The Measurement and Behavior of Airborne Radionuclides Since 1962

10 The Measurement and Behavior of Airborne Radionuclides Since 1962

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch010

C. W. THOMAS, J. A. YOUNG, N. A. WOGMAN, and R. W. PERKINS Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Wash. 99352

The atmospheric concentration of natural and bomb-pro duced radionuclides has been measured at ground level for several years at three locations throughout the world. The manner in which the concentration decreased suggested a half-residence time for stratospheric aerosols of 11.8 months at 46°N latitude. The annual spring concentration maxi mum occurred one to four months earlier at 71°Ν than at 46°Ν. Cosmogenic Be attained a maximum concentration before the bomb-produced radionuclides at 71°Ν and later than the bomb-produced isotopes at 46°N. The rate of in crease toward the annual peak concentration for most radio nuclides could be approximated by an exponential in which the concentration doubled every 60 days; likewise, the rate of decrease from the maximum concentration could be ap proximated by an exponential with a half-time of about 40 days for most radionuclides except Be at 46°N, which shows a half-time of about 60 days. 7

7

*"phe atmosphere contains many radionuclides which result from nuclear weapons testing and from natural processes. The nuclear weaponsproduced radionuclides include both fission products and activation products from the construction materials of the device. The natural radio nuclides include the decay products of radon and thoron, the natural radionuclides i n the airborne dust, and the cosmic-ray-produced radio nuclides which result from spallation reactions i n the atmosphere. Through the determination of the absolute and relative concentrations of this wide spectrum of radionuclides, it should be possible to define the rates of both the long term stratospheric processes and the shorter term tropospheric processes. A t the beginning of 1962 a ground-level 158

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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Airborne Radionuclides

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air sampling program was initiated at Richland, Wash., using highly developed instrumental analysis which has allowed the behavior of some 20 airborne radionuclides to be followed. More recently, sampling sta tions have been set up at Point Barrow, Alaska, and Rio de Janeiro, Brazil. Measurements at these locations are helping to define the temporal and spatial distribution of airborne radionuclides and providing infor mation for estimating surface deposition.


Sampling and Instrumental

Techniques

The ground-level air sampling was performed by filtering air through 5-μ pore size membrane filter papers at flow rates of 400-700 cu. ft./min. These membrane filters are essentially 100% efficient for the atmospheric aerosols on which airborne radionuclides are attached (8). The air is drawn through four 1-sq. ft. membrane filters which are mounted on the vertical sides of a cube-shaped air sampler (10). The air pumps

Figure 1.

Large crystal multidimensional γ-ray spectrometer system

were all positive displacement units with direct shaft linkage to their electric motors. Filters were changed semimonthly, depending on the sampling station, and pressed into a standard counting geometry of 1.25-cm. thick χ 5-cm. diameter. A l l of the radionuclide measurements were performed by direct instrumental analysis of the air filters. Most of the measurements were


160

RADIONUCLIDES

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THE

ENVIRONMENT

made with a multidimensional γ-ray spectrometer. The sample was counted between two 11-inch diameter by 6-inch thick N a l ( T l ) detec tors, while a third detector, a large plastic phosphor, served as the anti coincidence shield (15). This detector assembly, shown i n Figure 1, measures both the single and coincidence y-rays emitted from the sample and stores the events i n a 4096-word computer memory at locations which uniquely define the energies of the y-rays. Large volume coaxial G e ( L i ) diodes (2) with resolutions of 2.9 kev. ( F W H M ) or better at the 1332 kev. y-ray of C o were also used to measure certain radionuchdes which were, not resolved by the N a l ( T l ) system. Recently the anticoincidence shielded G e ( L i ) diode shown i n Figure 2 has been used for the fallout studies and has provided a new dimension i n selectivity for the measure ment of radionuchdes i n fresh fallout (3). It combines a reasonably high sensitivity with a peak-to-Compton edge ratio for C s of about 240 to 1.


6 0

1 3 7

Figure 2. Ground-Level

Anticoincidence spectrometer system

Radionuclide

Measurements

H i g h volume air sampling stations are presently i n operation at Point Barrow, Alaska ( 7 1 ° N ) , Richland, Wash. ( 4 6 ° N ) , and Rio de Janeiro, Brazil (23°S). The air sampling rates at Alaska and at Rio de Janeiro are 400 cfm, while a 700-cfm sampling rate is employed at Richland, Wash. The observed radionuclide concentrations i n ground level air at the three sampling locations are summarized i n Figures 3 through 6. The measurements at Richland ( 13 ) cover the six-year period from early 1962 to 1968 and provide excellent definition of the seasonal variations. The Alaskan measurements provide similar definition over



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Airborne Radionuclides

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Figure 3.

Ground level air concentration of radionuclides that show prominent seasonal variations

Top curve: 71° Ν htitude Middle: 46°N latitude Bottom: 23°S latitude

a shorter period. The Brazil measurements reflected the southern hemi sphere nuclear testing and d i d not provide good definition of the normal seasonal variations in the ground-level air concentrations. The seasonal concentration variations at Richland and Point Barrow were used to classify the radionuclides somewhat arbitrarily into four groups. The first group consists of the relatively long lived bomb-pro duced radionuchdes M n , C o , Ru, Ag, Sb, Cs, C s , and C e (Figure 3). They were inserted into the atmosphere mainly during the 1961-1962 nuclear test series and show prominent seasonal concen5 4

60

1 0 6

1 1 0

125

134

137

144


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RADIONUCLIDES IN

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ENVIRONMENT

tration variations with maximum concentrations occurring in the spring and minimum concentrations occurring in the fall. Their concentrations have decreased at a very nearly exponential rate during the last five years because of radioactive decay, vertical and meridional mixing in the atmosphere, and wet and dry deposition at the earth s surface. These radionuclides were mainly of stratospheric origin. Because relatively 1.0

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The Measurement and Behavior of Airborne Radionuclides Since 1962

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