Patent Number: 046997512
Section: summary

DESCRIPTION 1. Technical Field This invention relates generally to devices for measuring neutron fluence, neutron exposure dose in rad units, and neutron biological dose in rem units, and more particularly to devices to measure these quantities over a wide spectrum of neutron energies, e.g., from thermal to 10.sup.7 eV, including the difficult region of 10.sup.3 ev to 10.sup.6 ev, with sufficient sensitivity to cover the range from less than 1 mrad to many megarads. 2. Background Art Approximately thirty thousand workers are involved in the nuclear industry in the United States. Many thousands more are involved throughout the world. These workers wear a "badge" containing materials which serve as a dosimeter for the various types of possibly harmful radiation encountered by these persons. A personnel neutron dosimeter (PND) may be included in the badge to indicate the levels of neutron exposure. The measurement of very low levels of neutron exposure is a matter of renewed concern especially at low neutron energies below about 100 keV. New information on relative biological effectiveness calls for the reduction of the neutron exposures received by personnel in the nuclear industry. This is particularly true because large numbers of the occupational workers are involved in pressurized water reactor operations where low energy neutrons may be present. None of the known existing dosimeters or neutron measurement devices have the desired combination of sensitivity, rejection of gamma radiation, response to low energy neutrons and stability. Accordingly, it is the primary object of the present invention to provide a dosimeter which may be worn by personnel to measure extremely low doses of neutrons, even at low energies. It is also an object of this invention to provide a sensitive neutron dosimeter for low dose measurement which discriminates against electromagnetic radiation such as gamma radiation. It is also an object of the invention to provide a neutron dosimeter for low doses of low energy neutrons that has high stability and is easily fabricated. It is another object of the invention to provide a dosimeter which will permit the measurement of neutron doses over a wide range to levels as low as a few microrads of exposure to tissue. It is an additional object of this invention to provide a device to measure the neutron flux or the neutron fluence over a wide range of energy to be placed in a device which may or may not be worn by personnel. It is a further object of this invention to provide a device to measure the biological dose in rem units, particularly for the applications of a PND. Other applications of the fluence meter, the rad meter or the rem meter may not involve their use on personnel, since they can also be used as stationary field devices either for the protection of personnel, for research, and for testing applications. Further objects of the invention will become apparent upon a consideration of the drawings and the description of the embodiments hereinafter. DISCLOSURE OF THE INVENTION In accordance with the invention, a neutron dosimeter is provided for determining extremely low doses of exposure. In the broadest embodiment of the invention, the present dosimeter includes a source material containing atoms which are ejected, e.g., sputtered, from the source material in response to the neutrons. The sputtered material, which may be gaseous or solid, is then measured as a value of the dose of the impinging neutrons. By appropriate choice of source materials, together with appropriate radiator and absorber materials, dosimeters can be constructed whose sensitivity varies with neutron energy as the fluence, the rad dose, or the rem dose. Any of these choices of energy dependence may be desirable according to the application to be made of the dosimeter. In a specific embodiment of the invention, the dosimeter contains a radiator material which emits protons or other recoil atoms in proportion to the neutrons. These recoils impinge on a solid matrix in which a noble gas such as xenon is incorporated. A very small fraction of the xenon is emitted (sputtered) by the action of the protons and the quantity thereof is then measured as an indication of the dose received by the detector. The measurement can be performed, for example, by using a method based on Resonance Ionization Spectroscopy (RIS).