Minimum Detectable Activity of Pb-210 in Skull From In vivo Measurements: Insights From the Literature

To address the urgent need for the retrospective assessment of the health conditions of people with a history of appreciable radon exposure, a novel technique that directly measures the characteristic γ-rays emitted from Pb-210 in the living skull was developed. Since the first pioneering study in 1968, this technique has experienced continued advancement over more than half a century, where the limit of detection of Pb-210 is a common criterion to assess the performance of the measuring devices. However, researchers have defined the limit of detection in assorted ways, and the measurement conditions often greatly differ from study to study, both of which significantly challenge interstudy comparisons and obscure how various factors make their impacts. In this work, we reanalyze the reported results in the literature according to the minimum detectable activity (MDA) defined by Currie and investigate the effects of key elements therein. Firstly, we focus on the reported background count rates and analyze their dependence on detector’s energy resolution and active area. Secondly, we turn to the reported calibration factors and conduct analysis in the same manner. Thirdly, we calculate MDA for each study and monitor its dependence on the active area of detector and measurement duration. In the limit of the largest achievable active area (∼75000 mm2), it is found that the asymptotic MDA is approximately 6 (4) Bq and 15 (11) Bq under 30 (60) min measurement using NaI-CsI scintillator and HPGe semiconductor detectors, respectively. Finally, we discuss these asymptotic MDA in the context of estimated Pb-210 activity in the skull resulted from a hypothetical history of radon exposure.

Statistical criteria for limiting the measurement of radionuclide activity by plastic scintillators

The paper is devoted to the estimation of the characteristic limits (statistical criteria) for the detection of small amounts of ionizing radiation by a measuring device under conditions of a natural radioactivity background of the environment: the decision threshold, the detection limit, the minimum detectable activity and the confidence interval. The assessment procedures were carried out in accordance with the national harmonized standard DSTU ISO 11929-3:2009. The threshold for making a decision on the presence of 137Cs and 60Co radionuclides in objects of the external environment and the limit of their detection using a measuring device equipped with plastic scintillators manufactured by the Institute of Scintillation Materials of the National Academy of Sciences of Ukraine were estimated. The influence of the energy of the detected radiation, the dimensions of the scintillators and the geometry of the irradiation on the estimation of the characteristic limits were investigated. Keywords: scintillator; decision threshold; detection limit; confidence interval; minimum detectable activity.

Absence of Detectable Radionuclides in Breast Milk in Sendai, Japan in 2012 Even by High-Sensitivity Determination: Estimated Dose among Infants after the Fukushima Nuclear Disaster

The aim of this study was to estimate radionuclide levels in breast milk and the transferred dose to their infants in Sendai (100 km from Fukushima), Japan after the 2011 Fukushima nuclear disaster. Radionuclide concentrations were analyzed in 101 specimens of breast milk collected in 2012. Median values for minimum detectable activities were 0.39, 0.34, 1.1, 1.89, and 17.1 Bq/kg for 137Cs, 134Cs, 131I, 110mAg, and 40K, respectively. Only radionuclides from 40K were detected. To estimate potential exposure and radiocesium dose, we assumed that the samples contained each minimum detectable activity level. The mean minimum detectable activity concentrations (standard deviation) of 137Cs and 134Cs were 0.42 (0.15) and 0.37 (0.14) Bq/kg, respectively. Means of estimated dietary intakes of 137Cs and 134Cs among infants were 0.35 (0.12) and 0.31 (0.11) Bq/day, respectively. The committed effective doses of radiocesium in infants aged 3 and 12 months via breastmilk were estimated at 5.6 (2.1) and 3.3 (1.2) μSv/year, respectively. Dietary intakes of 137Cs and 134Cs in breastfeeding mothers were back-calculated at 1.9 (0.71) and 1.7 (0.65) Bq/day, respectively. The study verified no discernible exposure to radionuclides among infants. The most conservative estimates were below the Japanese internal exposure limit of 1 mSv/year.

Structural and Kinetic Characterization of Hyperthermophilic NADH-Dependent Persulfide Reductase from Archaeoglobus fulgidus

NADH-dependent persulfide reductase (Npsr) has been proposed to facilitate dissimilatory sulfur respiration by reducing persulfide or sulfane sulfur-containing substrates to H2S. The presence of this gene in the sulfate and thiosulfate-reducing Archaeoglobus fulgidus DSM 4304 and other hyperthermophilic Archaeoglobales appears anomalous, as A. fulgidus is unable to respire S0 and grow in the presence of elemental sulfur. To assess the role of Npsr in the sulfur metabolism of A. fulgidus DSM 4304, the Npsr from A. fulgidus was characterized. AfNpsr is specific for persulfide and polysulfide as substrates in the oxidative half-reaction, exhibiting k cat / K m on the order of 104 M-1 s-1, which is similar to the kinetic parameters observed for hyperthermophilic CoA persulfide reductases. In contrast to the bacterial Npsr, AfNpsr exhibits low disulfide reductase activity with DTNB; however, similar to the bacterial enzymes, it does not show detectable activity with CoA-disulfide, oxidized glutathione, or cystine. The 3.1 Å X-ray structure of AfNpsr reveals access to the tightly bound catalytic CoA, and the active site Cys 42 is restricted by a flexible loop (residues 60-66) that is not seen in the bacterial homologs from Shewanella loihica PV-4 and Bacillus anthracis. Unlike the bacterial enzymes, AfNpsr exhibits NADH oxidase activity and also shows no detectable activity with NADPH. Models suggest steric and electrostatic repulsions of the NADPH 2 ′ -phosphate account for the strong preference for NADH. The presence of Npsr in the nonsulfur-reducing A. fulgidus suggests that the enzyme may offer some protection against S0 or serve in another metabolic role that has yet to be identified.

Improvement of the minimum detectable activity of a free release monitor for small articles

This paper presents results of the development of a small-sized free release monitor designed for the release of materials, various hand tools, equipment and instruments of nuclear enterprises and laboratories staff that weight up to 50 kg, from radiation control. To increase the registration sensitivity of controlled radionuclides, 12 scintillation units based on a 3"x3" sized NaI (Tl) crystal were used as gamma-radiation detector. Volume of the measuring chamber of the monitor amounted to 200 liters, the thickness of the low-background shielding was chosen 50 mm. The values of the minimum detectable activity of the designed monitor for the point sources 123I, 131I, 99mTc, 18F were better than 100 Bq with measurement time not exceeding 60 s.