Validating a Methodology That Associates Minimum Detectable Activity with Detector Velocity.

ABSTRACT: Mobile radiation detection systems are used widely in remediation and nuclear security. However, their detection efficiency and thus their minimum detectable activity is not completely understood. It is recognized that the detector's velocity will affect its detection efficiency. At lower speeds, detection efficiency will be higher than a detector moving at higher speeds. The relationship describing how speed and efficiency are related was only recently quantified using a modified four-parameter logistic function (M4PL). The current research verifies the M4PL function in a controlled laboratory setting using a 5.08 cm × 5.08 cm sodium iodide detector at speeds between 20-120 cm s-1. As expected, the M4PL function indicates a detection system's highest efficiency at low speeds and its lowest efficiency at higher speed. In between is a transition region of decreasing detector efficiency. This decrease is gradual within initial speeds but quickly steepens and then approaches a minimum at higher detector speeds. This general shape was observed in the experimental data and validated the M4PL function as a predictive tool. To conclude this research and to demonstrate the function's usefulness, a relationship between speed and minimum detectable activity (MDA) was developed. Using this function and the methods described in this research, planners can now optimize surveys by controlling velocity to maintain MDA. It is also possible to use this technique to accelerate surveys while having the ability to predict the reduction in MDA. This foundational relationship between detector speed and detection efficiency has the potential to improve detector performance in various applications for both the academic and operational fields.

Validation of a Dose Assessment Method to be Used in 18F FDG Loose Contamination Exercises.

ABSTRACT: Radiological emergency response may require responders to operate in contaminated environments. To provide more realistic training to these individuals, it has been proposed to disperse low amounts of short-lived radioactive material in simulated emergency scenarios. To demonstrate the applicability and safety of such activities, a limited exercise was conducted where 18F was sprayed in a small area and survey activities were executed. A pre-job external radiation exposure dose assessment was performed in preparation for this training. The research presented here compares participant external recorded doses to assessment results in order to validate the dose estimates. Two individuals were used during the dispersion, search, and survey activities. First, a radiation worker mixed 200 MBq Fludeoxyglucose 18F with 470 mL H2O in a weed sprayer and distributed it over a 3 m × 3 m area. After evaporation, an exercise participant performed search and survey activities in the area. Actual whole-body doses measured with optically stimulated luminescence dosimeters were 10 ± 1 µSv for both personnel. Whole-body digital dosimeters read 4.3 ± 0.2 µSv and 3.3 ± 0.5 µSv for the radiation worker and exercise participant, respectively. Actual extremity doses were below the dosimeters' minimum detectable limits for the radiation worker (thermoluminescence dosimeter) and exercise participant (optically stimulated luminescence dosimeter). The dose assessment-predicted whole-body doses were 2.8 ± 0.4 µSv and 3.2 ± 0.1 µSv for the radiation worker and exercise participant, respectively. The estimated dose to the radiation worker's hand was 21.8 ± 3.8 µSv, and the estimated dose to the exercise participant's knee was 13.4 ± 0.6 µSv. The study provided substantial evidence for the validity of the dose assessment method, supporting its use for a larger training exercise.

Preliminary Dose Assessment for Emergency Response Exercise Using Unsealed Radioactive Contamination.

A preliminary dose assessment for an emergency response exercise using unsealed radioactive sources was performed based on conservative calculation methods. The assessment was broken into four parts: activation, distribution, exercise participation, and post-exercise monitoring. The computer code MicroShield was used to determine external exposure from the source during and after distribution. Internal exposure via inhalation and ingestion was estimated by assuming fractional intakes of activity and converting to dose using annual limits on intake and dose coefficients. It was determined from the dose assessment that a radionuclide-dependent range of 37 MBq to 1.5 GBq can be used to achieve detectable dose rates during the exercise without exceeding assumed administrative dose limits. Of the identified radionuclides, Tc results in the lowest dose and is recommended from a radiological safety standpoint. However, the choice of which radionuclide and what activity to use for an exercise should be made based on budget and the logistics of the actual exercise.

Radionuclide Selection for Emergency Response Exercise at Disaster City® Using Unsealed Radioactive Contamination.

The Department of Nuclear Engineering at Texas A&M University currently supports exercises at Disaster City, a mock community used for emergency response training that features full-scale, collapsible structures designed to simulate various levels of disaster and wreckage. Emergency response exercises can be enhanced by using unsealed radioactive sources to simulate a more realistic response environment following an incident involving the dispersion of radioactive material. Limited exercises are performed worldwide using unsealed radioactive sources, and most of that information is not publicly available. This research compiles the publicly available information along with additional information acquired through discussion with experts and presents the process for selection of a short-lived radionuclide for use at Disaster City. The historically-used radionuclides were F, Tc, Br, and La. These radionuclides were considered for the Disaster City exercise, as well as other short-lived radionuclides commonly used or capable of being produced at Texas A&M. The selection process described in this paper identified seven radionuclides that could be used in an unsealed contamination exercise at Disaster City. Radiopharmaceuticals Tc and F are suitable and available for purchase from nearby vendors. In addition, the Texas A&M Nuclear Science Center TRIGA reactor could be used to produce Na, Mn, Cu, Br, and La via thermal neutron activation.

Developing a Methodology for Determination of Elemental Composition of Shielding Materials.

Radiation transport simulation models can provide estimations of radiation effects such as detector response and detection capabilities. The objective of this research was to develop a methodology for quick, efficient, and effective determination of the composition of shielding materials to be used in radiation transport models. A C++ code, MatFit, was developed that used the concept of densitometry and the iterative method developed for the Spectrum Analysis by Neutron Detectors II (SAND II) computer program to estimate the elemental composition of shielding materials. These results were compared to previous neutron activation analysis (NAA) on the same samples. It was determined that densitometry provided an elemental approximation that yielded an attenuation rate within 10% of that found through NAA but requires much fewer resources, as well as less time. From this research, it is recommended that the developed method and C++ program be used when constructing models for detector response.

Signal Processing and Its Effect on Scanning Efficiencies for a Field Instrument for Detecting Low-energy Radiation.

Signal processing within a radiation detector affects detection efficiency. Currently, organizations such as private industry, the U.S. Navy, Army, and Air Force are coupling some detector systems with data collection devices to survey large land areas for radioactive contamination. As detector technology has advanced and analog data collection has turned to digital, signal processing is becoming prevalent in some instruments. Using a NIST traceable (241)Am source, detection efficiency for a field instrument for detecting low-energy radiation (FIDLER) was examined for both a static and scanning mode. Experimental results were compared to Monte Carlo-generated efficiencies. Stationary data compared nicely to the theoretical results. Conversely, scanning detection efficiencies were considerably different from their theoretical counterparts. As speed increased, differences in detection efficiency approached two orders of magnitude. To account for these differences, a quasi time-dependent Monte Carlo simulation was created mimicking the signal processing undertaken by the FIDLER detection system. By including signal processing, experimental results fell within the bounds of the Monte Carlo-generated efficiencies, thus demonstrating the negative effects of such processing on detection efficiencies.