Dd Rao

Application of Cherenkov radiation counting for determination of 90Sr in environmental samples.

Abstract Cherenkov radiation produced by the hard β -emitter 90Y was measured by using a commercial liquid scintillation analyser (LSA) for determination of 90Sr in environmental samples. The conventionally used chemical procedures for separation of 90Sr were simplified and a non-destructive Cherenkov assay method (NDCA) was developed. The analytical procedures and measurement techniques were tested by participating in the quality assurance programme conducted by the International Reference Centre (IRC, France) for the determination of 90Sr in diet and IAEA soil samples. The paper presents the methodologies developed for determination of 90Sr in several matrices such as fresh water, soil, grass, seaweed, diet and air filter samples. The results were compared with those by the conventional nitrate method, showing an excellent agreement.

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

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

Measurement of High Energy Gross Beta and 40K by Cerenkov Counting in Liquid Scintillation Analyser

The Environmental Survey Laboratory is responsible for the monitoring of radioactivity due to natural and artificially produced radionuclides in a variety of samples available around the nuclear facilities at Tarapur. Standard methods of radiochemical separation and counting are followed to determine β-emitters in various matrices. To make the measurement simple and detect lower levels of contamination in some of the matrices, a method was developed to measure the high energy gross β-activity primarily due to 90Y in borehole water and urine samples of occupational workers by Cerenkov counting in a liquid scintillation analyser. Cerenkov counting results of borehole samples were compared with the total gross β-activity measured by a low background beta-counter. The combined measurements were used to evaluate 90Sr and 137Cs levels without chemical separation or gamma-spectrometry. Cerenkov counting technique was also utilised to evaluate 40K in drinking water and intercomparison water samples. The paper presents the methodology and results of a few measurements using the technique.

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.

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.

Detection of low level gaseous releases and dose evaluation from continuous gamma dose measurements using a wavelet transformation technique.

Measurement of environmental dose in the vicinity of a nuclear power plant site (Tarapur, India) is carried out continuously for the years 2007-2010 and attempts have been made to quantify the additional contributions from nuclear power plants over natural background by segregating the background fluctuations from the events due to plume passage using a non-decimated wavelet approach. A conservative estimate obtained using wavelet based analysis has shown a maximum annual dose of 38 μSv in a year at 1.6 km and 4.8 μSv at 10 km from the installation. The detected events within a year are in good agreement with the month wise wind-rose profile indicating reliability of the algorithm for proper detection of an event from the continuous dose rate measurements. The results were validated with the dispersion model dose predictions using the source term from routine monitoring data and meteorological parameters.

Optimization of parameters of alpha spectrometry with silicon detector for low level measurements of actinides in environmental samples

Determination of actinides in environmental and biological samples is an important activity of radiation protection program at nuclear energy facilities. High resolution alpha spectrometry with passivated ion implanted Silicon detectors is widely used for the determination of actinides concentration. Low levels of activity concentrations in these samples often require long counting duration of a few days to obtain accurate and statistically significant data for further impact assessment. In alpha spectrometry, the chamber in which Si detector operated is a critical component and maintained at a desired vacuum for minimizing the alpha particle attenuation. Experimental evaluation of variations in energy resolution and tailing of alpha spectra was investigated under different chamber air pressures from about 6.7 Pa to more than 2700 Pa under the chamber hold mode and pump electrically switched off conditions. As part of validation, data collected on an IAEA inter-comparison exercise sample are presented under short and long counting durations with pump operating and switched off conditions respectively. It has been observed that the FWHM values do not significantly degrade, to impact the low and medium level concentration alpha spectra, for variations in vacuum chamber pressures from about 6.7 Pa to 2700 Pa.

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.

Creatinine as a normalization factor to estimate the representativeness of urine sample - Intra-subject and inter-subject variability studies

In-vitro bioassay monitoring generally involves analysis of overnight urine samples (~12 h) collected from radiation workers to estimate the excretion rate of radionuclides from the body. The unknown duration of sample collection (10-16 h) adds to the overall uncertainty in computation of internal dose. In order to minimize this, IAEA recommends measurement of specific gravity or creatinine excretion rate in urine. Creatinine is excreted at a steady rate with normally functioning kidneys therefore, can be used as a normalization factor to infer the duration of collection and/or dilution of the sample, if any. The present study reports the chemical procedure standardized and its application for the estimation of creatinine as well as creatinine co-efficient in normal healthy individuals. Observations indicate higher inter-subject variability and lower constancy in daily excretion of creatinine for the same subject. Thus creatinine excretion rate may not be a useful indicator for extrapolating to 24 h sample collection.

A rapid method for estimation of Pu-isotopes in urine samples using high volume centrifuge

The conventional radio-analytical technique used for estimation of Pu-isotopes in urine samples involves anion exchange/TEVA column separation followed by alpha spectrometry. This sequence of analysis consumes nearly 3-4 days for completion. Many a times excreta analysis results are required urgently, particularly under repeat and incidental/emergency situations. Therefore, there is need to reduce the analysis time for the estimation of Pu-isotopes in bioassay samples. This paper gives the details of standardization of a rapid method for estimation of Pu-isotopes in urine samples using multi-purpose centrifuge, TEVA resin followed by alpha spectrometry. The rapid method involves oxidation of urine samples, co-precipitation of plutonium along with calcium phosphate followed by sample preparation using high volume centrifuge and separation of Pu using TEVA resin. Pu-fraction was electrodeposited and activity estimated using 236Pu tracer recovery by alpha spectrometry. Ten routine urine samples of radiation workers were analyzed and consistent radiochemical tracer recovery was obtained in the range 47-88% with a mean and standard deviation of 64.4% and 11.3% respectively. With this newly standardized technique, the whole analytical procedure is completed within 9h (one working day hour).