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In this study the effect of detector aging in terms of increased dead layer thickness on detector efficiency has been studied using the Geant4 toolkit. Variation of energy deposition in the detector dead layer with the dead layer thickness has been quantified for various values of incident g-ray energy considering point isotropic as well as extended sources including the circular disk source and cylindrical volume sources. For the point isotropic source, the Geant4 computed values of energy loss per particle in the dead layer are found in good agreement with the corresponding published results with maximum deviation remaining below 2 %. New results for dependence of geometric, full-energy peak and total efficiency on dead layer thickness have been studied using Geant4 simulations for various values of g-ray energy, and for point isotropic and extended sources at various axial and lateral positions. These simulations yield an exponentially decreasing profile of detector aging sensitivity with an increase in g-ray energy for point isotropic, circular disk and cylindrical volume sources highlighting a larger decrease in efficiency due to aging for low energy photons.

In this work, comparative study on energy dependence of absorbed, intrinsic, photo-peak and absolute total efficiency of polystyrene plastic scintillation fiber and polyvinyl-toluene detectors with NaI(Tl) scintillation detectors has been performed using Geant4 version 9.6 toolkit. The effects of geometry parameters on various efficiencies were investigated by varying detector radii, thickness and various source-to-detector configurations. These studies were carried out for both cylindrical and slab geometries for photon energy range of 10 keV-20 MeV using point isotropic sources and parallel beams of photons. Comparisons of the Geant4.9.6 based simulations for polystyrene scintillation fiber intrinsic efficiency as a function of photon energy and corresponding results obtained by earlier versions Geant4 (version 5.1) and Geant4 (version 8.1) show good agreements. The variation of the intrinsic efficiency with energy for polyvinyltoluene is also found to match very well with respective earlier results. This work confirms that the plastic scintillator based fibers and slab detectors are suitable for X-ray and low energy g-ray applications with energies typically below 50 keV with the optimum length of polystyrene scintillation fiber equal to 10 cm. For high energy range, cross talk remains an issue for polystyrene scintillation fiber and it is prominent in fibers having longer lengths and small diameters. Also, until the fiber radius is smaller than the incident photon beam, the fiber intrinsic efficiency increases with an increase in the radius.

The energy dependence on the intrinsic efficiency, absolute efficiency, full energy peak absolute efficiency and peak-to-total ratio have been studied for various wide band gap semiconductor detectors using the Geant4 based Monte Carlo simulations. The detector thickness of 1-4 mm and the area in 16-100 mm2 range were considered in this work. In excellent agreement with earlier work (Rybka et al., [20]), the Geant4 simulated values of detector efficiencies have been found to decrease with incident g-ray energy. Both for the detector thickness and the detector area, the increasing trends have been observed for total efficiency as well as for full-energy peak efficiency in 0.1 MeV-50 MeV range. For Cd1-xZnxTe, the detector response remained insensitive to changes in relative proportions of Zn. For various wide band gap detectors studied in this work, the detection efficiency of TlBr was found highest over the entire range of energy, followed by the HgI2, CdTe, and then by CZT.

The Geant4-based comprehensive model has been developed to predict absorbed fraction values for both electrons and gamma photons in spherical, ellipsoidal, and cylindrical geometries. Simulations have been carried out for water, ICRP soft-, brain-, lung-, and ICRU bone tissue for electrons in 0.1 MeV-4 MeV and g-photons in the 0.02 MeV-2.75 MeV energy range. Consistent with experimental observations, the Geant4-simulated values of absorbed fractions show a decreasing trend with an increase in radiation energy. Compared with NIST XCOM and ICRU data, the Geant4-based simulated values of the absorbed fraction remain within a 4.2% and 1.6% deviation, respectively. For electrons and g-photons, the relative difference between the Geant4-based comprehensive model predictions and those of Stabin and Konijnenberg's re-evaluation remains within a 6.8% and 7.4% range, respectively. Ellipsoidal and cylindrical models show 4.9% and 10.1% higher respective values of absorbed dose fractions relative to the spherical model. Target volume dependence of the absorbed fraction values has been found to follow a logical behavior for electrons and Belehradek's equation for g-photons. Gamma-ray absorbed fraction values have been found to be sensitive to the material composition of targets, especially at low energies, while for elections, they remain insensitive to them.

This work is a study of changes in coolant activity due to corrosion products of extended burn-up cycles of 18-24 months duration, in a typical pressurized water reactor, under pH and boric acid variations. It deals with non-linearly changing corrosion rates coupled with pH effects. The CPAIR-P computer program was modified to accommodate for time-dependent rising corrosion and effects of coolant chemistry. These simulations suggest that the effect of an increase in pH value for an extended 24-month cycle on a specific activity, in the form of a decrease in the said activity, is smeared by the rising corrosion. The new saturation values for activity at the end-of-cycle are lower than with a reactor operated at constant low pH/natural boric acid in the coolant. For a non-linear rise in the corrosion rate coupled with a pH rise from 6.9 to 7.4 and the use of enriched boric acid (30%-40%), coolant activity first rises to a peak value during the cycle and then approaches a much smaller saturation value at the end of the cycle, when compared with the activity for the system having a constant low pH value (6.9) in the coolant. In this work, we have shown that the use of enriched boric acid as a chemical shim actually lowers primary coolant activity when higher pH values, rather than natural boric acid, are employed in the coolant. For multiple long-term operating cycles, the saturation value of corrosion product activity increases in the first two cycles and becomes constant in subsequent ones, due to the high operational pH value for enriched boric acid (40%) as a chemical shim.

GEANT4 based Monte Carlo simulations have been carried out for the determination of efficiency and conversion factors of a gas-flow ionization chamber for beta particles emitted by 86 different radioisotopes covering the average-b energy range of 5.69 keV-2.061 MeV. Good agreements were found between the GEANT4 predicted values and corresponding experimental data, as well as with EGS4 based calculations. For the reported set of b-emitters, the values of the conversion factor have been established in the range of 0.5×1013-2.5×1013 Bqcm-3/A. The computed xenon-to-air conversion factor ratios have attained the minimum value of 0.2 in the range of 0.1-1 MeV. As the radius and/or volume of the ion chamber increases, conversion factors approach a flat energy response. These simulations show a small, but significant dependence of ionization efficiency on the type of wall material.

The total efficiency of cylindrical scintillation γ-ray detectors has been determined using a novel, primary interaction based Monte Carlo algorithm. With the use of biasing in these simulations, this approach has been made computationally efficient, yielding converged results with standard errors less than a fraction of a percent for about 104 histories which is about two orders of magnitude smaller than the conventional stochastic techniques. This methodology has been implemented in a MATLbased computer program, DSEMC. For thin disk sources of various radii having coaxial configurations, the predictions of the DSEMC code have been found in excellent agreement with the corresponding results obtained by using the direct analytical technique. Similar agreement has also been found in the DSEMC calculated values of total efficiency and the corresponding results obtained by the direct analytical technique for coaxial thin disk sources of various radii over a wide range of γ-energies. The DSEMC program has been used for the determination of total detection efficiency for off-axial configurations. As the disk source radius increases, results show a gradual decreasing trend in total efficiency. For these configurations, energy dependency of the total efficiency is found to follow the variation of the corresponding total attenuation coefficient which is consistent with the expected behavior. For coaxial thin disk sources, the total detection efficiency has been found to approach a corresponding maximum value as length is increased in the 1-20 cm range for various values of radii of disk sources and γ-ray energy, while keeping the diameter of the detector fixed at 7.62 cm. A similar behavior is observed when the radius of the detector is increased from 1-20 cm, while keeping the length of the detector fixed at 7.62 cm for various values of radii of disk sources and γ-ray energy.

A primary-interaction based Monte Carlo algorithm has been developed for determination of the total efficiency of cylindrical scintillation g-ray detectors. This methodology has been implemented in a Matlab based computer program BPIMC. For point isotropic sources at axial locations with respect to the detector axis, excellent agreement has been found between the predictions of the BPIMC code with the corresponding results obtained by using hybrid Monte Carlo as well as by experimental measurements over a wide range of g-ray energy values. For off-axis located point sources, the comparison of the BPIMC predictions with the corresponding results obtained by direct calculations as well as by conventional Monte Carlo schemes shows good agreement validating the proposed algorithm. Using the BPIMC program, the energy dependent detector efficiency has been found to approach an asymptotic profile by increasing either thickness or diameter of scintillator while keeping the other fixed. The variation of energy dependent total efficiency of a 3'x3' NaI(Tl) scintillator with axial distance has been studied using the BPIMC code. About two orders of magnitude change in detector efficiency has been observed for zero to 50 cm variation in the axial distance. For small values of axial separation, a similar large variation has also been observed in total efficiency for 137Cs as well as for 60Co sources by increasing the axial-offset from zero to 50 cm.