Hyperfine structures and isotopic shifts of uranium transitions using tunable laser spectroscopy of laser ablation plumes

doi:10.1016/j.sab.2020.105828

Spectrochimica Acta Part B: Atomic Spectroscopy

Volume 169, July 2020, 105828

https://doi.org/10.1016/j.sab.2020.105828 Get rights and content

Highlights

•
Absorption and fluorescence spectroscopy of U plasmas.
•
Isotopic shift and hyperfine structures analysis of U.
•
Estimation of HFS constants of selected U transitions.
•
High absorbance in LIF measurements.
•
U isotopic ratio analysis.

Abstract

We report isotopic shifts and hyperfine structures of selected U transitions employing tunable spectroscopy viz: laser-induced fluorescence and laser absorption spectroscopy of laser ablation plumes. The plasmas were produced during ns laser ablation on a natural U metal target which contains 0.73% ²³⁵U. Our results show that isotopic shifts between ²³⁸U and ²³⁵U are entangled with hyperfine structures of ²³⁵U. Measurements obtained using laser-induced fluorescence are affected by the high absorbance of ²³⁸U. Time-resolved laser absorption spectroscopy is carried out for evaluating the optical absorption and estimating the hyperfine constants.

Graphical abstract

Introduction

Optical spectroscopy in conjunction with laser-produced plasma (LPP) is a very promising tool for in-field and non-contact isotopic analysis of solid materials [1]. Both emission and absorbance/fluorescence spectroscopy of laser ablation plumes can be used for isotopic analysis. However, the reported isotopic shifts of U I and U II transitions in the visible spectral regime are in the range ~ 1–25 pm which necessitates the requirement of an extremely high-resolution spectrograph with a resolution ⪖ 60,000 for using emission-based diagnostic tools (eg. laser-induced breakdown spectroscopy [[1], [2], [3]]) for isotopic analysis. In addition to this, the emission spectral analysis requires thermal excitation by electrons which happens at early times of plasma evolution when the lines are broader due to various line broadening mechanisms (Stark, Doppler etc.) [1]. Laser-absorption spectroscopy (LAS) and laser-induced fluorescence (LIF) tools can be used to marginalize the effect of instrumental broadening [1,4,5]. LAS and LIF probe the ground state atoms existing in the plasma when it is cooler, which inherently provides narrower lineshapes [[6], [7], [8], [9], [10], [11], [12]]. The reported linewidths of U transitions using LAS/LIF of laser-produced plasmas are ~ 1 pm which is significantly lower than the average isotopic shift of U atoms/ions (~ 9 pm) [13,14]. LIF of LPP is also suited for standoff detection [15]. Recently several groups explored resonance excitation of LIBS plumes specifically for boosting the emission signal [[16], [17], [18], [19]].

In addition to isotopic splitting, the hyperfine structures (hfs) may influence the lineshape of a transition. Hyperfine splitting is usually small; however, in certain cases, they can be larger than the isotope splitting. In that scenario, isotope shifts of atoms and molecules can be entangled with hyperfine structure. In addition to these, the high angular momentum and nuclear spin for ²³⁵U (I = 7/2) leads to many hyperfine levels. Hyperfine structures of atomic transitions are routinely observed using high-resolution laser spectroscopy with cooled atomic reservoirs or atomic beams [[20], [21], [22], [23]]. Several reports exist in the literature on isotopic and hyperfine analysis using LAS of LPP [1,9,[24], [25], [26], [27]]. Here we report the hyperfine structures of ²³⁵U using LIF of laser-ablation plumes. The results show that even in the relatively high-temperature laser-induced plasma environment, the ²³⁵U hyperfine structure is observed. Effects of high absorbance at the ²³⁸U peak are shown to influence the peak shape and area in the LIF signal, and the LIF results are compared with LAS measurements. The recorded hyperfine structures are also used for estimating the hfs constants.

Section snippets

Experimental setup

A schematic of the experimental setup used in the present study is given in Fig. 1. The plasma plumes were generated by focusing 1064 nm, 6 ns full-width half maximum (FWHM) pulses from an Nd: YAG laser (Continuum, Surelite) with a spot size of ~1 mm. The laser energy and fluence at the target surface were ~48 mJ and ~ 6 J/cm² respectively. The target used was natural uranium, containing ~ 0.73% ²³⁵U and ~ 99.27% ²³⁸U, which was positioned in a chamber with a 10 Torr nitrogen ambient gas during

Results and discussion

Uranium emission features are very congested. For example, there exist ~ 92,000 U I and U II transitions that originated from ~1600 energy levels in the ultraviolet-visible spectral region [1,29]. Hence resolving uranium transitions in the UV-VIS spectral region requires high-resolution spectroscopic tools. Both LIF and LAS probe ground state populations of the selected transition and offer a high spectral resolution which is defined by the bandwidth of the probe laser beam. In LAS, the

Conclusions

In this study, isotopic shift and hyperfine structure analysis of selected U transitions in a laser-produced plasma were carried out using tunable optical spectroscopic tools such as LIF and LAS. The results showed that the isotopic shifts between ²³⁸U and ²³⁵U are entangled with hfs of ²³⁵U. The time-integrated 2D-FS recorded using collinear pump-probe geometry showed effects due to high absorption at the ²³⁸U peak. TRAS was carried out for evaluating the magnitude of optical absorption and

Funding

Office of Defense Nuclear Nonproliferation (DNN); National Nuclear Security Administration (NNSA); U.S. Department of Energy (DOE) (DE-AC05-76RL01830), and Sandia National Labs' Lab Directed Research and Development Office (SAND2020-3141 J). This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Author statement

All authors listed in the manuscript contributed equally for this work. Specifically, SSH and MCP conceived of the original idea and developed the experimental plan. SSH and MCP conducted the experiments. All authors were part of the data analysis and writing of the draft manuscript. Finally, all authors reviewed and edited the manuscript.

Declaration of Competing Interest

The authors declare no competing interests.

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View full text

Hyperfine structures and isotopic shifts of uranium transitions using tunable laser spectroscopy of laser ablation plumes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental setup

Results and discussion

Conclusions

Funding

Author statement

Declaration of Competing Interest

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Spectrochim. Acta B

Optical spectroscopy of laser-produced plasmas for standoff isotopic analysis

Appl. Phys. Rev.

Monitoring uranium, hydrogen, and Lithium and their isotopes using a compact laser-induced breakdown Spctroscopy (LIBS) probe and high-resolution spectrometer

Appl. Spectrosc.

Argon fluoride LA-LEAF for rapid arsenic quantitation

J. Anal. At. Spectrom.

Development of laser ablation absorption spectroscopy for nuclear fuel materials: plume expansion behavior for refractory metals observed by laser-induced fluorescence imaging spectroscopy

Appl. Phys. A Mater. Sci. Process.

Absorption spectroscopy of uranium plasma for remote isotope analysis of next-generation nuclear fuel

Appl Phys A-Mater

Differential laser absorption spectroscopy of uranium in an atmospheric pressure laser-induced plasma

Opt. Lett.

Two-dimensional fluorescence spectroscopy of uranium isotopes in femtosecond laser ablation plumes

Sci. Rep.

Laser ablation absorption spectroscopy for remote analysis of uranium

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Opt. Lett.