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An international intercomparison was organized by Working Group 7, Internal Dosimetry, of the European Radiation Dosimetry Group in collaboration with Working Group 6, Computational Dosimetry, for measurement and Monte Carlo simulation of Am-241 in three skull phantoms. The main objectives of this combined exercise were (1) comparison of the results of counting efficiency in fixed positions over each head phantom using different germanium detector systems, (2) calculation of the activity of Am-241 in the skulls, (3) comparison of Monte Carlo simulations with measurements (spectrum and counting efficiency), and (4) comparison of phantom performance. This initiative collected knowledge on equipment, detector arrangements, calibration procedures, and phantoms used around the world for in vivo monitoring of Am-241 in exposed persons, as well as on the Monte Carlo skills and tools of participants. Three skull phantoms (BfS, USTUR, and CSR phantoms) were transported from Europe (10 laboratories) to North America (United States and Canada). The BfS skull was fabricated with real human bone artificially labeled with Am-241. The USTUR skull phantom was made from the US Transuranium and Uranium Registries whole-body donor (case 0102) who was contaminated due to an occupational intake of Am-241; one-half of the skull corresponds to real contaminated bone, the other half is real human bone from a noncontaminated person. Finally, the CSR phantom was fabricated as a simple hemisphere of equivalent bone and tissue material. The three phantoms differ in weight, size, and shape, which made them suitable for an efficiency study. Based on their own skull calibration, the participants calculated the activity in the three European Radiation Dosimetry Group head phantoms. The Monte Carlo intercomparison was organized in parallel with the measurement exercise using the voxel representations of the three physical phantoms; there were 16 participants. Three tasks were identified with increasing difficulty: (1) Monte Carlo simulation of the simple CSR hemisphere and the Helmholz Zentrum Munchen high-purity germanium detector for calculating the counting efficiency for the 59.54 keV photons of Am-241, in established measurement geometry; (2) Monte Carlo simulation of particular measurement geometries using the BfS and USTUR voxel phantoms and the Helmholz Zentrum Munchen high-purity germanium detector detector; and (3) application of Monte Carlo methodology to calculate the calibration factor of each participant for the detector system and counting geometry (single or multidetector arrangement) to be used for monitoring a person in each in vivo facility, using complex skull phantoms. The results of both exercises resulted in the conclusion that none of the three available head phantoms is appropriate as a reference phantom for the calibration of germanium detection systems for measuring Am-241 in exposed adult persons. The main reasons for this are: (1) lack of homogeneous activity distribution in the bone material, or (2) inadequate shape/size for simulating an adult skull. Good agreement was found between Monte Carlo results and measurements, which supports Monte Carlo calibration of body counters as an alternative method when appropriate physical phantoms are not available and the detector and source are well known.

Gamma ray spectroscopy enables the identification of radioactive sources, and, with proper calibration, their activities. In use since the 1950s, NaI(Tl) spectroscopic systems remain popular due to their affordability and high detection efficiency. The crystals and accompanying electronics may vary, and there are practical choices necessary to produce the best possible spectra for a given application or correctly interpret system performance. An overview of the scintillation mechanism as well as the common features of a gamma ray spectrum are presented in this paper. This includes a discussion of the impacts of the size and shape of detector crystals on the spectrum and counting efficiency. A description of supporting electronics is included along with techniques for arranging and optimizing them. Coaxial cables become part of the circuit and can degrade the detector signal if they have mismatched impedance or excessive length. A discussion is included of tradeoffs involved in selecting combinations of individual electronics components for NaI(Tl) spectroscopic applications. Lastly, a comprehensive energy calibration procedure is provided. This paper thus serves as a tutorial on several practical aspects of a NaI(Tl) gamma ray spectroscopy system.

The counting efficiencies obtained using a physical neck phantom are typically used in the measurement of I activity in the thyroid. It is well known, however, that the geometrical discrepancies between the physical neck phantom and the anatomy of the subject can significantly influence the counting efficiencies. Thus, it is necessary to consider the anatomical characteristics of individuals if we need to accurately determine the activity of I in the thyroid. This study aims to produce individualized counting efficiencies for thyroid measurement, considering the age, sex, and overlying tissue thickness of the subject being measured by Monte Carlo simulation. Simulations were performed using a series of computational human phantoms of different ages and sexes. The difference in counting efficiencies, depending on the age and sex of the phantom, were found to range from -26 to 3% for the phantoms and monitoring systems considered in the present study. The overlying tissue thickness of the computational phantoms was also modified to find the relationship between the counting ratio of the 80.2 and 364 keV gammas from I and the overlying tissue thickness. The equations for estimating the overlying tissue thickness of a subject were then derived from the relationships between counting ratios and overlying tissue thickness. Finally, in the present study, a set of equations representing the variation in counting efficiencies for the 364 keV peak as a function of the overlying tissue thickness were derived, which can be used to determine individualized counting efficiencies for the subject being measured. These individualized counting efficiencies considering the overlying tissue thickness given a subject's age and sex can provide accurate estimates of I activity for internal dosimetry.

An environmental radiation survey using a gamma-ray spectrometer is used to rapidly detect radioactive contamination over a wide area of ground that was released from nuclear events. For the successful application of a gamma-ray spectrometer to the calculation of the radioactivity concentration in the ground and the dose rate at 1 m above the ground, it is necessary to build a calibration procedure to obtain the counting efficiency at the in situ measurement, which means in situ calibration factor to report the calculation results from the measured net count rate according to the diverse detection geometries. This study is focused on the development of a program to calculate the in situ calibration factor and report the survey results in the environmental radiation surveys using three kinds of gamma-ray spectrometers, which have been widely used in the field of in situ measurements: a coaxial HPGe detector, cylindrical NaI(Tl), and rectangular NaI(Tl). The program is based on the results of diverse theoretical calculations of the unscattered photon fluence at the detector height, detector responses of three detectors, and their angular corrections. The developed program was successfully applied to the estimation of the radioactivity concentration of nuclides in the ground and the dose rate at 1 m height above the ground induced from them.

Dust loading on air sample filters is known to cause a loss of efficiency for direct counting of alpha activity on the filters, but the amount of dust loading and the correction factor needed to account for attenuated alpha particles is difficult to assess. In this paper, correction factors are developed by statistical analysis of a large database of air sample results for a uranium and plutonium processing facility at the Savannah River Site. As is typically the case, dust-loading data is not directly available, but sample volume is found to be a reasonable proxy measure; the amount of dust loading is inferred by a combination of the derived correction factors and a Monte Carlo model. The technique compares the distribution of activity ratios [beta/(beta + alpha)] by volume and applies a range of correction factors on the raw alpha count rate. The best-fit results with this method are compared with MCNP modeling of activity uniformly deposited in the dust and analytical laboratory results of digested filters. A linear fit is proposed to evenly-deposited alpha activity collected on filters with dust loading over a range of about 2 mg cm to 1,000 mg cm.

The examination of internal contamination is important for providing an adequate medical response during a radiological emergency. A whole-body counting system can assess gamma-emitting radionuclides in a human body when monitoring internal contamination. It is necessary to calibrate whole-body counting systems by using a calibration phantom, such as a Bottle Manikin Absorption phantom, to properly assess internal contamination. However, the total weight of the Bottle Manikin Absorber phantom is high, and there can be leakage of radioactive sources, which are disadvantages of using such a phantom. This study proposes a calibration phantom that is designed to overcome these disadvantages. The proposed phantom consists of rod sources that are inserted in each part of the phantom. The counting efficiency of the rod-source-inserted calibration phantom was acquired using a Monte Carlo simulation method, but the results were evaluated by comparing the experimental efficiencies with those of a conventional Bottle Manikin Absorption phantom by using two commercial whole-body counting systems (stand-up type and bed type). The efficiency curve of the rod-source-inserted phantom matched well that of the conventional calibration phantom. The relative deviation between the efficiencies of the conventional Bottle Manikin Absorption phantom and the proposed calibration phantom in both whole-body counting systems was less than 11%, and the total weight of the phantom was also reduced. These results suggest that the proposed phantom can be manipulated more easily and replace the conventional Bottle Manikin Absorption calibration phantom for these two types of whole-body counting systems.

The Human Monitoring Laboratory (Canada) has looked at parameters (lung volume, lung deposition pattern, etc.) that can affect the counting efficiency of its lung counting system. The calibration of the system is performed using the Lawrence Livermore National Laboratory (LLNL) torso phantom; however, the effect of respiratory motion cannot be accounted for using these phantoms. When measuring an internal deposition in the lungs of a subject, respiration causes a change in the volume of the lungs and the thoracic cavity and introduces a variable distance between the lungs and the detectors. These changes may have an impact on the counting efficiency and may need to be considered during a measurement. In this study, the HML has simulated the respiration motion using a 4D non-uniform rational b-spline (NURBS)-based Cardiac-Torso (NCAT) phantom and determined the impact of that motion on the counting efficiency of their lung counting system during measurement. The respiratory motion was simulated by a 16 timeframe cycled 4D NURBS-based NCAT phantom developed at the Department of Biomedical Engineering and Radiology, University of North Carolina. The counting efficiency of the four germanium detectors comprising the HML lung counting system was obtained using MCNPX version 2.6E for photon energies between 17 and 1,000 keV. The amount of uncertainty due to the breathing motion was estimated by looking at the efficiency bias, which was highest at low photon energies as expected due to attenuation and geometry effects. Also, to reduce the influence of the detectors' positioning, an array was calculated by adding the individual detector tallies for a given energy and timeframe. For photon energies of 40 keV and higher, the array efficiency bias showed an underestimation of about 5%. If compared to other parameters already studied by the HML, this value demonstrates the insignificant impact of the breathing motion.

Skull counting can be used to assess the activity of radionuclides internally deposited in the bone. The Human Monitoring Laboratory (HML) at Health Canada conducted the measurement of 241Am in the BfS (Bundesamt für Strahlenschuts) skull phantom on contact with the skull for various positions. By placing the detector in contact, the HML can improve the counting efficiency by over 20% compared to placing the detector 1 cm above the surface of the skull. Among all the positions tested, the forehead position is the preferred counting geometry due to the design of HML's counting facility and the comfort it would provide to the individual being counted, although this counting position did not offer the highest counting efficiency for the gamma rays (either the 59.5 keV or the 26.3 keV) emitted by 241Am.

Using demographic data, a series of BOMAB phantoms were developed to study the effect of size, at a fixed height, of a BOMAB phantom using Monte Carlo simulations in three different whole body counting systems: the HML's scanning detector whole body counter, a FastScan whole body counter, and a StandFast whole body counter. The latter has had two counting geometries simulated--one for the recommended position, and another simulating a constant detector-to-front-of-phantom distance. The six phantom sizes corresponded to the following masses: 48 kg, 73 kg, 98 kg, 123 kg, 148 kg, and 173 kg. The effect of size varies with photon energy, as might be expected, and at any given energy is an exponential function of the mass. An equation has been found that fits most cases very well and is still good in poorer cases. Persons lighter in mass than the normal calibration phantom (73 kg) will have their body burdens overestimated by as much as a factor of 1.3, depending on mass, photon energy, and counting geometry. Conversely, heavier individuals will have their body burdens underestimated by as much as a factor of 1.9, depending on mass, photon energy, and counting geometry.

Previously, this laboratory conceptualized a new phantom for calibration or performance testing of whole body counters using Monte Carlo simulations. This paper describes the physical reality that was created from the Monte Carlo design project and compares its counting efficiency to that of a conventional BOMAB phantom using two whole body counters. In one counter (NaI based) the agreement between the two phantoms was +/-8% and in the second counter (Ge based) the agreement was +/-5% at all the energies measured (126 keV, 661 keV, 1172 keV, 1330 keV). The advantage of the sliced phantom is that the sources are solid, sealed, and cannot leak activity thereby simplifying packing for shipment if the phantom is classified as a Dangerous Good. The new phantom is, therefore, ideal for uses that involve shipment, such as an intercomparison exercise. The phantom is also re-usable as the sources can be changed.

The W-chair whole body counting geometry, a derivative of the meter-arc counting geometry, has been examined using Monte Carlo simulations to investigate the effect of phantom size on the counting efficiency. Three detector positions were simulated (42 cm, 71 cm, and 100 cm from the chair), with one being very close to the physical counting geometry in use at the Whiteshell Laboratories, to investigate the effects of detector-chair distance. The agreement of the simulations with observed counting efficiencies was within 7% over the energy region of 122 keV to 1,173 keV. The optimum counting geometry was found to be the 71-cm detector-chair distance as this balances both sensitivity and counting efficiency. The range of relative counting efficiencies is 0.94 to 1.13 for all energies studied (126 keV to 2,754 keV) and selected phantom sizes relative to the Reference Man phantom. This range also represents the extent of the activity estimate's uncertainty if no size correction factors are applied during routine counting and shows the W-chair counting geometry to be relatively subject-size independent.

The performance characteristics of a commercially available whole body counting phantom have been examined experimentally and using Monte Carlo simulations. The counting efficiency as a function of photon energy (126 keV to 2,754 keV) obtained using this phantom has been compared with those obtained using a real or virtual BOMAB phantom in a scanning detector and a FastScan whole body counter. The results suggest that the commercially available whole body counting phantom is equivalent to a BOMAB phantom under certain circumstances but that under other measurement conditions it may be necessary to develop correction factors.

Lung counters are usually calibrated with an anthropometric phantom that has a fixed lung size; however, people have widely varying lung sizes (both volume and dimensions). This work uses a simple Monte Carlo simulation to investigate the effect on the counting efficiency of a lung counter based on a four detector array of 50 mm diameter, 70 mm diameter, or 85 mm diameter as lung size varies. The simulations were carried out at several photon energies (17, 60, 120, and 1,000 keV). Comparing the simulated efficiencies with a reference value close to the lung volume of Reference Man, biases in the range of -21% to 63% were discovered. The values from the Monte Carlo simulation have also been compared with some literature data based on experimental measurements, and the agreement was found to be comparable suggesting that lung volume is indeed a factor that should be considered when trying to make an accurate estimate of a lung burden.

New in-vivo calibration phantoms (anthropometric phantoms) were developed to meet the needs for Japanese standard phantoms. Two important characteristics of these phantoms were that (1) they were designed using Japanese body size survey data, and (2) they were designed so that they can be adapted to various positions or geometries. The performance of these phantoms was tested with respect to body size, activity distribution along the axis, and counting efficiency. The actual dimensions of the anthropometric phantoms were compared with the survey data. Most items (31 of 47) indicated good agreement between the actual values and the survey data for the adult anthropometric phantom. The activity distribution for the anthropometric phantoms was compared with that for block phantoms that simulate a uniform activity distribution. The anthropometric phantoms have some gaps in their joints. The measurement results, however, indicated that these gaps did not significantly affect the overall accuracy of the measurements. Differences in counting efficiency between the block phantoms and the anthropometric phantoms for the same age were no more than 6%.

A realistic computational 3D human body library, called MaMP and FeMP (Male and Female Mesh Phantoms), based on polygonal mesh surface geometry, has been created to be used for numerical calibration of the whole body counter (WBC) system of the nuclear power plant (NPP) in Doel, Belgium. The main objective was to create flexible computational models varying in gender, body height, and mass for studying the morphology-induced variation of the detector counting efficiency (CE) and reducing the measurement uncertainties. First, the counting room and an HPGe detector were modeled using MCNPX (Monte Carlo radiation transport code). The validation of the model was carried out for different sample-detector geometries with point sources and a physical phantom. Second, CE values were calculated for a total of 36 different mesh phantoms in a seated position using the validated Monte Carlo model. This paper reports on the validation process of the in vivo whole body system and the CE calculated for different body heights and weights. The results reveal that the CE is strongly dependent on the individual body shape, size, and gender and may vary by a factor of 1.5 to 3 depending on the morphology aspects of the individual to be measured.

As part of an ongoing laboratory survey process improvement program, we evaluated the addition of water to liquid scintillation cocktail to improve wipe test counting efficiency. Both polar and non-polar 3 H and 14 C-labeled compounds were used as model contaminants. Our results support the recommendations in the literature regarding the addition ofwater to scintillation cocktail. We found an increase in the counting efficiency of the watersoluble material as a function ofwater content of the cocktail, but also observed a decrease in the efficiency of detection ofthe non-polar compound. The offsetting effects are believed to be the result of increased solubility of the polar compounds in water and increased quench of the already solubilized non-polar compound. The finding that adding water to the cocktail brought counting efficiencies of both polar and non-polar molecules to roughly the same value is novel and allows the use of a single quench curve for each radionuclide, regardless of chemical form.

Carbon cartridge standards were prepared to assess the activity of 125 I incident on, and adsorbed in, cartridge samples during air sampling. Each cartridge standard consisted of an 125 I-spiked filter paper at a known depth, ranging from 0 to 19 mm, embedded in approximately 34 g of 20-30 mesh activated carbon contained within a 6.35 cm diameter by 2.22 cm deep metal cartridge with screened openings. The total counting efficiency values range from 17.8 to 20.8% for cartridges counted at 3.2 mm from a thin-crystal NaI(TI) detector. The standards were analyzed using a front/back counting technique, and fitting functions were developed relating the front/back net counts ratio and counting efficiency to the 125 I depth of burial. A method for determining sample activity that accounts for exponential radioiodine loading in cartridge samples is compared to a less complicated technique that assumes all the radioiodine is located at an equivalent depth of burial that is based on the sample front/back net counts ratio. In addition, methods are presented for determining airborne 125 I activity for constant and variable concentrations. Variable concentrations are assumed to occur in a fume hood duct by one or more bulk releases as a result of iodinations that are performed during a given sampling interval. The two methods are shown to have maximum relative deviations ranging from -16 to +16%.

Gamma-ray spectral analyses with a 5-cm × 5-cm LaBr3(Ce) detector and a NaI(Tl) detector of the same size show that the LaBr3(Ce) has much better gamma-ray peak resolution and full-energy peak counting efficiency but worse detection sensitivity. The LaBr3(Ce) detector has relatively high intrinsic radiation background due to the naturally occurring La radioisotope in lanthanum. Although this La background is entirely below the energy of 1,500 keV, additional background is in the energy region between 1,500 keV and 2,750 keV. The manufacturer attributes this radiation to alpha particles emitted by the five short-lived progeny of an Ac impurity. Comparative values for peak resolution, full-energy peak counting efficiency, and detection sensitivity are reported for Am, Co, and Cs. Results of counting Cs sources at two activity levels demonstrate the impact of background on detection sensitivity.

The National Internal Radiation Assessment Section's Human Monitoring Laboratory (HML) has the responsibility to measure persons who may become internally contaminated following an accidental or intentional release of radioactivity. In preparation for measuring individuals who may be highly internally contaminated, the HML has reconfigured and recalibrated its whole body counter for this event. The calibration was performed using Monte Carlo simulations and validated by experimental measurements. An equation was developed that related the counting efficiency as a function of photon energy and phantom-to-detector distance. The equation could predict efficiencies to within 10% or better. Dead time problems, as a result of high internal activities, have been minimized by having a variety of counting positions. Six example nuclides have been used ( 57 Co, 60 Co, 88 Y, 133 Ba, 137 Cs, and 241 Am) to show what is achievable and what is not.

A new continuous air monitor (CAM) sampler for assessing alpha-emitting transuranic aerosol particles has been developed. The system has been designed to permit collection of particles that can potentially penetrate into the thoracic region of the human respiratory system. Wind tunnel testing of the sampler has been used to characterize the penetration of aerosol to the collection filter. Results show that greater than or equal to 50% of 10-micrograms aerodynamic equivalent diameter (AED) particles are collected by the filter at wind speeds of 0.3 to 2 m s-1 and at sampling flow rates of 28 to 113 L min-1 (1 to 4 cfm). The deposition of 10-microns AED particles takes place primarily in the center of the filter, where the counting efficiency of the detector is highest.

This paper describes the implementation of the NORMAN phantom with the Human Monitoring Laboratory's Monte Carlo simulator, the problems that were encountered, and their solution. The NORMAN phantom has been compared with the reference man BOMAB phantom in three different whole body counting geometries: a scanning detector system (WBC1), and two stand-up whole body counters (WBC2, WBC3) that have different reference points for their counting geometry. The average agreement (taken over all energies) of the two phantoms is approximately a factor of 1.15 on any given counting system. For the first two systems (WBC1, WBC2) the BOMAB has the highest counting efficiency, whereas it is reversed on the third system (WBC3). Considering the differences between the two phantoms, the agreement is good.

The HML has investigated the effect the uncertainty introduced into an activity estimate from a lung count due to 1) replicate counts and 2) lung set variability. Replicate counts in the HML seem to only be affected by random statistics as the uncertainty can be predicted by Monte Carlo simulations. These findings from the lung set variability experiments suggest that a lung set has an unquantified uncertainty on its activity that adds a component to the uncertainty on the counting efficiency, and ultimately the activity estimate, as they can differ by as much as 30% at 17.5 keV or about 13% at 185.7 keV, when one is expecting only a 3% difference.

The StandFast whole body counter has been modeled using Monte Carlo simulations to examine the effect of phantom size, photon energy, and position of the phantom within the counting enclosure on the counting efficiency. The first geometry, the manufacturer's recommended positioning, was found to have the higher counting efficiencies and the most dependence on phantom size. The second position, where the phantom is at the back of the counting enclosure, had lower counting efficiencies, and hence higher minimum detectable activities, by a factor of between 1.3 to 2.1 when compared with the first geometry; however, for emergency response where accuracy is to be preferred over sensitivity, this geometry would be the better choice. A unified calibration equation was also developed for the StandFast so that it is possible to predict the counting efficiency as a function of photon energy and size to within 11%.

The Human Monitoring Laboratory has found an alternate use for a hyperpure germanium field deployable instrument that was originally designed to be used in a search and identify mode for contraband radioactive material. The instrument, the Ortec Detective, becomes a fully functional spectroscopy system when connected to a laptop computer. In this configuration it can be used as a high-resolution portable whole body counter. This work has determined that the instrument has adequate sensitivity for emergency response with respect to fission and activation products, but not actinides. The use of Monte Carlo simulations has allowed the HML to calibrate the instrument, partially optimize the counting geometry, and develop a calibration curve that is a function of photon energy and a person's size. Similarly for thyroid counting, a function has been found that fits counting efficiency to a person's height. The MDA's are a few kilo Becquerels for fission and activation products for a 5-min count in an unshielded environment using a male subject.

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.

Monte Carlo simulation has been used to model the Human Monitoring Laboratory's scanning detector whole body counter. This paper has also shown that a scanning detector counting system can be satisfactorily simulated by putting the detector in different places relative to the phantom and averaging the results. This technique was verified by experimental work that obtained an agreement of 96% between scanning and averaging. The BOMAB phantom family in use at the Human Monitoring Laboratory was also modeled so that both counting efficiency and size correction factors could be estimated. It was found that the size correction factors lie in the region of 2.4 to 0.66 depending on phantom size and photon energy. The efficiency results from the MCNP scanning simulations were 97% of the measured scanning efficiency. A single function that fits counting efficiency, size, and photon energy was also developed. The function gives predicted efficiencies that are in the range of +10% to -8% of the true value.

An approach is outlined for the evaluation of the efficiency of whole body counters using experimental data for a point source and Monte Carlo simulation of the efficiency of photon arrival at a large volume NaI(Tl) detector. The approach was applied to the measurement of an experimental whole body phantom. The experimental and the predicted efficiencies were found to differ from -2.5% to 8.4%. An investigation of the geometry dependence of the counting efficiency was made using theoretical dimensions of phantoms of different sizes. The relative geometry dependence was found to be less than 9% for the "Reference Man" and "Reference 10-y-old Child" phantoms.

A new in vivo thyroid monitoring system was devised with radioluminography using an imaging plate. The counting features of the system were investigated by using a neck-thyroid phantom. A clear thyroid image was obtained at thin prethyroid tissue thicknesses and short distances between IP and neck surface. The characteristic 125 I thyroid image made it possible to visually confirm that the thyroid definitely accumulated 125 I. The counting efficiency in 100×cps Bq -1 of the imaging plate system was determined by setting a region of interest on the thyroid image resulting in a maximum efficiency of 0.5%. The higher efficiency was obtained for thinner tissue thickness overlaying the thyroid, smaller thyroids, and shorter distances between the imaging plate and neck surface. The imaging plate thyroid monitoring system was sensitive to thyroid 125 I. Its minimum detection limit of 23 Bq for a 10-min measurement was considerably lower than the annual thyroid burden limit of 300 kBq.

Information on particle size is one of the important factors for internal dose estimation at accidents with airborne radioactive materials. An autoradiography method has been investigated as a technique for the sizing of alpha-emitting particles. Concerning nuclear emergency medicine, the waiting time for dose estimation is limited. For determining the shortest estimation time, the exposure time of autoradiography was examined using PuO2 particles captured on HEPA filters. In this study, the effective counting efficiency of tracks produced by alpha particles was evaluated to be 0.31 by a numerical simulation. The minimum exposure time for PuO2 with aerodynamic diameter of 5 μm was found to be only 10 min. When any star-like alpha particle track was not detected after 6 h of exposure, even if the sample had significant radioactivity, the aerodynamic diameter was assumed to be less than 1 μm. When the radioactivity of PuO2 particles detected by autoradiography within 1 h was dominant of total activity, the aerodynamic diameter would be estimated to be over 5 μm. These results indicate that the precise dose estimation is useful for the decision of medical treatment.

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 reliability of calculating the full-energy peak counting efficiency of germanium detectors by Monte Carlo simulation was evaluated by comparing MCNP-4 code results with measurements over a range of conditions. The measurements were performed for two detectors of 20% and 110% nominal efficiencies with a point source at two distances and with four different volume sources, including a reentrant beaker, placed on the end caps. The radionuclides were National Institute of Standards and Technology standard sources that emit photons between energies of 42.8 and 1,596.4 keV. Each detector was modeled in detail with respect to the dimensions of the detection volume and attenuation material in the can, dead layer, and crystal holder foil. The measurements of the 42.8-keV photon were used to check dimensions and to change slightly the value of the dead-layer thickness so that the simulation agreed with measurements at this energy. After this adjustment, the average ratio of simulation results to measurements for 13 photon energies above 42.8 keV was between 0.97 and 1.03 for all sets of point and volume source comparisons. Ratios at individual energies were between 0.92 and 1.06 for the point source and between 0.94 and 1.09 for volume sources. The observed differences were consistent with the estimated standard deviations of simulation and of measurement, which ranged from 1 to 6% and typically were 2 to 3% except for larger uncertainties at low energies. On the basis of this comparison, simulation with the MCNP-4 code is equivalent to measurement with photon standards if the detector and source configuration can be modeled within a tenth of a millimeter.

This paper describes the design and testing of an alpha probe detector for the continuous measurement of the activity concentrations of alpha emitters in the culture media of in vitro cell suspension irradiation systems. The probe detector consists of a pen-size body housing a small silicon surface-barrier detector with a Mylar window. Theoretical calculations were performed to study the dependence of the alpha-energy spectrum on 1) the thickness of the Mylar barrier; 2) the Mylar-detector distance; and 3) the size of the detector window. These design parameters were selected by taking a compromise between the counting efficiency, the integrity of the detector, and its required range of application. The probe detector was tested using both chelated and unchelated 212Bi and 212Pb standard solutions; plate-out of these radionuclides on the Mylar barrier was observed for unchelated solutions. Alpha energy spectra were analyzed using a total integration technique. The measured activity concentrations and the calibrated values agree to within 4% for the chelated 212Bi and to within 6% for the unchelated 212Bi. The alpha probe detector can be used throughout an entire exposure time period to determine the total dose received by suspended cells, or at different time intervals to determine the dose rate in real time.

A standard platinum-iridium needle that contains 374 MBq 226Ra was tested as a source for calibrating a portable germanium detector used with a gamma-ray spectrometer for environmental radioactivity measurements. The counting efficiencies of the 11 most intense gamma rays emitted by 226Ra and its short-lived radioactive progeny at energies between 186 and 2,448 keV were determined, at the full energy peaks, to construct a curve of counting efficiency vs. energy. The curve was compared to another curve between 43 and 1,596 keV obtained with a NIST mixed-radionuclide standard. It was also compared to the results of a Monte Carlo simulation. The 226Ra source results were consistent with the NIST standard between 248 and 1,596 keV. The Monte Carlo simulation gave a curve parallel to the curve for the combined radium and NIST standard data between 250 and 2,000 keV, but at higher efficiency.

This note discusses the use of blank or background counting data that are measured for times that differ from times used for the sample counts. The correct formula for the minimum detectable activity, under this condition, is given as follows: MDA = [3 + 3.29 square root of Rbtg(1 + tg/tb)]/epsilon tg, where Rb denotes background count rate, tb and tg denote background and gross count times, and epsilon denotes counting efficiency. Counting backgrounds for a long time reduces decision levels, uncertainties, and minimum detectable activities. These benefits are fully available only when there is no other source of variability than random fluctuations in count rates.

An array of four large-volume coaxial germanium detectors was installed in a shielded room for the in vivo measurement of gamma emitters in the body. All four detectors are in a single 50-L dewar of liquid nitrogen and are positioned underneath the supine subject with the axes of the detector elements parallel with the length of the body. This orientation gives approximately 15% more counting efficiency than having the axes of the detectors perpendicular to the body length. The detectors have resolutions that are less than 2 keV with an average of 1.86 keV for the 60Co 1.33-MeV gamma-ray peak. The efficiencies of the four detectors range from 59% to 77% of a 7.62- x 7.62-cm (3- x 3-in.) NaI (Tl) detector. The detectors are interfaced to a multichannel analyzer such that spectra from the detectors are stored both as a sum of all counts from the four detectors and as a spectrum from each detector. The sensitivity of the germanium array for measuring radioactivity in the body exceeds that of multiple large NaI (Tl) detectors for measuring single radionuclides in the body and is much superior for measuring internal depositions of radionuclide mixtures. Another major advantage of this system compared to NaI(Tl) systems is that the results can be calculated by computer analysis with minimal human interpretation. The sensitivity of the array configuration is described and applications of the array in other configurations, such as that used for measuring the very small quantities of 24Na induced in the body during air travel, are also discussed.

This paper describes the counting efficiency and detection limit of a thyroidal 125I monitoring system. Two systems were used: (1) M1 was composed simply of one survey meter for 125I (SM) having an NaI (Tl) crystal of 5.08-cm diameter by 5-mm thick, and (2) M2 was composed of the SM having an output terminal for spectroscopy and a multichannel pulse height analyzer. The counting efficiency was determined by using four simulated thyroids of 17, 20.5, 31, and 40 mL containing 125I solution. The simulated thyroids were embedded in an anthropomorphic neck phantom. The counting efficiency between 0 degrees and 45 degrees to the normal of the thyroid (front of the neck) changed by +/-2%. The efficiency of M1 ranged from 1.8 to 7.9% as the distance between the probe and the neck increased from 0 to 5 cm. Similarly, the efficiency of M2 ranged from 2.2 to 8.3%. The detection limit ranged from 7 to 34 Bq for M1 and from 1 to 5.1 Bq for M2. The M2 system was applied to monitoring a worker performing iodination with 74 MBq Na 125I. Both monitoring systems provided the necessary sensitivity to detect thyroidal 125I within the uncertainty of +/-10%.

A complete system is described for electronically processing and analyzing metabolic data which are used to estimate radiation protection guides for man. Counting data for multichannel liquid scintillation detectors, together with experimental information which is manually dialed into the punch control unit of the counter, are automatically punched onto paper tape or IBM data cards. The cards comprise the input for a computer program (RTW) which will c~culate elapsed time between radionuclide administration and time of measurement, correct all data for changes in counting efficiency relative to measurements made on standards at zero time and for physical decay, if desired, and express the observed counting information as effective or biological retention at any time after the initial measurement. Portions of the RTW program output are on IBM data cards in a format compatible with the input requirements for a least squares computing method programmed for the IBM 7090 which, in turn, will derive the parameters of the function which best fit the data. Lastly, a General-Dynamics Model SC-4020 tape-to-film device is used to produce on 35-mm film a plot of the observed retention data, the fitted curve, and ancillary labeling statements. (auth)