Wearable Active Directionally-sensitive Radiation Dosimeter

Submission note: A thesis submitted in total fulfillment of the requirements for the degree of Doctor of Philosophy to the Department of Engineering, College of Science, Health and Engineering, La Trobe University, Victoria, Australia.

The International Atomic Energy Agency (IAEA) has warned of the threat of “dirty “bomb attacks resulting from inadequate management of radioactive sources world-wide. Since 1996, approximately 1500 radioactive sources have been lost in America and Europe every year, which has led to a very high risk of radioactive material being used to produce dirty bombs for terrorist purposes. The consequence of such a bomb would be not only as deadly as a regular bomb but radiation spread out from a dirty bomb explosion could cause long-term health effects to the public as well as social disruption and economic crisis to society. Radiation is energy that travels in the form of waves or high-speed particles. It can exist naturally in the environment or is man-made for use in diagnostic X-rays, nuclear weapons, nuclear power plants, and cancer treatment. Radiation exposure can damage human tissues and organs, sometimes resulting in disease. The effects of radiation exposure to human health depend on the amount and duration of exposure, but are different for different body regions as well as the types of radioactive sources. Health effects can be chronic (long-term effects) or acute (short-term effects). The biggest risk for long-term human health after radiation exposure is cancer: the unborn, neonates, children and adolescents are particularly at risk, as they are more sensitive to radiation than adults. Ionizing radiation cannot be detected by human senses, so we are dependent on instruments to observe its existence in the environment. Radiation dosimeters are instruments designed for detecting and measuring particles emitted by radioactive materials produced by particle accelerators or observed in cosmic rays. Recently, the problem of detecting and locating radioactive sources has gained increased importance due to the threat to public safety and homeland security from radiation attacks by terrorists. In addition, the localization of radioactive sources is also a critical issue within the nuclear industry, including nuclear power plant decommissioning, radioactive waste management and radiation protection. In practice, measurements of radiation intensity acquired at multiple locations are often used to estimate the location and strength of a radioactive source. Currently, to improve the accuracy of detecting and locating radioactive materials, researchers have been attempting to design detectors with better measurement characteristics, as well as developing new mathematical models for automatic source localization. This thesis proposes a novel wearable directionally sensitive radiation dosimeter that can provide the real-time identification, direction estimation, and intensity of a radioactive source. It commences with a literature review of existing radiation detectors and the current state-of-the-art of directionally sensitive radiation dosimeters. Next, the proposed dosimeter is modelled in Geant4, a Monte Carlo simulation package for radiation dosimetry, followed by the development of an algorithm to estimate the emission direction of radioactive sources based on the radiation intensity measured by four closest detectors in an array of eight detectors, arranged in a circular pattern. The performance of the proposed system is then analysed in terms of radioactive source emission direction, estimation speed and accuracy at different source-to-detector (STD) distances. Finally, the ability of the proposed dosimeter to determine the source direction in a 3D environment is evaluated using different shaped detectors. Although the proposed dosimeter is intended for human use, with minor modifications, it can be adapted for use in other platforms (such as a robot) to meet the requirements of different situations.