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At least 685 records · Page 38Linked to original sources
PubMed1983

A nearly mono-energetic 6-7 MeV photon calibration source.

A photon source has been developed which delivers about 85% of its photon dose equivalent from photons with energies of 6.1,6.9 and 7.1 MeV produced in the 19F(p, alpha gamma)16O reaction. The source uses up to 50 muA of 2.7 MeV protons incident on a 6 mg/cm2 target of CaF2. It produces a photon field with a dose equivalent rate of up to 6 mSv/h (600 mrem/h) over a large area 100 cm from the target. The field can be calibrated in terms of photon fluence to within +/- 5%. In common with other high-energy photon sources, there is considerable contamination of the field by knock-on electrons and scattered photons. Experiments with various filter materials and detailed Monte-Carlo calculations with the EGS electron-photon transport code have been done to investigate the importance of these contaminants.

Calibration↗
PubMed1983

Recent muon fluence measurements at Fermilab.

Muon fields at Fermilab were measured during 1980-81 using a Mobile Environmental Radiation Laboratory (MERL). During the spring of 1980, measurements were made with the accelerator operating at 350 GeV; in the spring of 1981, measurements were made at 400 GeV. The measurements were used to obtain an understanding of muon dose-equivalent rates at various locations both on and off the Fermilab site. These were found to be less than 1 mrem/yr at any given location. The data indicate that more severe problems may be encountered during operations of the 1000 GeV accelerator presently being installed.

Elementary Particles↗
PubMed1983

The neutron sensitivity of a Geiger-Müller counter between 0.5 and 8 MeV.

Geiger-Müller counters are often used in mixed-field dosimetry to determine the photon dose fraction. For accurate dose specification, their neutron sensitivities must be known. The pulsed beam time-of-flight method was used to determine the neutron sensitivities of an energy-compensated type ZP1320/PTFE Geiger-Müller counter in the 0.5-8-MeV region. The flux of monoenergetic neutrons from the T(p,n)3He and D(d,n)3He reactions, which were used in these measurements, was determined from time-of-flight spectra with an NE213 scintillation counter of known efficiency.

Calibration↗
PubMed1983

Dose-rate conversion factors for external exposure to photons and electrons.

Dose-rate conversion factors for external exposure to photons and electrons have been calculated for approx. 500 radionuclides of potential importance in environmental radiological assessments. The three exposure modes considered are immersion in contaminated air, immersion in contaminated water, and irradiation at a height of 1 m above a contaminated ground surface. For each exposure mode, the source region is assumed to be effectively semi-infinite or infinite in extent with uniform radionuclide concentration. The dose-rate factors then give external dose-equivalent rates per unit radionuclide concentration in air, in water or on the ground surface. The results are tabulated in this paper in the form of effective dose-rate factors based on the definition of the effective dose equivalent given in ICRP Publication 26 (ICRP77). The effective dose-rate factors are obtained from photon dose-rate factors for 23 separate body organs and electron dose-rate factors for skin calculated with the revised DOSFACTER computer code (Ko81a). In addition to presenting the dose-rate factor equations and the tabulated results, this paper emphasizes the assumptions underlying the calculations for each exposure mode, differences between the organ dose-rate factors for photons used here and those used previously with the original version of the DOSFACTER code (Ko80a), and limitations inherent in application of the idealized external dose-rate factors to realistic environmental radiological assessments.

Air Pollutants↗
PubMed1984

Fluence to dose equivalent conversion factors calculated with EGS3 for electrons from 100 keV to 20 GeV and photons from 11 keV to 20 GeV.

The EGS3 Monte-Carlo electron-photon transport simulation package has been used to calculate dose equivalent per unit fluence vs depth curves for broad parallel beams of mono-energetic electrons, positrons and photons incident on a 30-cm-thick slab of ICRU four-element tissue. The electron kinetic energy range covered is 100 keV to 20 GeV and that for photons is 11 keV to 20 GeV. It was found that by making minor modifications, EGS3 is in reasonable agreement with other codes for electron energies down to 100 keV. Complete dose equivalent vs depth curves as a function of electron and photon energy are presented to allow proper calculations of the maximum dose equivalent for a mixed photon and electron spectrum since there are substantial variations in the locations of the peak dose equivalent. Explicit calculations demonstrate that l/r2 corrections give an accurate means to convert results for broad parallel beams to those for point source geometries. The relative contributions of various physical processes to the peak dose equivalent are presented.

Computers↗
PubMed1986

Effect of partition walls and neighboring buildings on the indoor exposure rate due to cosmic-ray muons.

Numerical calculations were done of the indoor exposure rate due to the cosmic rays. Only muons were considered, and the calculations were done within a two-dimensional framework under the non-scattering approximation. The effects of various structural parameters on the level and the distribution of the exposure rate were studied by a case control method. It was shown that a building's dimensions and the floor thickness were important in all cases. However, the effect of partition walls would become significant only when the partition number is large. The effect of neighboring buildings would be especially important in the lower floors of tall buildings. In such a case, attention is necessary not only to the buildings' internal structures but also to the relationship between that particular building and its neighborhood. The maximum amplitude of the possible variation of the exposure rate due to the change of any single structural parameter would be 30% in ordinary Japanese housing conditions.

Building Codes↗
PubMed1990

Energy deposition at the bone-tissue interface from nuclear fragments produced by high-energy nucleons.

The transport of nuclear fragmentation recoils produced by high-energy nucleons in the region of the bone-tissue interface is considered. Results for the differential flux and absorbed dose for recoils produced by 1-GeV protons are presented in a bidirectional transport model. The energy deposition in marrow cavities is seen to be enhanced by recoils produced in bone. Approximate analytic formulae for absorbed dose near the interface region are also presented for a simplified range-energy model.

Bone Marrow↗
PubMed2003

Assessment of the prompt radiation hazards of trapped antiprotons.

Investigators at several laboratories are seriously considering the storage and transport, perhaps over long distances, of very low energy antiprotons as a part of basic physics research programs and perhaps even for practical applications. To do this will require proper attention to the prompt radiation hazards due to the release of energy in the annihilations of antiprotons with nuclei, under either planned or accidental circumstances. In this paper, the potential storage of very low energy antiprotons is discussed, and the major features of the radiation fields produced by their annihilations are reviewed both qualitatively and quantitatively. Detailed Monte Carlo shielding calculations for a conceptual source of annihilating antiprotons nearly at rest are presented. It is concluded that these radiation fields are readily understood and that the radiation hazards can be mitigated using conventional means.

Air↗
PubMedDate not supplied

Dichromatic absorptiometry of vertebral bone mineral content.

A dichromatic photon absorptiometric technique for the in vivo measurement of the bone mineral of the spine is discussed. A high purity, high activity 153Gd source which has photons of predominantly 44 and 100 keV was used as the transmission source. The transmission scans were performed on a modified Ohio Nuclear whole body rectillinear scanner. The precision of four measurements on three individuals and three measurements on one individual over a one month period was 2.3% (coefficient of variation). The accuracy determined by measurements of phantoms with known concentrations of dipotassium hydrogen phosphate solutions was 1.2% (standard error of estimate). This technique can be applied in most well equipped nuclear medicine departments.

Absorption↗
PubMedDate not supplied

Vertebral and peripheral bone mineral content by photon absorptiometry.

Dual photon absorptiometry with 153Gd was used to determine the bone mineral content of the lumbar spine of 46 subjects. The bone mineral content of the radius and ulna were measured on these same subjects using single photon absorptiometry. The bone mineral content at the peripheral sites was not a very accurate indicator of absolute bone mineral content in the spine. However, the rate of bone loss with age was greater in the peripheral bones than in the spine. Measurement sites on the appendicular skeleton may provide a more sensitive indication of age changes in bone mineral content throughout the skeleton than would sites on the axial skeleton.

Absorption↗
PubMedDate not supplied

Tomographic imaging with nuclear magnetic resonance.

A technique is described for obtaining tomographic images of hydrogen distribution in animals using nuclear magnetic resonance (NMR). Resonant frequency is proportional to magnetic field strength, so that spatial resolution is achieved by frequency selection and magnetic field shaping. The results of scanning a phantom and two rats are presented.

Animals↗
PubMedDate not supplied

Photon scattering measurement of calcaneal bone density: result of in vivo cross-sectional studies.

The density of trabecular bone in the os calcis has been measured in 321 subjects using a gamma ray scattering technique. In normal subjects it was shown that density could be predicted from body weight and age with a standard error of 5.6%. It was also shown that the dependence of density upon weight and age was the same for both sexes. When the os calcis had been subjected to a greater than normal mechanical stress by either increased physical activity or excessive body weight, trabecular bone density was increased. Density was measured in 128 patients in whom the incidence of skeletal demineralization was expected to be greater than that in control subjects. Some significant reductions in density were observed. It is projected that density measurements might be of value in those situations where, in response to metabolic stress, the rate of loss of mineral from trabecular bone is greater than that from cortical bone.

Adult↗