Lloyd D. Stephens

An Intercomparison of Dosimetry Techniques in Radiation Fields at Two High-energy Accelerators

A COmParison of radiation intensity measurements outside the shielding of the Bevatron and the 20 GeV Electron Linear Accelerator at Stanford is described. Measurements were made using several different techniques by workers from Brookhaven National Laboratory, Lawrence Berkeley Laboratory, and the Stanford Linear Accelerator Center. The measurements indicate large discrepancies (factors of two or more) between different techniques of measurement and illustrate the need for improvement in high-energy accelerator dosimetric techniques.

An Analytical Approach to Environmental Radiation Measurements

Environmental radiation measurements have been made and the results reported for the past 15 years. These measurements have included both the neutron and gamma contribution to the environment due to Laboratory operations. In order to determine these values an understanding of the natural environmental radiation must be known. The techniques for measuring environmental gamma radiation have been improved upon during this time and the latest instrumentation and interpretation methods commonly used at this laboratory are reported. Not only is the natural radiation dose rate important but the isotopes responsible need to be identified for a complete understanding of the observed radiation. The techniques for this determination are described.

The design of an experiment to study leukemogenesis in mice irradiated by energetic heavy ions.

The design of an experiment to study the incidence of cancer and hematological effects in mice irradiated by heavy ions is described. A beam of fully stripped

SOME RECENT DEVELOPMENTS IN TECHNIQUE FOR MONITORING HIGH-ENERGY ACCELERATOR RADIATION

{\rm C}^{6+}

Population exposure from high-energy accelerators

ions of energy 250 MeV/amu was produced by the Bevatron. Mice were irradiated in groups of 12 by rotating them in a wide beam (11.4 cm FWHM) once per minute. At a beam intensity of 108 ions per pulse irradiations of 250 animals to absorbed doses of 200 rad were completed in a few hours, an efficient use of accelerator beam time. The average LET of the radiation was 16.6 keV/μm with a spread in the animal of ±2 keV/μm. Radial and longitudinal variations in absorbed dose were less than 30% and 15%, respectively. Estimates of tissue-entrance absorbed doses with an ionization chamber and with thermoluminescent dosimeters differed by less than 3%.

The efficiency of 7LiF thermoluminescent dosimeters to high LET-particles, relative to 63Co gamma-rays.

Abstract In order to accurately evaluate the exposures received by individuals working near high-energy accelerators it is necessary to measure the separate components in the radiation field and to determine their energy spectrum. Since no single instrument or detector will do this, a variety of different detectors and instruments must be used. Three recent developments in technique for monitoring particulate radiation above 20 MeV use nuclear emulsion, elemental mercury, and Be7 production in light elements; below 20 MeV, the use of moderated foils of In, Au, and Co has been extended to include Ta. When emulsion is used, the number of stars formed by high-energy inelastic collisions is counted, together with the number of gray prongs. The ratio of gray prongs per star was previously found to be linearly related to the energy of the neutron which formed the star, over the energy range 20 to 300 MeV. This technique is used to measure average neutron energy and to estimate spectrum shape for neutron energies above 20 MeV. A second technique makes use of the spallation of Hg to Tb149, an alpha-emitter of 4.12-hr half-life. The threshold for this reaction is near 500 MeV, and it therefore extends the use of threshold detectors for the estimation of spectrum shape to a higher energy domain and gives additional confidence in previous estimates of spectrum shape made with Bi fission, C12(n, 2n)C11, and A127(n, a)Na24. Thirdly, production of Be7 from C12, N14, and O16 has been studied; it offers a method of high-energy neutron threshold detection with practical thresholds extending from 30 to 40 MeV for carbon to 45 to 55 MeV for oxygen. The practical sensitivity can be arbitrarily high without making the extraction process either too lengthy or unwieldy. Another recent development involves the inclusion of Ta in the class of detectors which use a thermal-neutron-sensitive activation element inclosed in a Cd-clad hydrogenous moderator to allow an integration period of a few months and a sensitivity considerably greater than with Co. Finally, recent improvements in the performance of our large parallel-plate Bi fission chamber are discussed.

A measurement of the average energy required to create an ion pair in nitrogen by 250 MeV/amu C6+ ions.

There is increasing interest in estimating the magnitudes of population exposure due to the operation of nuclear facilities. This paper discusses the population exposures that might result from high energy accelerators. External whole body radiation is the dominant type of radiation exposure, while exposure resulting from the leakage of radionuclides into the atmosphere or ground water is negligible. At large distances from operating accelerators, neutrons are the predominant form of radiation. The transport of high-energy neutrons through the atmosphere is not well understood at distances greater than about 1000 m from the radiation source. Consequently, estimates of population exposure out to distances of 80 km from high energy accelerators may be quite inaccurate. It is extremely important that health physicists and administrators be aware of the limitations of estimates of population exposure. A model is described which facilitates the estimation of population exposure due to accelerators and its limitations are discussed. Suaaestions are made for experimental investigations -which would improve the model.