Background UV in the 300 to 400 nm region affecting the extended range detection of radioactive material
The desire to find alternative methods for the detection of radioactive material at extended ranges has resulted in an increased interest in the detection of the air fluorescence resulting from the alpha or beta radioactive particle's interaction with molecules of air. Air fluorescence photons travel further than the radioactive particles, allowing for detections at longer distances. However, any detection of the ultraviolet (UV) air fluorescence is dependent on overcoming natural and man-made background UV to achieve favorable signal to noise ratios. This research describes laboratory and field experiments conducted to determine the background UV in the 300 to 400 nm region of the electromagnetic spectrum for certain detection scenarios, and number of UV air fluorescence photons required to achieve detections with a certain confidence limit. The reflective, scintillation, and transmissive UV characteristics of some common materials are discussed and their contribution to a successful detection explored. Additionally, the contributions to the UV background from natural and man-made light sources are investigated. The successful outside optical detection of alpha and beta radioactive isotopes in the 300 to 400 nm region is possible in the lower part of the spectral region (i.e., near 316 nm), when there is no UV light from man-made sources in that band and only natural light exists. Alpha sources (i.e., ^241 Am) equal to or larger than 1.017 curies, theoretically can be detected with 95% confidence during nighttime scenarios with moonless overcast skies at a distances of 20 meters at 316 nm with the optical system assumed for these calculations. Additionally, where scintillators are available that can be employed near ^90 Sr radioactive sources, the detectable activities can be reduced by factors as high as 250. This allows for detections of sources in the millicuries. Tests results are presented for several common materials (e.g., polypropylene, high density polyethylene, low density polyethylene, etc.) that scintillate in the presence of ^90 Sr and can be used to achieve gains in the 100s in the air fluorescence bands centered on 316 nm and 337 nm.