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GEANT4 Simulation of the Radeye Response to Point Source and Ground Deposition for the Full-Scale Radiological Dispersal Device Field Trials

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GEANT4 Simulation of the Radeye Response to Point Source and Ground Deposition for the Full-Scale Radiological Dispersal Device Field Trials Defence Research and Recherche et de´veloppement Development Canada pour la de´fense Canada GEANT4 Simulation of the RadEye Response to Point Source and Ground Deposition for the Full-Scale Radiological Dispersal Device Field Trials Chuanlei Liu Calian, 340 Legget Drive, Suite 101, Ottawa, ON K2K 1Y6 Lorne Erhardt DRDC – Ottawa Research Centre Defence Research and Development Canada Scientific Report DRDC-RDDC-2016-R005 January 2016 GEANT4 Simulation of the RadEye Response to Point Source and Ground Deposition for the Full-Scale Radiological Dispersal Device Field Trials Chuanlei Liu Calian, 340 Legget Drive, Suite 101, Ottawa, ON K2K 1Y6 Lorne Erhardt DRDC – Ottawa Research Centre Defence Research and Development Canada Scientific Report DRDC-RDDC-2016-R005 January 2016 c Her Majesty the Queen in Right of Canada (Department of National Defence), 2016 c Sa Majesté la Reine en droit du Canada (Ministère de la Défense nationale), 2016 Abstract Radiological Dispersal Device (RDD) characterization is an area of active research for security and military purposes. Defence Research and Development Canada (DRDC) has been conducting research in this area since 2003. The evaluation of RDD con- sequences, however, is difficult to do in practice due to practical and safety issues around the use of radioactive isotopes that are considered threats in RDDs. At present, theoretical models are heavily relied upon to characterize and describe RDD dynamics, and to derive useful quantities such as airborne concentration and ground deposition. In order to build up experimental data on real-world RDD effects, DRDC Ottawa Research Centre led a series of Full-Scale RDD (FSRDD) trials in 2012 that used a short-lived radioactive source (140La) as a surrogate for threat isotopes. In this work, the count rate, energy spectrum, and dose rate were calculated using Geant4 simulations for the gamma sensor array that was used in trials. The buildup effect due to gamma-ray scattering in the soil was explored in detail, and the con- tribution proved to be significant in this case. The ambient dose rate was derived from the γ-ray flux results. Conversion coefficients were determined, allowing the calculation of dose rate and the ground deposition from count rate measurements. Significance for Defence and Security This work simulated the response of the RadEye personal radiation detector to a 140La source, and explored the gamma radiation signal and energy spectrum in detail. The simulation results reveal that the presence of other materials (i.e. the ground, in this case) near the detector or source can significantly affect the detector response when taking measurements in the field. The simulation results were geared to the analysis of the FSRDD trials, but have broader application for all radiation detection measurements taken in the field. In an operational situation, the hazard posed by radioactive contamination may be a critical consideration for decision makers. The correct interpretation of field mea- surements can have a large impact on the success of a military or a security mission. Therefore, thorough understating of the response of a detector system is critical for translating field measurements into an actionable hazard estimate. Simulations and field trials both aid in the interpretation of detector response in real-world scenarios, and ultimately aid decision-making. DRDC-RDDC-2016-R005 i Résumé La caractérisation des dispositifs de dispersion radiologique (DDR) est un domaine de recherche actif voué à la sécurité et à des fins militaires. Recherche et développe- ment pour la défense Canada (RDDC) mène des recherches dans ce domaine depuis 2003. Cependant, l’évaluation des conséquences liées aux DDR est difficile à réaliser dans les faits en raison de problèmes pratiques et de questions de sécurité relatifs à l’utilisation des isotopes radioactifs, lesquels sont considérés comme des menaces pour les DDR. À l’heure actuelle, on fait largement appel à des modèles théoriques pour caractériser et décrire la dynamique des DDR, et en tirer des quantités utiles telles que la concentration dans l’air et le dépôt au sol. Afin d’obtenir des données expérimentales sur les effets concrets des DDR, le Centre de recherches d’Ottawa de RDDC a mené une série d’essais de DDR à l’échelle réelle (FSDDR) en 2012 en utilisant une source radioactive de courte durée (140La) comme substitut pour les isotopes présentant un danger. Dans le cadre de ces travaux, on a calculé le taux de comptage, le spectre d’énergie et le débit de dose au moyen de simulations Geant4 pour le réseau de capteurs gamma utilisé au cours des essais. L’effet d’accumulation dû à la diffusion du rayonnement gamma dans le sol a été étudié en détail et cette contribution s’est avérée importante dans ce cas. Le débit de dose ambiant a été calculé à partir des résultats des mesures du flux de rayonnement. On a déterminé des coefficients de conversion, ce qui permet de calculer le débit de dose et le dépôt au sol à partir des mesures du taux de comptage. Importance pour la Défense et la Sécurité Ce travail a simulé la réponse du détecteur individuel de rayonnement RadEye à une source de 140La, et a exploré en détail le signal de rayonnement gamma et le spectre d’énergie. Les résultats des simulations révèlent que la présence d’autres matériaux (par exemple, le sol dans ce cas) à proximité du détecteur ou de la source peut influer de manière notable sur la réponse du détecteur lors de la prise de mesures sur le terrain. Les résultats de la simulation ont été liés à l’analyse des essais du FSDDR, mais ils ont une application plus étendue pour toutes les mesures de détection de rayonnement prises sur le terrain. Dans une situation opérationnelle, le danger posé par la contamination radioactive peut être un facteur déterminant pour les décideurs. L’interprétation correcte des mesures sur le terrain peut avoir une incidence importante sur le succès d’une mission militaire ou sur la sécurité. Par conséquent, une compréhension approfondie de la réaction d’un détecteur est essentielle pour traduire les mesures sur le terrain en ii DRDC-RDDC-2016-R005 estimation du danger pouvant donner lieu à des actions. Les simulations et les essais sur le terrain aident tous deux à interpréter la réponse des détecteurs dans les scénarios du monde réel et, finalement, facilitent la prise de décision. DRDC-RDDC-2016-R005 iii This page intentionally left blank. iv DRDC-RDDC-2016-R005 Table of Contents Abstract ....................................... i Significance for Defence and Security ....................... i Résumé ....................................... ii Importance pour la Défense et la Sécurité .................... ii Table of Contents .................................. v List of Figures . viii List of Tables .................................... x 1 Introduction ................................... 1 1.1 The Full-Scale Radiological Dispersal Device Field Trials . 1 1.2 Motivation for this Study ........................ 2 2 Ambient Dose Rate and Ground Deposition ................. 3 2.1 Fluence to Ambient Dose - H*(10)/Φ ................. 3 2.2 Air Kerma to Ambient Dose - H*(10)/Ka ............... 4 2.3 Fluence due to the Ground Deposition ................. 5 3 The RadEye Sensor Array (RSA) ....................... 6 4 GEANT4 Simulation .............................. 8 4.1 Radiation Source Simulation ...................... 8 4.1.1 Source Simulation for Exposure Studies ............ 9 4.1.2 Source Simulation for Ground Deposition Studies . 9 4.2 RadEye PRD Simulation ........................ 9 4.2.1 PRD Simulation for Exposure Studies ............ 9 4.2.2 PRD Simulation for Ground Deposition Studies . 11 4.3 Soil Simulation .............................. 11 DRDC-RDDC-2016-R005 v 4.4 PRD Response Simulation ....................... 12 5 Results and Discussion ............................. 13 5.1 Radiation Flux (Φ) ........................... 14 5.1.1 The Total Flux ......................... 14 5.1.2 The Exclusive Flux From Soil Buildup . 15 5.1.3 The Total Flux Excluding Soil Buildup ............ 16 5.2 Energy Flux (ΦE) ............................ 17 5.3 Hit and Detection Efficiencies ...................... 20 5.4 Ambient Dose Rate - H*(10) ...................... 23 5.5 Coefficients Converting Count Rate to Dose Rate . 24 5.6 Radiation from Ground Deposition ................... 26 5.6.1 Analytical Calculations ..................... 26 5.6.1.1 Fluence Result - Φunit . 27 5.6.1.2 Count Rate ...................... 28 5.6.1.3 Dose Rate - H*(10) . 28 5.6.2 Geant4 Simulations ....................... 28 5.6.2.1 Fluence Result - Φnorm . 29 5.6.2.2 Count Rate ...................... 30 5.6.2.3 Dose Rate - H*(10) . 30 5.6.3 Comparisons and Discussions ................. 31 5.6.3.1 Fluence Result .................... 31 5.6.3.2 Count Rate ...................... 31 5.6.3.3 Dose Rate ...................... 32 5.6.3.4 Conversion Coefficients between Dose Rate and Count Rate ...................... 32 vi DRDC-RDDC-2016-R005 6 Conclusions and Future Work ......................... 33 6.1 Conclusions ................................ 33 6.1.1 Soil Buildup Effect ....................... 33 6.1.2 Ambient Dose Rate Estimation ................ 34 6.1.3 Count Rate Efficiency and Conversion to Dose Rate . 34 6.1.4 Ground Deposition ....................... 35 6.2 Future Work ............................... 35 References ...................................... 37 List of Acronyms .................................. 39 DRDC-RDDC-2016-R005 vii List of Figures Figure 1: The coefficients converting
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