241Am Activity Research Articles

Calculation and Measurement of Calibration Factors for Bone-Surface Seeking Low Energy Gamma Emitters and Determination of 241Am Activity in a Real Case of Internal Contamination

A voxel phantom has been developed to simulate in vivo measurement systems for calibration purposes. The calibration method presented here employs a mathematical phantom, produced in the form of volume elements (voxels), obtained through magnetic resonance images of the human body. The voxel phantom has a format of 871 slices' each of 277 x 148 picture elements. The calibration method uses the Monte Carlo technique to simulate the tissue contamination, to transport the photons through the tissues and to simulate the detection of the radiation. The program was applied to obtain calibration factors for the in vivo measurement of 241 Am deposited on the cortical bone surface of a real contamination case, and to the simulation of in vivo measurements of 241 Am deposited on the cortical bone surface of three head phantoms, as measured with a germanium detector. The calculated and real activities in all four cases were found to be in good agreement. The results indicate that mathematical phantoms could eventually substitute for physical phantoms in the calibration of in vivo measurements for low energy radionuclides.

Development of the NIST bone ash standard reference material for environmental radioactivity measurement

The bone ash standard reference material (SRM), a blend of 4% contaminated human bone and 96% diluent bovine bone, has been developed for radiochemical method validation and quality control for radio-bone analysis. The massic activities of 90Sr, 226Ra, 230Th, 232Th, 234U, 235U, 238U, 238Pu, (239+240)Pu and (243+244)Cm were certified using a variety of radiochemical procedures and detection methods. Measurements confirmed undetectable radionuclide heterogeneity down to a sample size of 5 g, thereby implying adequate blending of particulate materials with dilution factors of up to 17,900. The results among most of the intercomparison laboratories and their methods were consistent. Disequilibrium was observed for decay chains: 234U(0.67 mBq/g)– 230Th(0.47 mBq/g)– 226Ra(15.1 mBq/g)– 210Pb(23 mBq/g)– 210Po(13 mBq/g) and 232Th(0.99 mBq/g)– 228Ra(6.1 mBq/g)– 228Th(7.1 mBq/g). The disequilibria were the results of mixing occupationally contaminated human bone with natural bovine bone and the fractionation during internal biological processes. The massic activity of 210Pb, 228Th and 241Am were not certified because of insufficient 228Ra and 241Pu data and lack of knowledge in how 222Rn and its daughters will be fractionated in the SRM bottle over time.

The influence of mixing timescales and re-dissolution processes on the distribution of radionuclides in Northeast Irish Sea sediments

The sediments of the northeast Irish Sea contain substantial inventories of contaminant radionuclides as a consequence of past discharges of low-level liquid radioactive waste from the Sellafield nuclear fuel reprocessing plant. Previous work has indicated that onshore transport of this contaminated sediment is the dominant mechanism of supply of Sellafield waste radionuclides to surrounding intertidal and floodplain areas, but an inherent, and previously unsubstantiated, assumption in this work was that past discharges from Sellafield are effectively homogenised in the surface offshore sediment, with respect to 238Pu 239,240Pu and 241Am 239,240Pu activity ratios, as the result of mixing on a timescale of a few years. Results are presented here for a study of three cores, of lengths ranging from 10 to 38 cm, collected from the deposit of contaminated offshore sediment in 1992. The activity ratios of the longlived radionuclides 238Pu, 239,240Pu and 241Am in the cores exhibited variations that were small relative to those in the Sellafield discharge, indicative of a high degree of mixing of the sediment. Moreover, the sediment activity ratios were found to be compatible with those of the Sellafield time- integrated discharge and with those of sediment deposited in surrounding coastal areas during the year after sample collection. In view of the heterogeneous nature of the sediments in the northeast Irish Sea, caution must obviously be applied in any generalisation based upon the analysis of only three cores, but for the locations sampled, the results are consistent with the surface sediment acting as a well mixed source of long-lived Sellafield waste radionuclides. 60Co and 134Cs were detected in the sediment to depths ranging from 10 to 18cm, implying transport to these levels by mixing on a timescale commensurate with their half-lives. Activities of the short-half-life nuclides 106Ru and 144Ce decreased with depth in the sediment, suggesting a timescale for mixing of the surface (< 13 cm depth) sediment of the order of 1 year. On the basis of the distribution of these short-half-life radionuclides, biodiffusion coefficients of the order of 10 −7 cm 2 s −1 were derived for the top 10 cm of sediment. The trends in 137Cs 241Am activity ratio with depth were consistent with re-dissolution of radiocaesium that had initially been taken up in the sediment during the period of high discharges from Sellafield. Comparison of the results with 137Cs 241Am activity ratios for Irish Sea sediment in the late 1970s suggests that up to 77% of the 137Cs had been re-dissolved from surface (< 10 cm depth) sediment and some 15% from deeper layers. Finally, it was observed that the 137Cs 241Am activity ratios for the three cores studied implied a K d of the order of 10 4–10 5 for radiocaesium in this system.

Americium, Curium and rare earths radionuclides in forest litter samples from Poland

Twenty seven samples of forest litter or humus layers ( A 0 or A 1) collected in 1991 in selected regions of Poland were analysed for the presence of 241Am, Cm isotopes and rare-earth alpha-emitters. Samples were originating from a large research project and were previously analysed for gamma-emitters and plutonium. The measurements were done using alpha spectrometry preceded by a radiochemical separation procedure. Additional low-level gamma spectrometric measurements were performed for the rare-earths fractions of samples with the aim to search for europium gamma-emitters. The applied radiochemical procedure, based on the IAEA recommendations, is described. The maximum observed specific activity of 241Am was found to be equal to (5.00 ± 0.19) Bq/kg, for 243+244Cm it was (0.25 ± 0.02) Bq/kg and for 147Sm it was (0.124 ± 0.009) Bq/kg. The possible origin of each isotope is discussed. The obtained results confirmed the presence of the non-volatile isotopes of the Chernobyl fallout in north-eastern Poland.

Inventories and fluxes of 210Pb, 137Cs and 241Am determined from the soils of three small catchments in Cumbria, UK

Inventories and vertical profiles of fall-out 210Pb, 137Cs (both weapons test and Chernobyl) and 241Am (a decay product of 241Pu) have been measured in the soils of three catchments in Cumbria, UK. Soil types varied from mineral soils to highly organic peat bog. The 210Pb inventories in different cores from the same catchment were relatively uniform, the standard deviation of measurements being around 30%. Mean annual fluxes of 210Pb calculated from the soil inventories were 63 Bq m −2 year −1 (Brotherswater catchment), 63 Bq m −2 year −1 (Blelham Tarn catchment), and 97 Bq m −2 year −1 (Devoke Water catchment) per metre of rainfall. From these and earlier data published in the literature, the mean UK 210Pb flux is estimated to be 77 ± 14 Bq m −2 year −1 per metre of rainfall. Inventories of 137Cs (weapons test and Chernobyl) were more variable, but mean values for each of the catchments were in agreement with independent studies. The mean weapons test deposition (decay corrected to 1986) was 2790 Bq m −2 per metre of rainfall, compared to a UK average of 3160 Bq m −2 derived from extensive national surveys. Although 241Am levels in Brotherswater soils were comparable with those expected from weapons test fall-out, significantly higher values were recorded at Blelham Tarn and Devoke Water. Well-resolved peaks in 241Am activity some distance below the present surface suggest that this radionuclide is relatively immobile within the soil column. The depth of penetration of 75% of the inventory into the soil column was similar for 210Pb, 137Cs (weapons) and 241Am, all being significantly greater than Chernobyl 137Cs. The vertical profiles of 137Cs (weapons) suggest that there has been little mobility during the 15 years since a previous study (Cawse 1983).

Radiological assessment of the Ribble Estuary—III. Redistribution of radionuclides

The Ribble Estuary in the UK is contaminated by radionuclides derived from BNFL Sellafield and Springfield. Following the temporary shutdown of the Springfields plant, beta dose and gamma air kerma rates were measured at a range of sites in the estuary. Sediment surface samples were analysed for 137Cs, 234mPa, 234Th and 241Am activity concentrations by gamma spectrometry, 230Th and Pu isotopes by alpha spectrometry and sediment grain size (percentage < 63 μm) by wet sieving. Temporal variations in Springfieldsderived radionuclides are explained by variations in discharges by radionuclides and the distribution of fine grained sediments in the estuary which is controlled mainly by the spring-neap tidal cycle and river runoff. Radionuclide dynamics, modelling variations in the 234Th 137Cs activity concentration ratio and calculating environmental beta dose rate and 234Th half-lives allow the estuary to be split into three areas: (a) upper estuary where fine grained sediments tend to be deposited during periods of low and average river flow; (b) middle reaches of the estuary which encompass most of the main channel sites where fine grained sediments are probably deposited and removed on most tides; (c) outer estuary including large areas of salt marshes which tend to accumulate fine grained sediment. This is confirmed by similar results derived from the 234Th 239,240Pu ratio.

A Comprehensive Appraisal of 241Am in Soils around Rocky Flats, Colorado

Soils east of Rocky Flats Plant (RFETS) near Golden, Colorado, were contaminated with actinides because of accidental release of oils laden with plutonium isotopes. Consequently, these soils were contaminated by 241Am due to radioactive decay of 241Pu (t1/2 = 14.4 y). A spatial analysis of 241Am activity in soils east of RFETS was conducted to elucidate the magnitude and the mode of 241Am dispersion in the soil environment. 241Am activity of 178 soil samples ranged from 0.037 Bq kg-1 to 10,004 Bq kg-1 with a mean of 214 Bq kg-1, median of 7.28 Bq kg-1, standard deviation of 942 Bq kg-1, and a coefficient of variation of 4.3. Spatial analysis of 241Am in soils around RFETS was conducted using indicator kriging, which is a nonparametric technique especially suitable to model a conditional cumulative distribution function (ccdf) of highly skewed environmental data such as 241Am. The ccdf was used to generate an E-type (mean of the conditional cdf) surface. The resulted surfaces were consistent with the hypothesis that the westerly winds were the dominant mechanism of americium dispersal. The spatial distribution and dispersal mechanisms of 241Am were similar to those of 239+240Pu. The ccdf was also used to construct probability of exceedence maps of 241Am in soils. For the purpose of this report two threshold values for the probability maps were selected: (1) the mean measured background activity of 241Am (0.4 Bq kg-1), and (2) the programmatic preliminary remediation goal for residential occupancy scenario (87.7 Bq kg-1). The probability-of-exceedance maps provide estimates of spatial uncertainty associated with each threshold. The E-type maps in conjunction with the probability-of-exceedance maps provide a robust framework for future cleanup options and land use decisions.

Radiometric dating (210Pb, 137Cs, 241Am) of recent ombrotrophic peat accumulation and evidence for changes in mass balance

The paper summarizes and evaluates the results of 210Pb, 137Cs and 241Am analysis on 37 recent ombrotrophic peat profiles. From these and previously published data, it is concluded that radiometric measurements alone cannot be relied on to give an accurate chronology of peat accumulation. Where constrained by profiles of 241Am activity in the upper part and one or more pollen-dated horizons below this, 210Pb can provide good chronologies of peat accumulation provided the constraint does not displace the calculated 210Pb dates by more than c. 10%. Profiles of 137Cs activity, in most cases, fail to contribute significantly to dating recent peat. At somewhat degraded sites, the residual dry mass of peat representing the last c. 150 years is reduced. Further analysis of well-dated profiles suggests that this may reflect loss of catolelm peat newly exposed to aerobic decomposition processes after a fall in water table. Several mechanisms are proposed for the displacement and net loss of lead (including 210Pb) from peat profiles.

The distribution and retention of plutonium, americium and uranium in CBA/H mice

Groups of male and female CBA/H mice were given intraperitoneal injections of 40 kBq kg-1 of 239Pu, 241Am and 233U citrate solutions and the retention and distribution of the three radionuclides compared at times up to 448 days. Similar results were obtained for males and females and showed that: 1. Whole body retention at 448 days was very similar for 239Pu and 241Am, accounting for about 20% of injected activity for each nuclide; retention of 233U was much lower at about 3%. 2. The skeleton accounted for 85% or more of retained 239Pu, 241Am and 233U activity from 6 weeks after injection. 3. The greatest concentrations of each radionuclide were measured in the main body of the spine, limb girdles and ribs, with lowest concentrations in the paw bones, head bones and caudal vertebrae. The inhomogeneity of distribution was in the order Pu > U > Am; with a trend to more uniform activity with time. 4. Average bone doses to 448 days were calculated as about 1.6 and 1.7 Gy for 239Pu and 241Am, respectively, and 0.3 Gy for 233U, with ranges for individual bones of 0.7-3.0 Gy, 1.1-2.5 Gy and 0.1-0.6 Gy, respectively. Average liver doses to 448 days were calculated as about 0.9 Gy, 0.6 Gy and 0.007 Gy for 239Pu, 241Am and 233U respectively, whilst the dose to the kidney for 233U was about 0.1 Gy. 5. Autoradiographic studies of the distribution of the nuclides in the femur showed differences in their initial distribution and subsequent movement. Initially, concentrations of 239Pu were greater on endosteal than periosteal surfaces while 241Am distributed more evenly on bone surfaces. The initial deposition of 233U on all surfaces was uneven with concentrations probably on active surfaces. Burial of all three nuclides in areas of bone growth was observed. Transfer of activity to the marrow was greatest for 239Pu and least for 233U.

Monitoring of the release of transuranium elements in airborne effluents from V-1 nuclear power plant at Jaslovské Bohunice in Czechoslovakia

The activity concentrations of 238Pu, 239,240Pu, 241Am, 242Cm and 244Cm in airborne effluents and discharged activities were determined for several time periods during 1988 and 1989 and for the whole of 1990 at a V-1 nuclear power plant. It was found that these radionuclides were present in airborne effluents in the range ∼ 1 to 50 μBq m −3. During 81 days' monitoring in 1988, 6·0 kBq of 238Pu, 5·6 kBq of 239,240Pu, 6·1 kBq of 241Am, 6·3 kBq of 242Cm, 4·0 kBq of 244Cm and during 303 days' monitoring in 1989, 16·7 kBq of 238Pu, 13·3 kBq of 239,240Pu, 12·3 kBq of 241Am, 49·0 kBq of 242Cm and 8·3 kBq of 24Cm were discharged by the nuclear power plant to air. The annual discharged activities of 238Pu, 239,240Pu, 241Am, 242Cm and 242Cm in 1990 were to 19·6, 15·1, 9·4, 44·3 and 9·5 kBq, respectively.

241Am and 137Cs activity in fine grained saltmarsh sediments from parts of the N.E. Irish sea shoreline

241Am and 137Cs activities in surface and core samples of saltmarsh sediment from parts of the Galloway and south Cumbria coastlines are reported. The study concentrates on areas of saltmarsh which have been colonised by Spartina anglica Hubbard., in view of its ability to trap fine grained sediments and to colonise previously bare mudflats beyond the outer edge of pre-existing marsh. At each site downprofile variations of 241Am and 137Cs are presented, and most appear to show a more or less complete record of the history of discharges from Sellafield. The total inventories of the radionuclides are calculated for core samples and compared with published results from previous studies of intertidal sediments in the immediate vicinity. The relationship between particle size and radionuclide activity and the use of magnetic measurements as a relatively simple way of normalizing for this effect are examined.

241Am Distribution and Retention in Pregnant Mice, in their Offspring and in Non-Pregnant Mice: Comparison Between Continuous Am Administration and Single Injection

Pregnant BALB/c mice and age and sex matched nulliparous controls were contaminated with 241Am (13 kBq per mouse) by two different procedures: (1) a single intraperitoneal injection at the 14th day of gestation, and (2) continuous adminstration between the 7th and 14th day of gestation and between the 14th and 19th day of gestation. Continuous administration was achieved by implanting osmotic pumps filled with 241Am which released the 241Am activity at a constant rate. Five days after the termination of contamination, 241Am incorporation was measured in the tissues of adults and in the liver and the femur of newborn and one-month-old mice. Pregnancy results in higher 241Am concentrations in bone but lower concentrations in the liver of the mothers. Protracted administration of 241Am compared to a single injection resulted in a lower concentration of 241Am in the livers of pregnant mice, their nulliparous controls and from newborn mice. The higher 241Am concentration in the femur at birth after protracted exposure before 14 days of gestation compared to protracted exposure after 14 days of gestation could reflect the increased placental transfer of 241Am with advancing gestational age. Radiation doses to the femur were estimated between 4 and 20 mGy. Haemopoietic changes were noticed at these dose levels in all groups until at least 6 months after birth.

Deposition and transport of radionuclides within an upland drainage basin in mid-Wales

The deposition of radiocaesium from nuclear weapons testing and the Chernobyl accident upon the Llyn Llygad Rheidol catchment in mid-Wales is described. Inventories of soil cores from the catchment support estimates of total atmospheric fallout. The mean inventory of weapons testing 137Cs in lake sediment cores is broadly similar to that in soil cores. The inventory of Chernobyl fallout in sediment cores is significantly lower and raises questions concerning the residence time of 137Cs in the catchment soils and lake waters. 137Cs and 241Am activities in a sediment core record the 1963 peak of fallout from nuclear weapons testing. The association of the peak activities of 137Cs and 241Am in the sediments with the falloot maximum is confirmed by 210Pb dating. The 210Pb dates also reveal a significant increase in sediment accumulation rates over the past 20 years.

The influence of grassland management on the radionuclide inventory of soils in West Cumbria, UK

The 137Cs, 239,240Pu, 238Pu, 241Am and 40K activities in soils and vegetation under grassland management in west Cumbria were investigated. Uptake of these radionuclides by the vegetation is affected by the land management practices employed. The more intensively utilised areas have lower concentrations in soils and the plants show reduced uptakes of the artificial radionuclides. It is proposed that this is due to differences in the physical mixing of the soil, natural and artificial chemical additives, contamination of the vegetation with soil by animal usage and the regular cropping of grass for silage. Comparison of the soil data with samples taken on Exmoor indicates that the west Cumbrian samples were enriched by factors of ∼2·5 for 137Cs and ∼10 for 239,240Pu.

In Vivo Measurement of 241Am Organ Burdens: Techniques and Results of Measurements after an Incident in an Industrial Research Institute

As a consequence of 241Am contamination in a German industrial institute extensive in-vivo measurements have been performed in the lung counter of the Karlsruhe Nuclear Research Center (KfK) starting in 1986. Because of poor energy resolution of phoswich detectors, measurements have been severely disturbed by Cesium body burdens due to the Chernobyl reactor accident. After development of a special background prediction method measurements showed the 241Am activity to be distributed over lungs, liver and skeleton. For assessment of respective 241Am organ burdens extensive calibration measurements have been performed using the Lawrence Livermore National Laboratory chest phantom and some bone phantoms from the New York University Medical Center. From April to June, 1987 about 130 involved persons were screened in the KfK lung counter with the detectors located over the lungs, this being about 150 Bq. In October, 1987 some of these persons have been counted in four different measuring geometries with the detectors over head, lungs, liver and knees, respectively. The ratio of estimated liver and skeleton burdens was found to be unity within limits of error. Using this result as boundary condition for evaluation of other measurements, the biological half-life of the pulmonary compartment was found to be about 200 d. For further evaluations the ICRP lung model parameters were interpolated for biological half-life of 200 d. Interpolated parameters were used to calculate the time dependence of the 241Am organ burdens after a single intake by inhalation of aerosols with different particle size distributions. The results of these calculations were found to be in good agreement with measurements when using a mean particle size of 1 µm AMAD. From March to July, 1987 some persons have been treated with DTPA. The measurements of these persons clearly indicate that 241Am activity is removed primarily from the liver rather than from the skeleton.

Cancer Incidence and Lifespan Vs. Alpha-particle Dose in Beagles

Young adult beagles were injected with graded activities of 239Pu, 241Am, 228Th, 228Ra or 226Ra and observed throughout their lifespans. The vast majority of the dose was from alpha particles. The lifetime incidence of bone sarcoma increased with average skeletal dose, more or less linearly up to high incidence for 239Pu, 241Am, 228Th and 226Ra, but sigmoid fashion for 228Ra. Based on average skeletal dose, the toxicity of the emitters relative to 226Ra = 1.0 was 239Pu = 16.6 +/- 4.5, 241Am = 5.4 +/- 1.6, 228Th = 8.5 +/- 2.3 and 228Ra = 2.0 +/- 0.5. At the lowest doses, the average lifespans were 97% +/- 3% of that in the controls. If beneficial effects occurred, they may have been overwhelmed by the destructiveness of the densely ionizing alpha particles. A cell nucleus 5 micron in diameter receives a mean dose of about 1 Gy (100 rad) when traversed by a single alpha particle. We found no evidence that alpha-particle doses suppressed cancer or lengthened lifespan in beagles.

The transport to and deposition of americium in intertidal sediments of the ravenglass estuary and its relationship to plutonium

The distributions of 241Am in four cores of intertidal sediment from the Ravenglass estuary near to the Windscale nuclear fuel reprocessing facility, northwest England, are reported. Maximum 241Am activities (up to ∼200 pCi g −1) are found at depth in the sediments, and this pattern of accumulation is compared with annual discharges to the sea from Windscale. The results indicate a slow transit of americium to the depositing sediments when the sedimentation rates derived from two independent methods are applied to the cores. This feature is interpreted, and further confirmed, by consideration of plutonium: americium isotopic ratios, as the result of a lag-time of transit from Windscale to the sediments of ∼2·5 years. This slow movement is consistent with earlier theoretical and field data which indicate that americium is rapidly removed to sediments on release, and subsequently transported with bed-loads in the coastal environment.

241Pu and 241Am in sediments from coastal basins off California and Mexico

In sediments deposited in the coastal basins off the western North American continent since the early 1960's, there is more 241Am activity than one would predict if its presence was solely due to the decay of 241Pu that was produced during the testing of weapons in 1961–1962 (taken as July 1, 1962 for calculations). This excess can be accounted for by the decay of 241Pu, if pre-1962 fallout (fallout from the pre-moratorium testing) has been continuously introduced into the recent sediments along with fallout from post-moratorium testing. This hypothesis is supported by the plutonium depth profiles which do not reflect direct fallout from the atmosphere, as characterized by two peaks corresponding to the two major weapons testing periods, but continuously increasing plutonium and americium concentrations from 1954 to 1974. Thus, the nuclides may be coming from a large reservoir that has accumulated and mixed fallout since 1952. Such a concentration gradient for all of the nuclides requires their immobilization in the sediment after deposition.

Development of a plutonium-americium monitor for in situ soil surface and pond bottom assay

An intrinsic germanium detector has been found to provide in situ monitoring of the depth and concentration of 239Pu and 241AAm in soil surfaces and pond bottoms. The detector can simultaneously measure the three uranium X-ray peaks from plutonium decay and the 26 and 60 keV gamma ray peaks from 241Am. Average 241Am depths are determined from the change in the 26 to 60 keV peak ratio relative to the ratio observed for a thin source of 241Am on the surface of the soil. The 241Am activity is then determined from the depth-corrected intensity of the 60 keV photopeak, and the 241Am contribution to the uranium X-ray measurements of 239Pu is calculated. The average depth and activity of the 239Pu deposit is calculated from the corrected X-ray peaks in the same manner as for 241Am. Detection limits for the method are 6.1 dpm/cm 2 241Am and 72 dpm/cm 2 239Pu for soil depths up to several millimeters. Americium-241 can be measured at soil depths of several centimeters at 10 dpm/cm 2. The effect of soil composition was also examined. The intrinsic Ge detector was determined to be most suitable for this type of monitoring after comparison with the response of NaI(Tl) scintillators, gold surface barrier alpha detectors, and gas proportional ionization chambers.

Gamma-Ray Spectrometric Determination of Isotopic Ratios of Plutonium

Abstract Plutonium samples (3–30 μg) of various isotopic compositions were measured with a high-resolution Ge(Li) detector. The γ-rays counted are the 43.5 keV of 238Pu, the 38.7, 51.6 and 129.3 keV of 239Pu, the 45.2 keV of 240Pu, and the 148.6 keV of 241Pu. To estimate the relative photopeak efficiencies of the γ-rays, an empirical curve was derived from measurements of the three γ-rays of 239Pu adopted as internal standard. Data treatment is described in detail as well as the method of correction to account for the activities of 237U and 241Am inherently accumulating in the plutonium sample. The branching ratios of the γ-rays of plutonium were also evaluated by comparing the γ-ray spectrometric results with mass-spectrometric data.