UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes
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SOURCES AND EFFECTS OF IONIZING RADIATION United Nations Scientific Committee on the Effects of Atomic Radiation UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes VOLUME I: SOURCES UNITED NATIONS New York, 2000 NOTE The report of the Committee without its annexes appears as Official Records of the General Assembly, Fifty-fifth Session, Supplement No. 46 (A/55/46). The designation employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the United Nations concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. The country names used in this document are, in most cases, those that were in use at the time the data were collected or the text prepared. In other cases, however, the names have been updated, where this was possible and appropriate, to reflect political changes. UNITED NATIONS PUBLICATION Sales No. E.00.IX.3 ISBN 92-1-142238-8 ANNEX B Exposures from natural radiation sources CONTENTS Page INTRODUCTION.................................................... 84 I. COSMICRADIATION ........................................... 84 A. COSMICRAYS............................................. 84 1. Exposureatgroundlevel.................................. 86 2. Exposures at aircraft altitudes .............................. 87 B. COSMOGENICRADIONUCLIDES............................. 89 II. TERRESTRIALRADIATION...................................... 89 A. EXTERNALEXPOSURES.................................... 89 1. Outdoors.............................................. 89 2. Indoors ............................................... 92 3. Effectivedose .......................................... 92 B. INTERNAL EXPOSURES OTHER THAN RADON ................. 93 1. Inhalation ............................................. 93 2. Ingestion.............................................. 93 3. Effectivedose .......................................... 94 C. RADON AND DECAY PRODUCTS ............................. 96 1. Sourcesofradon ........................................ 97 2. Concentrationsinair.................................... 102 3. Effectivedose ......................................... 105 III. ENHANCED EXPOSURES FROM INDUSTRIAL ACTIVITIES .......... 108 IV. WORLDWIDEAVERAGEEXPOSUREFROMNATURALSOURCES..... 111 CONCLUSIONS.................................................... 112 Tables............................................................ 113 References......................................................... 141 84 ANNEX B: EXPOSURES FROM NATURAL RADIATION SOURCES INTRODUCTION 1. The exposure of human beings to ionizing radiation geographic variations more accurately evaluated, allowing from natural sources is a continuing and inescapable feature some issues to be addressed in greater detail. There remain, of life on earth. For most individuals, this exposure exceeds however, some questions that are not yet satisfactorily that from all man-made sources combined. There are two resolved. For example, there are difficulties in evaluating main contributors to natural radiation exposures: high-energy cosmic ray exposures in aircraft because of the complex cosmic ray particles incident on the earth's atmosphere and neutron and ionizing radiation fields, and the dosimetry of radioactive nuclides that originated in the earth's crust and are inhaled radon is complicated by the complexities and present everywhere in the environment, including the human variations of the interacting factors and processes involved. body itself. Both external and internal exposures to humans arise from these sources. These exposures were reviewed in 5. Many exposures to natural radiation sources are previous reports of the Committee, the most recent being the modified by human practices. In particular, natural UNSCEAR 1993 Report [U3]. radionuclides are released to the environment in mineral processing and uses, such as phosphate fertilizer production 2. In assessing exposures to the natural radiation and use and fossil fuel combustion, causing enhanced natural background, the Committee has considered the properties of radiation exposures. In a few cases, for example, by paving the sources and the transport of both radionuclides and roads or building houses over water, radiation exposures may radiation in the environment. Estimates have been made of be decreased, but these seem to be rather isolated cases. The typical exposures to the world population and the range of the general topic of enhanced exposures from natural radiation components of such exposures under various environmental sources was considered in detail in the UNSCEAR 1982 conditions, and note has been taken of the unusually high Report [U6], and some aspects were further evaluated in the natural radiation exposures that occur in some locations. This UNSCEAR 1988 and 1993 Reports [U3, U4]. The topic is information has been combined with relevant dosimetric discussed further, with updated information, in Chapter III of quantities to estimate the absorbed doses in tissues and the this Annex. Many persons are also exposed to enhanced effective doses from the various sources of exposure. levels of natural radiation at their places of work. Such workers include underground miners, some workers involved 3. In this Annex, the Committee continues its general in processing of minerals, and aircraft flight crew. review of the various components of the natural radiation Occupational radiation exposures from both man-made and background. To broaden the database, an attempt has been natural sources are considered in Annex E, “Occupational made to gather representative levels of exposure in as many radiation exposures”. countries as possible. Many scientists and representatives of national institutions have responded to the questionnaire on 6. Thebroad relevanceofnatural background exposures natural radiation exposures, UNSCEAR Survey of Natural to the world population makes the evaluations of this Radiation Exposures, which was widely distributed by the Annex particularly pertinent. For most individuals, the Committee. Respondents to the questionnaire are listed in natural background exposures are much more significant Part A of the References. The Committee acknowledges with than the exposures caused by man-made sources. appreciation their useful contributions to its work. Exceptions that apply to certain individuals are some exposures caused bymedical radiation procedures, through 4. The database on natural radiation exposures has become mishandling of radiation sources, in accidents allowing extensive enough to allow quite detailed analysis. For radionuclides to be released to the environment, and at example, the distributions of populations within various dose some workplaces. In all cases, however, the natural intervals from the different components of exposure can be background source forms the baseline upon which all other examined within and between countries. The processes giving exposures are added, and it is a common level against rise to the exposures can be better described and the time and which other exposures may be compared. I. COSMIC RADIATION 7. The earth is continually bombarded by high-energy radionuclides. Best known of these are 3Hand14C. Exposures particles that originate in outer space. These cosmic rays from cosmic rays and from cosmogenic radionuclides are interact with the nuclei of atmospheric constituents, considered in this Chapter. producing a cascade of interactions and secondary reaction products that contribute to cosmic ray exposures that decrease A. COSMIC RAYS in intensity with depth in the atmosphere, from aircraft altitudes to ground level. The cosmic ray interactions also 8. Galactic cosmic rays incident on the top of the produce a number of radioactive nuclei known as cosmogenic atmosphere consist of a nucleonic component, which in ANNEX B: EXPOSURES FROM NATURAL RADIATION SOURCES 85 aggregate accounts for 98% of the total, and electrons, which particles of higher energies can penetrate at lower geo- account for the remaining 2%. The nucleonic component is magnetic latitudes. This produces the geomagneticlatitude primarily protons (88%) and alpha particles (11%), with the effect, with minimum intensities and dose rates at the remainder heavier nuclei [G11]. These primary cosmic equator and maximum near the geomagnetic poles. particles have an energy spectrum that extends from 108 eV to over 1020 eV. Below 1015 eV the shape of the energy spectrum 14. Thehigh-energyparticlesincident on theatmosphere can be represented by a power function of the form E2.7, interact with atoms and molecules in the air and generate where E is expressed in eV. Above that point, known as the a complex set of secondary charged and uncharged knee, the spectrum steepens to a power of 3. The highest particles, including protons, neutrons, pions and lower-Z energy thus far measured is 3.2 1020 eV, which was inferred nuclei. The secondary nucleons in turn generate more from ground measurements of the resulting cascade nucleons, producing a nucleonic cascade in the atmo- interactions in the atmosphere [O7]. sphere. Because of their longer mean free path, neutrons dominate the nucleonic component at lower altitudes. As 9. It is thought that all but the highest energy cosmic rays a result of the various interactions, the neutron energy that reach earth originate within the earth’s own galaxy. The