Detection of Uranium Using Laser-Induced Breakdown Spectroscopy

Abstract

The goal of this work is a detailed study of uranium detection by laser-induced breakdown spectroscopy (LIBS) for application to activities associated with environmental surveillance and detecting weapons of mass destruction (WMD). The study was used to assist development of LIBS instruments for standoff detection of bulk radiological and nuclear materials and these materials distributed as contaminants on surfaces. Uranium spectra were analyzed under a variety of different conditions at room pressure, reduced pressures, and in an argon atmosphere. All spectra displayed a high apparent background due to the high density of uranium lines. Time decay curves of selected uranium lines were monitored and compared to other elements in an attempt to maximize detection capabilities for each species in the complicated uranium spectrum. A survey of the LIBS uranium spectra was conducted and relative emission line strengths were determined over the range of 260 to 800 nm. These spectra provide a guide for selection of the strongest LIBS analytical lines for uranium detection in different spectral regions. A detection limit for uranium in soil of 0.26% w/w was obtained at close range and 0.5% w/w was achieved at a distance of 30 m. Surface detection limits were substrate dependent and ranged from 13 to 150 microg/cm2. Double-pulse experiments (both collinear and orthogonal arrangements) were shown to enhance the uranium signal in some cases. Based on the results of this work, a short critique is given of the applicability of LIBS for the detection of uranium residues on surfaces for environmental monitoring and WMD surveillance.

Persistence of uranium emission in laser-produced plasmas

“…Argon was chosen as the ambient gas due to its inert properties; a reactive ambient gas such as air could affect plasma emission properties due to quenching; 17 moreover, uranium is oxygen-reactive, causing increased background emission in the presence of air. 13 Table I lists the U I lines identified in the analyzed emission region (350 nm to 370 nm, chosen due to the absence of interference from matrix emission lines) and their spectroscopic parameters, obtained from Ref. 18; the U I 356.18 nm line was used for studying analytical merits of spectral emission and OTOF studies.…”

“…Additionally, uranium's multi-electron system means that the spectral line emission is very crowded, with significant line overlapping. Chinni et al 13 attributed increased background emission to overlapping of the high density U lines and observed that the background decays with the same behavior as both neutral and ionic line emission, indicating that the background emission is increased by line emission itself. However, our OTOF studies showed that (given in Sec.…”

“…[10][11][12] Various ambient environments have also been explored for detection of bulk SNMs, including air and Ar at various pressures. 13 As U has a very complex atomic structure with closely packed emission lines, high spectral resolution is a crucial figure of merit. The presence of ambient pressure during plasma generation can help to enhance line intensity and persistence through collisional and confinement effects; however, its presence may also lead to superficial effects such as line broadening thereby hindering the identification of already crowded spectra.…”

Spatio-temporal evolution of uranium emission in laser-produced plasmas

“…According to Chinni et al, [7] the presence of strong background emission seen in U plasmas could be due to overlapping of high density U lines.…”

“…LA-Laser Absorption Spectroscopy (LA-LAS) is an extremely sensitive method capable of probing elemental as well as isotopic ground and excited state populations [4,5]. LA -Optical Emission Spectroscopy (LA-OES), commonly called Laser Induced Plasma Spectroscopy (LIPS) or Laser Induced Breakdown Spectroscopy (LIBS), provides another dimension to LA-hyphenated techniques in nuclear forensics where light emission from the plasma plume is analyzed [6][7][8][9][10][11][12][13][14].…”