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F H Attix

Publications and source records attributed to F H Attix.

At least 55 records · Page 3Linked to original sources
PubMedDate not supplied

Displacement correction factor for fast-neutron dosimetry in a tissue-equivalent phantom.

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

Fast Neutrons↗
PubMedDate not supplied

Composition of A-150 tissue-equivalent plastic.

In recent years, the use of tissue-equivalent materials has become quite common in fast-neutron dosimetry, with the A-150 plastic developed by Shonka et al. probably the most popular. Information on this specific plastic is scantily reported in the literature and as a consequence a preponderance of authors unknowingly reference an article by Shonka describing an early version of a tissue substitute plastic but having a different elemental composition than the present A-150 formulation. We have reviewed the results of 21 chemical analyses which have occurred over a time span of four years on a total of 14 samples of A-150 plastic and based on these data and the formulation of the plastic, have arrived at a suggested composition for A-150 tissue-equivalent plastic. The ambiguities of water absorption by nylon, one of the components of the plastic, and the uncertainty this reflects in the composition of the plastic were evaluated.

Fast Neutrons↗
PubMedDate not supplied

Scattered radiation from a neutron collimator.

Fast-neutron beams are being employed in radiotherapy trials and associated radiobiology studies at numerous centers in the U.S., Europe, and Japan. Since collimated beams of various sizes and shapes are employed, it is desirable to know the composition of the scattered radiation component contributed by the collimator. A simple method is shown for deducing the field composition in terms of a three-component model, from measurements made with three ionization chambers (tissue-equivalent, graphite, and magnesium). The dose contributed by the scattered radiation in the present example was found to be predominantly due to fast neutrons indistinguishable from those in the primary spectrum (from 35-MeV D+ on Be). This method may prove useful for measurements in phantoms as well.

Fast Neutrons↗
PubMedDate not supplied

A solid water phantom material for radiotherapy x-ray and gamma-ray beam calibrations.

The formulation, manufacture and testing of an epoxy resin-based solid substitute for water is presented. This "solid water" has radiation characteristics very close volumetrically to those of water. When it is used as a dosimetry phantom for x- and gamma-ray beams in the radiotherapy range, phantom-to-water corrections and density corrections are eliminated. Relative transmission measurements have shown that the transmission through 10 cm of solid water is within 0.2% of that through an equal thickness of water for x and gamma rays. The use of this material for calibration phantoms can help achieve the goal of radiotherapy beam calibrations within +/- 1.0% of the true dose rate, easier to achieve.

Calibration↗
PubMedDate not supplied

Characteristics of A-150 plastic-equivalent gas in A-150 plastic ionization chambers for p(66)Be(49) neutrons.

The evaluation of a gas mixture having an atomic composition similar to that of A-150 tissue-equivalent (TE) plastic has been extended to a high-energy neutron therapy beam. "A-150" gas, air, and methane-based TE gas were each flowed through A-150 plastic-walled ion chambers of different sizes and irradiated with p(66)Be(49) neutrons. A tentative value for W(A-150) of 27.3 +/- 0.5 JC-1 was derived for this beam. The W value of the A-150 gas mixture is compared to those of methane-based TE gas and of air for the p(66)Be(49) neutron beam as well as to corresponding values found in similar experiments using 14.8-MeV monoenergetic neutrons.

Beryllium↗
PubMedDate not supplied

Electron contamination in 60Co gamma-ray beams.

All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron "contamination." The present study of a 60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5 X 5, 20 X 20, and 35 X 35-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a "clear" 60Co gamma-ray beam of 1-cm diameter.

Cobalt Radioisotopes↗
PubMedDate not supplied

Calculational methods for estimating skin dose from electrons in Co-60 gamma-ray beams.

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

Cobalt Radioisotopes↗
PubMedDate not supplied

Performance characteristics of A 150 plastic-equivalent gases in A 150 plastic proportional counters for 14.8-MeV neutrons.

Two recently developed A 150 plastic-equivalent gas mixtures have been tested for suitability in proportional counter applications. Methane- and propane-based "tissue-equivalent" gases were also included for comparison purposes. Event-size weighted dose distributions were measured in a 14.8-MeV neutron beam. Resolution was found to be independent of gas mixture. Moreover the gains of the two A 150 mixtures were the same, and comparable to that of the methane-based gas mixture. The ionization yield per event size was also independent of the hydrogenous gas mixture employed. Neutron doses determined with the proportional counter were in reasonable agreement with those obtained from an ionization chamber.

Fast Neutrons↗
PubMedDate not supplied

A simple derivation of Ngas, a correction in Awall, and other comments on the AAPM Task Group 21 protocol.

The new AAPM Task Group 21 dosimetry protocol is a useful document that will no doubt result in improved accuracy in radiotherapy beam standardization when it is generally understood and widely adopted. It does however, require some "debugging." The present paper clarifies the derivation of Ngas, points out an error in the Awall data, and discusses the impracticality of applying the gradient approach in correcting for ion chamber perturbation of dose in phantoms irradiated by electron beams.

Humans↗
PubMedDate not supplied

A method for matching NBS x-ray beam qualities with a half- or full-wave rectified generator.

A method has been devised to enable users of half- and full-wave rectified x-ray sets to match the revised National Bureau of Standards (NBS) x-ray beam qualities obtained with a constant potential generator. The method provides a rapid technique for determining the added filter necessary to attain the same first half-value layers and nearly the same homogeneity coefficients as NBS. Beam qualities from 100 to 250 kVcp have been duplicated and used to perform blind calibrations at four beam qualities on each of two ion chambers owned by the National Bureau of Standards.

Aluminum↗
PubMedDate not supplied

Charge storage in electron-irradiated phantom materials.

A recent article by Galbraith et al. [Med. Phys. 11, 197 (1984)] revealed the existence of dose errors due to charge storage in electron-irradiated plastic phantoms. We have subsequently studied the same effect using similar materials, plus some others including "solid water," which is an epoxy-based phantom material manufactured by Radiation Measurement, Inc. Our work shows that there is minimal charge storage in solid water, as compared to polymethylmethacrylate (PMMA) and polystyrene. Since existing dosimetry protocols allow PMMA and polystyrene to be used for calibration phantoms, users should beware of the possible dosimetry errors resulting from charge storage in those plastics, and consider choosing other water-substitute media, such as solid water, that do not display this effect.

Electrons↗