Numerical estimation of calibration matrices for 241Am measurements using HPGe based in-vivo monitoring system.

In-vivo measurements of 241Am using HPGe detectors become complicated when the active adjacent source organs interfere with the target organ measurements. It is important to calculate the contribution of confounding organs to estimate the activity of the target organ accurately. In the current study, numerical simulations were performed using FLUKA Monte Carlo code and International Commission on Radiological Protection (ICRP) computational reference phantoms to determine the calibration matrices consisting of the calibration and cross-talk coefficients for three interfering organs namely, lungs, liver and skeleton. It was found that the interference from adjacent organs contaminated with 241Am was found to be significant in the case of lungs and liver. Knee monitoring was least influenced by the activity possessed by other source organs due to their anatomical distance from the knee. A comparison between lung and liver coefficients obtained from the Lawrence Livermore National Laboratory (LLNL) torso phantom and thorax phantom derived from ICRP adult male voxel phantom was performed. It was found that variations in coefficients obtained from simulations and experiments range between 2% and 48%. The differences were attributed to the uncertainties arising from the composition of the phantoms and detectors, size and shape of organs, positional errors, and source distribution. A comparison of calibration matrices of adult male and adult female thorax voxel phantoms revealed that all the coefficients except knee as the target organ were larger for the female thorax phantom owing to the lesser bulk of attenuating tissues on its chest. The coefficients obtained from simulations for different phantoms also showed that the organ activity estimation can be significantly affected by the subject morphology.

Determination of calibration matrix for HPGe-based in vivo monitoring system for estimation of 241Am in lungs and liver.

The detection of internal contamination may be carried out by direct or indirect methods. The lung counting technique using an array of High Purity Germanium (HPGe) detectors is one of such direct detection methods. It is known from the literature that the estimation of activity by organ counting can lead to erroneous results if an amount of activity is possessed by an adjacent organ. In the case of HPGe-based lung monitoring, the estimation could be misleading if the activity is possessed by the liver, which is a proximal organ. In such cases the measured activity should be modified using cross-talk coefficients which account for the contribution from adjacent organs. The determination of cross-talk coefficients for 241Am was carried out by placing the detectors over inactive lungs of an Lawrence Livermore National Laboratory phantom when the source activity was contained in the liver and vice versa. A calibration matrix was formulated with calibration coefficients as diagonal elements and cross-talk coefficients as off-diagonal elements. The measured activities may be modified by matrix multiplication with the inverse of the calibration matrix to nullify the contribution from adjacent organs. The current work has empirically determined the fitting equations which relate calibration and cross-talk coefficients for lungs and liver measurement geometries with muscle-equivalent chest wall thickness (MEQ-CWT) values. The values of these coefficients were determined for an average MEQ-CWT of 1.77 cm for lungs and 1.33 cm for liver. The calculations showed that the activity contribution from liver to lungs was 29% higher than that of lungs to liver. A verification exercise was conducted to demonstrate this method. For the given calibration source, the percent overestimation was reduced for lung activity, while the liver activity was slightly underestimated. In the case of old exposure follow-up monitoring cases, the 241Am activity built up in liver could interfere with the lung monitoring results and this method using the calibration matrix may be used for estimation of more accurate results.

Studies on pelletised lithium magnesium borate TL material for eye lens dosimetry.

Recent epidemiological studies in various cohorts confirm that radiation induced cataract may occur at a threshold dose as low as 0.5 Gy. ICRP has recognised the higher radiosensitivity of eye lens and recommended to reduce annual eye lens dose limit from 150 to 20 mSv. Present dosimetry for eye lens is largely based on LiF based dosimeters. The present work is an attempt towards using near tissue equivalent lithium borate material namely lithium magnesium borate doped with terbium (LMB:Tb) in eye lens dosimetry. The material in powder form was synthesised using solid state sintering method and pelletised using poly tetra fluoro ethylene (PTFE) as binding agent. It was observed that 130 mg of 1 mm thick LMB:Tb pellet bound with PTFE showed stable glow curve structure and attractive dosimetric features in terms of sensitivity, fading, linearity, reusability etc. A proposal of 1.5 mm Teflon encapsulation corresponding to tissue equivalent thickness of 3 mm was put forward for an ideal dosimeter for the measurement in terms of Hp(3). Energy and angular dependence studies based on FLUKA simulations suggest a flat response for the prototype design. In addition to PTFE, various tissue equivalent encapsulations such as polyimide, polyamide 6 and PMMA were also evaluated using FLUKA code.