Development of Long-Range Optical Detection Systems for Alpha Emitters Using Radioluminescence

Abstract

Heavy nuclides, such as uranium and plutonium, are commonly found in nuclear industries and can cause significant health risks due to their alpha-emitting properties. Traditional alpha detectors require close contact with contaminated surfaces, usually a few millimeters. This process is time-consuming, labor-intensive, and operators are exposed to potential hazards. Therefore, the need for long range detection methods of alpha emitters is urgent in nuclear industry. One promising candidate is through alpha-induced radioluminescence (RL), which detects photons emitted from excited nitrogen molecules and locate alpha emitters from a distance. However, previous experiments have mostly been conducted under controlled lighting conditions, where ambient light interference is minimized. This limits their applicability in real-world environments.
In this project, the spectrum and yield of alpha RL in both air and nitrogen at different pressures were studied. Additionally, a 3D distribution of alpha particles in air was constructed using tomographic techniques. CCD based cameras including ICCD and EMCCD was tested for their ability to detect alpha RL. Based on these evaluations, the development of a long-range alpha camera system is demonstrated under various lighting conditions.
The detection of alpha sources in UV-free environments such as LED room light and inside gloveboxes with acrylic window is important for routine monitoring. It was achieved using a "sandwich" filter structure, which includes an absorptive filter between two reflective filters. The filter system, combining with a deep-cooled CCD camera and a lens system, successfully enabled the imaging of a 29 kBq Am-241 source at 3 meters in 10 minutes. The lowest detection limit was
calculated as 3.8 kBq/cm2 in 1 meter within 10 minutes. This represents a 1000-fold improvement in minimum detectable activity over prior alpha RL systems, which typically required MBq-level sources in dark environments. Crucially, the imaging system resolves spatial features inaccessible to traditional approaches: it discriminates a 29 kBq source positioned 7 cm from a 3 MBq emitter at 2 m distance, which is unattainable with non-imaging RL systems.
The detection of alpha sources in open environment is vital for nuclear emergency monitoring. The detection at night time is preferred as the UV background it not high. The tests in open environment was accomplished using a dual-filter system. It used a sandwich filter assembly centered at 337 nm for capturing the main alpha RL emission and a sandwich filter assembly centered at 343 nm for background subtraction. A compact UV fused silica lens system with low-f number was also deigned to reduce the blue shift of the filters. A 3 MBq alpha source was
successfully imaged at 1 meter distance in 1 minute. The lowest detection limit was calculated as 173 kBq/cm2
. The system was also tested under LED, fluorescence and incandescent light
conditions.
If night time detection is not feasible, the detection of alpha sources at day time in open environment was achieved by using a stack of five tilted 276 nm short-pass filters. It blocked the sunlight background and detected alpha RL in UVC region. However, the detection ability is limited as the alpha signal in this wavelength region is low. A 3 MBq alpha source was successfully imaged at 70cm in 10 minutes under indirect sunlight condition.
This study demonstrates the feasibility of long-range optical detection of alpha emitters via alpha-induced radioluminescence (RL), presenting a transformative approach for remote alpha radiation monitoring in complex environments. The methodology holds significant promise for applications in nuclear forensics, radioactive waste management, and decommissioning operations. Beyond practical implementation, the technique provides capabilities for fundamental RL
research such as three-dimensional tomographic reconstruction of alpha particle distributions in air. The technology has applications including pre-decontamination radiation hotspot identification, non-destructive monitoring of alpha activity within shielded facilities (e.g., gloveboxes and hot cells), and integration with emerging robotic and drone systems equipped with LiDAR for large-scale contamination mapping. These advancements position alpha RL detection as a critical tool for enhancing safety and efficiency in nuclear industry workflows while reducing human exposure risks.

Date of Award	10 Dec 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	David Megson-Smith (Supervisor) & John C C Day (Supervisor)