Application of high-sensitivity radon monitor on walk-in type radon calibration chamber at KRISS.

This paper introduces the KRISS-Rn4, a high-sensitivity radon monitor with four detection cells, installed within a walk-in type radon calibration chamber at KRISS. The KRISS-Rn4 exhibits enhanced energy resolution through channel-by-channel signal processing and data acquisition. Results reveal that it achieves lower statistical fluctuations and faster response times in monitoring test atmospheres compared to commercial devices.

Optimal Coverage of Full Frequency Reuse in FFR Networks in Relation to Power Scaling of a Base Station.

As a strategy to coordinate inter-cell interference in cellular networks, a fractional frequency reuse (FFR) system is proposed, in which the frequency bandwidth is split into two orthogonal bands; users staying near the center of a FFR cell use the band with a frequency reuse (FR) factor of one (i.e., full FR), and users located close to the cell edge utilize the band with a FR factor greater than one (i.e., partial FR). Full FR coverage, which identifies full FR and partial FR regions (that is, near-center and near-edge regions) within a FFR cell, has a crucial effect on system performance. Some of the authors of this paper recently investigated the optimization of full FR coverage to maximize system throughput. They analytically showed that under the constraint of satisfying a specified target outage probability, the optimal full FR coverage is a non-increasing function of base station power when all base station powers in the cellular network are scaled at an equal rate. Interestingly, in this paper, it is proven that as the power of a single base station is scaled, the optimal full FR coverage in that cell is a non-decreasing function of base station power. Our results provide useful insight into the design of full FR coverage in relation to the transmit power of a base station. It gives a deeper understanding of the intricate relationship between important FFR system parameters of base station power and full FR coverage.

Performance of a segmented HPGe detector at KRISS.

A 24 segmented HPGe coaxial detector was set up with a digitized data acquisition system (DAQ). The DAQ was composed of a digitizer (5 × 107sampling/s), a Field-Programmable Gate Array (FPGA), and a real time operating system. The Full Width Half Maximum (FWHM), rise time, signal characteristics, and spectra of a 137Cs source were evaluated. The data were processed using an in-house developed gamma-ray tracking system.

Compensation Method of Natural Head Movement for Gaze Tracking System Using an Ultrasonic Sensor for Distance Measurement.

Most gaze tracking systems are based on the pupil center corneal reflection (PCCR) method using near infrared (NIR) illuminators. One advantage of the PCCR method is the high accuracy it achieves in gaze tracking because it compensates for the pupil center position based on the relative position of corneal specular reflection (SR). However, the PCCR method only works for user head movements within a limited range, and its performance is degraded by the natural movement of the user's head. To overcome this problem, we propose a gaze tracking method using an ultrasonic sensor that is robust to the natural head movement of users. Experimental results demonstrate that with our compensation method the gaze tracking system is more robust to natural head movements compared to other systems without our method and commercial systems.

Clarification of the calculation of minimum detectable activity in low-level radioactivity measurements.

We discuss a new method to determine the value of minimum detectable activity (MDA) in low-level measurements. The method is based on the Feldman-Cousins unified approach which effectively combines the construction of confidence intervals with the determination of the decision threshold and MDA. The new method requires a single probability to be pre-specified for the determination of MDA as well as decision threshold and confidence interval, in contrast with the ISO 11929 method requiring three probabilities.

Absolute measurement of (198)Au activity in gold foil using plastic scintillators and a well-type NaI(Tl) detector.

A beta-gamma coincidence system has been developed for measuring (198)Au activity in gold foils. The system was validated by Monte Carlo simulations and by measuring the activity of a (60)Co point-source. To study effects such as self-shielding of beta particles in gold foils, (198)Au activity measurements and simulations were performed for various scintillators and foil sizes. The measured (198)Au activities were ~1% above the reference activity, which might be due to self-shielding of beta particles. The measured and simulated (198)Au activities agreed, suggesting feasibility of precise activity measurement.

New method to incorporate Type B uncertainty into least-squares procedures in radionuclide metrology.

We discuss a new method to incorporate Type B uncertainty into least-squares procedures. The new method is based on an extension of the likelihood function from which a conventional least-squares function is derived. The extended likelihood function is the product of the original likelihood function with additional PDFs (Probability Density Functions) that characterize the Type B uncertainties. The PDFs are considered to describe one's incomplete knowledge on correction factors being called nuisance parameters. We use the extended likelihood function to make point and interval estimations of parameters in the basically same way as the least-squares function used in the conventional least-squares method is derived. Since the nuisance parameters are not of interest and should be prevented from appearing in the final result, we eliminate such nuisance parameters by using the profile likelihood. As an example, we present a case study for a linear regression analysis with a common component of Type B uncertainty. In this example we compare the analysis results obtained from using our procedure with those from conventional methods.

A preliminary study for the development of reference material using oyster for determination of (137)Cs, (90)Sr and plutonium isotopes.

A new reference material for the determination of (137)Cs, (90)Sr and Pu isotopes ((238)Pu and (239,240)Pu) is being developed using dried oyster matrix by Korea Research Institute of Standards and Science (KRISS). The oyster was collected from Tongyoung harbour, southern part of Korea and the artificial radionuclides ((137)Cs, (90)Sr, (238)Pu and (239,240)Pu) were spiked into the material. After pretreatment and processing, the material was tested for homogeneity and massic activities were determined by measuring (137)Cs, (90)Sr, (238)Pu and (239,240)Pu. The reference value and extended uncertainty for those isotopes will be reported later.

A new gaze estimation method considering external light.

Gaze tracking systems usually utilize near-infrared (NIR) lights and NIR cameras, and the performance of such systems is mainly affected by external light sources that include NIR components. This is ascribed to the production of additional (imposter) corneal specular reflection (SR) caused by the external light, which makes it difficult to discriminate between the correct SR as caused by the NIR illuminator of the gaze tracking system and the imposter SR. To overcome this problem, a new method is proposed for determining the correct SR in the presence of external light based on the relationship between the corneal SR and the pupil movable area with the relative position of the pupil and the corneal SR. The experimental results showed that the proposed method makes the gaze tracking system robust to the existence of external light.

Nonwearable gaze tracking system for controlling home appliance.

A novel gaze tracking system for controlling home appliances in 3D space is proposed in this study. Our research is novel in the following four ways. First, we propose a nonwearable gaze tracking system containing frontal viewing and eye tracking cameras. Second, our system includes three modes: navigation (for moving the wheelchair depending on the direction of gaze movement), selection (for selecting a specific appliance by gaze estimation), and manipulation (for controlling the selected appliance by gazing at the control panel). The modes can be changed by closing eyes during a specific time period or gazing. Third, in the navigation mode, the signal for moving the wheelchair can be triggered according to the direction of gaze movement. Fourth, after a specific home appliance is selected by gazing at it for more than predetermined time period, a control panel with 3 × 2 menu is displayed on laptop computer below the gaze tracking system for manipulation. The user gazes at one of the menu options for a specific time period, which can be manually adjusted according to the user, and the signal for controlling the home appliance can be triggered. The proposed method is shown to have high detection accuracy through a series of experiments.

Atmospheric input of ¹³7Cs and 239,240Pu isotopes in Korea after the Fukushima nuclear power plant accident.

Caesium isotopes ((134)Cs and (137)Cs) and (239,240)Pu in rainwater and dry deposition have been analyzed by Korea Research Institute of Standards and Science (KRISS) since the Fukushima nuclear power plant (NPP) accident in March 2011. The concentrations of (239,240)Pu and (137)Cs in the rainwater are 2.6±1.0 to 15±3 µBq/kg and 0.01 to 0.36 mBq/kg, respectively. The concentrations are concordant to those observed before the Fukushima NPP accident, on the other hand, the monthly depositional flux of (239,240)Pu and (137)Cs are much lower than the amounts observed after Fukushima NPP accident and in Monaco in 1998-2001. This confirms that the Fukushima NPP accident caused no significant impact in Korea.

Realization of radioactive equilibrium in the KRISS radon chamber.

The maintenance of radioactive equilibrium between radon and its decay products in a radon chamber is necessary to calibrate radon decay product monitors. In this study, the activity concentrations of radon decay products have been measured, and mosquito-repellent incense has been used to produce aerosol particles in the chamber. Filter papers with 8 µm pore size were used to collect aerosol in the chamber. The activity concentrations of radon decay products have been evaluated by the Modified Tsivoglou Method. The correction factors due to the differences in counting time requirements of the Modified Tsivoglou Method and the time delay between consecutive measurements have been determined. Finally, the radioactive equilibrium has been confirmed by applying the Bateman equation.

New method to determine the decision threshold for low-level radioactivity measurements.

We discuss a new method to determine the value of a decision threshold that can be used to decide whether to choose a one-sided confidence interval or a two-sided confidence interval. The method is based on the Feldman-Cousins unified approach providing a unique confidence region for estimated parameters. We apply this method to a net count rate measurand in low-level radioactivity measurements which is physically restricted to nonnegative values. We tabulate the values of the decision threshold and detection limit of the measurand for some typical coverage probabilities. The decision threshold in this method does indeed enable a decision on whether or not the physical effect quantified by the measurand is present.

Standardization of 18F by digital coincidence counting.

The radioactivity of (18)F has been measured by a digital coincidence counting (DCC) system. The main advantages of the digital coincidence counting technique are a shortening of the measurement time as compared with conventional coincidence counting and an ability to obtain activities with various experimental parameters through off-line analysis. The measurement results of radioactivity for (18)F solution were compared with those of a conventional coincidence counting technique and a reference ion chamber method.

Calibration of KRISS reference ionization chamber for key comparison of (99m)Tc measurement.

KRISS, as the national metrology institute of Korea, has used a reference ionization chamber system to certify the activity of (99m)Tc aqueous sources, but could only recently participate in a comparison exercise by the BIPM (BIPM.RI(II)-K4.Tc-99m) to secure the international equivalence of (99m)Tc radioactivity measurement by way of the BIPM transfer instrument (SIRTI). The KRISS ionization chamber system was calibrated about 100 days before the comparison with a (99m)Tc solution source standardized by the 4πß(LS)-γ(NaI(Tl)) coincidence counting method. During the comparison, beginning with a higher activity mother solution, the KRISS ionization chamber measured its specific activity without a dilution. The activity of a diluted-solution source was measured by the SIRTI at the same time.

Comparison of (99m)Tc activity measurements at the KRISS using the new SIRTI of the BIPM.

In 2010, a comparison of activity measurements of (99m)Tc was carried out at the KRISS using the BIPM's Transfer Instrument of the International Reference System (SIRTI). The transfer instrument and the KRISS coincidence measurement method are briefly described. The degrees of equivalence with the key comparison reference value and between the present KRISS result and the earlier participants in the SIRTI and SIR comparisons have been evaluated. World-wide consistency of activity measurements of (99m)Tc is demonstrated.

Construction of classical confidence regions of model parameters in nonlinear regression analyses.

Feldman-Cousins' unified approach provides an unique confidence region for parameters under estimation and assures an exact coverage for the constructed confidence region. We present a procedure to implement this approach in least-squares regression analyses. The procedure is based on a series of the most powerful likelihood-ratio tests of hypothesis using a single number as a test statistic. The procedure thereby avoids the complications of the Feldman-Cousins method arising when the number of free parameters is more than one. Applying the procedure to a case of nonlinear regression problems where the estimated parameters are not generally Gaussian distributed, we show that one has to use the procedure when the results of the regression analysis are to be carefully investigated near a boundary of the physical region.

Development of a low-level background gamma-ray spectrometer by KRISS.

A new low-level background and high-efficiency gamma-ray spectrometric system, to be used mainly for the activity certification of natural-matrix certified reference materials (CRMs) and environmental reference materials (RMs) that has been developed on the grounds of the Korea Research Institute of Standards and Science (KRISS). The spectrometer consists of a low-background high-purity germanium detector with a relative efficiency of 120% and various shielding devices to reduce radiation background. The cabinet-shaped device made of 10ton of shielding materials encloses the germanium detector for protection against background from natural radioactivity and neutrons. Three plates of 50-mm-thick plastic scintillation detectors on top of the passive shielding cabinet suppress cosmogenic background by detecting high-energetic cosmic muons bombarding the germanium detector. The measured background rate of the spectrometer for the energy range 50-3000keV was 1.72s(-1).

Standardization of radionuclide by beta(LS)-gamma coincidence counting using the geometry-efficiency variation method.

A liquid scintillation counting method consisting of three photomultiplier tubes for beta counters and a NaI(Tl) gamma counter has been developed for the standardization of radionuclides with the beta-gamma coincidence technique. The beta detection efficiency functions are obtained by means of a geometry-variation method developed in the present work; an array of beta detectors is moved uniformly at the same time from a centrally located counting vial to 50 mm. The method has been applied in the standardization of 60Co and 134Cs. Unquenched liquid scintillation samples with nominal count rates from 1000 to 6000 s-1 were prepared. The observed beta detection efficiencies with this method are from 90 to 45% in the case of 60Co, and from 84 to 50% for 134Cs. The output of each beta channel is summed together and compared with gamma data by the coincidence analyzer. The dead time of each counting channel is adjusted to be 20 micros, sufficiently long to suppress the afterpulses in the beta counting channel. The activity of each sample is determined by using the Cox and Isham formula. The obtained results are in good agreement with KRISS certified values.

Development of radon calibration chamber at KRISS.

Radon calibration chamber has been constructed to test and calibrate radon and radon progeny detectors at various environmental conditions and to study the characteristic behavior of radon decay products. The size of main room in the chamber is 3.2 m x 3.3 m x 2.4 m (25.3 m3) and the total volume of the chamber is 39.1 m3. The maximum concentration of radon in the chamber can be maintained up to 250 kBq m(-3) and temperature and humidity are controlled in the ranges of (10-40) degrees C and (30-80)%, respectively. The equilibrium factor and unattached fraction of radon progeny are determined at the radon concentration of 2 kBq m(-3).