S. Revis “Determination and Mitigation of Precipitation Effects on Portal Monitor Gamma Background Levels”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2012).
The purpose of this project is to establish a correlation between precipitation and background gamma radiation levels at radiation portal monitors (RPM) deployed at various ports worldwide, and to devise a mechanism by which the effects of these precipitation-induced background fluctuations could be mitigated. The task of detecting special nuclear materials (SNM) by passive gamma spectroscopy is very difficult due to the low signal-to-noise ratio observed in an uncontrolled environment. Due to their low activities and the low energies of their characteristic gamma rays, the signals from many types of SNM can easily be obscured by background radiation. While this can be somewhat mitigated by taking regular background radiation measurements, even this cannot resolve the issue if background levels change suddenly and dramatically. Furthermore, any increase in background count rate will increase the statistical uncertainty of the count rate measurement, and thus decrease the minimum quantity of SNM that can be reliably detected. Existing research suggests that the advent of precipitation is the culprit behind many such large and sudden increases in background radiation. The correlation between precipitation and background levels was explored by in-situ testing on a full-scale portal monitor at Oak Ridge National Laboratory, and by comparing previously recorded background radiation and weather data from portal monitors located at ports worldwide. The first was utilized to determine the frequency and magnitude at which precipitation introduces background activity, and the second was used to quantify the effects of various quantities and types of precipitation in various parts of the world. Once this analysis was complete, various methods of mitigating these changes in background radiation were developed based on the collected data. Precipitation was found to be the most common culprit for rapid increases in background count rate, and was attributable to 85.6% of all such events. Based on extensive simulation via the Origen-ARP and MCNP software, a response function for the portal monitor was developed, and an algorithm designed to predict the contribution of the precipitation to the background count rate was developed. This algorithm was able to attenuate the contribution of precipitation to the background count rate by an average of 45% with very minimal over-correction. Such an algorithm could be utilized to adjust the alarm levels of the RPM to better allow it to compensate for the rise and fall in background count rate due to precipitation. Additionally, the relative contribution of precipitation which landed at various distances from the portal monitor to the increase in background count rate was measured via simulation. This simulation demonstrated that 37.2% of all background counts were due to the radon daughters which landed within a 2.76 m radius from the center of the portal monitor. This radius encompasses the area between the two portals. Based on this, several designs for shielding were simulated, the most successful of which was a concrete structure that was able to attenuate 71.3% of the background radiation caused by a given precipitation event at a materials cost of approximately $6,000 per RPM. This method is recommended as the primary means of mitigating this issue.