Methods for nuclear spectroscopyLow-level beta counting with an automatic sample changer
Abstract
Beta-particle emitters present in the snow of polar ice caps which are being analyzed for dating purposes, are due to fission products from thermonuclear tests in the atmosphere (90Sr, 137Cs) and to natural radioactivity. The measurement of many snow samples containing these radio-isotopes was made possible by the use of an automatic sample changer located inside a nuclear shielding. For detection, a surface-barrier silicon diode is connected in anti-coincidence with a gas-flow detector. The background radiation reaches 5 counts per hour (cph) above the energy of 120 keV and the efficiency for 40K is 33%. The nuclear detection characteristics are studied in detail.
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4π low level beta counter using two SiAu detectors
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Spatial characteristics of snow accumulation in Dronning Maud Land, Antarctica
2004, Global and Planetary ChangeAn accurate knowledge of spatial variations in snow accumulation on ice sheets is vital for numerical reconstructions and future projections of ice sheet dynamics, with implications for changes in global climate and sea level. This study considers net snow accumulation in Dronning Maud Land, East Antarctica. Ground-penetrating radar (GPR) soundings were carried out along two ground traverses, 800 and 1000 km long, extending from the ice shelves to the high plateau in order to map snow accumulation rates continuously along the route. Comparisons between the net accumulation rates obtained by GPR and those obtained by stake-line measurements exhibited a close correlation. Large dissimilarities in the accumulation distribution were observed between the two traverses. The ice sheet surface topography and the regional distribution of air pressure systems and cyclone tracks can explain these differences to a large extent. Spatial trends in snow accumulation rate were determined for characteristic traverse sections. The spatial variability was calculated as the standard deviation in snow accumulation rate over 5 km, expressed as percentages of the average accumulation within the 5-km section. On ice shelves, the spatial variability was less than 10%. In grounded coastal areas, it ranged between 20% and 40%, and on the high plateau, it ranged up to 20%. In areas of intense wind erosion, the spatial variability often exceeded 50%. The generally high spatial variability demonstrates that the spatial representativeness of individual point measurements is of vital importance in Antarctic snow accumulation studies. This is a problem that is presently not properly attributed.
Radionuclides deposition over Antarctica
2003, Journal of Environmental RadioactivityA detailed and comprehensive map of the distribution patterns for both natural and artificial radionuclides over Antarctica has been established. This work integrates the results of several decades of international programs focusing on the analysis of natural and artificial radionuclides in snow and ice cores from this polar region. The mean value (37±20 Bq m−2) of 241Pu total deposition over 28 stations is determined from the gamma emissions of its daughter 241Am, presenting a long half-life (432.7 yrs). Detailed profiles and distributions of 241Pu in ice cores make it possible to clearly distinguish between the atmospheric thermonuclear tests of the fifties and sixties. Strong relationships are also found between radionuclide data (137Cs with respect to 241Pu and 210Pb with respect to 137Cs), make it possible to estimate the total deposition or natural fluxes of these radionuclides. Total deposition of 137Cs over Antarctica is estimated at 760 TBq, based on results from the 90–180° East sector. Given the irregular distribution of sampling sites, more ice cores and snow samples must be analyzed in other sectors of Antarctica to check the validity of this figure.
Radioactivity measurements applied to glaciers and lake sediments
1995, Science of the Total EnvironmentThe behaviour of glaciers, polar ice-caps and lakes can be studied by means of natural and artificial radioactivity measurements conducted on snow (Alps, Arctic and Antarctic) and sediments samples. The nuclear decay of elements (210Pb and 238U filiation products) and nuclear events (atmospheric thermonuclear tests: 1954 and 1962–1963; Chernobyl accident: 1986) allow an absolute dating of corresponding layers. These determinations need radiochemical separations (electro-plating, ion exchange filters), followed by ultra low level alpha and gamma spectrometries, or beta counting (137Cs, 90Sr). The high purity — N type — germanium detector (Compton-suppressed) allows the 210Pb analysis at 46.52 KeV, enabling a direct comparison with 210Po alpha spectrometry. Typical applications concern primarily dating, and the determination of mean annual accumulation rates of glaciers, sedimentation rate and mixing time in lakes, with their associated spatio-temporal variations. These measurements give access to the global fallouts of radionuclides and to meteorological parameters: air to snow (or sediment) transfer, deposition processes and atmospheric circulation.
Natural and artificial radioactivity in the Svalbard glaciers
1994, Journal of Environmental RadioactivityNatural and artificial radioactivity in the snow of 10 Svalbard glaciers has been measured from 31 ice core samples, drilled between 1981 and 1993. Of these ice cores, seven exhibit the well-known level arising from the fallout of the 1961–1962 atmospheric thermonuclear tests. The second level, due to the Chernobyl accident (26 April 1986), has been detected in all the studied glaciers; the maximum 137Cs fallout reaches 22 Bq kg−1 and shows a high variability. The natural radioactivity, mostly due to 210Pb, shows an in-depth variation which is not governed by its half-life (22.2 years). These measurements serve many glaciological purposes: absolute dating of the snow layers; air-snow transger and fallout studies; the determination of mean annual mass balances in the accumulation area of glaciers and their associated spatio-temporal variations.
Ultra-clean underground counting facility for low-level environmental samples
1988, Science of the Total Environment, TheAn underground counting facility with cleanroom conditions has been designed and constructed to improve detection of low-level radioisotopes in the environment. The 3.0m × 4.3m × 2.4m counting chamber is placed 14.3m below ground, has 10.2cm thick walls of pre-WWII naval armour plate, and is further shielded by a minimum of 1.2m of specular hematite. The total overburden of shielding is equivalent to 31.7m of water. Careful selection of building materials and a special air filtering system maintain a cleanroom environment with minimum contamination potential.
Radioactivity levels in a firn core from the Yukon Territory, Canada
1984, Atmospheric Environment (1967)Gross β-activity and tritium concentration measurements made on a 26.8m length of consolidated snow (firn) core retrieved from an altitude of 5340 m on Mt. Logan (Yukon Territory, Canada) have revealed a detailed chronology of natural and anthropogenic radioactivity levels extending from about 1943 to 1980. Oxygen isotope measurements were used to assist in the dating of the core and a reliable time scale spanning 1950–1980 has been established with an accuracy of at least ± 0.5 y, and in some cases ± 0.25y.
An immediate result is the establishment of mean annual accumulation rates, which are, for the years 1963–1980: 0.36m y−1 and for 1951–1980: 0.39m y− water equivalent. Another result is the estimation of the ‘stratospheric residence time’ for the fission fallout products (mainly 90Sr and 137Cs). A value of the stratospheric residence time of about 1 y is derived from data spanning 1963–1967.
Reference to the history of atmospheric thermonuclear device testing shows that individual or specific groups of tests by different countries can be identified.
Comparison of the gross β-activity data with a similar set from Station Centrale (Greenland) shows that although the same trends exist in the two data sets, the amplitude of most corresponding major peaks in the Greenland data is significantly lower. Since the accumulation rates at the two sites are almost identical and since the deposition of radioactive aerosols in any case is expected to be dominated by dry fall-out, the differences are assumed to be related to the trajectory path and to the diffusion rates of the radionuclide clouds.