Differences in grid performance exist. At 25 and 30 kVp, the cellular grid exhibited superior contrast improvement factor performance, whereas one of the linear grids exhibited superior Bucky factor performance. Measured contrast improvement and Bucky factors are dependent on nongrid technique. Cassette tunnels introduce scatter and should not be used with nongrid or magnification techniques.

No abstract

We carried out photometric and spectroscopic observations of comet 29P/Schwassmann-Wachmann 1 at the 6-m BTA telescope (SAO RAS, Russia) and the 1.6-m telescope of the National Laboratory for Astrophysics (LNA, Brazil) on February 20, 2012, and on May 31, 2011, respectively. The spectra revealed the presence of CO þ and N 2 þ emissions in the cometary coma at a distance of 5.25 AU from the Sun. The ratio [N 2 þ ]/[CO þ ] within the projected slit is 0.013. The images obtained through BVR filters showed a bright, dust coma, indicating a high level of activity. We estimated a colour index and a colour excess for the comet. The parameter Afρ, which is used as an indicator of a cometary activity, was measured to be 2584 750 cm in the reference optical aperture of ρ¼10 4 km. The dust production constituted 33 kg/s and 9.3 Á 10 3 kg/s, it was obtained using different methods. We also investigated the morphology of the comet using image enhancement techniques and found two jets in the coma.

Dosimetric measurements in small therapeutic x-ray beam field sizes, such as those used in radiosurgery, that have dimensions comparable to or smaller than the build-up depth, require special care to avoid incorrect interpretation of measurements in regions of high gradients and electronic disequilibrium. These regions occur at the edges of any collimated field, and can extend to the centre of small fields. An inappropriate dosimeter can result in an underestimation, which would lead to an overdose to the patient. We have performed a study of square and circular small field sizes of 6 MV photons using a thermoluminescent dosimeter (TLD), Fricke xylenol gel (FXG) and film dosimeters. PMMA phantoms were employed to measure lateral beam profiles (1 x 1, 3 x 3 and 5 x 5 cm2 for square fields and 1, 2 and 4 cm diameter circular fields), the percentage depth dose, the tissue maximum ratio and the output factor. An ionization chamber (IC) was used for calibration and comparison. Our results demonstrate that high resolution FXG, TLD and film dosimeters agree with each other, and that an ionization chamber, with low lateral resolution, underestimates the absorbed dose. Our results show that, when planning small field radiotherapy, dosimeters with adequate lateral spatial resolution and tissue equivalence are required to provide an accurate basic beam data set to correctly calculate the absorbed dose in regions of electronic disequilibrium.

Observed visual magnitudes of the short‐period comets 2P/Encke, 81P/Wild 2, 6P/d'Arrest and 73P/Schwassmann–Wachmann 3 and the long‐period comet C/1984 V1 Levy–Rudenko are converted into water release rates. These H2O release rates are used to derive the nuclear surface areas, minimum nuclear radii and gas mass loss of these comets. Combined with estimated gas release rates, we make an uniform and systematic analysis of the observed continuum fluxes in the comae of these comets with the objective to find dust release rates, effective dust particle sizes and total mass release rates. The nuclei of Comets 2P/Encke, 6P/d'Arrest and 73P/Schwassmann–Wachmann 3, which has split into as many as four fragments, are potential targets for the flyby of the CONTOUR (COmet Nucleus TOUR) spacecraft, scheduled for launch in 2002 July, while Comet 81P/Wild 2 has been chosen as a target by the NASA spacecraft STARDUST, which was launched on 1999 February 7.

No abstract

To the best of the authors' knowledge, the authors directly measured for the first time the instantaneous speed profile of a radiation source in a HDR brachytherapy unit traveling from the unit safe to the end of the catheter and between interdwell distances. The method is feasible and accurate to implement on quality assurance tests and provides a unique database for efficient computational simulations of the transient dose.

The A 2&`and X 2% electronic states of the SiP species have been investigated theoretically at a very high level of correlation treatment (CASSCF/MRSDCI). Very accurate potential energy curves are presented for both states, as well as the associated spectroscopic constants as derived from the vib-rotational energy levels determined by means of the numerical solution of the radial Schro dinger equation. Electronic transition moment function, oscillator strengths, Einstein coefficients for spontaneous emission, and Franck-Condon factors for the A 2&`ÈX 2% system have been calculated. Dipole moment functions and radiative lifetimes for both states have also been determined. Spin-orbit coupling constants are also reported. The radiative lifetimes for the A 2&`state, taking into account the spin-orbit diagonal correction to the X 2% state, decrease from a value of 138 ms at v@ \ 0 to 0.48 ms at v@ \ 8, and, for the X 2% state, from 2.32 s at v@@ \ 1 to 0.59 s at v@@ \ 5. Vibrational and rotational transitions are expected to be relatively strong.