Results 1 - 10 of 5287
Results 1 - 10 of 5287. Search took: 0.03 seconds
|Sort by: date | relevance|
[en] 90Sr+90Y clinical applicators are used for brachytherapy in Brazilian clinics even though they are not manufactured anymore. Such sources must be calibrated periodically, and one of the calibration methods in use is ionometry with extrapolation ionization chambers. 90Sr+90Y clinical applicators were calibrated using an extrapolation minichamber developed at the Calibration Laboratory at IPEN. The obtained results agree satisfactorily with the data provided in calibration certificates of the sources. - Highlights: • 90Sr+90Y clinical applicators were calibrated using a mini-extrapolation chamber. • An extrapolation curve was obtained for each applicator during its calibration. • The results were compared with those provided by the calibration certificates. • All results of the dermatological applicators presented lower differences than 5%
[en] Two secondary standard systems of beta radiation were used to calibrate a PTW extrapolation chamber Model 23391. Three 90Sr+90Y sources of different activities were used in this calibration procedure. Medium-term stability of the response of the chamber was also studied. The calibration was performed with and without field-flattening filters. The relative standard deviation of the obtained calibration factors was 8.3% for the aluminum collecting electrode and 4.1% for the graphite collecting electrode. - Highlights: ► 90Sr+90Y standard sources were used to calibrate a PTW extrapolation chamber. ► Characterization tests of the chamber response were performed. ► Chamber response showed very good short- and medium-term stabilities. ► Linear extrapolation curves were obtained. ► Calibration factors of the chamber were acceptable.
[en] We adapt a method introduced by Fuchs for calculating evaporation rates from small droplets to the evaluation of the linear extrapolation distance of the neutron density at the surface of a black body immersed in an infinite, absorbing medium. Explicit results are obtained for spheres and cylinders and, by comparison with some very accurate calculations carried out by others, these results are shown to be accurate to within about 9% for a range of parameters. Modifications are introduced which enable the method to deal with absorbing and scattering bodies which are not black. (author)
[en] A scheme is presented for calculating confidence contours for the parameters of the saturating exponential function fitted to experimental TL-dose data. The contours are accurate but, because of the non-linearity of the function, the corresponding probabilities are not. The conditions for obtaining good precision without excessive effort are explored. A reasonable prescription is six equally-spaced doses, with the largest about six times the intercept on the TL-dose plot, and with eight and seven replicates at the lowest doses of the first- and second-glow data sets respectively, and two replicates at all other doses. (author)
[en] We examine several recent lattice-simulation data sets, asking whether they are consistent with infrared conformality. We observe, in particular, that for an SU(3) gauge theory with 12 Dirac fermions in the fundamental representation, recent simulation data can be described assuming infrared conformality. Lattice simulations include a fermion mass m which is then extrapolated to zero, and we note that this data can be fit by a small-m expansion, allowing a controlled extrapolation. We also note that the conformal hypothesis does not work well for two theories that are known or expected to be confining and chirally broken, and that it does work well for another theory expected to be infrared conformal.
[en] The extrapolation method, which is one of the two methods currently employed by standardizing laboratories for the weighing of drops of radioactive solutions, easily leads to errors. However, it is very convenient to store active solutions in rubber-capped vials and to use disposable syringes for extractions and depositions; it is then also convenient to use the extrapolation technique for weighing. Studies have been made with the aim of attaining accuracies for the extrapolation method of not less than the ± 10 μg attainable with the pycnometer method. The following procedures were adopted: (a) The standard method, but with weighings started sufficiently early to account as much as possible for non-equilibrium effects; (b) The high humidity method; it was expected that the non-equilibrium effects would be reduced by operating as close as practicable to a relative humidity (rh) of 100%; and (c) The sleeve method where the non-equilibrium effects are reduced by cooling the active solution prior to the depositions. Within the range rh 50% - 75%, weighings with method (c) yield radioactive concentrations identical (within ± 0.1%) to the results obtained with the pycnometer method. Method (a) is somewhat less reliable than method (c), method (b) offers no advantages over measurements within the readily attainable range of rh 50% - 75%. (author)
[en] The central axis depth dose in the build-up region (surface to d_m_a_x) of single open field photon beams (cobalt-60 through 24 MV) has been measured utilizing parallel plate and extrapolation chamber methodology. These data were used to derive, for a prescription dose of 100 cGy, values of surface dose, the maximum value of dose along the central axis (D_m_a_x) and the depth (nearest the surface) at which 90% of the prescription dose occurs (d_9_0). For both single and parallel opposed pair (POP) open field configurations, data are presented at field sizes of 5 × 5, 15 × 15 and 25 × 25 cm"2 for prescription depths of 10, 15 and 20 cm (midplane for POP). For the treatment machines, field sizes, and prescription depths studied, it is possible to conclude that: for single open field irradiation, surface dose values (as a percentage of the prescription dose) can be either low (<10%) or comparable to the prescription dose itself; for POP open fields, surface dose values are relatively independent of photon energy and midplane depth, and range between 30% and 70% of prescription dose, being principally dependent on field size; the depth of the initial 90 cGy point for a prescription dose of 100 cGy, d_9_0, was larger for POP fields. For either single or POP open field treatments, d_9_0 was always less than 22 mm, while for 6 MV or less, values of d_9_0 were less than 4 mm; D_m_a_x values can be very large (e.g., above 300 cGy) for certain treatment situations and are reduced significantly for POP treatments; for open field POP treatments, the percent reduction in D_m_a_x with each increment in beam energy above 10 MV is reduced over that seen at 10 MV or less and, possibly, this further reduction may be clinically insignificant; for open field POP treatments, changes in surface dose, d_9_0 and D_m_a_x with beam energy above 10 MV do not suggest, with regard to these specific build-up curve parameters, any obvious advantage for treatment with beam energies greater than 10 MV for the specific machines and situations studied.
[en] An extrapolation chamber for measuring beta absorbed dose in tissue and its main properties are introduced. The device is developed with reference to ISO related recommendation in 1983. The diameter of the extrapolation chamber collected electrode is 30 mm. The distance between electrodes is variable from 0.15 to 10mm. The unparallel degree between electrodes is less than 0.005mm. The memory effect of the chamber lasts about 48 hours as it is irradiated to a β source with dose rate of 25 Gy/h for half an hour. The surface dose rates of natural uranium and some beta sources have been measured with the device and the measuring range of dose rate is 1.27 x 10-3 - 2.37 x 10 Gy/h. The system unaccuracy and random error of the measured values are less than 4% and 1%, respectively
[en] 90Sr+90Y applicators are commonly utilized in brachytherapy, including ophthalmic procedures. The recommended instruments for the calibration of these applicators are extrapolation chambers, which are ionization chambers that allow the variation of their sensitive volume. Using the extrapolation method, the absorbed dose rate at the applicator surface can be determined. The aim of the present work was to develop a mini-extrapolation chamber for the calibration of 90Sr+90Y beta ray applicators. The developed mini-chamber has a 3.0 cm outer diameter and is 11.3 cm in length. An aluminized polyester foil is used as the entrance window while the collecting electrode is made of graphited polymethylmethacrylate. This mini-chamber was tested in 90Sr+90Y radiation beams from a beta particle check source and with a plane ophthalmic applicator, showing adequate results