H. Wade Patterson

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

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.