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At least 451 records · Page 25Linked to original sources
PubMed1985

Measurement of yield and cross section for the 27Al (gamma,2p) 25Na reaction with a clinical linear accelerator.

A clinical electron linear accelerator was adapted for continuous energy selection and used to measure bremsstrahlung yield for the photonuclear reaction 27Al (gamma,2p) 25Na from 24.0 to 32.0-MeV in 0.25 MeV steps. A germanium detector was used to measure the number of photoproduct gamma-rays emitted. The yield values were analyzed for photonuclear cross section using PHONUC II, a program especially adapted for use with yield data from thick-target bremsstrahlung. The cross section shows distinct resonance peaks at 26.3 +/- 0.3, 27.3 +/- 0.4 and 30.3 +/- 0.5 MeV. Possible theoretical origins of these resonances and applications of the reaction in accelerator energy calibration are discussed.

Aluminum↗
PubMed1998

Investigation and modelling of the surface dose from linear accelerator produced 6 and 10 MV photon beams.

The radiation reaction from the surface dose to a patient's skin is clinically important as an indicator of tolerance to treatment and is frequently the most visible side-effect from external beam radiotherapy. To quantify surface dose, its build-up was investigated in a solid water phantom over a range of field sizes, source to skin distances, wedge angles and beam shaping conditions. These data were analysed and used to produce an empirical model of the effect of these factors which predicted surface dose to within 5% under test conditions. The effects of changing treatment techniques was investigated by comparing fixed to dynamic wedges, and multileaf collimator (MLC) with alloy blocks. It was found that the MLC reduced the surface dose relative to alloy blocks but that dynamic wedges made no change relative to open fields. Fixed wedges reduced the relative surface dose as a consequence of beam hardening. It is hoped that routine clinical use of the model to calculate skin dose will increase awareness of situations in which the skin sparing effect of modern linear accelerators may be compromised.

Equipment Design↗
PubMedDate not supplied

Computer-controlled direct TMR measurement.

It is common practice to derive tissue maximum ratios (TMRs) indirectly by calculation from percent depth dose (PDD) data which are more convenient to measure using currently available automated scanners. A system has been developed, however, for the completely automatic direct measurement of TMR, using an ionization chamber which is rigidly supported at the isocenter of a teletherapy machine. The chamber is immersed in a water phantom which is moved up and down by computer control of the motorized treatment couch. Ionization current is sampled by the computer which simultaneously regulates both the radiation field size and chamber depth. This system provides a means of rapidly measuring and processing complete sets of TMR and output factor values for both square and elongated field shapes. The results of such measurements performed on the Clinac 18 accelerator are described in this paper.

Computers↗
PubMedDate not supplied

An equivalent-square formula for head-scatter factors.

A simple formula is evaluated for calculating the equivalent square collimator setting that gives the same headscatter factor as a given rectangular field. The expression requires that one parameter is determined experimentally. It is found that one single value of this parameter can be used for the six x-ray beams studied (two accelerators, two energies, with and without wedge).

Humans↗
PubMed1995

A novel comprehensive quality control instrument for medical accelerators.

A comprehensive quality control instrument for calibration of medical accelerators that use photon, electron, or proton beams in teleradiotherapy is described. It employs a fluorescence screen, mounted on a central stage with four degrees of freedom, monitored by a CCD camera. A single set-up of the instrument enables one to perform mechanical, light laser, and radiation tests, at arbitrary angles of the accelerator gantry. The new device provides for quantitative evaluation of the tests performed and provides for documentation of the test results in real time. The device provides significant time savings with concurrent improvement in accuracy for tests performed during installation and acceptance processes, and the implementation of quality control procedures for medical accelerators.

Electrons↗
PubMed1998

Startup performance of the traveling wave versus standing wave linear accelerator.

The startup performance of medical linear accelerators is of increasing importance for modern radiotherapy techniques. The traveling wave-type linear accelerator of the SL series of Philips (now Elekta Oncology Systems) has been modified in its flight tube design to meet this goal of a fast rise time of the radiation field. The new slitless flight tube combined with a redesigned gun servo electronic now achieves start up times of the radiation comparable with those of a standing wave linear accelerator (Siemens Mevatron) according to our measurements.

Equipment Design↗
PubMed2004

Strength of higher-order spin-orbit resonances.

When polarized particles are accelerated in a synchrotron, the spin precession can be periodically driven by Fourier components of the electromagnetic fields through which the particles travel. This leads to resonant perturbations when the spin-precession frequency is close to a linear combination of the orbital frequencies. When such resonance conditions are crossed, partial depolarization or spin flip can occur. The amount of polarization that survives after resonance crossing is a function of the resonance strength and the crossing speed. This function is commonly called the Froissart-Stora formula. It is very useful for predicting the amount of polarization after an acceleration cycle of a synchrotron or for computing the required speed of the acceleration cycle to maintain a required amount of polarization. However, the resonance strength could, in general, only be computed for first-order resonances and for synchrotron sidebands. When Siberian Snakes adjust the spin tune to be 1/2, as is required for high-energy accelerators, first-order resonances do not appear and higher-order resonances become dominant. Here we will introduce the strength of a higher-order spin-orbit resonance, and also present an efficient method of computing it. Several tracking examples will show that the so computed resonance strength can indeed be used in the Froissart-Stora formula. HERA-p is used for these examples which demonstrate that our results are very relevant for existing accelerators.

Journal Article↗
PubMed2001

Capillary force on colloidal particles in a freely suspended liquid thin film.

The dynamic behavior of micron-sized polystyrene latex particles confined in a free-standing liquid film is experimentally studied. When the thickness of the film is less than the particle diameter and varies depending on position, the particles are accelerated toward the thicker region. Using a simple geometrical model and hydrodynamic theory, we calculate the capillary force on the particle. The drag coefficient of the particle is found to depend on the thickness of the film from the value of near zero to the Stokes' drag coefficient.

Journal Article↗