A low background gamma-ray spectrometer with a large well HPGe detector.

A low background gamma-ray spectrometer was established in a ground-based laboratory. It consists of a large well (diameter: 28 mm, depth: 40 mm) HPGe detector and an anti-cosmic shielding system. The photon background of the present system was measured with and without the anti-cosmic mode, and compared with each other, and the detection capability with the detection efficiency calibration was estimated. The background with passive and active shields for the energy region between 40 keV and 3000 keV was reduced by 78% compared to one with just passive shield. The full energy peak efficiency for cylindrical samples with three different heights and the minimum detectable activity of the present system were tabulated for main radionuclides to be measured in environmental monitoring.

A large buoy-based radioactivity monitoring system for gamma-ray emitters in surface seawater.

A buoy (shallow water light type) -based in situ gamma-ray spectrometry system with a 7.6 cmØ × 7.6 cm NaI(Tl) detector for remote real-time monitoring of gamma-ray emitting radionuclides in surface seawater is presented. To convert measured count rates to radioactivity, the full energy peak efficiency of the detector for radionuclides in seawater was estimated using Monte Carlo simulation with the MCNP code. The efficiency calibration was validated by comparing the results with a sampling analysis of 40K in seawater at the sites where the monitoring systems were deployed. The minimum detectable activity of the system for 137Cs, 134Cs and 131I with gamma-ray measurement time is discussed.

IN-SITU GAMMA-RAY SPECTROMETRY FOR RADIOACTIVITY ANALYSIS OF SOIL USING NaI(Tl) AND LaBr3(Ce) DETECTORS.

In this study, the full energy peak count rates to radioactivity conversion factors of 3â€³Ø × 3″ NaI(Tl) and 2â€³Ø × 2″ LaBr3(Ce) detectors for radioactivity analysis in the soil were determined on site using a semi-empirical method with point-like gamma-ray sources. To validate the conversion factors derived for the detectors, in-situ gamma-ray measurements were performed in wide open fields with almost flat surface and compared with the sampling analysis for the radioactivity of U-series, Th-series, and 40K in the soil. As a result, radioactivity concentrations of 40K, 208Tl and 214Bi by in-situ and laboratory measurements agreed well with each other within 5%, and the MDAs for artificial radionuclides were estimated under the condition of fresh deposition considering a radiation emergency situation.

Characteristics of artificial radionuclides in sedimentary soil cores from a volcanic crater lake.

The distributions of 137Cs, 237Np, and 239+240Pu activity concentrations in sedimentary soil cores of the volcanic crater-lake have been studied. The 240Pu/239Pu atom ratios measured by MC-ICP-MS and mutual activity ratios were examined. These results were used to evaluate the sedimentation characteristics of the crater-lake (Baengnokdam of Mt. Halla, Korea). The 137Cs, 237Np, and 239+240Pu activity concentrations showed similar distribution patterns and one maximum peak was observed in each sediment core, except at St.10. For all sediment cores, the activity concentrations were in the range 1.03 × 100-1.92 × 102 Bq·kg-1 dw for 137Cs, 7.56 × 10-3 - 7.15 × 100 mBq·kg-1 dw for 237Np, and 5.20 × 10-3 - 5.13 × 100 Bq·kg-1 dw for 239+240Pu, respectively. The averaged 240Pu/239Pu atomic ratio (0.159) was slightly less than the global fallout ratio (0.176). The averaged inventories were estimated to be 9.21 × 103±5.34 × 103 Bq·m-2 for 137Cs, 2.27 × 102±1.58 × 102 Bq·m-2 for 239+240Pu, and 3.22 × 10-1±1.78 × 10-1 Bq·m-2 for 237Np. The averaged 239+240Pu/137Cs and 237Np/239+240Pu activity ratios were 2.21 × 10-2 and 2.21 × 10-3, respectively. The mean sedimentation rates calculated using 239+240Pu activity concentrations at the central area (St.30 - St.45) and at all stations (St.5 - St.75) were estimated to be 0.844 cm yr-1, and 0.767 cm yr-1, respectively. In addition, the sedimentation rates calculated using 210Pb and 226Ra were 0.856 cm yr-1 at depths of 0-35 cm and 0.204 cm yr-1 at depths of 35-55 cm. These results imply that the sedimentation in Baengnokdam was relatively slow (0.204 cm yr-1) until about 44 years ago and then became faster (0.856 cm yr-1) to the present. The excess 210Pb dating is consistent with the sedimentation rate calculated from the vertical 239+240Pu profile.

A calibration transmission method to determine the gamma-ray linear attenuation coefficient without a collimator.

It is shown that the gamma-ray linear attenuation coefficient of a sample with unknown chemical composition can be determined through a systematic calibration of the correlation between the linear attenuation coefficient, gamma-ray energy and the relative degree of attenuation. For calibration, H2O, MnO2, NaCl, Na2CO3 and (NH4)2SO4 were used as reference materials. Point-like gamma-ray sources with modest activity of approximately 37kBq, along with an HPGe detector, were used in the measurements. A semi-empirical formula was derived to calculate the linear attenuation coefficients as a function of the relative count rate and the gamma-ray energy. The method was applied to the determination of the linear attenuation coefficients for K2CrO4 and SiO2 test samples in the same setup used in calibration. The experimental result agreed well with the ones calculated by elementary data.

An anticoincidence-shielded gamma-ray spectrometer for analysis of low level environmental radionuclides.

We developed an ultralow-level background gamma-ray spectrometer, using active and passive shield devices at the same time. Cosmic-ray-induced background is suppressed by means of active shield devices consisting of plastic scintillating plates of 50mm thick and anti-coincidence electronic system. The observed background rate was 0.34 s(-1) (=0.12s(-1) per 100 cm(3) Ge volume) for energy regions between 50 and 3000 ke V. The detection efficiency curve for 10(3)ml Marinelli beaker samples is obtained over all the energy regions. The advantages of the method are demonstrated by measuring the activity of 137Cs in powdered milk sample prepared without taking any chemical procedure. The MDA for 137Cs is estimated to be (17+/-1.7)mBq at a confidence level of 95% and it is about a factor of 10 lower than the MDA obtained from the previous cryostat assembly with 10-cm thick lead shielding.

Mathematical approach to determine the linear attenuation coefficient without collimator in gamma-ray spectrometry.

The linear attenuation coefficient is an important factor in the correction of self-attenuation. In this study, a cone beam from a source of ∼1 µCi was utilised, not equipped with the collimator, and a complete geometric configuration was mathematically modelled. Samples of NaCl, Na2CO3 and (NH4)2SO4 were used to verify the mathematical model. The linear attenuation coefficient can be calculated within ∼4 % of difference in the cylindrical sample at energies of 59.5, 121.8, 244.7, 344.2 and 444.0 keV.

Radiological impact in Korea following the Fukushima nuclear accident.

The pathway and radiological impact of the radioactive plume released at the Fukushima nuclear accident site on the Korean Peninsula has been studied. On March 28, 2011, only (131)I was first observed at all 12 regional monitoring stations in Korea, and the highest activity concentrations in aerosol were 3.12 mBq/m(3) for (131)I at Gunsan, 1.19 mBq/m(3) for (134)Cs and 1.25 mBq/m(3) for (137)Cs at Busan on April 7, 2011. These radionuclides were also detected in dry and wet deposition samples, while the significant increase of (131)I, (134)Cs and (137)Cs activity concentrations were not observed in seawater and marine biota samples.

7Be in ground level air in Daejeon, Korea.

(7)Be concentrations in the ground level air in Daejeon, Korea were determined during the period of January 1998 to December 2009 by gamma-ray spectrometric analysis of particulate samples collected on filter paper with a high-volume air sampler. The monthly concentrations of (7)Be in the ground level air were in the range of 1.3-7.7 mBq m(-3) with strong seasonal trends of low values in the summer and high values in the spring and autumn. The annual mean values of (7)Be concentrations showed weak reverse correlation with the annual average sunspot number.

A 4-point in-situ method to locate a discrete gamma-ray source in 3-D space.

The determination of the source position (x,y,z) of a discrete gamma-ray source using peak count rates from four measurement points was studied. We derived semi-empirical formulas to find the position under the condition to neglect attenuation effects by obstacles between the target source and the detector. To validate the methodology, we performed the locating experiments for a (137)Cs small volume source placed at 10 different positions on the floor of a laboratory using the formulas derived in this study. In this study, a portable HPGe gamma spectrometry system with a virtual point detector concept was used. The calculation results for the source positions were compared with reference values measured with a rule. The applicability of the methodology was estimated based on the differences of the results.

Indoor radon distribution of subway stations in a Korean major city.

The overall survey on indoor radon concentration was conducted at all subway stations in a major city, Daejeon in the central part of Korea. It was quarterly performed from September 2007 to August 2008. The annual arithmetic mean of indoor radon concentration of all the stations was 34.1+/-14.7 Bq m(-3), and the range of values was from 9.4 to 98.2 Bq m(-3). The radon concentrations in groundwater (average 31.0+/-0.8 Bq m(-3)) were not significantly high in most stations, but the concentration (177.9+/-2.3 Bq L(-1)) of one station was over the level of 148 Bq L(-1) in drinking water proposed by U.S. EPA. Based on indoor survey results, the approximate average of the annual effective dose by radon inhalation to the employees and passengers were 0.24 mSv y(-1), and 0.02 mSv y(-1), respectively. Although the effective dose based on the UNSCEAR report was potentially estimated, for more accurate assessment, the additional survey on the influence by indoor radon will be necessary.