The article discusses the anomalies in using gamma spectrometry for the determination of ²³²Th in environmental samples against other methods. ²³²Th in environmental samples (soil, sediment, etc.,) is measured using several techniques, the most commonly used are gamma spectrometry, neutron activation analysis (NAA), and inductively coupled plasma-mass spectrometry. However, gamma spectrometry measurement is based on the assumption that ²³²Th is in secular equilibrium with its progenies, which is often not true, particularly in the case of vegetation and biological samples. Food samples collected from a high background radiation area in Tamil Nadu, India, were analyzed for ²³²Th by gamma spectrometry and NAA. This paper gives details of measurements and results obtained from both NAA and gamma spectrometry. ²³²Th activity concentration determined using gamma spectrometry was an order or two higher than those measured using NAA indicating an extreme overestimation of ²³²Th by gamma spectrometry technique.

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Natural and artificial radioactivity appears to be different in different geological regions. Moreover, the radionuclides may migrate to the deeper region of the earth after deposition, and the depth profile of a radionuclide reveals information on the rate of migration. Countrywide background gamma radiation mapping program has been initiated in Bangladesh by dividing the whole country primarily into ~2 km × ~2 km (1' ×1' in Global Positioning System scale) systematic square grids to formulate the radioactivity profile. About 28 km × 30 km area of forest land of Gazipur district called the “BhawalGahr” including the Bhawal National Park was selected for sampling. Soil samples from 0 to 5 cm, 5 to 15, cm and 15 to 30 cm depth were collected from the 27 grid node points of the square grids. A total of 81 samples were collected for the current study. The samples were subjected to elemental and radioactivity analyses. The elements found in the samples were in decreasing order as Fe>K>Ti>Ca>Co>Mn>V>Cr>Ni. The calculated activity concentration of natural radionuclides ²²⁶Ra, ²³²Th, and ⁴⁰K for surface soil samples (0–5 cm depth) ranged from 44.2 ± 5.7 to 84.5 ± 10.2 Bq/kg, 75.6 ± 8.3 to 126.5 ± 8.3 Bq/kg, and 263.7 ± 92.3 to 606.7 ± 107.9 Bq/kg, respectively and there is no significant variation for other depths. The radiological hazard indices such as radium equivalent activity, radiation external hazard index, absorbed dose rate, and annual effective dose were also estimated from the calculated values of activity concentrations.

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High-purity germanium (HPGe) detectors are commonly used for high-resolution gamma spectrometry. A HPGe detector considered for this study was in continuous operation for the past 15 years in our laboratory. The performance of the detector system has been monitored periodically using energy and efficiency calibration data. It is observed from the experimental efficiency calibration that there is a reduction in absolute efficiency of the detector for practical geometries after long period of operation. The main reason for this reduction in efficiency is due to increase in inactive Ge dead layer thickness of the detector. A study has been carried out using Monte Carlo techniques to assess the dead layer thickness of the detector after 15 years of operation and compared with the value quoted by the manufacturer while procuring the system and find out how much the dead layer thickness of the detector has varied from the manufactured value. The study shows that dead layer thickness has increased from 0.5 to 1.028 mm.

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The mountains' composition, especially the granitic mountains, contains a certain amount of natural radioactivity due to the decay of uranium, thorium, and potassium isotopes. As a part of the impact assessment study, thirty rock samples were collected from natural mountains in the Red Sea coast, Egypt, considered as the most popular ones, and were measured for their natural radioactivity to assess the radiological impact when they are used as building materials. Rock samples were examined by high-resolution γ-spectrometry. The average activity concentration of ²³⁸U, ²²⁶Ra, ²³²Th, and ⁴⁰K were 192 ± 24, 178 ± 27, 66 ± 7 and 193 ± 23, and 287 ± 31 Bq/kg, respectively. The annual effective dose rate (mSv/y), the mean of the absorbed dose rates (D), radium equivalent (Ra_eq), the external hazard index (H_ex) and the internal hazard index (Hin) and the representative level index (I_γr, I_αr) were; 0.25 mSv/y, 205.64 nGy/h, 286.9 Bq/kg, 0.81, 1.33, 1.97 and 0.89, respectively. The specific activity ratios of ²²⁶Ra/²³⁸U and ²³²Th/²³⁸U were evaluated to analyze the behavior of these radionuclides.

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The induced radioactivity concentrations of ⁴¹Ar, ¹³N, and ¹⁵O in air in 30 MeV proton accelerator vaults has been estimated using Fluka Monte Carlo simulations and National Council on Radiation Protection (NCRP) 144 based analytical methods. The results obtained by these methods are compared and discussed. It is found that analytical calculations using NCRP 144 guidelines underestimate the ⁴¹Ar concentrations about three times while ¹⁵O concentrations are overestimated about ten times when compared to Monte Carlo estimates. However, the ¹³N concentrations estimated by both the methods are found to be in good agreement. It is seen that the analytical calculations ignore the significantly large contribution of ⁴¹Ar formation by the neutrons above thermal energy. The overestimation of ¹⁵O is caused by the large cross-section value assigned in the analytical calculations.

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Gamma spectrometry for environmental samples of low specific activities demands low minimum detection levels of measurement. An attempt has been made to lower the gamma detection level of measurement by optimizing the sample geometry, without compromising on the sample size. Gamma energy of 50–200 keV range was chosen for the study, since low energy gamma photons suffer the most self-attenuation within matrix. The simulation study was carried out using MCNP based software “EffCalcMC” for silica matrix and cylindrical geometries. A volume of 250 ml sample geometry of 9 cm diameter is optimized as the best suitable geometry for use, against the in-practice 7 cm diameter geometry of same volume. An increase in efficiency of 10%–23% was observed for the 50–200 keV gamma energy range and a corresponding lower minimum detectable activity of 9%–20% could be achieved for the same.

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Calibration check of radiation survey instruments needs to be carried out periodically so as to verify that the calibration of instruments holds good during its use. Multipoint calibration check of every radiation survey instruments is carried out on quarterly basis. Conventionally, calibration check was done by aligning the source or the instrument manually at different distance from each other to obtain readings at different dose rates or radiation fields. This incurs radiation exposure to the radiation worker, which is avoidable. An automatic distancing system was developed in-house at Tarapur Atomic Power Station - 1 and 2 which minimized the exposure of handling radiation source by automatically aligning the source to the instrument and distance between them remotely and also increased the precision in measurement.

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