the time resolution achievable with two different photo sensor combinations, PMT-PMT and PMT-SiPM Even if a time resolution of 50 ps may be possible to achieve in a PbF2 system, this requires very special photo sensors Our results with standard photo sensors indicate time resolutions of the order of ~250 ps If a 50 ps time resolution can be achieved, this may be equivalent to a 500 ps LSO system with 10 times higher sensitivity One must consider, however, the minimum number of counts needed in a 50 ps system to achieve reasonable image quality In addition to use very fast, photo sensors, these have to have high quantum efficiency from 250 nm up to 600 nm Possible photo sensors for Cherenkov PET may be MCP PMTs

Wavelength Shifting Phoswich Detectors for Superior Depth-of-Interaction Resolution

Abstract: In order to simultaneously achieve both high spatial resolution and high sensitivity in small Positron Emission Tomography (PET) systems, scintillation detectors must be long in the radial direction as well as able to provide depth-of-interaction (DOI) information. DOI information is typically provided by constructing detectors from two or more layers of scintillators that are identifiable due to their different decay times. This approach has worked well in tomographs such as the High Resolution Research Tomograph (HRRT, CTI PET Systems, Inc.) in which the emission and excitation bands of the scintillator layers do not overlap each other. However, many potentially important pairs of scintillator crystals exist in which the emission of one crystal is, in fact, absorbed and re-emitted by the second crystal, thus impacting the pulse shape discrimination process used to identify the scintillator layers. These potentially useful pairs of scintillators are unlikely to be implemented in phoswich detectors without a comprehensive understanding of the complex emission that results when the light of one crystal is absorbed by the second crystal and then reemitted. Our objective is to develop a fundamental understanding of the optical phenomena that occur in phoswich detectors and to exploit these phenomena to achieve improved spatialmore » resolution in small high sensitivity PET scanners.« less