EURADOS coordinated action on research, quality assurance and training of internal dose assessments

EURADOS working group on 'Internal Dosimetry (WG7)' represents a frame to develop activities in the field of internal exposures as coordinated actions on quality assurance (QA), research and training. The main tasks to carry out are the update of the IDEAS Guidelines as a reference document for the internal dosimetry community, the implementation and QA of new ICRP biokinetic models, the assessment of uncertainties related to internal dosimetry models and their application, the development of physiology-based models for biokinetics of radionuclides, stable isotope studies, biokinetic modelling of diethylene triamine pentaacetic acid decorporation therapy and Monte-Carlo applications to in vivo assessment of intakes. The working group is entirely supported by EURADOS; links are established with institutions such as IAEA, US Transuranium and Uranium Registries (USA) and CEA (France) for joint collaboration actions.

EURADOS intercomparison on measurements and Monte Carlo modelling for the assessment of americium in a USTUR leg phantom

A collaboration of the EURADOS working group on 'Internal Dosimetry' and the United States Transuranium and Uranium Registries (USTUR) has taken place to carry out an intercomparison on measurements and Monte Carlo modelling determining americium deposited in the bone of a USTUR leg phantom. Preliminary results and conclusions of this intercomparison exercise are presented here.

Field deployable technique for 90Sr emergency bioassay

Rapid bioassay is very important for immediate and near-term consequence management, which includes identifying contaminated individuals and providing necessary medical intervention during a radiological or nuclear emergency. This paper reports the application of a newly developed bioassay technique for (90)Sr in urine on a field deployable instrument, the Triathler. Performance of this field technique for sensitivity, accuracy and repeatability is evaluated against bioassay criteria (ANSI N13.30). This field technique offers the following analytical merits: (1) minimum detectable activity of 121 Bq l(-1) when 20 ml of urine is used; (2) relative bias of 11.1 % and relative precision of 3.2 % at the level of 45 Bq per 20 ml of urine and (3) sample turnaround time of less than 1 h. The technique meets the requirements for emergency bioassay when a committed effective dose of 0.5 Sv is used as the action dose threshold for medical intervention. Sample throughput can be significantly improved if this technique is automated.

Internal dosimetry: towards harmonisation and coordination of research

The CONRAD Project is a Coordinated Network for Radiation Dosimetry funded by the European Commission 6th Framework Programme. The activities developed within CONRAD Work Package 5 ('Coordination of Research on Internal Dosimetry') have contributed to improve the harmonisation and reliability in the assessment of internal doses. The tasks carried out included a study of uncertainties and the refinement of the IDEAS Guidelines associated with the evaluation of doses after intakes of radionuclides. The implementation and quality assurance of new biokinetic models for dose assessment and the first attempt to develop a generic dosimetric model for DTPA therapy are important WP5 achievements. Applications of voxel phantoms and Monte Carlo simulations for the assessment of intakes from in vivo measurements were also considered. A Nuclear Emergency Monitoring Network (EUREMON) has been established for the interpretation of monitoring data after accidental or deliberate releases of radionuclides. Finally, WP5 group has worked on the update of the existing IDEAS bibliographic, internal contamination and case evaluation databases. A summary of CONRAD WP5 objectives and results is presented here.

Use of group monitoring data in lung dose estimation for intakes of uranium

This paper describes an internal dosimetry program developed for Canadian uranium processing facilities. The recently adopted recommendations of ICRP Publication 60 have made it extremely difficult to detect intakes of insoluble forms of natural uranium by in vivo methods (lung counting). A type S intake of UO2 corresponding to a 20 mSv effective dose has a lung burden that is a factor of 2-3 lower than the MDA, 6 months after the intake occurred. A new methodology, approved in principle by the Canadian Nuclear Safety Commission, has been designed to overcome this problem by summing sequential, but separate, lung counts for an individual, or summing lung counts from a group of workers performing similar tasks. This summing technique effectively increases the counting time and, therefore, reduces the MDA to a value below the dose limit. Doses will be assigned to the individual or work group based on the average lung burden.

Monte Carlo simulations: a useful tool to extend in vivo calibrations and explore alternative approaches

Designing and testing new equipment can be an expensive and time consuming process or the desired performance characteristics may preclude its construction due to technological shortcomings. Cost may also prevent other types of scenario being tested. An alternative is to use Monte Carlo simulations to make the investigations. This paper exemplifies how Monte Carlo code calculations can be used to fill the gap by describing two investigations: (1) the possible self-attenuation of homogeneously distributed natural uranium in a lung phantom; and (2) the effect of activity deposited in the ribs on the activity estimate from a lung count.

Comparison of the 1st, 2nd and 3rd generation Lawrence Livermore National Laboratory Torso phantoms

The Lawrence Livermore National Laboratory (LLNL) Torso phantom, which is the de facto standard for lung counter calibration for low energy photon emitters, has undergone a number of revisions since its development. The first generation used real human bone: the second generation used synthetic bone and had a major design change; the third generation had more subtle material and mould changes. This work has compared a first generation, two second generation, and two third generation LLNL phantoms to see if there are any differences between the phantoms. The comparison of five LLNL phantoms using the same counting regions has shown that the second and third generation phantoms are essentially equivalent at low photon energies, but the first generation phantom shows an increased counting efficiency at low photon energies due to a design flaw. It is also apparent that these phantoms have maintained their performance characteristics over an extended period of time.

Biological half-life of iodine in adults with intact thyroid function and in athyreotic persons

A joint project between the Human Monitoring Laboratory (HML) and the Ottawa Hospital has measured the retention of 131I in patients who have received the radioiodine diagnostically. Thirty-nine subjects with intact thyroid glands and nine athyreotic subjects were measured in the HML's whole-body/thyroid counter to determine the retention of 131I following its medical administration. The average biological half-life of 131I in 26 euthyroid subjects was found to be 66.1+/-6.3 days which may he statistically significantly lower than the ICRP recommended value of 80 days. Nine hyperthyroid patients had a mean biological half-life of 38.2+/-8.6 days and in three hypothyroid patients the corresponding value was 29.3+/-8.8 days. Thyroid 131I uptake was measured in a conventional clinical fashion at the Ottawa Hospital Civic campus 24 h after oral administration of the radioiodine using a collimated thick sodium iodide detector placed over the neck anteriorly. Measured values were 10.144+/-0.009, 0.314+/-0.035 and 0.045+/-0.010 of the administered dose in euthyroid, hyperthyroid and hypothyroid patients respectively. The euthyroid range at the hospital is 0.06 - 0.22. Uptake was significantly lower for the euthyroid group than the ICRP value of 0.3. The radioiodine retention in athyreotic subjects followed a two compartment model with biological half-lives of 1.0+/-1.2 days and 18.4+/-1.1 days.

An evaluation of germanium detectors employed for the measurement of radionuclides deposited in lungs using an experimental and Monte Carlo approach

A study was undertaken to evaluate the performance of an advanced design broad energy germanium detector for the in vivo measurement of radionuclides in lungs. Relative counting efficiency, background, and sensitivity for lung counting arrays consisting of four, three, and two 80-mm-diameter by 20-mm-thick (80 x 20 mm) broad energy germanium detectors were simulated by collecting spectra with the single 80 x 20 mm broad energy germanium at each of four locations over a humanoid torso phantom. Regions of interest were evaluated for photon energies ranging from 17 to 1,500 keV. The 80 x 20 mm detector arrays were then benchmarked against a standard array of four 70-mm-diameter by 20-mm-thick (70 x 20 mm) broad energy germanium detectors. Since testing new equipment can be an expensive and time consuming process, an alternative approach, using Monte Carlo simulations instead of physical measurements, was also evaluated and compared to experimental data. With this approach, counting efficiency and minimum detectable amount were simulated for two sizes of germanium detectors (70 mm and 80 mm diameter) at four different crystal thicknesses (15, 20, 25, and 30 mm). For the experimental measurements, arrays consisting of three and four 80 x 20 mm broad energy germanium detectors resulted in an increase in counting efficiencies, relative to the standard array, at all photon energies. The greatest relative increase was observed for the four-detector array (24-35%). In contrast, counting efficiency decreased, relative to the standard array, by 24-28% with a two-detector array. Arrays consisting of two and three 80 x 20 mm broad energy germanium detectors resulted in decreased relative background at all photon energies, with the exception of the 946 keV photon for the three-detector array. The most significant decrease in background occurred with the two-detector array (28 to 40%), while background was increased by 18-43% for the four-detector array. Arrays consisting of three and four 80 x 20 mm broad energy germanium detectors resulted in increased relative sensitivity at all photon energies. The three-detector array provided the greatest sensitivity at photon energies below 344 keV. The four-detector array provided slightly better measurement sensitivity at photon energies greater than 344 keV. The two 80 x 20 mm detector array provided sensitivity unexpectedly comparable to the standard array. Monte Carlo predictions on how size affects counting efficiency and minimum detectable amount agreed well with the experimental results. From the Monte Carlo predictions, the effect of detector thickness on counting efficiency was unimportant at photon energies up to 60 keV and independent of detector diameter. At higher photon energies for both detector diameters, the counting efficiency decreased as the thickness decreased. The values of minimum detectable amount for the 70-mm and 80-mm diameter detectors did not differ by more than 15% at 17 keV or 20% at 60 keV when compared to detectors of equivalent thickness. Minimum detectable amount increased slightly at 17 keV and rose by approximately 52% at 660 keV, with decreases in thickness from 30 mm to 15 mm.

The second international in-vivo monitoring intercomparison program for whole body counting facilities by Canadian and United States agencies

The Canadian National Calibration Reference Centre for In-Vivo Monitoring and the United States Department of Energy collaborated to offer a second international in vivo intercomparison program to whole body counting facilities in 1996. This program used a Reference Female phantom shell filled with radioactive tissue-substitute polyurethane to simulate a uniform fission-product distribution in soft tissues. The nuclides used were 137Cs and 60Co. The phantom also contained 40K homogeneously distributed in an amount similar to a Reference Female to produce a representative Compton background in the resulting spectra. Participants were asked to identify the nuclides and report activities for all except 40K. They were also asked to measure the precision of counting and supply the MDA for 137Cs and 60Co. The bias results were in the range of -30% to +80% with most facilities falling inside the range of -25% to +50% (Canadian and U.S. acceptable performance criteria). Results indicated that there was no measurable size dependency for this phantom. All reported precisions were less than 5% but NaI detector based systems seemed to have a systematic uncertainty in addition to Poisson variability. Contrarily, this was not found for Ge detector based systems. MDA data was scattered (14-3,500 Bq for 137Cs and 9-460 Bq for 60Co) and only suggested that lengthening the counting time improves MDA.

Chest wall thickness measurements and the dosimetric implications for male workers in the uranium industry

The Human Monitoring Laboratory has measured the chest wall thickness and adipose mass fraction of a group of workers at a Canadian uranium refinery, a conversion plant, and a fuel fabrication site using ultrasound. A site-specific biometric equation has been developed for these workers, who seem to be somewhat larger than other workers reported in the literature. Chest wall thickness is a very important modifier on lung counting efficiency and these data have been put into the perspective of the impending Canadian dose limits that will reduce the limit of occupationally exposed workers to 100 mSv in a 5-y period with a maximum of 50 mSv in any one year. The sensitivity of the germanium and phoswich based lung counting systems have been compared. Over a range of chest wall thickness of 1.6 cm to 6.0 cm and using a 30-min counting time, the achievable MDA's lie in the range of 6.7 mg to 19.1 mg or 6.7 mg to 30 mg with a two-phoswich-detector array or a germanium lung counting system, respectively. Depending on chest wall thickness, these achievable MDA's are close to, or exceed, the predicted amounts of natural uranium that will remain in the lung (absorption type M and S) after an intake equivalent to the Annual Limit on Intake that corresponds to 20 mSv. Neither system is sufficiently sensitive to detect an intake of Type S natural uranium in a worker with a chest wall thickness that corresponds to the average (3.73 cm) if it occurred more than 7 d prior to the lung count.

Comparison of two JAERI phantoms and the problems discovered

During the course of an intercomparison exercise it was possible to compare two JAERI phantoms with each other by using a multi-energy photon emitting lung set (241Am/152Eu). One belonged to the IAEA (Vienna), the other belonged to the Human Monitoring Laboratory (Ottawa). The intercomparison of the phantoms showed that they were statistically distinct from each other, although the differences were small. The counting efficiencies varied from each other by about 4% at 17 keV and 2% at photon energies above 17 keV. It was concluded that these difference were either due to small variations of chest wall thickness during the manufacturing process or positioning errors. The intercomparison also revealed a serious problem with one of the overlay plates of the HML's phantom. The adipose mass fraction of the overlay plate was found to be much greater (approximately 40%) than the manufacturer's stated value (10%).

Chest wall thickness measurements and the dosimetric implications for male workers in the south Korean uranium industry

Using ultrasound techniques, the Human Monitoring Laboratory has measured chest wall thicknesses of a group of male workers at the Korea Atomic Energy Research Institute. A site-specific biometric equation has been developed for these workers, who are somewhat smaller than other workers reported in the literature. Chest wall thickness is an important modifier on lung counting efficiency. These data have been put into the perspective of the ICRP recommended dose limits for occupationally exposed workers: 100 mSv in a 5-year period with a maximum of 50 mSv in any one year. For measured chest wall thicknesses of 1.9 cm to 4.1 cm and a 30 min counting time, the achievable MDAs for natural uranium in the KAERI lung counter vary from 6.6 mg to 13.2 mg. These values are close to, or even exceed, the predicted amounts of natural uranium that will remain in the lung (absorption type M and S) after an intake equal to the Annual Limit on Intake corresponding to a committed dose of 20 mSv. This paper shows that the KAERI lung counter probably cannot detect an intake of Type S natural uranium in a worker with a chest wall thickness equal to the average value (2.7 cm) under routine counting conditions.

Chest wall thickness measurements of the LLNL phantom for small area germanium detector counting

The Lawrence Livermore National Laboratory (LLNL) phantom was developed to calibrate lung counting systems that are used to estimate plutonium and other low energy photon emitting radionuclides deposited in the lung. Originally, low energy photon counting systems consisted of sodium iodide or phoswich detectors, but they have been largely replaced by smaller germanium detector arrays. The average chest wall thicknesses of the LLNL phantom's torso plate and its overlay plates provided by the manufacturer refer to the regions covered by phoswich detectors; however, germanium detectors are of a different size and are placed in different locations on the phantom's torso plate. Previous work has shown that the manufacturer's data were not applicable for large area germanium detectors. The lung counting system at the Korea Atomic Energy Institute (KAERI) is a small area germanium detector array. Although the detectors are placed within the phoswich circles, only about 25% of the area is covered by the detectors. The LLNL phantom at KAERI has been examined to determine if the manufacturer's data are valid or if new chest wall thickness values must be determined. This paper presents chest wall thickness data for the LLNL phantom with and without its B-series overlay plates at 17 keV, 60 keV, 200 keV, and 1,500 keV and shows that these values are different from the manufacturer's values.

The uncertainty in the activity estimate from a lung count due to the variability in chest wall thickness profile

Calibration of a lung counter requires the use of a realistic torso phantom. The depth profile of both torso phantoms' (LLNL and JAERI) chest plate covers is fixed and assumed to be equivalent to a person's chest wall; however, ultrasound measurements of humans have shown this to be an approximation. When the depth profile of a calibration phantom is different from that of a subject, then a systematic uncertainty will be introduced into the activity estimate. Monte Carlo simulation has shown that changes in the depth profile of the chest wall thickness affect the counting efficiency. Ultrasound measurements have suggested that the coefficient of variation in the depth profile of the chest wall thickness lies between 13% and 26% for male workers; therefore, the added uncertainty to an activity estimate will be an over or underestimate of about a factor of 1.07 resulting from the different depth profile. The factor will be somewhat higher for females, probably about 1.2 at the extreme. These additional uncertainties resulting from depth profile differences are small compared with other uncertainties commonly encountered in lung counting: detector positioning, deposition patterns of the activity, measurement of the chest wall thickness, etc.

The assessment of the effect of thyroid size and shape on the activity estimate using Monte Carlo simulation

Monte Carlo simulations have been used to assess the uncertainty introduced into an activity estimate of radioiodine (125I and 131I) in the thyroid when the size and shape of the gland differs from that of the calibration phantom. The detector dimensions for the 125I simulations were small (diameter 2.54 cm, thickness 0.2 cm); medium (diameter 7.62 cm, thickness 0.2 cm); large (diameter 30.48 cm, thickness 0.2 cm). The detector dimensions for the 131I simulations were small (diameter 2.54 cm, thickness 3.2 cm); medium (diameter 7.62 cm, thickness 6.4 cm); large (diameter 30.48 cm, thickness 11.0 cm). Shapes simulated have included thyroid glands with a third (pyramidal) lobe, no isthmus, and rotated lobes. Sizes simulated have been 10 g, 20 g, and 40 g. The results show that the size of the uncertainty is dependent on the detector size, the neck-to-detector distance, and the type of radioiodine being measured (i.e., 131I or 125I). The worst case bias (on contact counting) for either 125I or 131I using the different sized detectors is as follows: small is between -40% and 40%; medium is between -33% and 17%; large is between -20% and 5%. If the detectors are placed at about 15 cm from the neck the bias values drop so that the uncertainty introduced if the subject has a smaller than standard thyroid becomes insignificant and becomes much improved if the thyroid is larger than the standard. The bias values for the detectors are as follows: small is between -20% and 5%; medium is between -23% and 2%; large is between -21% and 2%.

Lung counting: a function to fit counting efficiency of a lung counting germanium detector array to muscle-equivalent-chest-wall-thickness and photon energy using a realistic torso phantom

A function has been developed that will fit counting efficiency to chest wall thickness, adipose content of the chest wall, and photon energy. The adipose content of the chest wall is removed as a variable by use of the derived quantity: muscle equivalent chest wall thickness. The function has been tested on experimental data sets obtained from the LLNL and JAERI torso phantoms using a variety of germanium detector lung counting systems. The function is applicable over the range of existing phantoms and the range of equipment types tested. Subtle differences between the detector systems are shown by differences in the fit parameters. This analysis could be a useful addition to lung counting analysis software. The LLNL and JAERI phantoms have very similar counting characteristics as evidenced by the similarity of the function parameters when measured using a single detector system.

The effect of lung deposition patterns on the activity estimate obtained from a large area germanium detector lung counter

Lung counters for in vivo detection of low energy photon emitters are typically calibrated using phantoms containing lung tissue equivalent material with the radioactivity homogeneously distributed throughout the material. If the activity in a measurement subject is heterogeneously distributed, the activity estimate for that subject will be uncertain due to the assumptions of distribution. The magnitude of the uncertainty for a four-detector germanium array, using the Lawrence Livermore National Laboratory torso phantom with a newly designed lung set that allows the activity to be localized in one or more of 16 areas, was estimated. The results show that detector arrays will reduce the uncertainties arising from the geometry of the lung deposition compared to single detectors. The estimated activity of an internal deposition that emits 17.5 keV photons can be overestimated by a factor of three, or underestimated by a factor of infinity (i.e., the activity is missed completely). As the photon energy rises to 59.5 keV the uncertainty in the activity decreases so that the maximum overestimate (underestimate) will be a factor of two (five). As the energy rises to 344.3 keV only the maximum underestimate changes: it becomes a factor of three.

Comparison of sliced and whole lung sets for the LLNL and JAERI torso phantoms using GE detectors

The performance characteristics of sliced lung sets were compared to lung sets with activity homogeneously distributed throughout the lung tissue substitute material. The activity estimate from planar sources differs from the estimate from homogeneous sources by a factor of 0.88 to 1.09 depending on the photon energy. This error is small compared to other uncertainties commonly encountered in lung counting, i.e., activity deposition in the lung, detector placement, size difference between individuals, etc. Sliced lung sets could be used instead of homogeneous lung sets to test or provide an interim calibration for a lung counting system.

A joint HML-KAERI project--comparison of the LLNL and JAERI torso phantoms using four 50 mm Ge detectors

The Health Physics Department of the Korea Atomic Energy Research Institute and the Human Monitoring Laboratory have collaborated to compare the LLNL and JAERI torso phantoms. The counting efficiencies of the phantoms at 17.7 keV, 59.5 keV, 121.8 keV, and 344 keV were measured with KAERI's germanium lung counting system. The data were made comparable by converting the chest wall thicknesses and adipose mass fractions of the phantoms to muscle equivalent chest wall thicknesses. The counting efficiencies of the two phantoms are within 12% to 17% of each other at 17.7 keV, 15% to 22% at 59.5 keV, 10% to 15% at 121.8 keV, and 7% to 10% at 344 keV. This joint study has shown that the LLNL and JAERI phantom are essentially equivalent for the purposes of calibrating a lung counting system that consists of two ACTII germanium detectors.

Comparison of the LLNL and JAERI torso phantoms using Ge detectors and phoswich detectors

The Human Monitoring Laboratory has compared the LLNL and JAERI torso phantoms using its germanium detector lung counting system by measuring the counting efficiencies for radioactive materials in the phantoms at photon energies of 17.7 keV, 59.5 keV, 121.8 keV, and 344 keV to assess the similarity (or differences) in performance characteristics. The counting efficiencies obtained from the two phantoms were compared by converting the Chest Wall Thickness data and Adipose Mass Fractions of the phantoms to Muscle Equivalent Chest Wall Thicknesses. The counting efficiencies for the two phantoms were found to be within a factor of 1.44 of each other at 17.7 keV, 1.30 at 59.5 keV, 1.25 at 121.8 keV, and 1.17 at 344 keV when using a four detector array (JAERI efficiency divided by LLNL efficiency). However, individual detector responses show that the counting efficiencies from the two phantoms differ considerably in the region of the heart (up to a factor of 6 at 17 keV). Other areas above the lungs give counting efficiencies that are similar to each other. A routine intercomparison exercise with Cameco Corporation has shown that the counting efficiencies derived from the LLNL and JAERI phantoms were found to be within a factor of 1.18 (JAERI/LLNL) when a natural uranium lung set was used to calibrate a lung counter consisting of phoswich detectors. This work has also shown that over the energy range 63 keV-185 keV the LLNL phantom can be used to calibrate phoswich detector systems that are positioned on the back of the subject.

Considerations in assigning dose based on uncertainties from in vivo counting

Lung counting is highly geometry dependent, especially at low photon energies. Monte Carlo simulations have been used to determine the magnitude of the errors obtained if it is assumed that the deposition is homogeneous, when in fact it is not. Simulation for a germanium lung counting system consisting of four, 70 mm x 30 mm diameter thick detectors have been performed for 70 deposition patterns. The detector efficiencies for 20, 40, 60, 120, 240, 660, and 1000 kiloelectron volts were calculated for a homogeneous deposition and these efficiencies were used to estimate the bias when the deposition was heterogeneous. A bias of 800% was not unusual. Whole-body counting is prone to errors that can arise from the activity distribution and/or size of the subject. The latter are geometry dependent and, for example, a bias result of 200% can be obtained when measuring children in a chair geometry using a calibration factor based on reference man. Thyroid counting is the least prone to error. If the measurement is done correctly, biases can usually be kept below 20%. However, if the measurement is made with a collimator or if the detector is in contact with the subject's neck, biases exceeding 200% can be obtained.

A study of thyroid radioiodine monitoring by Monte Carlo simulations: implications for equipment design

Monte Carlo simulations have been performed to evaluate the design of collimated detectors used to measure 125I or 131I in the thyroid gland. Two detector sizes were simulated for each radioisotope: (i) for 125I monitoring 2.54 cm diameter and 7.62 cm diameter and 0.2 cm thickness and (ii) for 131I monitoring 2.54 cm diameter, 3.2 cm thickness and 7.62 cm diameter, 6.4 cm thickness. The virtual thyroid gland was 20 g. Activity was placed in both the gland and the remainder of the body in varying amounts to assess the efficacy of collimation. The results show that the detector should be sufficiently large so that its solid angle of acceptance when placed 15 cm anterior to the skin surface will include the whole of a moderately enlarged thyroid gland. Heavy collimation to reduce the contribution of extrathyroidal radioiodine within the subject's body is not normally required. It may be of more value as a positioning device and spacer ensuring an appropriate and constant neck to detector distance than in cutting down counts from extrathyroidal activity. In specifying a sensitive detector system for monitoring intrathyroidal radioiodine, a wide angle of acceptance and sufficient detector crystal thickness take precedence over collimation and shielding.

Radiocesium body burdens in residents of northern Canada from 1963-1990

Measurements of 137Cs body burdens in over 1,100 people from five northern Canadian communities were carried out with a portable whole body counting system during the winters of 1989 and 1990. These results are compared with over 3,000 similar measurements carried out during 1967-1969. Community mean body burdens and body concentrations had decreased by approximately a factor of 30 between the two survey periods. The dependence of body concentrations on the sex and age of the subjects has also changed significantly. This can be related to changes in the patterns of caribou consumption in the northern communities. Measurements of 137Cs in urine are also available for an earlier period (1963-1966) when world-wide fallout was at its highest level. A normalization procedure was developed to calculate the average radiocesium body concentration in each community from the concentrations in urine. From data spanning a period of nearly 30 y (1963-1990), lifetime radiation doses have been estimated for most communities in the Yukon and Northwest Territories. These cumulative doses vary from 0.3 to nearly 40 mSv, with an Arctic-wide average of about 12 mSv. No health effects would be expected at these levels.

Examination of the effect of counting geometry on 125I monitoring using MCNP

This paper theoretically investigates how the counting efficiencies of three selected detector (NaI) sizes are affected by neck-detector distance, detector misplacement, thickness of overlaying tissue, and size of the thyroid gland. The detector sizes were small, 2.54 cm diameter and 0.2 cm crystal thickness; medium, 7.62 cm diameter and 0.2 cm crystal thickness; large 30.48 cm diameter and 0.2 cm crystal thickness. The evaluations were determined using a Monte Carlo technique. The simulations show that the large detector minimizes the uncertainties for all problems except the thickness of overlaying tissue.

Chest wall thickness measurements: the alternative approach extended for 241Am

The Human Monitoring Laboratory has extended the technique of determining the chest wall thickness of an individual using information from the spectrum produced by internally deposited radionuclides. The technique has been investigated both theoretically and practically using germanium detectors and the Lawrence Livermore Torso Phantom. The phantom was used with a lung set containing homogeneously distributed 241Am. Chest wall thicknesses were varied by using a series of muscle equivalent overlay plates that gave a range of 1.6 cm to 3.9 cm thickness. It was found that a 3-cm chest wall thickness can be estimated to within 18%. Using a spectral addition technique 1 kBq was estimated to be the "practical" lower limit of activity for this method.

The BRMD thyroid-neck phantom: design and construction

The Human Monitoring Laboratory, which acts as the Canadian National Calibration Reference Center for In Vivo monitoring, has constructed a robust neck-thyroid phantom for use in the Canadian Thyroid Intercomparison Program. The phantom is rugged and capable of being distributed through the mail with no expectation of a leak of radioactive materials; it is anthropomorphic; the thyroid inserts simulate 125I and 131I; the phantom can be used to mimic different layers of adipose tissue over the thyroid insert; and it is cost effective. This paper describes the design criteria and the manufacturing process; the performance characteristics have been described in an earlier publication.

Comparison of the ANSI, RSD, KKH, and BRMD thyroid-neck phantoms for 125I thyroid monitoring

The Human Monitoring Laboratory, which acts as the Canadian National Calibration Reference Centre for In Vivo Monitoring, has determined the performance characteristics of four thyroid phantoms for 125I thyroid monitoring. The phantoms were a phantom built to the specifications of the American National Standards Institute Standard N44.3; the phantom available from Radiology Support Devices; the phantom available from Kyoto Kagaku Hyohon; the phantom manufactured by the Human Monitoring Laboratory and known as the BRMD phantom. The counting efficiencies of the phantoms for 125I were measured at different phantom-to-detector distances. The anthropomorphic characteristics of the phantoms have been compared with the average man parameters. It was concluded that the BRMD, American National Standards Institute, and Radiology Support Devices phantoms have the same performance characteristics when the neck-to-detector distances are greater than 12 cm and all phantoms are essentially equivalent at 30 cm or more. The Kyoto Kagaku Hyohon phantom showed lower counting efficiencies at phantom-to-detector distances less than 30 cm. This was attributed to the design of the phantom. This study has also shown that the phantom need not be highly anthropomorphic provided the calibration is not performed at short neck-detector distances. Indeed, it might be possible to use t simple point source of 125I placed behind a 1.5 cm block of lucite at neck detector distances of 12 cm or more.

Dosimetry of an incident involving 14C

High level contamination was observed on two workers during a routine hand check of personnel leaving a laboratory where barium carbonate labeled with 14C was handled. A third worker was found to have been exposed to airborne 14C (carbonate) aerosol during the investigation. Urinary excretion analysis and in vivo monitoring were carried out. This paper presents the results of more than one year of lung counting using phoswich detectors and gives some indication about the metabolism of the 14C over that period. Doses were difficult to estimate due to lack of information in the literature. Estimates were based on default parameters and information gathered in this study using ICRP 30 and the new ICRP lung models. These ranged from 0.3 mSv to 3 mSv depending on the assumptions and models.

The Canadian whole body counting intercomparison program: a summary report for 1989-1993

The Human Monitoring Laboratory, which is the Canadian National Calibration Reference Centre for In-Vivo Measurements, has been conducting an annual intercomparison program for whole body counting facilities since 1989. Each year a number of phantoms are prepared for participants to measure the accuracy, precision, size dependency, and identification capabilities of their whole body counters. Other tests are designed to measure the sensitivity of the whole body counter to geometry factors. Some phantoms are prepared with the same radionuclide from year to year so that time dependent data can be acquired, whereas other phantoms are prepared as unknowns. This article describes the tests that were performed, gives the results for Canadian participants, and compares the results to interim performance criteria. Measurement results from a human volunteer who ingested 137Cs contaminated caribou meat and was counted at a number of facilities in 1990 are also presented.

Radiocesium body burdens in immigrants to Israel from areas of the Ukraine, Belarus and Russia near Chernobyl

Of the 500,000 immigrants from the former Soviet Union who came to Israel during 1990-1993, about 100,000 are estimated to have come from radiocontaminated areas near Chernobyl. These people were subject to chronic uptake of environmental radiocesium over protracted periods. During October-November 1991, a joint Israeli-Canadian investigation measured radiocesium body burdens in immigrants to Israel from the Ukraine, Belarus, and the southern Russian republic in order to provide factual information on radiocesium levels to concerned immigrants and to relate the body burdens to the geographic area of residence before coming to Israel. Assessments were made of 137Cs body burdens in 1,228 volunteer men, women, and children. These measurements were accompanied by medical assessments based on clinical histories and examinations. Radiocesium levels were strongly dependent on the duration of residence in Israel, with the highest levels being found in the most recent immigrants. The maximum level, extrapolated back to the time of leaving the former Soviet Union, was estimated to be about 0.83 kBq (10.3 Bq kg-1). Of the most recent immigrants from the Kiev region (< 101 days in Israel), only 15% had back extrapolated body burdens > 50 Bq, whereas 53% of those coming from Gomel and other towns in the contaminated zones (> 3.7 x 10(10) Bq km-2 of radiocesium) had detectable levels > 50 Bq. People coming from the latter region had significantly higher body burdens as compared to those from the former, in accordance with the higher degree of ground radiocesium contamination reported for the latter region. Women and children showed considerably lower total radiocesium content in comparison to men. All radiocesium body burdens at the time of measurement were too low to be of health concern.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring: thyroid monitoring. Part V: Minimizing placement error in a thyroid monitoring system

This article is the last of a five-part series covering various aspects of occupational thyroid monitoring. This part describes the techniques for minimizing errors due to improper placement of the detector. The impact of counting time and minimum detectable activity as a function of detector position are also discussed. The importance of minimum detectable activity is exemplified by showing how it can be used to ensure that the thyroid monitoring system can detect an amount of radioactivity below the derived investigation level.

The Canadian National Calibration Reference Center for Bioassay and in-vivo Monitoring: a program summary

The Canadian National Calibration Reference Center for Bioassay and in-vivo Monitoring is part of the Radiation Protection Bureau, Department of Health. The Reference Center operates a variety of different intercomparison programs that are designed to confirm that workplace monitoring results are accurate and provide the necessary external verification required by the Canadian regulators. The programs administered by the Reference Center currently include urinalysis intercomparisons for tritium, natural uranium, and 14C, and in-vivo programs for whole-body, thorax, and thyroid monitoring. The benefits of the intercomparison programs to the participants are discussed by example. Future programs that are planned include dual spiked urine sample which contain both tritium and 14C and the in-vivo measurement of 99mTc.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring: thyroid monitoring. Part IV: Optimizing a counting system that uses a single-channel analyzer

This article is the fourth of a five-part series covering various aspects of occupational thyroid monitoring. This article describes the energy calibration of the monitoring system with particular emphasis on techniques for optimizing a system that is based on a single-channel analyzer, or any system that does not have a multi-channel analyzer. These systems cannot directly show the operator the photopeak of the calibration source. The article also briefly discusses quality control and problem solving.

Measurement of the natural uranium content of fabricated, tissue substitute lung sets by dual photon absorptiometry

The assessment of tissue substitute materials by dual photon absorptiometry has been extended to detect the presence of high atomic weight materials. Commercial dual photon systems dedicated to the measurement of bone mineral mass can be used to detect other elements in fabricated tissue phantoms. The technique is sufficiently sensitive to quantitate the mass of natural uranium that had been distributed throughout the lungs of a thorax phantom.

Chest wall thickness measurements for enriched uranium: an alternative approach

The Human Monitoring Laboratory has developed a technique to determine the chest wall thickness of an individual using information from the spectrum produced by internally deposited radionuclides. The technique has been investigated both theoretically and practically using phoswich detectors and the Lawrence Livermore Torso Phantom. The phantom was used with lung sets containing homogeneously distributed 93% enriched uranium, 20% enriched uranium, natural uranium, and 241Am. It was found that a 3-cm chest wall thickness can be estimated to within 9% when measuring 93% enriched uranium. The technique does not work for the latter two radionuclides because of an insufficient separation in the photon energies and poor resolution of the phoswich detectors. The technique is only of value for activity levels well above the detection limit.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring: thyroid monitoring. Part III: A basic calibration procedure for thyroid monitoring

This article is the third of a five-part series covering various aspects of occupational thyroid monitoring. This article introduces the basic concepts required to understand the procedure for determining the counting efficiency of a thyroid detector. The B.R.M.D. thyroid-neck phantom is used as the calibration source. A procedure for personnel monitoring is also discussed and the concept of Minimum Detectable Activity (MDA) is introduced. The last two articles in this series discuss energy calibration, counting system optimization based on a single-channel analyzer and placement error minimization.

Radiocesium in children from Belarus

The body burdens of 137Cs were measured in 74 children from Belarus who had been exposed to fallout from the Chernobyl accident. The children were between the ages of 8 and 12 y old and were visiting the Ottawa area during the summers of 1991 and 1992. The body burdens varied from 0.04-2.25 kBq, which are less than or comparable to the amount of natural 40K in the children's bodies. There was no apparent dependence on age or sex. The body burdens were related, to some extent, with recorded fallout levels in the region of origin but did not appear to change significantly from 1991 to 1992. During their stay in Canada, radiocesium was being cleared from the children's bodies with a mean half-time of 33 d (range = 12-77 d). The highest intake rate of 137Cs was estimated to be 18 Bq d-1 and the highest dose rate was 0.13 mSv y-1.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring. Part II: Sources of errors in thyroid monitoring of occupationally exposed personnel

This article, the second of a five-part series covering various aspects of occupational thyroid monitoring, addresses the sources of error that can affect the final result obtained from thyroid monitoring, such as geometry effects (thyroid size, thyroid depth, precision and accuracy of the detector placement, and neck-detector distance). The article also suggests ways in which these errors can be minimized and identifies those errors that are difficult to quantify.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring: thyroid monitoring. Part I: The national inter-comparison programme

This article is the first part of a five-part series covering various aspects of occupational thyroid monitoring. The Canadian National Calibration Reference Centre for In-Vivo Monitoring conducts a thyroid inter-comparison programme that now includes more than 100 facilities. The scope of the programme, begun in 1988, has greatly expanded in the last two years following a considerable effort to locate and inform facilities. This article presents the details of the programme, its results, and the lessons learned. Subsequent articles will discuss sources of errors, methodology, instrumental configuration, and counting geometry optimization.

Evaluation of the Lawrence Livermore National Laboratory (LLNL) torso phantom by bone densitometry and x-ray

The recent Workshop on Standard Phantoms recommended that the LLNL torso phantom be adopted as a calibration standard for the quantitation of in vivo radioactivity. This phantom was designed for the calibration of systems for the detection of x-rays of less than 20 keV. The anthropomorphic characteristics and tissue substitute composition of the phantom were assessed with techniques using photons of higher energy. Dual photon absorptometry at 42 and 100 keV showed that the phantom was representative of in vivo tissue composition. Chest radiography showed that the phantom was representative of a human even though the stomach, GI tract and the scapulae were not present and air gaps were observed at organ boundaries.

The optimization of the analysis of 63Ni in urine

A method has been developed that separates 63Ni from urine. The subsequent estimation of activity is by liquid scintillation counting. The urine is wet-ashed by a new procedure which is much faster than conventional ashing methods. Interfering phosphates are removed before a precipitation of Ni as the dimethylglyoxime complex. The effects of the carrier weight, the pH of the phosphate removal step and the pH of the dimethylglyoxime precipitation have been investigated and optimized to give a mean recovery of 97 +/- 8% for 63Ni. The detection limit as defined by Currie (1968) is estimated to be 4.0 pCi (147 mBq) per sample.