Vertical migration of 60Co, 137Cs and 226Ra in agricultural soils as observed in lysimeters under crop rotation

In most studies quantifying the migration parameters - apparent migration velocity and apparent dispersion coefficient - of radionuclides in the soil by model calculations, these parameters are determined for undisturbed soils. For soils disturbed by ploughing, however, no such data are available in the literature. Therefore, in the present study, the migration parameters of (137)Cs, (60)Co and (226)Ra were estimated for ploughed soils by means of a convection-dispersion model. The depth distributions of the radionuclides were determined in four lysimeters (area: 1m(2), depth of soil monolith: 0.75m) filled with artificially contaminated soils of different types in July 1990. The lysimeters were cropped with agricultural plants. The soil in each lysimeter was ploughed manually once a year until 1996 (plough depth 20cm). In July 1999, soil samples were collected from three pits in each lysimeter. The depth distributions of all radionuclides proved to be very similar in each soil pit. The spatial variability of the depth distributions of a given radionuclide within the lysimeters was about the same as their variability between the four lysimeters. Evaluation of the migration parameters revealed that the convective transport of the radionuclides was always rather small or even zero, while the dispersive transport caused a "melting" process of the initially sharp activity edge at the lower border of the Ap horizon. These results are explained by the high evapotranspiration (80-90% of the total precipitation plus irrigation) and the small amounts of seepage water during the observation period of 9 years.

Evaluation of the dose rate distribution for an air-type 60Co irradiation facility

The dose rate distributions in the 29,000-Ci 60Co irradiation facility in National Tsing Hua University were investigated by measurements and calculations. The dose rate measurements were performed using radiochromic dye films and an Exradin A2 ion chamber mounted on a PC-controlled motorised vertical translation stage. The calculations were made by using the three-dimensional point kernel code QAD-CGGP with detailed source composition and geometry modelling. The scattered gamma rays from the walls of the irradiation cell were also evaluated by using the Monte Carlo code MCNP.

Proposed definitions for isodose flatness and symmetry in clinical radiotherapy beams

In clinical radiotherapy it is important that beam intensity be as homogeneous as possible to reduce the probability of treatment failure. As an extension of the well established concepts of beam flatness and symmetry for characterizing radiotherapy beams, the concepts of isodose flatness and symmetry are introduced. The definitions are tested with actual data obtained from a Co-60 unit and a linear accelerator and results are presented. The concepts as defined appear to provide a useful quantitative indication of beam homogeneity.

Evidence for using Monte Carlo calculated wall attenuation and scatter correction factors for three styles of graphite-walled ion chamber

The basic equation for establishing a 60Co air-kerma standard based on a cavity ionization chamber includes a wall correction term that corrects for the attenuation and scatter of photons in the chamber wall. For over a decade, the validity of the wall correction terms determined by extrapolation methods (K(w)K(cep)) has been strongly challenged by Monte Carlo (MC) calculation methods (K(wall)). Using the linear extrapolation method with experimental data, K(w)K(cep) was determined in this study for three different styles of primary-standard-grade graphite ionization chamber: cylindrical, spherical and plane-parallel. For measurements taken with the same 60Co source, the air-kerma rates for these three chambers, determined using extrapolated K(w)K(cep) values, differed by up to 2%. The MC code 'EGSnrc' was used to calculate the values of K(wall) for these three chambers. Use of the calculated K(wall) values gave air-kerma rates that agreed within 0.3%. The accuracy of this code was affirmed by its reliability in modelling the complex structure of the response curve obtained by rotation of the non-rotationally symmetric plane-parallel chamber. These results demonstrate that the linear extrapolation technique leads to errors in the determination of air-kerma.

Dosimetric quality control measurements of 60Co teletherapy units in Nigeria

Measurements of output dose rates of four Co teletherapy machines located in three hospitals in Nigeria have been made using the facilities of the Secondary Standard Dosimetry Laboratory at Federal Radiation Protection Service in Ibadan, Nigeria, and by applying an International Atomic Energy Agency dosimetric protocol. The results show that the percentage deviation between our measured dose rates and dose rates employed by the users ranged generally between 0.15% and 10.6% relative to our measured dose rates. For one of the machines, the deviation was consistently within the acceptable limit of +/-5%, while for another it was within this range 75% of the time. For the other two machines the deviation was outside this limit for all of the few measurements that were possible.

Measurement of peak and total efficiencies of low-energy gamma-ray detectors with sources emitting photons in cascade

A method for measurement of peak and total efficiencies for point sources measured on low-energy gamma-ray detectors is presented. The efficiencies are measured with radionuclides emitting gamma-rays in a two-step cascade or in a crossover transition. One of the photons emitted in the cascade must have an energy which is much lower than the other in order to attain stronger attenuation as compared to the higher energy photon. The method is illustrated by measurements of the efficiencies with 179Ta and 57Co.

Absolute radioactivity measurements by the use of a 4πβ–4πγ detector configuration

The radioactivity of 60Co and 134Cs sources were measured using a 4pibeta-4pigamma detector configuration with a well-type NaI(Tl) crystal and a sandwich type 4pibeta detector composed of two sheets of NE102A plastic scintillator coupled to a slender photomultiplier tube. The beta-detector was inserted into the well of the gamma-detector. Since counting efficiencies in both the beta- and gamma-channels can be kept high, excellent counting statistics were attainable even when weak sources were measured. This configuration can also be used for radioactivity measurements based upon the direct integral counting of 4pibeta + 4pigamma logic sum signals, and two independent results can be obtained for every measurement. This technique is especially useful for the standardization of complex decaying nuclides.

Temperature dependence of spurious pulses in use of plastic scintillation detectors

Pulse-height and time spectra of afterpulses from a scintillation detector with NE102A scintillator sandwiching a 60Co source were observed at several different temperatures. From time analysis with a slow amplifier followed by a conventional TAC, spurious pulses are grouped into two types; time-dependent afterpulses and random noise. The intensity of afterpulses decreased considerably with increasing temperature, while the intensity of random noise pulses increased abruptly above 50 degrees C. Experiments, in which the temperatures of scintillator sheets and of the photomultiplier tube were changed separately, suggested that the time-dependent afterpulses were produced in the scintillator itself, while the random component arose from photomultiplier tube.

Measurements of 60Co in spoons activated by neutrons during the JCO criticality accident at Tokai-mura in 1999

Neutron activated items from the vicinity of the place where the JCO criticality accident occurred have been used to determine the fluence of neutrons around the facility and in nearby residential areas. By using underground laboratories for measuring the activation products, it is possible to extend the study to also cover radionuclides with very low activities from long-lived radionuclides. The present study describes gamma-ray spectrometry measurements undertaken in a range of underground laboratories for the purpose of measuring (60)Co more than 2 years after the criticality event. The measurements show that neutron fluence determined from (60)Co activity is in agreement with previous measurements using the short-lived radionuclides (51)Cr and (59)Fe. Limits on contamination of the samples with (60)Co are evaluated and shown to not greatly affect the utility of neutron fluence determinations using (60)Co activation.

[Radioactivity for 137Cs, 125I, 131I, 59Fe, y 57Co windows from foods included in the basic alimentary basket and in the water, consumed in the state of Carabobo, Venezuela]

One of the input ways of radionucleids into the organism is through food intake. The aim of the present study is to measure the radioactivity levels in food and water samples within energy windows corresponding to 137Cs, 125I, 131I, 59Fe, and 57Co. Samples were taken from local and imported food belonging to the venezuelan basic alimentary basket and included: beef meat, hen egg, chicken bone, tomato, black bean, rice, powder milk from local dealers or imported from Italy and New Zeeland, potable water from the Valencia city aqueduct and bottled water from local sources or imported from Portugal. Radioactivity was measured with a well type Nal (TI) scintillation counter. Analyzed foods and water presented levels lower than the minimal detectable activity for 137Cs, 131I, 59Fe, 57Co, but it was detected in the Valencia city aqueduct water and in bottled water imported from Portugal, levels greater than the minimal detectable activity for the 125I energy window. These results strongly suggest the need of repeated multienergy windows monitoring of radioactivity of basic alimentary basket foods and potable water.

Dose reconstruction for residents living in buildings with moderate and minor 60Co contamination in rebar

Previously, we have reconstructed cohort dependent individual doses for residents living in rebar buildings of high 60Co contamination. These reconstructions were carried out using intensively collected TLD data on exposure rates at locations of 1 m height and 1 m x 1 m intersections. The present work deals with dose reconstructions for residents living in rebar buildings of moderate and minor 60Co contamination. Since only limited data on exposure rates from survey meters were available, dose reconstructions were based on these data using interpolations. To utilize such data, we examined them with respect to all factors that influenced the dose uncertainties. The interpolated results were given in terms of contour plots (isodose curves) and compared with corresponding results derived from TLD data and Monte Carlo simulations. The comparison revealed that survey meter data could be used to provide reasonable and conservative estimates of residential doses. By applying the cohort-dependent room occupancy factor and the site-dependent area occupancy factor, we reconstructed cohort dependent individual doses and associated uncertainties. Results of dose reconstructions for all residents living in contaminated rebar buildings were provided to the Atomic Energy Council and health authorities for epidemiologic and medical uses.

Monte Carlo aided room scatter corrections in the air-kerma strength standardization of 169Yb and 60Co brachytherapy sources

For accurate evaluation of air-kerma strength, S(k), of 169Yb and 60Co brachytherapy sources, the present study reports Monte Carlo (MC) based corrections for (1) room scatter, and (2) departure from constant room scatter for rooms of various sizes. Correction for exponential attenuation of effective primary in air is also reported for the above sources. Values of S(k) per contained mCi, S(k)/A(c) predicted by MC calculations for 169Yb source (model X1267) with and without Ti K x-rays are 1.302 +/- 0.03% (this value is in excellent agreement with the published value reported by Piermattei et al) and 1.260 +/- 0.03% cGy cm2 h(-1) mCi(-1) respectively, and in the case of Cathetron 60Co source the value of S(k)/A(c) is 11.015 +/- 0.01% cGy cm2 h(-1) mCi(-1). It is observed that depending upon the position of the source with respect to the surrounding concrete scattering surfaces and set of d values, the assumption of constant room scatter has resulted in overestimation of S(k) that varied between 0.30% and 1.5% for the 169Yb source and only between 0.10% and 0.20% for the 60Co source.

Monte Carlo dosimetry of 60Co HDR brachytherapy sources

Monte Carlo based dosimetry is presented for three 60Co HDR sources with a unique configuration of two active pellets in contact or spaced 9 and 11 mm apart. Results are presented in Cartesian "away and along" as well as polar coordinates following the AAPM TG-43 dosimetric formalism. Iso-dose rate contours around the sources in Cartesian coordinates reveal that significant differences between the three source designs exist only close to the source centers where dose rate distributions bear the effect of the unique source configurations. Dose rate constants of all three sources are accurately described by an equation of the form: lambda(cGyh(-1)U(-1))=lambda*pointG(1 cm,90 degrees) = 1.094*G(1 cm,90 degrees) where lambdapoint is the dose rate constant of a bare 60Co point source and G(1 cm,90 degrees) is the "exact" source geometry factor as defined by the TG-43. Radial dose and anisotropy function data extracted using the point source approximated geometry factors are tabulated for all three source designs. Finally, the dependence and variation of the above TG-43 parameters are discussed and it is demonstrated that the dosimetric properties of high-energy photon emitters are largely dependent on radionuclide source distribution.

Angle dependence of signal currents from cylindrical ionisation chambers

A signal current from a cylindrical ionisation chamber with an ionisation volume of 62.7 cm3, 40 mm in diameter and 50 mm long, peaked when the chamber was lixed at 0 degrees and at 90 degrees in 137Cs and 60Co gamma ray fields for source-chamber distances of 1 m and 2 m. A smaller ionisation chamber showed a small peak at 0 degrees in both fields but not at 90 degrees. However, calculations indicated that the signal current from the smaller chamber would also show a peak at 90 degrees in a 137Cs point-source gamma ray field. Peaks occur because gamma rays attenuate along the cylindrical side wall or along the end walls when a chamber is tilted slightly from 0 degrees or 90 degrees and the direction of the gamma ray beam agrees with the plane of one of these walls. These facts suggest the need for care in the common practice of measuring and calculating responses for cylindrical ionisation chambers fixed perpendicular to gamma ray beams.

Validating riverine transport and speciation models using nuclear reactor-derived radiocobalt

Risk assessment of intentional or accidental discharges of toxic substances into river systems requires combined hydraulic and chemical modeling. Periodic discharges of known volumes with low radioactivity by the Beznau nuclear reactor (Switzerland) serve as validation tracers for both river flow and chemical speciation simulation. Validation of the former has been achieved by comparison of modeled and measured arrival times of radiocobalt along a 65 km transect with a maximum reaction period of 24 hours. Modeled breakthrough curves coincide well with measurements collected during three field campaigns, in spite of the fact that sorption and sedimentation processes were not activated during simulation. This gives indirect evidence of inefficient cobalt sorption. Particle/solution distribution measurements carried out during breakthrough allow further validation of our speciation approach, which is based on Tableau setup of inorganic reactions combined with sorption and organic complexation. Modeled and measured speciation results confirm recent observations of enhanced complexation of cobalt with dissolved organic substances, which significantly reduces particle sorption. The large variability of conditional stability constants for sorption and complexation reactions, for sorption site densities, and for organic ligand concentrations explains the variability of published particle-solution distribution coefficients.

Behavior of a surface applied radionuclide and a dye tracer in structured and repacked soil monoliths

There has been increasing evidence in recent years about the impact of soil structure on vadose zone hydrology and the distribution of surface applied chemical substances. We have carried out a combined dye and radionuclide tracer study on two monoliths from the same location, one structured and one repacked, as part of an ongoing study to investigate the link between preferential flow, leaching of surface applied substances and their distribution within the soil.A tracer solution containing 1300 Bq/L (58)Co and 0.31 micromol/L Sulforhodamine B (SB) was added with roughly constant irrigation during a period of three weeks. The dye served as a tracer for water movement within the soil and thus allowed linkage of the radiotracer ((58)Co) with the flow pattern. Both were monitored in the outflow and measured within profile sections after monolith disassembly. Preferential flow in the structured monolith promoted the bypass and transport of both tracers, although transport was impeded at depths greater than 30 cm by compacted soil and reduced hydraulic conductivity. Eighty four percent of radiocobalt and 8% of SB were found in the upper 4 cm of the structured monolith. The homogenized monolith, on the other hand, showed mostly chromatographic infiltration and a more efficient soil filtering capacity with 91% of radiocobalt and 20% SB residing in the upper 4 cm. Furthermore no tracer was found in the outflow of the homogenized monolith during normal to high irrigation or at greater depth within the monolith. We have related flow characteristics and sorption of radiotracers by quantifying dye distributions and radionuclide activities throughout the profiles. Activities within the flow paths are up to 20-times higher than those measured in the soil matrix, and a fraction of radiocobalt follows the dye tracer in spite of cobalt's low mobility. The dye can thus be used to trace radionuclide distribution within the soil block.

Electron spectra measurements and Monte Carlo calculations for a 60Co standardised hot particle source

A benchmark set of measured beta particle spectra for a standardised 60Co hot particle source is presented. The spectra were obtained for conditions similar to those encountered in practical dosimetric applications. The measured spectra were compared with Monte Carlo calculations using the MCNP code. These comparisons provided information to guide the selection of the optimal set-up parameters of the code. Important differences were observed in the MCNP calculated spectra when ITS and the default indexing style algorithm were used. Overall the calculations using the default mode of MCNP version 4B provide the best agreement with the measured electron spectra.

The response of lif thermoluminescence dosemeters to photon beams in the energy range from 30 kV x rays to 60Co gamma rays

The energy response of standard (TLD-100) and high-sensitivity (TLD-100H) LiF thermoluminescence dosemeters (TLDs) has been studied for photon beams with mean energies from about 25 keV to 1100 keV. Canadian primary standards for air kerma were used to establish the air kerma rates for each of the photon beams. TLDs were mounted in a PMMA holder and the air kerma response was measured as a function of energy. The EGSnrc Monte Carlo code was used to model the TLD holder and calculate the absorbed dose to the TLD chip per unit air kerma for each beam. The measured and calculated results were combined to obtain the intrinsic dose response of the TLD chip. Broadly, our results are consistent with existing data, which show a marked difference in the energy dependence of the two materials. However, the precision of our measurements (standard uncertainty of about 0.6%) has permitted the identification of features that have not been noted before. In particular, the energy dependence of the two materials is quite different in the important energy region delimited by 137Cs and 60Co gamma rays.

The measurement of 60Co contaminated rebar buildings with thermoluminescence dosemeters

Three contaminated housing units, yielding annual dose levels of 6, 22 and 57 mSv, were selected randomly from identified radiation contaminated buildings (RCBs) to perform this experiment. The results are presented of a study to investigate the probability of finding RCBs by using eight thermoluminescence dosemeters (TLDs) for each suspected radiation contaminated housing unit. The results revealed that, for these three housing units, the probabilities of detecting contaminated housing units using eight TLDs were 93%, 86% and 99%, respectively. The number of TLDs required for such a determination depended strongly on both the radiation contaminated level (i.e., the activity) in the housing unit and the distribution of contaminated columns and beams in the house.

Decommissioning of a 60Co unit and estimation of personal doses

Chang-hua Christian Hospital needs to uninstall the 60Co unit. The mode of this 60Co teletherapy unit is SHIMADZU RTGS-10. The original lead head was taken as the source container of this 60Co unit. The source head was dismantled and put into the prepared wooden box, after the source was sealed. This study describes the planning and dismantling of the retirement and transport of the 60Co unit, and personal doses measured during the procedure. This work estimates the doses of radiation received by exposed workers during the dismantling of the machine. The workers received doses of approximately 53 microSv. This study shows that the original lead head can be used as the source container of this 60Co unit. The 60Co machine was smoothly dismantled and transported by conscientious and careful workers, using planned and controlled radiation protection, following the ALARA (as low as reasonably achievable) rule.

Measurement of residual 60Co activity induced by atomic-bomb neutrons in Nagasaki and background contribution by environmental neutrons

Residual 60Co activity in five steel samples induced by neutrons from the Nagasaki atomic bomb has been measured within about 1000 m from the hypocenter. The chemical separation of cobalt and nickel from steel samples was performed, and cobalt-enriched samples were prepared for all samples. Gamma-ray measurements were carried out with a low-background well-type germanium detector. The gamma-ray spectra for five samples were compared with the spectrum of a control sample to ensure that the observed 60Co was actually induced by A-bomb neutrons. The activation of cobalt by environmental neutrons was also investigated. It has been shown that the present 60Co data are consistent with earlier Hashizume's data.

Radiation surveys and dose equivalent assessments for 60Co-contaminated rebar buildings

The first 60Co-contaminated rebar building was found in Taipei City in 1992. In order to provide effective remedial suggestions to the building owner and residents of contaminated housing units, and to evaluate the dose equivalent to the residents, the Atomic Energy Council (AEC) organized a survey team on October 17, 1992. The main object of this team was to perform a detailed survey and dose equivalent evaluation of the 60Co-contaminated buildings. By the end of 2000, 181 buildings with 1607 housing units were identified as being contaminated with radioactive rebars at different levels. This article presents the detailed radiation survey and dose equivalent evaluation procedures employed by the survey team.

Leachability of radionuclides from cement-solidified waste form produced at Korean nuclear power plant

The leach test of radionuclide in cement-solidified waste form was performed. After leach test, compressive strength of waste form was measured. Cement-solidified waste form produced at Korean nuclear power plant, Kori (PWR) was chosen for the leach test. Specimens were cored out from a full-scale waste form (2001 drum size). The leach test procedure used for this study was ANSI/ANS 16.1 procedure. The leach ability indexes for cesium and cobalt isotopes were determined. Semi-infinite model was used for analysis of the controlling mechanism in the release of isotopes. Release mechanism of cesium was dominated by diffusion but release of cobalt behaved somewhat differently from the diffusion controlled release. Averaged leachability indexes were 8.6 for cesium and 11.4 for cobalt. Compressive strength after leach/ immersion test was 7.34 MPa for the sample immersed in simulated seawater and 8.34MPa for the sample immersed in deionized water.

Evaluation of 20 years of environmental monitoring data around Swedish nuclear installations

Twenty years of environmental monitoring data around the Swedish nuclear power plants and the Studsvik research facilities have been evaluated. In the marine environment, Fucus vesiculosus generally has high activity concentrations and the presence of a large variety of radionuclides in comparison with other bioindicators. However, for single nuclides the detection frequency was higher for 110mAg in Littorina spp and for 152Eu in Macoma baltica in comparison with other bioindicators. Close to the discharge point the activity concentration of 60Co in F. vesiculosus and in the discharge water were correlated. In the terrestrial environment, few radionuclides were detected and the activity concentrations were generally low. Of the terrestrial indicators, mosses had the highest activity concentrations and also comprised the largest variety of radionuclides. The radiation doses to humans based on measurements of released activity were small. Based on the results from the evaluation, alternative sampling strategies for the monitoring program are discussed.