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At least 199 records · Page 11Linked to original sources
PubMed1983

Electron depth-dose dependence on energy spectral quality.

Electron depth-dose curves and spectra have been measured from an experimental accelerator on which both energy and spectral shape could be varied independently. The measurements were made at nominal energies of 6 and 12 MeV, with the energy spread (FWHM) varied from 1.7% to over 10% of average energy. Changes in depth-dose fall-off characteristics correlated only weakly with the width of the accelerator's spectrum. The use of a single accelerator and experiment set-up eliminated the differences that have complicated previous comparisons of beams of different spectral quality, which have been based upon data from different accelerator designs.

Particle Accelerators↗
PubMedDate not supplied

Off-axis beam quality change in linear accelerator x-ray beams.

The effective energy of the x-ray beam from linear accelerators changes as a function of the position in the beam due to nonuniform filtration by the flattening filter. In this work, the transmittance through a water column was measured in good geometry and the beam quality characterized in units of HVL in water. Measurements were made on a variety of linear accelerators from 4 to 10 MV. The beam energy decreased with increasing distance from the central ray for all accelerators measured.

Particle Accelerators↗
PubMedDate not supplied

Electron beam simulation applicators.

A system for simulating electron beam treatment portals using low-temperature melting point alloy is described. Special frames having the same physical dimensions as the electron beam applicators used on the Varian Clinac 20 linear accelerator were designed and constructed.

Electrons↗
PubMedDate not supplied

Determination of the source position for the electron beams from a high-energy linear accelerator.

We have investigated the energy and field-size dependence of the source position of the electron beams from a Varian Clinac-2,500 accelerator. Three independent experimental methods were used: (1) multipinhole camera (MPC), (2) back projection of the full width at half maximum (FWHM), and (3) the inverse square law (ISL). The positions of the virtual and effective sources were calculated using the multiple Coulomb scattering (MCS) formalism. The results obtained from the MPC agree, within the experimental uncertainties, with the calculated values for the virtual source position. Similarly, the results from the FWHM method agree with the calculations with the exception of those for small field sizes at the lower energies. This is consistent with the fact that both kinds of measurements are not very sensitive to scattering in the photon and electron collimators. In contrast, the source position determined by the ISL method shows strong dependence on field size and energy, and does not agree with the values predicted by the MCS formalism. This is due to contamination from electrons scattered in the x ray and electron collimation system. The techniques and results reported here should be generally applicable to other scatter foil linear accelerators.

Electrons↗
PubMed1988

[Radioactivity in dismantling a betatron].

The authors report on the specific radioactivity of construction components of a 42 MeV betatron measured 28 days after the last run by means of a Ge spectrometer (ultra-pure germanium detector employed in a semiconductor spectroscopic method) recording the gamma spectra of various components of the betatron. This examination served to find out which components would have to be considered as radioactive waste requiring special treatment in accordance with Federal German legislation on radioactive waste disposal (section 47 of the Radiation Protection Ordinance). The examination showed that such special treatment would be required only for the target (main material: platinum) and the Platness-filter (chief constituent: lead) of the decommissioned electron accelerator.

Humans↗
PubMed1986

Quality assurance procedures in radiotherapy--IEC specifications for equipment.

The International Electrotechnical Commission (IEC) worked out international standards for requirements and tests of electrical, mechanical and radiation safety as well as for definition and tests of functional performance characteristics of radiotherapy equipments (medical electron accelerators, gamma beam teletherapy and afterloading equipments, simulators and accessories) and for clinical dosimeters and terminology for medical radiology. A survey is given on the actual state of standardization projects. The problems of such standards are shown for the standard for functional performance characteristics of medical electron accelerators as example.

Humans↗
PubMed1999

Size analysis of a pressurized metered dose inhaler-delivered suspension formulation by the API Aerosizer time-of-flight aerodynamic particle size analyzer.

Aerosizer time-of-flight (TOF) aerodynamic particle size analyzers (TSI-Amherst, Amherst, MA) are widely used for the rapid assessment of aerosols from a wide variety of drug delivery devices, including pressurized metered dose inhalers (pMDIs). This technique offers significant advantages in terms of rapid measurement times in comparison with the more time-consuming compendial methods such as the cascade impactor or multistage liquid impinger. Particle size analysis takes place by determining the TOF of individual particles following acceleration to supersonic velocity. No drug assay is performed; thus, the resulting size distribution also includes particles that do not contain any medication such as the excipients and surfactant that are present in most pMDI-based formulations. Illustrative data are presented for one particular formulation (Pulmicort: 200 micrograms of budesonide per dose; Astra Draco; Lund, Sweden) and demonstrate that bias from this source can significantly shift the reported particle distribution to finer sizes compared with impactor-based analysis in which direct assay for drug has taken place. In this case, the mass median aerodynamic diameter (MMAD) determined by an Aerosizer-LD was close to 2.4 microns, but was found to be approximately 4 microns using the cascade impactor-based procedure. Such a shift results in an overestimation of the fine particle fraction of the emitted dose, which may lead to misleading conclusions about the therapeutic benefit of a particular drug delivery system when making use of this formulation. TOF aerosol measurement techniques appear to be vulnerable to this type of bias for any suspension formulation in which the drug content is not homogeneously distributed within all particle sizes.

Bronchodilator Agents↗
PubMed2000

Dynamics of inelastic collapse.

We consider a particle randomly accelerated by Gaussian white noise on the half-line x>0. The collisions of the particle with the wall at x=0 are inelastic. The velocities just before and after reflection are related by v(f)=-rv(i), where r is the coefficient of restitution. Cornell, Swift, and Bray have shown that for r<r(c)=e(-pi/sqrt[3])=0.163, there is inelastic collapse, i.e., boundary collisions localize the particle at x=0. The probability that the particle is not yet absorbed at the boundary after a time t decays, for long times, as t(-straight theta(r)). The exponent straight theta(r) is calculated exactly. We also consider the case of "partial-survival" boundary conditions, i.e., elastic reflection with probability p and absorption with probability 1-p, and derive the analogous persistence exponent straight phi(p). The exact exponents satisfy the Swift-Bray conjecture straight theta(r)=straight phi(r(2straight theta(r))).

Journal Article↗
PubMed1992

Possible "accelerated striatal aging" induced by 56Fe heavy-particle irradiation: implications for manned space flights.

The present experiments were carried out to determine the effects of energy deposition from energetic iron (56Fe particles, an important component of cosmic rays) on motor behavioral performance and to determine if the observed deficits were caused by alterations in the neostriatum (an important motor control area). Neostriatal function was assessed with two correlated parameters, i.e., motor behavioral performance (wire suspension task), and oxotremorine-enhanced K(+)-evoked release of dopamine from perifused striatal slices. Rats were exposed to one of several doses of 56Fe-particle irradiation (0.10-1.0 Gy) and tested on a wire suspension task at 3-180 days postirradiation. Results indicated that profound decrements occurred in both of these indices. The effects on K(+)-evoked release of dopamine were evident for as long as 180 days after irradiation, and a subsequent experiment indicated that these effects appeared as early as 12 h postirradiation. Since similar findings have been observed in aged rats, the results are discussed in terms of these particles producing a possible accelerated striatal aging effect.

Aging↗