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Results for “Elementary Particles”

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At least 613 records · Page 34Linked to original sources
PubMed1975

Study of contrast and modulation mechanisms in X-ray/photon transverse axial transmission tomography.

The ultimate capabibity and a detailed study of contrast and modulation in the context of photon transmission tomography (or computerized transverse axial tomography (CTAT)) is presented. Quantitative evaluation of atomic number, density and electron density of compound materials were made by X-ray fluorescence and combustion analysis, and the attenuation coefficients of photoelectric and Compton effect were measured with high resolution Ge(Li) and Si(Li) detectors. This study provides a guide to an understanding of the high resolution of the currently existing X-ray transmission CTAT scanner (EMI scanner) and to the future development of high resolution scanners.

Brain↗
PubMed1975

Secondary radiation from a spherical tissue-equivalent phantom irradiated with 60Co gamma radiation and 6 MV X-rays.

The angular dependence and the dependence on the phantom mass of the secondary exposure rate from spherical tissue-equivalent phantoms irradiated with 60Co gamma radiation and 6 MV X-rays have been investigated both by measurement and by calculation using the Monte Carlo method and approximate methods. A nonlinear dependence on the phantom mass was found. Calculation of the differential photon fluence was also made. These measurements and calculations can be used when calculating the required barrier thickness for walls which are not hit by primary radiation.

Cobalt Radioisotopes↗
PubMed1976

The energy response of LiF, film and a chemical dosemeter to high energy photons and electrons.

The physical responses of LiF, film and a chemical dosemeter in high energy photon and electron beams has been studied. It is shown that the response for film and the chemical dosemeter does not change significantly up to 42 MeV, while for LiF it decreases by about 10%. Analysis of calculations and measurements showed that this decrease could not be explained by existing theories.

Electrons↗
PubMed1976

A technique for measuring the variation of photon spectrum from X-ray generators over the mains voltage cycle.

The variation in current and accelerating voltage across an X-ray tube, that occurs over the mains voltage waveform cycle, produces changes in photon flux and spectrum shape. A knowledge of these changes is required to provide an understanding of the parameters affecting X-ray output. Variation in the photon flux will cause distortion of a measured 'mean' spectrum if the dead time of the spectrometry system employed varies over the waveform cycle. A system has been developed that enables the spectrometer to be synchronized to the mains voltage cycle so that variations in photon flux and the instantaneous spectra at selected parts of this cycle can be measured. The variation in photon flux is dependent upon the type of power supplies (both high and low tension). the tube current and the degree of filtration employed. Examples are given of the ripple of photon output due to the voltage and current ripple for a nominally constant potential generator and a half-wave rectified type. The way in which they interact to produce the measured variation is shown.

Elementary Particles↗
PubMed1976

Bone density measured by photon scattering. II. Inherent sources of error.

The origins of inherent sources of error in results of measurements of bone density by a photon scattering technique are described and their effects are predicted theoretically. The predictions are confirmed experimentally. The prime source of error is multiple scattering. A procedure has been developed to correct the observed densities for effects of multiple scattering.

Bone Diseases↗
PubMed1977

Three-dimensional imaging in the position camera using Fourier techniques.

A mathematical algorithm for three-dimensional reconstructions in the positron camera is described. Fouier techniques have been adapted for use in analysing the data from cameras with limited detector configuration. Noise instabilities from random fluctuations in the data are discussed and treated. The technique is tested on a computer-generated phantom and the results are presented. Ways of incorporating the effects of Compton scattering and detector response into the algorithm are discussed. It is concluded that the method is feasible and practical for obtaining accurate reconstructions.

Brain Neoplasms↗
PubMed1977

Determination of some parameters for pion radiobiology studies.

An experimental investigation of the central axis depth-dose and stopping rate distribution of the SIN biomedical pion beam is reported. The pion stopping rate in a thin disc of tissue-equivalent plastic was determined using a counter telescope. The dose rate at the position of this dic 'target' was measured using a specially designed parallel-plate tissue-equivalent ionization chamber. Both dose rate and pion stopping rate are given as a function of depth for beams of two different momenta spread. The energy deposition required per pion stop to fit the measured dose rate curves was calculated and found to be between 37 and 40 MeV. From the stopping rate measurements the depth-dose distribution of pion interaction dose (star-dose) and the dose due to the pion slowing down have been evaluated. The maximum star-dose is found at a depth of about 2 g cm-2 beyound the maximum of the ionization dose.

Elementary Particles↗
PubMed1978

Dose outside the treatment volume for irradiation with negative pions.

Irradiation of humans with negative pions requires a knowledge of the absorbed dose and radiation quality outside the primary pion beam. In conjunction with early clinical trials at LAMPF, experimental data have been obtained with microdosimetric techniques and multiwire proportional counters. Theoretical calculations have been made for the neutron contribution to the dose and are consistent with these data. Measurements were made with in 40 cm x 51 cm x 76 cm water phantom for a negative pion beam of initial momentum of 170 MeV/c, deltap = +/- 3MeV/c. The absorbed dose outside the treatment volume is the result of: (1) neutrons and photons from the pion interactions,(2) treatment room background and (3) peripheral muons, electrons and pions in the primary beam. The first two components are nearly isotropic and are congruent to 0.02% of the peak dose at a distance of 24 cm from the treatment volume; the third component is anisotropic and varies from 0.01 to 0.1% of the peak dose. Collimation of the bean increases the dose outside the treatment volume typically by 50%.

Elementary Particles↗