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Biomedical subjects

F H Attix

Publications and source records attributed to F H Attix.

59 records · Page 4Linked to original sources
PubMedDate not supplied

Skin-sparing effects of neutron beam filtering materials.

The skin-sparing effects of several filtering materials for fast neutron beams were studied under various conditions. A parallel-plate ionization chamber was used for the measurements. The parameters which were studied included field size, distance from filter to ion chamber, filter material, and filter thickness. On the basis of this work, Teflon (polytetrafluoroethylene) was chosen for fabrication of flattening filters and wedges.

Humans↗
PubMedDate not supplied

Equations for Ngas and Nair in terms of Nx and Nk.

Task Group 21 of the American Association of Physicists in Medicine defined the cavity-gas calibration factor Ngas for ionization chambers in a protocol for radiotherapy dosimetry published in this journal [Med. Phys. 10, 741 (1983)]. Later, Schulz et al., [Med. Phys. 13, 755 (1986)] published a letter of clarification that proposed a revised equation for Ngas in which the effect of atmospheric humidity was specifically dealt with. The present report points out errors in both of those presentations of the Ngas equation, and derives formulations for cases where the standardization laboratory does or does not correct for ambient humidity during chamber calibration. In the case where humidity is corrected for, the resulting quantity is more properly called "Nair." The Ngas equation is stated in terms of the air kerma calibration factor Nk as well as the more familiar exposure calibration factor Nx.

Air↗
PubMedDate not supplied

A proposal for the calibration of plane-parallel ion chambers by accredited dosimetry calibration laboratories.

A procedure is described by which the AAPM-accredited dosimetry calibration laboratories could offer calibrations of plane-parallel ionization chambers for radiotherapy dosimetry applications, by comparison with a cylindrical ion chamber in a phantom irradiated by 60Co gamma rays. Ngas can thus be determined for the plane-parallel chamber under uniform conditions of photon scatter, and without the need for an electron fluence correction.

Calibration↗
PubMedDate not supplied

Calibration of 192Ir high-dose-rate afterloading systems.

A method is described for calibration of 192Ir high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average 192Ir energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.

Brachytherapy↗