Laboratory studies of the diffusive transport of 137Cs and 60Co through potential waste repository soils

Tests using reconstituted samples have been performed to assess the diffusive transport of (137)Cs and (60)Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion (D(e)) and sorption coefficients (K(d)) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D(e) values obtained in this study were of the order of 10(-6) cm(2) s(-1) for both species, higher than previously published data. The K(d) values from the diffusion and batch sorption tests were in reasonable agreement for (137)Cs, but an order of magnitude different for (60)Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.

Evaluation and assessment of 25 years of environmental radioactivity monitoring data at Tarapur (India) nuclear site

The evaluation and assessment of monitoring data generated over a period of 1983-2007 (25 years) of a nuclear facility is presented. Time trends of particulate radioactivity, correlation between (137)Cs in discharge canal seawater and station discharged activity and correlation of (137)Cs, (60)Co, and (131)I in marine species such as sponge and Nerita (gastropod) and corresponding discharged activity are discussed. The concentration of (137)Cs and (131)I in seawater versus biota are discussed. A good correlation between (137)Cs in seawater and (137)Cs in liquid waste discharged was observed (R(2) = 0.8, p < 0.001). Similarly, correlation was good for Nerita and discharged concentration of (137)Cs, (131)I and (60)Co (R(2) = 0.55-0.73 and p < 0.001). The measurements over the years indicated that there is no accumulation of radionuclides in either the terrestrial or aquatic environments. The mean (137)Cs decreased from the pre-operational levels: 7.0-3.6 Bq kg(-1) in soil, 0.91-0.016 Bq L(-1) in milk and 0.28-0.036 Bq kg(-1) in vegetation. Similarly, the mean (90)Sr in these matrixes decreased from 3.9 to 0.26 Bq kg(-1); 0.37-0.011 Bq L(-1) and 0.34-0.022 Bq kg(-1) respectively. Cesium-137 of about 700 microBq m(-3) was measured in the air filter disks during 1986 and there was a decrease of three orders of magnitude in concentration over the 25 years. The evaluation of environmental data indicated that the radionuclide concentrations and potential impacts, in terms of effective dose to the members of public, have significantly reduced since 1969.

Comparison of acidic polymers for the removal of cobalt from water solutions by polymer assisted ultrafiltration

In this study, three sulfonated water-soluble polymers based on poly(vinyl alcohol) of different molecular weights (10,000, 50,000 and 100,000 Da) were prepared and tested against commercially available poly(acrylic acid) for the removal of cobalt using polymer assisted ultrafiltration. High rejection rates were obtained between pH 3 and 6 with sulfonated poly(vinyl alcohol) (PVA 10,000 and 50,000 Da) whereas poly(acrylic acid) (PAA) of similar molecular weights performed rather poorly in this pH range. Sulfonation improved significantly sorption capability of PVA. Sulfonated PVA 10,000 was the best complexing agent with rejection rate above 95% between pH 3 and 6. For unmodified PVA the rejection rate was only 30-45% at pH 6 and there was no rejection at pH 3 at all. PAA rejection rate was above 90% at pH 6 and only about 10% at pH 3. Large scale experiment in cross-flow, continuous apparatus conducted by using PVA-SO(3)H 10,000 Da to remove (60)Co radioisotope from water solutions showed excellent results demonstrating the potential of this polymer to purify acidic radioactive wastes containing cobalt radioisotopes.

Characterization of a 60Co unit at a secondary standard dosimetry laboratory: Monte Carlo simulations compared to measurements and results from the literature

PURPOSE

To compare a Monte Carlo (MC) characterization of a 60Co unit at the Swedish Secondary Standard Dosimetry Laboratory (SSDL) with the results of both measurements and literature with the aims of (1) resolving a change in the ratio of air-kerma free in air Kair and absorbed dose to water Dw in a water phantom noted experimentally after a source exchange in the laboratory and (2) reviewing results from the literature on similar MC simulations. Although their use in radiotherapy is decreasing, the characteristics of 60Co beams are of interest since 60Co beams are utilized in calibrating ionization chambers for the absolute dosimetry of radiotherapy beams and as reference radiation quality in evaluating the energy dependence of radiation detectors and in studies on radiobiological effectiveness.

METHODS

The BEAMnrc MC code was used with a detailed geometrical model of the treatment head and two models of the 60Co source representing the sources used before and after source exchange, respectively. The active diameters of the 60Co sources were 1.5 cm in pellet form and 2.0 cm in sintered form. Measurements were performed on the actual unit at the Swedish SSDL.

RESULTS

Agreement was obtained between the MC and the measured results within the estimated uncertainties for beam profiles, water depth-dose curve, relative air-kerma output factors, and for the ratios of Kair/Dw before and after source exchange. The on-axis energy distribution of the photon fluence free in air for the unit loaded with its present (1.5 cm in diameter) source agreed closely with the results from the literature in which a source of the same make and active diameter, inside a different treatment head, was simulated. The spectrum for the larger (2.0 cm in diameter) source was in close agreement with another published spectrum, also modeling a 60Co source with an active diameter of 2.0 cm inside a different treatment head.

CONCLUSIONS

The reduction in the value of Kair/Dw following source exchange was explained by the spectral differences between the two sources that were larger in the free in-air geometry used for Kair calibrations than at 5 g/cm2 depth in the water phantom used for Dw calibrations. Literature review revealed differences between published in-air 60Co spectra derived for sources of different active diameters, and investigators in need of an accurately determined 60Co in-air spectrum should be aware of differences due to source active diameter.

Assessing deposition levels of 55Fe, 60Co and 63Ni in the Ignalina NPP environment

Two RBMK-1500 reactor units operated in Lithuania in the 1987-2004 period (one of them was stopped for decommissioning in 2004). This study presents a preliminary investigation of surface deposition density levels of (55)Fe and (63)Ni in moss samples collected in the close vicinity of the Ignalina NPP. Non-destructive analysis by the HPGe gamma-spectrometry was followed by radiochemical separation. Radiochemical analysis was based on anion-exchange and extraction chromatography. (55)Fe and (63)Ni activities were measured by liquid scintillation counting (LSC). The results indicate that the deposition values of (55)Fe are generally higher than those of (60)Co and (63)Ni.

A review of the (60)Co internal dosimetry at Devonport Royal Dockyard

The physico-chemical properties of (60)Co contaminants arising from the UK Naval Nuclear Propulsion Programme (NNPP) pressurised water reactor (PWR) plants have been investigated in order to review individual monitoring requirements at Devonport Royal Dockyard (DRD). This has been achieved through laboratory tests on NNPP primary component samples and interpretation of direct bioassay measurements using internal dosimetry modelling software. Interpretation of lung measurements was completed for two inhalation events involving material originating from a PWR plant and post-primary circuit decontamination. Initial estimates of intake and dose were calculated using International Commission on Radiological Protection default parameter values. However, a good fit could only be achieved by fitting the data to alternative absorption parameters where 90-95% of the material dissolved and absorbed rapidly at a rate of 1 day(-1). As a consequence of this review, a number of improvements have been made to monitoring arrangements at DRD. A minimum of three direct measurements are now taken during the 0-30 day period after an intake, the capability of the Canberra Accuscan has been enhanced and dissolution tests are being carried out by the Health Protection Agency (HPA) on samples taken from PWR plants.

Persistence and decontamination of surrogate radioisotopes in a model drinking water distribution system

Contamination of a model drinking water system with surrogate radioisotopes was examined with respect to persistence on and decontamination of infrastructure surfaces. Cesium and cobalt chloride salts were used as surrogates for cesium-137 and cobalt-60. Studies were conducted in biofilm annular reactors containing heavily corroded iron surfaces formed under shear and constantly submerged in drinking water. Cesium was not detected on the corroded iron surface after equilibration with 10 and 100mgL(-1) solutions of cesium chloride, but cobalt was detected on corroded iron coupons at both initial concentrations. The amount of adhered cobalt decreased over the next six weeks, but was still present when monitoring stopped. X-ray absorption near-edge spectroscopy (XANES) showed that adhered cobalt was in the III oxidation state. The adsorbed cobalt was strongly resistant to decontamination by various physicochemical methods. Simulated flushing, use of free chlorine and dilute ammonia were found to be ineffective whereas use of aggressive methods like 14.5M ammonia and 0.36M sulfuric acid removed 37 and 92% of the sorbed cobalt, respectively.

Secondary charged particle equilibrium in 137Cs and 60Co reference radiation fields

During the calibration or irradiation of dosemeters, typical irradiation geometries (collimated beams) and source-to-detector distances (1-5 m) lead to the fact that for photon energies above a few hundred keV, the secondary charged particle equilibrium is usually not ensured. The reason is that the effective beam radius at the detector position is smaller than the range of the secondary electrons produced in air whose maximum particle energy is as large as the maximum photon energy. Therefore, the International Organization for Standardization (ISO) recommends putting a build-up plate (BUP) made of polymethyl methacrylate in front of the dosemeter to be calibrated in ISO 4037-3. In this paper, the effect of the thickness of the BUP and its distance from the dosemeter at different source-to-dosemeter distances were investigated by means of measurement and calculation. It turned out that the geometrical arrangement of the source, dosemeter and BUP recommended by ISO mostly does not ensure secondary charged particle equilibrium. The consequence is to always place the BUP directly in front of the dosemeter to be calibrated or irradiated.

Probabilistic distribution coefficients (K(d)s) in freshwater for radioisotopes of Ag, Am, Ba, Be, Ce, Co, Cs, I, Mn, Pu, Ra, Ru, Sb, Sr and Th: implications for uncertainty analysis of models simulating the transport of radionuclides in rivers

The objective of this study was to provide operational probability density functions (PDFs) for distribution coefficients (K(d)s) in freshwater, representing the partition of radionuclides between the particulate and the dissolved phases respectively. Accordingly, the K(d) variability should be considered in uncertainty analysis of transport and risk assessment models. The construction of PDFs for 8 elements (Ag, Am, Co, Cs, I, Mn, Pu and Sr) was established according to the procedure already tested in Durrieu et al. [2006. A weighted bootstrap method for the determination of probability density functions of freshwater distribution coefficients (K(d)s) of Co, Cs, Sr and I radioisotopes. Chemosphere 65 (8), 1308-1320]: (i) construction of a comprehensive database where K(d)s values obtained under various environments and parametric conditions were collected; (ii) scoring procedure to account for the 'quality' of each datapoint (according to several criteria such as the presentation of data (e.g. raw data vs mean with or without replicates), contact time, pH, solid-to-liquid ratio, expert judgement) in the construction of the PDF; (iii) weighted bootstrapping procedure to build the PDFs, in order to give more importance to the most relevant datapoints. Two types of PDFs were constructed: (i) non-conditional, usable when no knowledge about the site of concern is available; (ii) conditional PDFs corresponding to a limited range of parameters such as pH or contact time; conditional PDFs can thus be used when some parametric information is known on the site under study. For 7 other radionuclides (Ba, Be, Ce, Ra, Ru, Sb and Th), a simplified procedure was adopted because of the scarcity of data: only non-conditional PDFs were built, without incorporating a scoring procedure.

New best estimates for radionuclide solid-liquid distribution coefficients in soils. Part 3: miscellany of radionuclides (Cd, Co, Ni, Zn, I, Se, Sb, Pu, Am, and others)

New best estimates for the solid-liquid distribution coefficient (K(d)) for a set of radionuclides are proposed, based on a selective data search and subsequent calculation of geometric means. The K(d) best estimates are calculated for soils grouped according to the texture and organic matter content. For a limited number of radionuclides this is extended to consider soil cofactors affecting soil-radionuclide interaction, such as pH, organic matter content, and radionuclide chemical speciation. Correlations between main soil properties and radionuclide K(d) are examined to complete the information derived from the best estimates with a rough prediction of K(d) based on soil parameters. Although there are still gaps for many radionuclides, new data from recent studies improve the calculation of K(d) best estimates for a number of radionuclides, such as selenium, antimony, and iodine.

[Soil sorption and transfer from it to plant of the radioactive and stable isotopes of chemical elements: methodology and experimental approach]

The adsorption of pair nuclides 60Co/Co and 65Zn/Zn by solid phase and their availability to plants were studied in soil suspension conditions and in the sod podzolic soil under controlled moisture. The situation when the radionuclide in constant activity is entered to system (surface water, soil) with different heavy metal contamination has been modeled. Was observed that soil contamination with heavy metal Co (Zn) significantly reduce sorption of the radionuclide 60Co (65Zn) by solid phase. As a result, the activity concentration of the 60Co or 65Zn in soil solution and, therefore, their mobility and potential availability to plants increases in 2-4 times with the total metal concentration increasing. The difference between two elements is that high Co concentration increase root uptake of the 60Co, whereas soil contamination with Zn reduce activity of 65Zn in the 14-days barley plants that may reflects diverse plant necessity of two elements and more important role of the isotopic exchange effect in the case of Zn.

Fate of 60Co at a sludge land application site

Vertical distributions of 60Co are determined in soil cores obtained from a 10-ha grassland, where anaerobically digested sludge was applied by surface spraying from 1986 to 1995 on the U.S. Department of Energy's Oak Ridge Reservation. These results, along with historical application records, are used to estimate vertical-migration rates and perform a mass balance. The presence of 60Co results solely from the sludge-application process. Soil, vegetation, and surface-water samples were collected. Eleven soil cores were sectioned into 3-cm increments and analyzed by gamma-ray spectrometry. No 60Co was detected in the vegetation or water samples. The downward migration rate of 60Co in the upper 15 cm of soil ranged from 0.50 to 0.73 cm/yr. About 98%, 0.020+/-0.011 Bq/cm2, of 60Co remained in the upper 15 cm of soil, which compared favorably with the expected 60Co activity based on historical records of 0.019+/-0.010 Bq/cm2.

Evaluation of sprayable fixatives on a sandy soil for potential use in a dirty bomb response

After the events of 11 September 2001, the possibility of a dirty bomb being detonated within the United States seems more realistic. Development of tools for use in response to a dirty bomb detonation has become a topic of both discussion and research. While it has been reported that the health risk to the public from such an event would likely be small, it is thought that the psychological impact could be considerable. One response option that has been considered is adapting sprayable solutions for the purpose of fixing contamination in place, thereby limiting the spread of contamination by wind and rain and facilitating subsequent cleanup. This work evaluated two commercially available particle fixatives (IsoFIX-HT and IsoFIX-RC) for their effectiveness in preventing dispersal of simulated contamination. Nonradioactive cesium chloride and cobalt oxide particles were selected as the simulated contamination and applied to the surface of three outdoor test plots. Two test plots were treated with fixatives; the third plot provided a control. Samples were collected over 95 days to observe changes in tracer concentration on the surface of the test plots. One fixative (IsoFIX-RC) effectively held the tracer in place with no net loss of tracer, while the other fixative (IsoFIX-HT) had no impact on the loss of tracer relative to the control. Under the conditions tested, IsoFIX-RC appears capable of fixing surface contamination in place for at least several months.

Dose rate dependency of electronic personal dosemeters measuring X- and gamma-ray radiation

Three models of electronic personal dosemeters (EPDs)-Siemens Mk 2.3, Rados RAD-60S and Vertec Bleeper Sv-were irradiated with seven photon beam qualities: 60Co, 137Cs and the ISO narrow spectrum series X-ray qualities N-250, N-200, N-150, N-60 and N-20. The personal dose equivalent rates delivered to the devices varied between 0.002 and 0.25 mSv s(-1). Measurements were made with the EPDs mounted free-in-air as well as against Lucite and water phantoms. Results for all models of EPDs showed differences in personal dose equivalent energy response for different energies covered by this range of radiation qualities, with different models showing variations from 15 to 65%. In some cases, the personal dose equivalent rate response of these devices varied by a factor of 3 between irradiations at typical calibration dose rates and those normally encountered by nuclear energy workers.

Transfer of radiocobalt from soil to selected plant species in tropical environments

Soil-to-plant transfer factors (TFs) of radiocobalt (60Co) were determined in pot experiments for leafy vegetation, root crops and rice grown in the tropical environment of Bangladesh. Soil properties were also measured to establish a relationship between these properties and TF values. Measured TF values of 60Co for leafy vegetation (average of 2.2 x 10(-2)) were slightly higher than the values obtained for root vegetation (average of 1.6 x 10(-2)). However, TF values obtained for rice (average of 1.17 x 10(-2)) were about a factor of 2 lower than the values obtained for leafy vegetation. TF values of 60Co for leafy vegetation and root crops were observed to decrease with increasing pH, exchangeable K+ and clay content in the soil, even though poor correlations were estimated statistically. No consistent relationship between the TF value for 60Co and organic matter content could be deduced. The results presented here provide a useful addition to existing databases on soil-plant transfer for 60Co, since this information is still rather sparse for tropical environments.

Radionuclide contamination of sediment deposits in the Ob and Yenisey estuaries and areas of the Kara Sea

The Ob and Yenisey rivers are major contributors to total riverine discharge to the Arctic Ocean. Several large nuclear facilities discharge into these rivers, which could affect actual and potential discharges of radionuclides to the Arctic region. This article presents new radionuclide concentration and grain-size data resulting from analyses of several sediment samples collected during research cruises in the Ob and Yenisey estuaries and adjacent areas during 2000 and 2001. Results indicate that discharges from the main nuclear facilities do not constitute a major contribution to the level of radioactive contamination in the marine areas studied, though Co-60 was detected at low concentrations in some sediment horizons. However, the aggregate contamination from different sources is not radioecologically significant in sediments within the study area, maximum Cs-137 levels being approximately 80 Bq kg(-1) dry weight.

Extremity dosimetry trial: Devonport Royal Dockyard

This trial was undertaken to assess extremity dosemeters, which were made available to Devonport Royal Dockyard and determine the most suitable to the site. The trial included operational and laboratory-based exposures. Operational exposures were within a submarine reactor compartment and a waste storage area. Laboratory exposures were undertaken using (241)Am, (137)Cs and (60)Co sources to compare and contrast the dosemeters energy response. In addition, the low dose response and the response if placed in the incorrect orientation were also assessed. Ten passive and two active dosemeters were tested, with three highlighted as the most technically suitable, DSTL Harshaw DXT-RAD, HPA Harshaw EXT-RAD and the AMEC Panasonic UD-807A. The most technically suitable dosemeter was the DSTL Harshaw DXT-RAD, due to good responses within all aspects of the trial and the user's preference for the ring type design. The John Caunt ED2 electronic dosemeter 2 (ED2) also performed well, but suffered radio frequency interference.

The air-kerma rate constant: application to air-kerma measurements for homeland security

Air-kerma rate measurements from 57Co, 60Co, and 137Cs radioactive sources were performed. These measurements were motivated by the development of new sources at the National Institute of Standards and Technology (NIST) for radiological testing of equipment for homeland security applications. The testing of radiation detection equipment relies on knowing the values of the air-kerma rate for the radioactive sources at a fixed distance from the source. The air-kerma rate can be measured or alternatively estimated by using published values of the air-kerma rate constant. Although there are a large number of published values of the air-kerma rate constant for radionuclide sources based on theoretical calculations, strong disagreement is observed throughout the literature. Furthermore, most of the published values have no uncertainties assigned, and therefore their use for testing radiological equipment is limited. In this work we report experimentally-measured values of the air-kerma rate for three radionuclides with well defined source geometries and activities. The results are compared to estimates based on published values of the air-kerma rate constant. Such values are easily found in the literature from the last three decades and are used commonly by the scientific community.

Dosimetry study comparing NCS report-2 versus IAEA report TRS-398 for high energy photon beams

In this work a dosimetry study is presented in which the results of absorbed dose data determined under reference condition according to the IAEA TRS-398 protocol and the NCS report-2 are compared. The IAEA TRS-398 protocol for absorbed dose calibration is based on ionization chamber having an absorbed dose to water calibration factor N(D,W), while the NCS-2 report for absorbed dose calibration is based on an ionization chamber having an air- kerma calibration factor N(K). This study shows that the absorbed dose calculated with the IAEA TRS-398 formalisms is higher than that calculated with the NCS Report-2 formalism within a range of 0.4 to 0.9% in a cobalt-60 beam, and from 0.2 to 1.1% for photon beams of 6, 8 and 18 MV. The chambers used are PTW 30001, 30004, and NE-2571, which have calibration factors N(K) and N(D,W) traceable to the BIPM (Bureau International des Poids et Mesures).

A novel technique to determine cobalt exchangeability in soils using isotope dilution

The environmental risk posed by Co contamination is largely a function of its oxidation state. Our objective was to assess the potential biological availability of Co and the reactions and fate of soluble Co(II) after addition to soils with varying physical and chemical characteristics. A potential risk in quantifying exchangeable Co in soils using isotope dilution techniques is the possible presence of two species of Co in soil solution and adsorbed on soil solid phases [Co(II) and Co(III)], coupled with the possibility that when an isotope of Co is added it may undergo a change in oxidation state during the measurement phase. In this study, we have utilized an isotope dilution technique with cation exchange and high-performance liquid chromatography-inductively coupled plasma-mass spectrometry to determine the isotopically exchangeable Co fraction in several soils with varying characteristics such as differing Al, Fe, and Mn oxide content; pH; and organic carbon content. The application of the cation exchange procedure adjusts measurements of isotopically exchangeable Co to correct for the presence of non-exchangeable 57Co not in equilibrium with the solution phase. Results indicated that oxidation of added 57Co(II) to 57Co(III) or precipitation of 57Co(II) may occur on the surfaces of some soils, particularly those with a high pH or substantial quantities of Mn oxide minerals. No detectable Co(III)(aq) was found in the aqueous extracts of the soils examined.

Investigation of the air kerma response of spherical ionization chambers for unfolded pulse height distributions of (60)Co and (137)Cs using the EGS4 Monte Carlo code

Data required for the determination of the absolute air kerma rate for (60)Co and (137)Cs gamma-rays using spherical cavity chambers were calculated using the EGS4 Monte Carlo system. Mass energy-absorption coefficient ratio and the stopping power ratio were calculated for a 10 cm(3) primary standard graphite-walled ionization chamber from the unfolded energy pulse height distributions of (60)Co and (137)Cs sources. Wall correction factors and non-uniformity correction factors for two graphite and one air equivalent plastic walled ionization chambers were also calculated with EGS4 code. The wall correction factors were compared with those determined by an experimental extrapolation method. To check the accuracy of the calculations the results were compared with those obtained from other primary standard laboratories such as NIST and NRCC. For a 10 cm(3) graphite ionization chamber, the mass energy-absorption coefficient ratios were 0.99917 for (60)Co and 1.0004 for (137)Cs. The values differed by 0.02-0.05 % for (60)Co and 0.11 % for (137)Cs from those of two laboratories. The stopping power ratios were 0.99984 for (60)Co and 1.0087 for (137)Cs. Comparison with NIST values showed differences of 0.06 % for (60)Co and 0.04 % for (137)Cs. The wall correction factors were obtained and they were different by 0.6-1.1 % for (60)Co and (137)Cs compared to the experimental linear extrapolation method. These values were compared with Monte Carlo derived values from other laboratories. The non-uniformity correction factors were also calculated and they differed from unity, the traditional value used in most standard national metrology laboratories.

Past radioactive particle contamination in the Columbia river at the Hanford site, USA

One closed-loop and eight single-pass plutonium production reactors originally operated on the Columbia river. During the 26 years of single-pass reactor operations, small amounts of radioactive particles were released in liquid discharges to the Columbia river and were deposited in sediment and cobble along the shoreline and on islands in the river. Islands located adjacent to D island and immediately downstream of D island had the greatest density of particles. In 1979, the small particles contained between 63 and 890 kBq of cobalt-60 activity. Dose rates emanating from those particles ranged from 1 to 14 microGy h(-1). Because of the short half-life of cobalt-60 (5.3 y), the hot-particle problem at Hanford has taken care of itself through radiological decay. Some scientists have proposed that it is economically and environmentally advantageous to manage isolated low-level contaminated sites with institutional controls until the activity decays and the sites can be released rather than to pursue expensive clean-up options.

Accumulation and transport behaviour of 241americium, 60cobalt and 134cesium by eggs of the spotted dogfish Scyliorhinus canicula

An experimental study examined the 96-h net influx from seawater of the anthropogenic radionuclides (241)Am, (60)Co and (134)Cs through the egg-case of the spotted dogfish Scyliorhinus canicula. Net influx directly through the wall of the egg-case was greatest for (134)Cs, then (241)Am and lastly (60)Co. Within the egg-case wall itself the measured concentration factors (CFs) and their gradients in the external, median and internal layers showed that for both (241)Am and (60)Co they were >10(3) in the external layer and declined by an order of magnitude in the interior layer. In contrast (134)Cs had a CF of only about three in the external layer which declined by a factor of 2 towards the two more internal layers of the egg-case. The egg-case apertures, that open within the prehatching stage of embryological development, significantly (P<0.05) increased the net influx of (241)Am and (60)Co to the interior of the egg-case, although their water concentrations were still lower than those in the labelled seawater bath. In contrast, the aperture did not increase the net influx of (134)Cs whose water concentrations equilibrated with those in seawater. Together these results indicate that the egg-case wall is very permeable to (134)Cs, representing little barrier to its movement, and hence consistent with the lack of importance of the aperture in determining its internal water concentrations in the egg-case. In contrast, (241)Am and (60)Co show much higher rates of accumulation by the egg-case, consistent with the measured reduced permeability of its wall, and therefore giving greater prominence to its aperture in the net transfer of these two radionuclides to the egg case's interior. The presence of the embryo within its egg-case did not significantly (P>0.05) affect the rates of influx of radioisotopes, with the exception of an interactive effect for (60)Co with the egg-case aperture (P<0.05). The CF of only (241)Am in the embryo itself relative to the external seawater concentration was significantly (P<0.05) enhanced by the presence of the aperture.

Tenth value layers for 60Co gamma rays and for 4, 6, 10, 15, and 18 MV x rays in concrete for beams of cone angles between 0 degrees and 14 degrees calculated by Monte Carlo simulation

The calculation of shielding barrier thicknesses for radiation therapy facilities according to the NCRP formalism is based on the use of broad beams (that is, the maximum possible field sizes). However, in practice, treatment fields used in radiation therapy are, on average, less than half the maximum size. Indeed, many contemporary treatment techniques call for reduced field sizes to reduce co-morbidity and the risk of second cancers. Therefore, published tenth value layers (TVLs) for shielding materials do not apply to these very small fields. There is, hence, a need to determine the TVLs for various beam modalities as a function of field size. The attenuation of (60)Co gamma rays and photons of 4, 6, 10, 15, and 18 MV bremsstrahlung x ray beams by concrete has been studied using the Monte Carlo technique (MCNP version 4C2) for beams of half-opening angles of 0 degrees , 3 degrees , 6 degrees , 9 degrees , 12 degrees , and 14 degrees . The distance between the x-ray source and the distal surface of the shielding wall was fixed at 600 cm, a distance that is typical for modern radiation therapy rooms. The maximum concrete thickness varied between 76.5 cm and 151.5 cm for (60)Co and 18 MV x rays, respectively. Detectors were placed at 630 cm, 700 cm, and 800 cm from the source. TVLs have been determined down to the third TVL. Energy spectra for 4, 6, 10, 15, and 18 MV x rays for 10 x 10 cm(2) and 40 x 40 cm(2) field sizes were used to generate depth dose curves in water that were compared with experimentally measured values.

Monte Carlo study of a 60Co calibration field of the Dosimetry Laboratory Seibersdorf

The gamma radiation fields of the reference irradiation facility of the Dosimetry Laboratory Seibersdorf with collimated beam geometry are used for calibrating radiation protection dosemeters. A close-to-reality simulation model of the facility including the complex geometry of a 60Co source was set up using the Monte Carlo code MCNP. The goal of this study is to characterise the radionuclide gamma calibration field and resulting air-kerma distributions inside the measurement hall with a total of 20 m in length. For the whole range of source-detector-distances (SDD) along the central beam axis, simulated and measured relative air-kerma values are within +/-0.6%. Influences on the accuracy of the simulation results are investigated, including e.g., source mass density effects or detector volume dependencies. A constant scatter contribution from the lead ring-collimator of approximately 1% and an increasing scatter contribution from the concrete floor for distances above 7 m are identified, resulting in a total air-kerma scatter contribution below 5%, which is in accordance to the ISO 4037-1 recommendations.