Activity median diameter

The working level concept evaluates the unattached fraction and the activity median diameter in an indirect way, through the dose conversion factor. This paper will show that in the domestic environment this is mostly inaccurate to estimate the dose. 

The fraction of unattached daughters (fp), the equilibrium factor (F) and the activity median diameter (AMD) are plotted in Figure 6 for all the measurements. The AMD is derived from the aerosol measurements. These three parameters are important in the dosimetric models. At the top of Figure 6 the effective dose equivalent is plotted, computed with two models called the J-E (Jacobi-Eisfeld) and J-B (James-Birchall) models in the NEA-report (1983, table 2.9, linear interpolation between AMD=0.1 and 0.2 ym). The figure also shows the effective dose equivalent calculated from the equilibrium equivalent radon concentrations with the NEA dose conversion factor (NEA,1983, table 2.11). 

Figure 6a. Evolution of the activity median diameter (A.M.D.), the equilibrium factor (F), the unattached fraction and the effective dose equivalent (AJ-B, V J-E, + NEA) during the case studies.

Table I. Calculated activity median diameter for attached radon daughters from measured aerosol characteristics (Tu, 1986).

Site Activity median diameter (jam) geometric standard deviation... 

The calculated size distribution of newly attached decay products is shown as curve C in Fig. 1.9. The activity median diameter is 0.16 /zm. With passage of time, the distribution would be shifted to larger particle sizes, as coagulation proceeds. George (1972) used diffusion batteries to measure the size distribution of nuclei carrying radon decay products and found activity median diameters (AMD) averaging 0.18,0.11, and 0.30 /um in a city basement, fifth floor room, and rural outside air, respectively. 

Plutonium is so toxic that processing and fabrication are always done in sealed cells or glove boxes, but accidental dispersions of aerosol occur from time to time. Following combustion of Pu metal chips in a production area at Rocky Flats, Colorado, in 1964, airborne contamination was widespread. Alpha tracks from individual particles caught on membrane filters were detected on nuclear film, and the Pu content, and hence the particle size, was deduced (Fig. 5.2, curve E). The activity median diameter was 0.3 /urn (Mann Kirchner, 1967). The same method, used during normal operations in a production area at Los Alamos, gave activity median diameters in the range 0.15 to 0.65 /urn (Moss et al., 1961). However, when a spill occurred, followed by clean-up operations, the Pu particles were found to be associated with inert dust particles of mass median diameter 7 /urn. 

Activity Median Aerodynamic Diameter (AMAD)—The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the entire size distribution of the aerosol. 

The mean values of the activity and aerosol median diameters together with the best estimate of the standard deviation, an, based on the total number of measurements made for each parameter, are listed in Table V. Figures 5-8 show representative size distributions ... 

Table IV. Activity and Aerosol Size Distributions, Count Median Diameter (CMD) and the Geometric Standard Deviation (indicated in brackets)...

Based on 46 measurements, the activity median aerodynamic diameter of Pb-212 averaged 0.13 pm (0g = 2.97), while Pb-214 averaged 0.16 pm (Og = 2.86). The larger median size of Pb-214 was attributed to a-recoil depletion of smaller aerosols following decay of aerosol-associated Po-218. 

Aerodynamic Size Distributions of Naturally-Radioactive Aerosols. Measurements of radionuclide distributions using cascade impactors indicate that Be-7 and Pb-210 are associated with larger aerosols than Pb-212 and Pb-214 (Robig et al., 1980 Papastefanou and Bondietti, 1986). Measurements of Pb-210 associations over oceans indicated activity median aerodynamic diameters (AMAD) near 0.6 pm (Sanak et al., 1981). The impactor measurements of Moore et al. (1980) on Pb-210, Bi-210, and Sr-90 sizes in continental air indicated that about 80% of the activity from all three nuclides was associated with aerosols below 0.3 pm. That work also determined that the mean age of aerosol Pb-210 was about a week. Knuth et al. (1983) compared Pb-210 and stable Pb sizes at a continental location and found that 78% of the Pb-210 found below 1.73 pm was smaller than 0.58 pm. Young (1974) reported that the most of the Be-7 in the atmosphere was associated with submicron aerosols. 

Table II. Summary of Mean Monthly Activity Median Aerodynamic Diameters (AMAD) and Geometric Standard Deviations (tfg) of Radon and Thoron Daughter Size Distributions in Ambient Aerosols...

The aerodynamic size distributions of Pb-214, Pb-212, Pb-210, Be-7, P-32, S-35-SoJ , and stable SO4 were measured using cascade impactors. Pb-212 and Pb-214, measured by alpha spectroscopy, were largely associated with aerosols small than 0.52 11m. Based on over 46 low-pressure impactor measurements, the mean activity median aerodynamic diameter (AMAD) of Pb-212 was found to be 0.13 11m, while for Pb-214 the AMAD was larger—0.16 lim. The slightly larger size of Pb-214, confirmed with operationally different impactors, was attributed to a-recoil-driven redistribution of Pb-214 following decay of aerosol-associated Po-218. A recoil model was presented that explained this redistribution. Low-pressure impactor measurements indicated that the mass median aerodynamic diameter of SoJ ... 

Activity median aerodynamic diameter h Geometric standard deviation. 

The half-life of nickel in the lungs of rats exposed by inhalation has been reported to be 32 hours for nickel sulfate (mass median aerodynamic diameter [MMAD] 0.6 pm) (Hirano et al. 1994b), 4.6 days for nickel subsulfide ( Ni3S2 activity, median aerodynamic diameter [AMAD] 1.3 pm), and 120 days for green nickel oxide ( NiO, AMAD 1.3 pm) (Benson et al. 1994). Elimination half-times from the lung of rats of 7.7, 11.5, and 21 months were calculated for green nickel oxide with MMADs of 0.6, 1.2, and 4.0 pm, respectively (Tanaka et al. 1985, 1988). 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

Most of the activity reaching western Europe was carried on particles of diameter less than 2 jum (Jost et al., 1986 Winkelmann et al., 1987) but larger particles reached Scandinavia (Devell et al., 1986 Persson et al., 1987) and these carried a higher proportion of refractory elements. Spherical particles, condensed from the vapour state, were found with activity dominated by individual elements, for example Ce, Ru, Ba or Mo (U.S.S.R. State Committee, 1986 Persson et al., 1987). With longer distances of travel, the larger particles, and most of the refractory elements, were lost from the plume. Activity reaching Tennessee was found to be in particles with median diameter about 0.4//m, similar to that of cosmogenic 7Be (Bondietti Brantley, 1986). 

A mean value of 3 x 10-10 m-1 for Kr was deduced by Shinn et al. (1983) from measurements of airborne Pu over a bare field near the Savannah River Processing Plant. Here the activity median aerodynamic diameter was 3 //m. The main reason why Kr measured in Nevada and North Carolina was much lower than Kr measured at Maralinga and Monte Bello is the effect of ageing on the characteristics of the deposited material. 

Resuspension of dust from the floor is an important source of indoor pollution. Jones Pond (1966) studied resuspension of plutonium from the floor of a laboratory which had been deliberately contaminated. Droplets were dispersed over the floor and allowed to dry out. Pu was present in the droplets either as a suspension of particles, mass median diameter 15 /mi, or as nitrate in solution. The activity per unit area was measured with a floor probe. After 16 h had elapsed, airborne Pu was measured while operators performed various tasks in the laboratory. 

Woods DA. 1986. Final product dust activity median aerodynamic diameters in the product packing areas of two Australian uranium mills. Health Phys 51 352-356. 

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