Measurement of uranium isotope ratio by laser ablation absorption spectroscopy

Abstract

<sec>High precision measurement of uranium isotope ratio (<sup>235</sup>U/<sup>238</sup>U) has important application in the field of nuclear energy safety. In this paper, based on high sensitivity tunable absorption spectroscopy technology, combined with the sample processing method of pulsed laser ablation plasma, high-precision measurement of uranium <sup>235</sup>U/<sup>238</sup>U isotope ratio in solid material is realized. In the experimental measurement, transitions near 394.4884 nm/394.4930 nm (vacuum) are selected as the <sup>235</sup>U/<sup>238</sup>U analytical lines. The influence of buffer gas and its pressure on the persistence time of uranium atom in laser ablated plasma are studied in detail. </sec><sec> The experimental results show that different buffer gases have different ability to restrict the movement of particles in the plasma, which leads to different longitudinal expansion velocity of the plasma (perpendicular to the surface of the sample), and increases the persistence time of uranium atoms in the laser beam. The effect of pressure change on plasma evolution can be reduced by adding buffer gas. When helium is used as the buffer gas, the persistence time of uranium atoms in the plasma is longer, which can improve the selection space of data acquisition delay. In the ablation environment with helium, the electron number density of laser ablated plasma is relatively low, which can reduce the influence of Stark broadening effect and obtain narrower absorption lines, which is more conducive to the measurement of uranium atomic absorption spectrum. In order to reduce the influence of Doppler shift effect on absorption spectrum measurement and avoid misjudgment in spectrum analysis, it is more appropriate to carry out experimental measurement after 3μs sampling delay. Through experiments, the optimal conditions for measuring atomic absorption spectrum of uranium are obtained. Under these conditions, five different samples with <sup>235</sup>U content of 4.95%, 4.10%, 3.00%, 1.10% and 0.25% respectively are measured, and the high-resolution absorption spectrum signals of <sup>235</sup>U and <sup>238</sup>U are obtained. The absorption spectra of samples with different content are measured and statistically analyzed, the <sup>235</sup>U absorption signal has high linearity, the fitting correlation coefficient can reach 0.989, and the limit of detection is 0.033% (3σ). The stability test of absorption spectrum signal shows that the relative standard deviation of <sup>238</sup>U, <sup>235</sup>U and <sup>235</sup>U / <sup>238</sup>U signals are 2.054%, 2.152% and 0.524% respectively. The wavelength scanning mode is superior to the fixed wavelength spectrum measurement, and the influence of the energy fluctuation between different ablation pulses on the spectrum measurement is weakened by the wavelength scanning mode to a certain extent. The results show that laser ablation combined with absorption spectroscopy technology is suitable for uranium isotope ratio analysis and has great potential applications in rapid isotope analysis of nuclear fuel.</sec>