Scintillation

About: Scintillation is a research topic. Over the lifetime, 14022 publications have been published within this topic receiving 187694 citations.

Model for a partially coherent Gaussian beam in atmospheric turbulence with application in lasercom

Abstract: Analytic expressions for the mutual coherence function (MCF) and the scintillation index of a partially coherent lowest order Gaussian beam wave propagating through the atmosphere (based on Kolmogorov spectrum model) are developed for the pupil plane of a receiving system. Partial coherence of the beam is modeled as a thin (complex) phase screen with Gaussian spectrum (Rytov theory and ABCD ray matrices are applied). The relation between the second- and fourth-order statistics for a beam with any degree of coherence in the atmosphere is introduced with the help of ''effective'' beam parameters, deduced from the free-space MCF. In particular, the scintillation (in weak and strong atmospheric conditions), based on these parameters, is stud- ied as a function of the diffuser's strength and that of the atmosphere. The model is applied for the calculation of the SNR and bit error rates (OOK modulation) of the communication link with diffuser at the trans- mitter and slow detection system. The improvement of bit error rates is observed in weak and strong atmospheric turbulence. In the weak re- gime, the optimal diffuser can be found. © 2004 Society of Photo-Optical Instru-

Diffractive Interstellar Scintillation Timescales and Velocities

Abstract: We derive general relationships between the observed timescale of diffractive interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively to saturated scintillations of point sources in the strong scattering regime. We show how scintillation observations may be combined with other observables (proper motion and dispersion measure) to yield (1) improvements in galactic models for the free-electron density and (2) estimates of the distance and transverse velocity of individual pulsars. We explicitly consider cases of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion measures alone. We also briefly consider scintillations of extragalactic sources, including gamma-ray burst sources at great distances from the Galaxy.

Measurement and correction of variations in interstellar dispersion in high-precision pulsar timing

Abstract: Signals from radio pulsars show a wavelength-dependent delay due to dispersion in the interstellar plasma. At a typical observing wavelength, this delay can vary by tens of microseconds on 5-yr time-scales, far in excess of signals of interest to pulsar timing arrays, such as that induced by a gravitational wave background. Measurement of these delay variations is not only crucial for the detection of such signals, but also provides an unparalleled measurement of the turbulent interstellar plasma at astronomical unit (au) scales. In this paper we demonstrate that without consideration of wavelength-independent red noise, ‘simple’ algorithms to correct for interstellar dispersion can attenuate signals of interest to pulsar timing arrays. We present a robust method for this correction, which we validate through simulations, and apply it to observations from the Parkes Pulsar Timing Array. Correction for dispersion variations comes at a cost of increased band-limited white noise. We discuss scheduling to minimize this additional noise, and factors, such as scintillation, that can exacerbate the problem. Comparison with scintillation measurements confirms previous results that the spectral exponent of electron density variations in the interstellar medium often appears steeper than expected. We also find a discrete change in dispersion measure of PSR J1603−7202 of ∼2 × 10^(−3) cm^(−3) pc for about 250 d. We speculate that this has a similar origin to the ‘extreme scattering events’ seen in other sources. In addition, we find that four pulsars show a wavelength-dependent annual variation, indicating a persistent gradient of electron density on an au spatial scale, which has not been reported previously.

System and method for x-ray fluoroscopic imaging

Abstract: A system for x-ray fluoroscopic imaging of bodily tissue in which a scintillation screen and a charge coupled device (CCD) is used to accurately image selected tissue. An x-ray source generates x-rays which pass through a region of a subject's body, forming an x-ray image which reaches the scintillation screen. The scintillation screen re-radiates a spatial intensity pattern corresponding to the image, the pattern being detected by the CCD sensor. In a preferred embodiment the imager uses four 8×8-cm three-side buttable CCDs coupled to a CsI:T1 scintillator by straight (non-tapering) fiberoptics and tiled to achieve a field of view (FOV) of 16×16-cm at the image plane. Larger FOVs can be achieved by tiling more CCDs in a similar manner. The imaging system can be operated in a plurality of pixel pitch modes such as 78, 156 or 234-μm pixel pitch modes. The CCD sensor may also provide multi-resolution imaging. The image is digitized by the sensor and processed by a controller before being stored as an electronic image. Other preferred embodiments may include each image being directed on flat panel imagers made from but not limited to, amorphous silicon and/or amorphous selenium to generate individual electronic representations of the separate images used for diagnostic or therapeutic applications.

LaBr3:Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time

Abstract: The use of time-of-flight (TOF) information in positron emission tomography (PET) enables significant improvement in image noise properties and, therefore, lesion detection. Silicon photomultipliers (SiPMs) are solid-state photosensors that have several advantages over photomultiplier tubes (PMTs). SiPMs are small, essentially transparent to 511 keV gamma rays and insensitive to magnetic fields. This enables novel detector designs aimed at e.g. compactness, high resolution, depth-of-interaction (DOI) correction and MRI compatibility. The goal of the present work is to study the timing performance of SiPMs in combination with LaBr3:Ce(5%), a relatively new scintillator with promising characteristics for TOF-PET. Measurements were performed with two, bare, 3 mm × 3 mm × 5 mm LaBr3:Ce(5%) crystals, each coupled to a 3 mm × 3 mm SiPM. Using a 22Na point source placed at various positions in between the two detectors, a coincidence resolving time (CRT) of ~100 ps FWHM for 511 keV annihilation photon pairs was achieved, corresponding to a TOF positioning resolution of ~15 mm FWHM. At the same time, pulse height spectra with well-resolved full-energy peaks were obtained. To our knowledge this is the best CRT reported for SiPM-based scintillation detectors to date. It is concluded that SiPM-based scintillation detectors can provide timing resolutions at least as good as detectors based on PMTs.