Results 1 - 10 of 15489
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[en] The structure of the experimental setup, the technique of its adjustment and organization of measurements under an on-line control of the beam and setup parameters and also the main details of off-line data processing by Σ-asymmetry are presented
[en] The irradiation of small targets in the brain in a singe fraction (radiosurgery) or with a fractionated approach (stereotactic radiosurgery) with small beams of photons requires specific conditions to measure and to model the dosimetric data needed for treatment planning. In this work we present the method and materials adopted in our institution since 1988 to perform the dosimetry of high energy (6-23) circular photon beams with diameters ranging from 10 to 40 mm at the isocenter of linear accelerators, and its evolution as new dosimetric material became commercially available. in circular ionization chambers of small dimensions. We want to answer the following questions: Which are the minimal basic data needed to model small circular beams of high energy photons? Can we extrapolate or convert data from conventional data of larger beams? Which are the detectors well adapted for these kind of measurements and for which range of beam sizes?
[en] The six Centers for Radiological Physics performed periodic thermoluminescent dosimetry (TLD) reviews for photon beams at 276 facilities across the country. The purpose of the reviews and the techniques in use are briefly described. Results from 2413 mailed reviews of megavoltage units indicate that, in 92% of the reviews, a specified dose was delivered within +/- 5% of the TLD-measured dose. A comparison is made to other mailed TLD review systems
[en] The phenomenon of self-pumped slow light, where a single beam appears to be slowed by a solid-state media, is both subtle and controversial. Here, we reply to a comment on our recent work, which uses an observation of enhanced photon drag to distinguish between group delay and pulse reshaping. (reply)
[en] As the quantification of dose in homogeneous media is now better understood, it is necessary to further quantify effects from heterogeneous media. The most extreme case is related to air cavities. Although dose corrections at large distances beyond a cavity are accountable by attenuation differences, perturbations at air-tissue interfaces are complex to measure or calculate. These measurements help understand the physical processes that govern these perturbations. A thin window parallel-plate chamber and a special diode were used for measurements with various air cavity geometries (layer, channel, cubic cavity, triangle) in x-ray beams of 4 and 15 MV. Underdosing effects occur at both the distal and proximal air cavity interfaces. The magnitude depends on geometry, energy, and field sizes. As the cavity thickness increases, the central axis dose at the distal interface decreases. Increasing field size remedied the underdosing, as did the introduction of lateral walls. Following a 2.0 cm wide air channel for a 4 MV, 4 x 4 cm2 field there was an 11% underdose at the distal interface, while a 2.0 cm cubic cavity yielded only a 3% loss. Measurements at the proximal interface showed losses of 5% to 8%. For a 4 MV parallel opposed beam irradiation the losses at the interfaces were 10% for a channel cavity (in comparison with the homogeneous case) and 1% for a cube. The losses were slightly larger for the 15 MV beam. Underdosage at the lateral interface was 4% and 8% for the 4 MV and 15 MV beams, respectively. Although reports suggest better clinical results using lower photon energies with the presence of air cavities, there is no reliable dose calculation algorithm to predict interface doses accurately. The measurements reported here can be used to guide the development of new calculation models under nonequilibrium conditions. This situation is of clinical concern when lesions such as larynx carcinoma beyond air cavities are irradiated. 17 refs., 9 figs., 2 tabs
[en] We demonstrate, both theoretically and experimentally, a kind of fan-shaped optical beam propagating along the arbitrary trajectories (such as parabolic, hyperbolic and three-dimensional spiraling trajectories). With a controlled profile, this fan-shaped optical beam can be obtained from superposition of the Bessel-like beam and vortex Bessel-like beam. Also, the ability of guiding and transporting microparticles along its lobes is explored. These beams may find a variety of applications in optical trapping and manipulation. (paper)
[en] We present a status update on the CMU group PWA efforts in the strangeness (K+Λ/K+Σ0) sector. The bulk of the currently available data comes from recently published CLAS g11a results, with extensive polarization data expected soon from the CLAS g8 and g9 set of experiments. We give a brief description of issues pertaining to different sign conventions for the polarization observables in the literature, and normalization discrepancies between the CLAS-g11a results and older high energy data from SLAC/CEA/DESY that used an untagged photon beam.
[en] In this letter we reply to the letter by Professor Andreo (Andreo 2010 Phys. Med. Biol. 55 L9-16) regarding the values of the replacement correction factor for cylindrical chambers, which in turn commented on our recent paper on this (Wang and Rogers 2009 Phys. Med. Biol. 54 1609-20). (letter to the editor)