Monte Carlo simulation of enriched uranium in the lungs of thorax voxel phantom for assessment of enrichment and its effect on dose.

In-vivo lung monitoring is an important technique for the assessment of internal dose of radiation workers handling actinides. At BARC, counting efficiencies (CEs) of detection systems used for estimation of natural uranium in the lungs are evaluated using realistic thorax physical phantoms or computational voxel phantoms. The quantification of 238U and 235U in lungs is done using CEs determined at 63.3 keV and 185.7 keV photon energies respectively. These CEs can also be used for assessment of enriched uranium in the lungs of the workers. In this study, spectra are generated for HPGe array detectors using Monte Carlo simulations of various enriched uranium compositions distributed in the lungs of thorax voxel phantom. A methodology is developed to predict the 235U enrichment from lung spectrum analysis using the ratio of net counts in 185.7 keV and 63.3 keV energy regions. It is possible to estimate enrichments in the range of 2%-30% using the developed method with less than ±9% error. Finally, effect of 235U enrichment on dose assessment using lung monitoring method is studied.

Evaluation of Electron Specific Absorbed Fractions in Organs of Digimouse Voxel Phantom Using Monte Carlo Simulation Code FLUKA.

BACKGROUND: For preclinical evaluations of radiopharmaceuticals, most studies are carried out on mice. Electron specific absorbed fractions (SAF) values have had vital role in the assessment of absorbed dose. In past studies, electron SAFs were given for limited source target pairs using older reports of human organ compositions. OBJECTIVE: Electron specific absorbed fraction values for monoenergetic electrons of energies 15, 50, 100, 500, 1000 & 4000 keV were evaluated for the Digimouse voxel phantom incorporated in Monte Carlo code FLUKA. From the latest report (International Commission on Radiological Protection ICRP) 110, organ compositions and densities were adopted. MATERIAL AND METHODS: We have used the Digimouse voxel phantom which was incorporated in Monte Carlo code FLUKA. Simulation studies were performed using FLUKA. The organ sources considered in this study were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal, eye and brain. The considered target organs were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal and brain. Eye and brain were considered as target organs only for eye and brain as source organs. RESULTS: The electron SAF values for self-irradiation decreases with increasing electron energy. The electron SAF values for cross-irradiation are also found to be dependent on the electron energy and the geometries of source and target. Organ masses and electron SAF values are presented in tabular form. CONCLUSION: The results of this study will be useful in evaluating the absorbed dose to various organs of mice similar in size to the present study.

Effective dose conversion coefficient for gamma ray exposure from an overhead plume.

The external radiation exposure from an overhead plume containing gamma emitting radionuclides can contribute substantial dose to the ground receptor during normal operations as well as accidental release conditions of nuclear facilities. In order to estimate the effective dose conversion coefficients (DCCs) directly, a finite plume Monte Carlo model along with the reference phantom at the ground receptor location needs to be implemented. In the present study, a comprehensive simulation of radiation transport from the Gaussian plume source to the ICRP reference adult voxel phantoms (receptor) is carried out using the FLUKA Monte Carlo code. The organ absorbed doses as well as the effective DCCs of the adult reference phantom are computed for different meteorological parameters and downwind distances. To illustrate the application of this model, an overhead Gaussian plume containing two different gamma emitting radionuclides, 135Xe and 41Ar are considered. From these simulations, the ratio of the effective dose rate to the kerma rate are estimated as 0.6 Sv Gy-1 and 0.65 Sv Gy-1 for the exposure from 135Xe and 41Ar, respectively. This ratio is constant irrespective of the meteorological conditions and cloud models. Further results show that the effective DCCs as a function of the downwind distance vary by an order of magnitude for an unstable weather category; however, the variations are very small in the case of a stable category. This study demonstrates an accurate method for calculating the effective dose to the ground receptor from an external plume which can be further applied for any radionuclide under any meteorological condition.

COMPARING LUNGS, LIVER AND KNEE MEASUREMENT GEOMETRIES AT VARIOUS TIMES POST INHALATION OF 239Pu AND 241Am.

In-vivo measurement of Pu/241Am in workers is carried out by placing suitable detector above lungs, liver and skeleton, as major fraction of Pu/Am is transferred to liver and skeleton, after its retention in entry organ. In this work, committed effective dose (CED) corresponding to minimum detectable activity for Type M and Type S 239Pu/241Am deposited in these organs are presented and a monitoring protocol of organ measurement giving lowest CED at different time intervals post inhalation is described. We have observed, for Type M compounds, lung measurement is most sensitive method during initial days after exposure. Liver measurement yields lowest CED between 100 and 5000 d and beyond that bone measurement gives lowest CED. For Type S compounds lung measurement remains most sensitive method even up to 10 000 d post inhalation. This study will be useful for the assessment of CED due to internally deposited 239Pu/241Am in the workers.

Estimation of Photon Specific Absorbed Fractions in Digimouse Voxel Phantom using Monte Carlo Simulation Code FLUKA.

BACKGROUND: Most preclinical studies are carried out on mice. For internal dose assessment of a mouse, specific absorbed fraction (SAF) values play an important role. In most studies, SAF values are estimated using older standard human organ compositions and values for limited source target pairs. OBJECTIVE: SAF values for monoenergetic photons of energies 15, 50, 100, 500, 1000 and 4000 keV were evaluated for the Digimouse voxel phantom incorporated in Monte Carlo code FLUKA. The organ sources considered in this study were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal, eye and brain. The considered target organs were lungs, skeleton, heart, bladder, testis, stomach, spleen, pancreas, liver, kidney, adrenal and brain. Eye was considered as a target organ only for eye as a source organ. Organ compositions and densities were adopted from International Commission on Radiological Protection (ICRP) publication number 110. RESULTS: Evaluated organ masses and SAF values are presented in tabular form. It is observed that SAF values decrease with increasing the source-to-target distance. The SAF value for self-irradiation decreases with increasing photon energy. The SAF values are also found to be dependent on the mass of target in such a way that higher values are obtained for lower masses. The effect of composition is highest in case of target organ lungs where mass and estimated SAF values are found to have larger differences. CONCLUSION: These SAF values are very important for absorbed dose calculation for various organs of a mouse.

Monte Carlo simulation of skull and knee voxel phantoms for the assessment of skeletal burden of low-energy photon emitters.

In case of internal contamination due to long-lived actinides by inhalation or injection pathway, a major portion of activity will be deposited in the skeleton and liver over a period of time. In this study, calibration factors (CFs) of Phoswich and an array of HPGe detectors are estimated using skull and knee voxel phantoms. These phantoms are generated from International Commission of Radiation Protection reference male voxel phantom. The phantoms as well as 20 cm diameter phoswich, having 1.2 cm thick NaI (Tl) primary and 5cm thick CsI (Tl) secondary detector and an array of three HPGe detectors (each of diameter of 7 cm and thickness of 2.5 cm) are incorporated in Monte Carlo code 'FLUKA'. Biokinetic models of Pu, Am, U and Th are solved using default parameters to identify different parts of the skeleton where activity will accumulate after an inhalation intake of 1 Bq. Accordingly, CFs are evaluated for the uniform source distribution in trabecular bone and bone marrow (TBBM), cortical bone (CB) as well as in both TBBM and CB regions for photon energies of 18, 60, 63, 74, 93, 185 and 238 keV describing sources of (239)Pu, (241)Am, (238)U, (235)U and (232)Th. The CFs are also evaluated for non-uniform distribution of activity in TBBM and CB regions. The variation in the CFs for source distributed in different regions of the bones is studied. The assessment of skeletal activity of actinides from skull and knee activity measurements is discussed along with the errors.

Counting efficiency of whole-body monitoring system using BOMAB and ANSI/IAEA thyroid phantom due to internal contamination of 131I.

In this study, the effect of Indian reference BOttle MAnnikin aBsorber (BOMAB) neck with axial cavity and American National Standards Institute (ANSI)/International Atomic Energy Agency (IAEA) thyroid phantom using pencil sources of (133)Ba ((131)I simulant) on counting efficiency (CE) is seen experimentally in static geometry for whole-body monitoring system comprising 10.16-cm diameter and 7.62-cm-thick NaI(Tl) detector. The CE estimated using the neck part of BOMAB phantom is 50.2% lower in comparison with ANSI phantom. In rest of the studies FLUKA code is used for Monte Carlo simulations using ANSI/IAEA thyroid phantom. The simulation results are validated in static geometries with experimental CE and the differences are within 1.3%. It is observed that CE for pencil source distribution is 3.97% higher for (133)Ba in comparison with CE of (131)I source. Simulated CE for pencil source distribution is 4.7% lower in comparison with uniform source distribution in the volume of thyroid for (131)I. Since the radiation workers are of different physique; overlying tissue thickness (OTT) and neck-to-detector distance play an important role in the calculation of activity in thyroid. The CE decreases with increase in OTT and is found to be 5.5% lower if OTT is changed from 1.1 to 2 cm. Finally, the simulations are carried out to estimate the variation in CE due to variation in the neck-to-detector distance. The CE is 6.2% higher if the neck surface-to-detector distance is decreased from 21.4 to 20.4 cm and it goes on increasing up to 61.9% if the distance is decreased to 15.4 cm. In conclusion, the calibration of whole-body monitoring system for (131)I should be carried out with ANSI/IAEA thyroid phantom, the neck-to-detector distance controlled or the CE corrected for this, and the CE should be corrected for OTT.

Monte Carlo simulation of NaI(TL) detector in a shadow-shield scanning bed whole-body monitor for uniform and axial cavity activity distribution in a BOMAB phantom.

This study presents the simulation results for 10.16 cm diameter and 7.62 cm thickness NaI(Tl) detector response, which is housed in a partially shielded scanning bed whole-body monitor (WBM), due to activity distributed in the axial cavities provided in the Indian reference BOMAB phantom. Experimental detection efficiency (DE) for axial cavity activity distribution (ACAD) in this phantom for photon emissions of (133)Ba, (137)Cs and (60)Co is used to validate DEs estimated using Monte Carlo code FLUKA. Simulations are also carried out to estimate DEs due to uniform activity distribution (UAD) as in the standard BOMAB phantom. The results show that the DE is ∼3.8 % higher for UAD when compared with ACAD in the case of (40)K (1460 keV) and this relative difference increases to ∼7.0 % for (133)Ba (∼356 keV) photons. The corresponding correction factors for calibration with Indian phantom are provided. DEs are also simulated for activity distributed as a planar disc at the centre of the axial cavity in each part of the BOMAB phantom (PDAD) and the deviations of these DEs are within 1 % of the ACAD results. Thus, PDAD can also be used for ACAD in scanning geometry. An analytical solution for transmitted mono-energetic photons from a two-dimensional slab is provided for qualitative explanation of difference in DEs due to variation in activity distributions in the phantom. The effect on DEs due to different phantom part dimensions is also studied and lower DEs are observed for larger parts.

Monte Carlo simulation of embedded 241Am activity in injured palm.

This paper describes a methodology to estimate embedded activity of (241)Am and Pu isotopes in a wound at an unknown depth. Theoretical calibration of an array of high-purity germanium detectors is carried out using the Monte Carlo code 'FLUKA' for a (241)Am source embedded at different depths in a soft tissue phantom of dimension 10 × 10 × 4 cm(3) simulating the palm of a worker. It is observed that, in the case of contamination due to pure (241)Am, the ratio of counts in 59.5 and 17.8 keV (Ratio 1) should be used to evaluate the depth, whereas the ratio of counts in 59.5 and 26.3 keV (Ratio 2) should be used when the contamination is due to a mixture of Pu and (241)Am compounds. Variations in the calibration factors (CFs) as well as in the Ratio 1 and Ratio 2 values are insignificant when source dimensions are varied from a point source to a 15-mm diameter circle. It is observed that tissue-equivalent polymethyl methacrylate material can be used in the phantom to estimate the embedded activity, when the activity is located at a depth of <1 cm, as the corresponding CFs do not show much variation with respect to those estimated using the phantom containing soft tissue material. In all other cases, an appropriate soft tissue-equivalent material should be used in the phantom for the estimation of CFs and ratios. The CFs thus obtained will be helpful in an accurate estimation of the depth of the wound and the activity embedded therein in the palm of a radiation worker.

Estimation of specific absorbed fractions for selected organs due to photons emitted by activity deposited in the human respiratory tract using ICRP/ICRU male voxel phantom in FLUKA.

The ICRP/ICRU adult male reference voxel phantom incorporated in Monte Carlo code FLUKA is used for estimating specific absorbed fractions (SAFs) for photons due to the presence of internal radioactive contamination in the human respiratory tract (RT). The compartments of the RT, i.e. extrathoracic (ET1 and ET2) and thoracic (bronchi, bronchioles, alveolar interstitial) regions, lymph nodes of both regions and lungs are considered as the source organs. The nine organs having high tissue weighting factors such as colon, lungs, stomach wall, breast, testis, urinary bladder, oesophagus, liver and thyroid and the compartments of the RT are considered as target organs. Eleven photon energies in the range of 15 keV to 4 MeV are considered for each source organ and the computed SAF values are presented in the form of tables. For the target organs in the proximity of the source organ including the source organ itself, the SAF values are relatively higher and decrease with increase in energy. As the distance between source and target organ increases, SAF values increase with energy and reach maxima depending on the position of the target organ with respect to the source organ. The SAF values are relatively higher for the target organs with smaller masses. Large deviations are seen in computed SAF values from the existing MIRD phantom data for most of the organs. These estimated SAF values play an important role in the estimation of equivalent dose to various target organs of a worker due to intake by inhalation pathway.

Monte Carlo calculations for efficiency calibration of a whole-body monitor using BOMAB phantoms of different sizes.

Internal contamination due to high-energy photon (HEP) emitters is assessed using a scanning bed whole-body monitor housed in a steel room at the Bhabha Atomic Research Centre (BARC). The monitor consists of a (203 mm diameter × 102 mm thickness) NaI(Tl) detector and is calibrated using a Reference BOMAB phantom representative of an average Indian radiation worker. However, a series of different size physical phantoms are required to account for size variability in workers, which is both expensive and time consuming. Therefore, a theoretical approach based on Monte Carlo techniques has been employed to calibrate the system in scanning geometry with BOMAB phantoms of different sizes characterised by their weight (W) and height (H) for several radionuclides of interest ((131)I, (137)Cs, (60)Co and (40)K). A computer program developed for this purpose generates the detector response and the detection efficiencies (DEs) for the BARC Reference phantom (63 kg/168 cm), ICRP Reference male phantom (70 kg/170 cm) and several of its scaled versions. The results obtained for different size phantoms indicated a decreasing trend of DEs with the increase in W/H values of the phantoms. The computed DEs for uniform distribution of (137)Cs in BOMAB phantom varied from 3.52 × 10(-3) to 2.88 × 10(-3) counts per photon as the W/H values increased from 0.26 to 0.50. The theoretical results obtained for the BARC Reference phantom have been verified with experimental measurements. The Monte Carlo results from this study will be useful for in vivo assessment of HEP emitters in radiation workers of different physiques.

Selected organ dose conversion coefficients for external photons calculated using ICRP adult voxel phantoms and Monte Carlo code FLUKA.

The adult reference male and female computational voxel phantoms recommended by ICRP are adapted into the Monte Carlo transport code FLUKA. The FLUKA code is then utilised for computation of dose conversion coefficients (DCCs) expressed in absorbed dose per air kerma free-in-air for colon, lungs, stomach wall, breast, gonads, urinary bladder, oesophagus, liver and thyroid due to a broad parallel beam of mono-energetic photons impinging in anterior-posterior and posterior-anterior directions in the energy range of 15 keV-10 MeV. The computed DCCs of colon, lungs, stomach wall and breast are found to be in good agreement with the results published in ICRP publication 110. The present work thus validates the use of FLUKA code in computation of organ DCCs for photons using ICRP adult voxel phantoms. Further, the DCCs for gonads, urinary bladder, oesophagus, liver and thyroid are evaluated and compared with results published in ICRP 74 in the above-mentioned energy range and geometries. Significant differences in DCCs are observed for breast, testis and thyroid above 1 MeV, and for most of the organs at energies below 60 keV in comparison with the results published in ICRP 74. The DCCs of female voxel phantom were found to be higher in comparison with male phantom for almost all organs in both the geometries.

Monte-Carlo simulation of uncertainty in the estimation of 125I in the thyroid.

At the Bhabha Atomic Research Centre, a thin (76 mm diameter x 2 mm thickness) NaI (Tl) detector is used for the assessment of (125)I in the thyroid of the radiation workers engaged in the preparation of radio-immunoassay kits. The detector was calibrated using a REMCAL (radiation equivalent manikin calibration) phantom with a known amount of the (125)I activity filled in its thyroidal cavity. Since (125)I emits low-energy photons ranging from 27 to 35.4 keV, its detection efficiency depends on several parameters such as neck-to-detector distance, detector size, unknown tissue thickness overlying (OTT) the thyroid and the shape and size of the thyroid. To account for uncertainties introduced by these factors in the estimation of (125)I, a computer program based on the Monte Carlo photon transport techniques was developed. The program simulates the detector response and the corresponding detection efficiencies using two thyroid models: (1) revised MIRD head phantom and (2) Ulanvosky model. The program has been validated with experimental measurements carried out using a REMCAL phantom. The computed values of uncertainties due to placement errors (+/-0.5 cm) for different detector sizes, differences in the OTT of the thyroid (0.6-2.0 cm) and different thyroid shapes are presented in this paper. The computed values of the calibration factors, determined for the revised MIRD phantom, varied from 5.23 to 1.06 x 10(-2) counts per photon for detector distance of 3-12 cm and from 7.53 to 3.66 x 10(-2) counts per photon for OTT varying from 0.6 to 2.0 cm keeping the detector at a distance of 3 cm. This study shows that the variations in OTT constitute a major source of uncertainty. The computed uncertainties due to various parameters should be taken into account while estimating the thyroidal burden of (125)I in the radiation workers. The feasibility of using coincidence method for absolute determination of the (125)I activity in the thyroid is also discussed in this paper.