Numerical simulations of radon exhalation from surrounding rocks of single and double underground roadways.

The radon exhalation rate of surrounding rocks in underground roadways is an important parameter in determining radon exhalation capacity and ventilation flowrate for radon removal. By approximating the roadways as thick-walled, porous cylinders, this study investigates radon exhalation from their surrounding rocks via simulations using computational fluid dynamics (CFD). Radon exhalation rates of single and double underground roadways were computed and analysed under different pressure differences, radon diffusion coefficients, permeabilities of rocks, single roadway locations and additional parallel roadway orientation. The radon regulating zone was presented and the effect of pressure difference on it was analysed. By fitting the data from simulation results, an estimation model was obtained for the radon exhalation rate of a single roadway. For two adjacent parallel roadways with a distance greater than or equal to 50m, the model is also suitable for estimating the radon exhalation rate when the rock permeability is less than 1 × 10-14 m2 and the ratio of permeability to diffusion coefficient is less than 5 × 10-9.

Simulations and Experimental Verifications of an Algorithm for Radiation Source Mapping and Navigational Path Generation.

ABSTRACT: Accurate and efficient mapping and localization of both ionizing and non-ionizing radiation sources are important across many different fields. As such, a versatile mapping and navigational path generation algorithm, which can be applied to any point source measurements that follow an inverse-square characteristic, was developed using non-linear least squares methods. Forty thousand simulations were performed on the algorithm, which located sources successfully in a 10 m × 10 m × 10 m three-dimensional space with a success rate of over 80% across different noise functions, given a proportional constant of 10 to 1,000. The algorithm was also verified experimentally with small-scale radioactive decontamination of a 70 cm × 70 cm surface and localization of a lost Wi-Fi router in a 70 m × 70 m open field. One hundred twenty-one measurements were taken from each experiment, which were then fed into the algorithm for navigation. For the radioactive 137Cs source, the estimated locations were within 7 cm × 7 cm of the answer in 79.3% of the scenarios, while the Wi-Fi router was located to within 7 m × 7 m in 57.9% of the tests. In general, the method requires much less information and data than a geographically comprehensive survey and thus shows a lot of potential for practical applications, such as lost source retrieval with unmanned aerial vehicles, small-scale decontamination, mapping undocumented Wi-Fi routers or radio towers, and radiation simulation with radio signals. Different failure modes, desirable features, and potential improvements were also identified but remain as future work.

Radon Kinetics in a Basement Space Measured with Different Devices.

ABSTRACT: Although indoor monitoring of radon and benchmarking of radon measurement devices remain important research topics, few intercomparisons of active radon measurement devices have been performed under realistic conditions, let alone dynamic ones enabling comparison of their transient behavior. Five different radon monitors were therefore placed in a poorly ventilated basement space under three different conditions: 24 h under a steady, elevated radon level, 24 h with fans turned on to produce a radon washout transient, and 9 d with fans turned off for a radon buildup transient. Resulting radon concentrations varied between ~200 and ~2,000 Bq m-3. Accuracy of the devices were evaluated using root-mean-square error, and ventilation data were fit to first order linear compartmental models. To more accurately model behaviors such as cyclic diurnal variations, the source term corresponding to entry of radon from soil into the basement was considered to be non-constant, as it is likely to vary drastically with both the indoor-outdoor pressure differential and soil concentration variations. The improved radon washout model fit very well with the measurements. Despite a wide variety in list prices, all devices performed similarly during transients and at different radon concentrations.

Preliminary Experiences with the Rexon UL-320-FDR: An Automated Thermoluminescent Dosimeter Reader with Removable Contact Heating Planchets and an Infrared Temperature Feedback System.

ABSTRACT: The Rexon UL-320 FDR is a novel resistive-heating thermoluminescent dosimeter reader with a unique temperature measurement system and an automated dosimeter processing mechanism. The removable contact heating planchets have black-body adhesives on the back for capturing temperature information with infrared sensors. A heating cycle feedback loop ensures accurate, precise, and reproducible heating sequences. Heating rates between 0.8 and 40°C s-1 for up to 1,000 s are possible. Photomultiplier tube sensitivity and drift, dark current counts, and planchet glow were measured experimentally. Additionally, 25 LiF:Mg,Ti dosimeters were calibrated to demonstrate reader performance. Sensitivity was optimized at 1,200 V, which produced the highest reference light count to dark current count ratio while extending photomultiplier tube life. Dark current counts measured with typical time-temperature profiles for LiF:Mg,Ti were below 10 counts per channel but increased by up to 2.5% for more extreme heating cycles. Reader sensitivity drifts of up to 10% were observed during extended automated operations with typical time-temperature profiles. Total counts resulting from planchet glow decreased with faster heating rates. Calibrations performed with LiF:Mg,Ti dosimeters yielded results comparable to more established reader designs. Spikes were observed in ~3% of the glow curves from planchet dust and oil burning off at elevated temperatures. The use of N2 gas and sensitivity drift corrections are recommended to improve dosimetry performance for the UL-320 FDR reader.

Radiation Mapping for an Unmanned Aerial Vehicle: Development and Simulated Testing of Algorithms for Source Mapping and Navigation Path Generation.

ABSTRACT: Image reconstruction algorithms were developed for radiation source mapping and used for generating the search path of a moving radiation detector, such as one onboard an unmanned aerial vehicle. Simulations consisted of first assuming radioactive sources of varying complexity and estimating the radiation fields that would then be produced by that source distribution. Next, the "measurements" that would result from a pair of adjacent spatial locations were computed. A crude estimate of the source distribution likely to have produced such "measurements" was reconstructed based upon the limited measurements. Location of the next "measurement" was then determined as halfway between the location of the estimated source and the current "measurement." With each additional sample, improved source distribution reconstructions were made and used to inform the immediate direction of detector motion. Source reconstruction or mapping was formulated as an inverse problem solved with either maximum a posteriori or least squares (LS) regression deconvolution methods. Different amounts of noise were added to the simulated "measurements," allowing evaluation of the methods' performances as functions of signal-to-noise ratio of the measured map. As expected, methods that promote sparsity were better suited in reconstructing point sources. Reliable prior information of the source distribution also improved the reconstruction results, especially with distributed sources. With a non-negative least square algorithm and the suggested paths it generated, location of sources was successfully estimated to an accuracy of 0.014 m within nine iterations in a single-source scenario and 12 iterations in a two-source scenario, given a 10% error on the integrated counts and a Poisson distribution of the noise associated with the measured counts.

Radon kinetics in a natural indoor radon chamber.

An unusual 180 m3 storage room in the basement of a two-story laboratory building is unventilated, and separated from occupiable rooms by double steel doors. The space completely borders on soil through the concrete floor and two of its concrete walls. The room also contains a separate inner chamber with 1 m thick concrete walls designed to damp vibrations in the room above it. The space boasts a relatively high radon level, 1083 Bq m-3, which varies with local outdoor environmental conditions. Measurements were made of radon concentrations at various locations and heights within the facility. More than a year of continuous radon concentration data corresponding to a single location are also available, along with measurements of indoor and outdoor pressure, temperature, and humidity. Data were also collected with as many as five fans placed in different locations and cycled on for variable time periods. First order linear kinetic models were created to explain the observed approaches to steady state due to changing conditions and wash-out resulting from intentional ventilation. Results demonstrate a good fit between changes in the radon concentration level and the developed compartmental models. However, no significant differences were observed between radon concentration at different locations or heights in the chamber.

Design and Characterization of an Extremely-Sensitive, Large-Volume Gamma-Ray Spectrometer for Environmental Samples.

A large volume gamma spectrometer was designed and constructed to analyze foodstuffs and environmental samples having low radionuclide concentrations. This system uses eight 11-cm × 42.5-cm × 5.5-cm NaI(Tl) detectors, chosen due to their relatively high sensitivity and availability and arranged in an octagonal configuration. The sensitive volume of the system is ~28 cm in diameter and ~42 cm deep. Shielding consists of an 86-cm × 86-cm square, 64-cm-tall lead brick enclosure with 18-cm-thick lead walls lined by 0.3-cm-thick copper plates. An aluminum top was machined to suspend the detectors within this shield. The shielding reduces background counts by 72% at 100 keV and 42% at 1,000 keV. The positional variability in sensitivity of the well was determined by both simulation and experiment. A 2.1-L volume of nearly uniform sensitivity, varying less than 10%, exists in the well's center. Energy resolutions of 14.6% and 7.8% were measured for Am and Cs, respectively. Energy resolution shows a 0.2% variation for both Am and Cs as a function of position within all regions of the well's central sensitive volume. Dead time was also determined to be less than 35% for all sources measured in the system, the largest of which had an activity of 1,760 kBq. Simulated results for various source geometries show higher counts for smaller samples, especially at lower energies due to less attenuation of low energy photons. Minimum detectable activities were determined for all source energies used, less than 5.1 Bq kg for reasonable background and sample counting times.

A laboratory method for concurrently determining diffusion migration parameters and water saturation effects of thoron in uranium tailings.

Environmental humidity has a significanteffect on changes in free 220Rn (thoron) production rate and effective diffusion coefficient of 220Rn in uranium tailings. To understand such changes, a closed cavity containing porous 220Rn media with finite thickness was studied. Based on the 220Rn diffusion migration theory in porous media with finite thickness, a model for calculating the uniform 220Rn activity concentration in a closed container with porous media of finite thickness was established. A laboratory method for concurrently determining free 220Rn production rate and effective diffusion coefficient of 220Rn was proposed and a corresponding experimental setup was made. With samples taken from a uranium tailing impoundment in southern China, water content, free 220Rn production rate, and effective diffusion coefficient of 220Rn in uranium tailings were determined under certain environmental temperature and humidity conditions. Results show that: (1) The method and experimental setup presented in this study can simultaneously determine free 220Rn production rate and effective diffusion coefficient of 220Rn in porous media such as uranium tailing; (2) The free 220Rn production rate in uranium tailings increases linearly with water saturation. Effective diffusion coefficient of 220Rn, on the other hand, decreases exponentially with the increase in water saturation.

Design of a Do-It-Yourself Geiger-Muller Counter With Smartphone Mapping Application.

Legacy Geiger-Muller (GM) survey meters recovered from fallout shelters have been used by several nuclear scientific societies as part of high school outreach programs. A donated antique instrument helps teachers demonstrate radiological principles, but fails to develop student's electronics skills, generate excitement for nuclear careers, or provide individuals with their own devices to explore the radioactive planet. A simple, affordable GM survey meter built by each student would increase direct engagement while providing hands-on experience with circuit-building, soldering, and computer programming. The inclusion of an affordable single-board computer as a component in the survey meter would enable students to tackle more various computer science and electronics projects, thereby potentially recruiting more students into technology and engineering. This paper details the challenges faced by an interdisciplinary undergraduate team designing an easy-to-assemble smart GM survey meter. Their iterative research, design, and testing process included modification to a basic circuit to enable use of different tube types, component cost reduction, application development, and data communication. The ultimate product of the team's efforts, a survey meter with affordable components and a smartphone application capable of creating radiation maps, is detailed in full.

The Effects of Radiation and Emitted Light Transport on the Positional Response of 11 cm × 42.5 cm × 5.5 cm NaI(Tl) Detectors.

Experiments were performed with 30 11 cm × 42.5 cm × 5.5 cm NaI(Tl) detectors to better understand their positional response. Spectra were collected using 0.02 to 0.15 MBq point sources of Am, Cs, Co, and Ba positioned on lines parallel and perpendicular to the long axis of the crystal along both the narrow and wide detector faces as well as at different distances from them. A greater density of positions was sampled at the ends of the detector, and repeated measurements were made to examine potential gain drifts during the experiment. Spectroscopic peak counts, spectroscopic pulse heights, and net counts were analyzed. Empirical equations were fit to the aforementioned data for each specific source energy as a function of source position. In addition, a Monte Carlo radiation transport code was used to simulate the expected positionally variable response based solely upon radiation absorption. The simulated radiation transport efficiency functions were compared to the experimental data. The effects of the geometric radiation efficiency, the attenuation and scattering of emitted light within the scintillation crystal, and combined effects such as nonuniformity of the photomultiplier tube, photocathode response, and crystal irregularities were then distinguished. Functions describing each effect were derived. The results suggest potential new corrections to data obtained with large scintillation detectors as well as a novel approach to partial positional gamma-ray detection with minimal collimation, given that the energy resolution is within reason for particular photopeaks.