Latent variable modeling of gamma-ray background in repeated measurements.

We propose a novel approach for background subtraction in repeated gamma-ray spectrometric measurements. This entirely data-driven method eliminates the need for Monte Carlo detector simulation. To accomplish this, we utilized the framework of Latent Variable Modeling, incorporating various matrix factorization techniques and artificial neural networks. Subsequently, we applied this method to estimate radionuclide activity through spectrum unmixing. Significant improvements in sensitivity, surpassing traditional methods, were observed for the test case scenario of aerosol filter measurements.

Radiocesium in the swash zones off the coast of the Japan Sea.

Radiocesium concentrations were measured in seawater and sediment samples collected in the swash zones in Ishikawa and Niigata prefectures, off the coast of Japan Sea opposite to the side where TEPCO Fukushima dai-ichi Nuclear Power Plant (FDNPP) is located in September 2016 and August 2017, five to six years after the accident. Cs-134 in the seawater samples was detected, suggesting the intrusion of FDNPP-derived radiocesium in both swash zones. FDNPP-derived radiocesium was appeared to be transported by the Tsushima Warm Current. In the surface sediments only 137Cs was detected during the sampling period. We could not find out the presence of the FDNPP-derived radiocesium in the corresponding sediment on the swash zones; however, detected radiocesium in those sediments was assumed to be influenced by 137Cs of FDNPP-derived radiocesium little for Ishikawa area and some for Niigata area.

Evaluation of absolute measurement using a 4π plastic scintillator for the 4πß-γ coincidence counting method.

Absolute measurement by the 4πß-γ coincidence counting method was conducted by two photomultipliers facing across a plastic scintillator to be focused on ß ray counting efficiency. The detector was held with a through-hole-type NaI(Tl) detector. The results include absolutely determined activity and its uncertainty especially about extrapolation. A comparison between the obtained and known activities showed agreement within their uncertainties.

Comparison of intercomparison results of gamma ray spectrometry of spiked and real samples.

The quality and comparability of results of different comparison schemes depend strongly on how the samples were prepared and how the reference values were determined. To test the influence of different approaches to preparing comparison samples and determination of reference values on the final evaluation of participants' results, analyses of S-shapes for Cs-134 and Cs-137 in different comparison schemes in time period 1986-2016 were performed. This study pointed out significant improvement in the quality of performance of comparison tests of selected providers.

Estimation of 134Cs activity by a new approximation referred to the sum-peak method via a well-type Ge detector.

The activity of a sample containing 134Cs and 137Cs was estimated by means of a new approximation referred to the sum-peak method using a well-type Ge detector. The contribution of 137Cs to the total count rate was estimated from its peak count rate, and subtracted from the total count rate. Then the total count rate originating from 134Cs was estimated. Finally, the new method was applied and the 134Cs activity was estimated.

Trophic transfer of 134Cs in the Manila clam Ruditapes philippinarum.

Bioaccumulation of radiocaesium in many marine organisms occurs through complex trophic transfer mechanisms. The present study addresses the trophic transfer of 134Cs in the widely distributed marine bivalve, the Manila clam Ruditapes philippinarum, by experimentally determining the assimilation efficiency (AE) and the specific role of food quality or diet on the AE in this marine invertebrate. Pulse-chase feeding experiments were carried out on this clam using the phytoplankton species Tetraselmis chuii, Phaeodactylum tricornutum and Isochrysis galbana. Depuration kinetics of 134Cs over 21 days were analysed using a two-component exponential model. Observed assimilation efficiencies were consistently less than 10% but slightly varied among individuals fed on the three different phytoplankton species diets (T. chuii: AE = 8.4 ± 0.6%; P. tricornutum: AE = 9.8 ± 0.5%; I. galbana: AE = 5.3 ± 0.6%), although no statistical differences were observed. Comparing results from these experiments with existing data from the literature on the same species exposed to caesium through seawater, it appears that trophic transfer processes are the main accumulation pathway, contributing up to 96% of the global 134Cs bioaccumulation in this bivalve species.

Proficiency testing with uncertainty evaluation for measuring activities per unit mass of 134Cs and 137Cs in brown rice in Japan.

In Japan, we conducted proficiency testing of activity measurement by using high-purity germanium detectors for 134Cs and 137Cs in brown rice grains. Among 176 reported results, 86 % (for 134Cs) and 93 % (for 137Cs) of the results satisfied |En| â‰¦ 1. However, 58 reports for 134Cs and 51 reports for 137Cs had some failures in their evaluations of uncertainties. The proficiency testing was effective to improve the ability of uncertainty evaluation.

Estimated association between dwelling soil contamination and internal radiation contamination levels after the 2011 Fukushima Daiichi nuclear accident in Japan.

OBJECTIVES: Measurement of soil contamination levels has been considered a feasible method for dose estimation of internal radiation exposure following the Chernobyl disaster by means of aggregate transfer factors; however, it is still unclear whether the estimation of internal contamination based on soil contamination levels is universally valid or incident specific. METHODS: To address this issue, we evaluated relationships between in vivo and soil cesium-137 (Cs-137) contamination using data on internal contamination levels among Minamisoma (10-40 km north from the Fukushima Daiichi nuclear power plant), Fukushima residents 2-3 years following the disaster, and constructed three models for statistical analysis based on continuous and categorical (equal intervals and quantiles) soil contamination levels. RESULTS: A total of 7987 people with a mean age of 55.4 years underwent screening of in vivo Cs-137 whole-body counting. A statistically significant association was noted between internal and continuous Cs-137 soil contamination levels (model 1, p value <0.001), although the association was slight (relative risk (RR): 1.03 per 10 kBq/m(2) increase in soil contamination). Analysis of categorical soil contamination levels showed statistical (but not clinical) significance only in relatively higher soil contamination levels (model 2: Cs-137 levels above 100 kBq/m(2) compared to those <25 kBq/m(2), RR=1.75, p value <0.01; model 3: levels above 63 kBq/m(2) compared to those <11 kBq/m(2), RR=1.45, p value <0.05). CONCLUSIONS: Low levels of internal and soil contamination were not associated, and only loose/small associations were observed in areas with slightly higher levels of soil contamination in Fukushima, representing a clear difference from the strong associations found in post-disaster Chernobyl. These results indicate that soil contamination levels generally do not contribute to the internal contamination of residents in Fukushima; thus, individual measurements are essential for the precise evaluation of chronic internal radiation contamination.

Cadmium and other elements in tissues from four ungulate species from the Mackenzie Mountain region of the Northwest Territories, Canada.

Tissue samples from four ungulate species from the south Mackenzie Mountain region of the Northwest Territories (NT), Canada, were analysed for stable and radioactive elements and (15)N and (13)C stable isotopes. Elevated Cd concentrations in moose (Alces americanus) kidney have been observed in the region and are a health care concern for consumers of traditional foods. This study examined the factors associated with, and potential renal effects from, the accumulation of cadmium, and interactions with other elements in four sympatric ungulate species. Mean renal Cd concentration was highest in moose (48.3mg/kg ww), followed by mountain caribou (Rangifer tarandus caribou) (13.9mg/kg ww) and mountain goat (Oreamnos americanus) (5.78mg/kg ww). No local sources of Cd were evident and the elevated levels in moose are considered to be natural in origin. Conversely, total Hg concentration was significantly higher in mountain caribou kidney (0.21mg/kg ww) than in moose (0.011mg/kg ww). (134)Cs (t½=2.1 y) in mountain goat and Dall's sheep (Ovis dalli) muscle is evidence of deposition from the Fukushima reactor accident in 2011. (137)Cs (t½=30.2 y) in all four ungulates is primarily a remnant of the nuclear weapons tests of the 1960s. The levels of both nuclides are low and the risk to the animals and people consuming them is negligible. Stable isotope δ(15)N and δ(13)C signatures in muscle showed a separation between the mountain caribou, with a lichen-dominated diet, and moose, which browse shrubs and forbs. Isotope signatures for mountain goat and Dall's sheep showed generalist feeding patterns. Differences in elemental and radionuclide levels between species were attributed to relative levels of metal accumulation in the different food items in the diets of the respective species. Kidneys from each species showed minor histological changes in the proximal tubule and glomerulus, although glomerular changes were rare and all changes were rare in mountain goat kidney. Kidney function was not expected to be affected in any species. Provisional Monthly Intake recommendations from the WHO indicate that Cd in moose organs will continue to be a public health care concern. However, traditional foods continue to be an important nutritional component of northern diets, particularly in consideration of the shift towards store-bought food.

Development of the modified sum-peak method and its application.

As the sum-peak method requires the total count rate as well as the peak count rates and the sum peak count rate, this meets difficulties when a sample contains other radionuclides than the one to be measured. To solve the problem, a new method using solely the peak and the sum peak count rates was developed. The method was theoretically and experimentally confirmed using (60)Co, (22)Na and (134)Cs. We demonstrate that the modified sum-peak method is quite simple and practical and is useful to measure multiple nuclides.

Fukushima derived radiocesium in subsistence-consumed northern fur seal and wild celery.

In July 2014, our investigative team traveled to St. Paul Island, Alaska to measure concentrations of radiocesium in wild-caught food products, primarily northern fur seal (Callorhinus ursinus). The 2011 Fukushima Daiichi Nuclear Power Plant accident released radiocesium into the atmosphere and into the western Pacific Ocean; other investigators have detected Fukushima-derived radionuclides in a variety of marine products harvested off the western coast of North America. We tested two subsistence-consumed food products from St. Paul Island, Alaska for Fukushima-derived radionuclides: 54 northern fur seal, and nine putchki (wild celery, Angelica lucida) plants. Individual northern fur seal samples were below minimum detectable activity concentrations of (137)Cs and (134)Cs, but when composited, northern fur seal tissues tested positive for trace quantities of both isotopes. Radiocesium was detected at an activity concentration of 37.2 mBq (134)Cs kg(-1) f.w. (95% CI: 35.9-38.5) and 141.2 mBq (137)Cs kg(-1) f.w. (95% CI: 135.5-146.8). The measured isotopic ratio, decay-corrected to the date of harvest, was 0.26 (95% CI: 0.25-0.28). The Fukushima nuclear accident released (134)Cs and (137)Cs in roughly equal quantities, but by the date of harvest in July 2014, this ratio was 0.2774, indicating that this population of seals has been exposed to small quantities of Fukushima-derived radiocesium. Activity concentrations of both (134)Cs and (137)Cs in putchki were below detection limits, even for composited samples. Northern fur seal is known to migrate between coastal Alaska and Japan and the trace (134)Cs in northern fur seal tissue suggests that the population under study had been minimally exposed Fukushima-derived radionuclides. Despite this inference, the radionuclide quantities detected are small and no impact is expected as a result of the measured radiation exposure, either in northern fur seal or human populations consuming this species.

Five-minute resolved spatial distribution of radiocesium in sea sediment derived from the Fukushima Dai-ichi Nuclear Power Plant.

The spatial distributions of radiocesium concentration in sea sediment to a core depth of 14 cm were investigated in the offshore region from the Fukushima Prefecture to the northern part of the Ibaraki Prefecture in February and July 2012, at a spatial resolution of 5 min of latitude and longitude. The concentrations in the area south of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) were generally higher than those in the area north of it. In the southern area, a band of especially high concentration with a width about 20 km was present in the region shallower than 100 m, and a narrow minimal concentration band was found along the 200-m isobaths. In more than half of all cases, the vertical core profiles of radiocesium concentration generally showed an exponential decreasing trend with depth. However, in the area north of the FDNPP, where the radiocesium concentrations tended to be very low, radiocesium concentrations that had similar or larger magnitude compared with those of the most-surface layer were often found in deeper layers. Relatively good correlations were found between radiocesium concentrations and grain sizes of the most-surface sediment. The vertical profile of radiocesium concentration also had a relationship with grain size. In other case, the radiocesium concentration in the sediment seems to have had a dependence on the radiocesium concentration in bottom seawater, suggesting that the quantity of radiocesium supplied and the grain size were major factors determining the spatial distribution pattern of the radiocesium concentration after the FDNPP accident.

Aerosol sample inhomogeneity with debris from the Fukushima Daiichi accident.

Radionuclide aerosol sampling is a vital component in the detection of nuclear explosions, nuclear accidents, and other radiation releases. This was proven by the detection and tracking of emissions from the Fukushima Daiichi incident across the globe by IMS stations. Two separate aerosol samplers were operated in Richland, WA following the event and debris from the accident were measured at levels well above detection limits. While the atmospheric activity concentration of radionuclides generally compared well between the two stations, they did not agree within uncertainties. This paper includes a detailed study of the aerosol sample homogeneity of (134)Cs and (137)Cs, then relates it to the overall uncertainty of the original measurement. Our results show that sample inhomogeneity adds an additional 5-10% uncertainty to each aerosol measurement and that this uncertainty is in the same range as the discrepancies between the two aerosol sample measurements from Richland, WA.

Activity standardization of ¹³4Cs and ¹³7Cs.

The paper presents the results from a primary standardization of (137)Cs using two independent methods - efficiency tracing using PC-NaI coincidence and the TDCR method. The nuclides (60)Co and (134)Cs were used as the tracers. Primary standardization of the (134)Cs is also discussed. The efficiency extrapolation was carried out by measuring samples of varying mass and using the wet extrapolation method. The results obtained are in good agreement; the differences did not exceed 0.5%. The advantages, pitfalls and also possibilities for improvement of the procedures are discussed.

Characterization of brown rice as a certified reference material for Fukushima accident-related radioactivity measurements.

We developed a certified reference material of brown rice to measure radioactivity from the Fukushima Daiichi Nuclear Power Plant accident. The rice was planted in the spring of 2011, just after the Fukushima accident occurred, and it was harvested in the autumn of 2011. The certified value of radioactivity concentration in the rice was 33.6 Bq kg(-1) of Cs-134 and 51.8 Bq kg(-1) of Cs-137 on August 1, 2012. The reference material is being widely distributed by the National Metrology Institute of Japan. To determine the radioactivity and its uncertainties in the brown rice, we employed gamma-ray spectrometry with a high-purity germanium detector and Monte Carlo simulation.