Measurement of uranium in a glass matrix based on spatial confinement using fiber-optic laser-induced breakdown spectroscopy

Abstract

The storage and management of nuclear waste materials require the detection of uranium, but traditional analytical methods are unsuitable for radioactive environments. The enhancement methods of uranium in glass matrices using fiber-optic laser-induced breakdown spectroscopy are still underdeveloped. Using an optical diagnostic system coupled with fast photography, shadowgraphy, and optical emission spectroscopy, the evolution of laser-induced plasma generated from a glass matrix under spatial confinement is studied. The plasma evolution image illustrates the temporal consistency between the compression of plasma width and the enhancement of luminescence intensity. Two enhancements were observed when the plate spacing was smaller than the plasma, which might be due to the high density of the core plasma or a synergistic effect of plasma expansion and shockwave confinement. Under spatial confinement, there is a 3–4-times enhancement in the intensity of uranium spectral lines and a 2–4 times enhancement in the signal-to-noise ratio. Several calibration curves are established under spatial confinement based on U II 409.01 nm, U II 367.01 nm and U I 358.48 nm. The lowest limit of detection (LOD) of uranium reaches 95 ppm, which supports the application of FO-LIBS in the detection of uranium-containing nuclear waste materials.

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1 Introduction

Analytical techniques for component analysis in nuclear waste materials can provide support for storage management.¹ Traditional analytical methods include laser ablation inductively coupled plasma atomic emission spectroscopy (LA-ICP-AES) and mass spectrometry (LA-ICP-MS).² However, the highly radioactive environment complicates in situ detection and the transportation of samples to laboratories for analysis.

Laser-induced breakdown spectroscopy (LIBS) is a technique used to qualitatively or quantitatively analyze sample elemental content. It operates by employing a pulsed laser to generate high-temperature plasma on the sample and then collecting the emissions from the laser-induced plasma.^3,4

Various types of uranium-containing materials have been investigated for detection. Barefield et al. studied remote detection of uranium in geological samples from Mars, archiving a LOD of 272 ppm.⁵ Chinni et al. developed remote detection methods for uranium on surfaces and in soil samples, ranging from 5 to 30 meters with a LOD of 2600 ppm.⁶ Sarkar et al. investigated the self-absorption effects in the calibration of uranium content in simulated nuclear fuel.⁷ Uranium-containing and high-level radioactive nuclear waste is vitrified with a plasma torch to form a glass matrix for long-term storage.^8–10 Jung et al. analyzed the typical spectral lines of uranium and europium in a glass matrix using an Echelle spectrometer and an intensified charge-coupled device (ICCD) camera. They reported a detection limit of ∼150 ppm for uranium.² However, the excitation energy density available must be spread out over the excited state distributions for all of the elements in the plasma. High-Z elements such as uranium, which has a high density of states and yields over 100 000 observable atomic emission transitions, form dense and weak spectral lines compared to other elements like Si and Fe.¹¹ In 2016, Skrodzki et al. studied long–short double-pulse enhanced uranium glass plasma, comparing double-pulse laser-induced breakdown spectroscopy (DPLIBS) and single-pulse LIBS (SPLIBS) for the U(I) 356.18 nm line, and found that the signal-to-noise ratio (S/N) and signal-to-background ratio (S/B) were improved by 1.5 times and 1.7 times, respectively.¹²

Utilizing flexible, long optical fibers to guide the laser and the emitted light, fiber-optic LIBS (FO-LIBS) systems allow important equipment such as lasers and spectrometers to remain distant from radiative environments.¹³ Due to its advantages of in situ rapid detection and sample pre-treatment-free operation, it is a potential material analysis technique for nuclear applications. Applied Photonics in the UK developed a fiber-optic LIBS (FO-LIBS) device for nuclear power applications. With a fiber length of 75 meters, they successfully completed copper element measurements on the steam superheater pipes of the Hinkley Point B nuclear power plant.¹⁴ In Japan, FO-LIBS was developed to examine the elemental composition of debris from the Fukushima nuclear power plant accident, demonstrating limits of detection (LOD) for Zr/Ce and Fe/Ce of 0.0161 and 0.0139, respectively.¹⁵ However, there have been no studies on detecting uranium using a FO-LIBS system.

Using a constrained wall-reflected shockwave to compress the plasma, spatial confinement is a usual method to enhance spectroscopy, serving as a passive enhancement technique that is relatively simple and easy to implement, requiring only minor modifications to the mechanical structure of the front-end sensor.¹⁶ Commonly utilized in various complex field scenarios, Yin investigated the increase in signal stability attributed to spatial confinement. Guo Lianbo et al. studied a 2.8–4.2-fold enhancement of V, Cr, and Mn spectral lines on a steel target under a hemispherical cavity.¹⁷ Qiu Y et al. examined the relationship between plasma emission enhancement and the compression width of various spatial constraint walls on nuclear power plant steel.¹⁸ Zhao Shangyong et al. researched the improvement of the LOD and accuracy in soil samples.¹⁹ The effect of spatial confinement is related to the compression of the plasma width, and the expansion speed and size of plasma on different materials affect the duration and enhancement. Multiple studies have focused on reducing the plate spacing to achieve the enhancement of strong spectral lines. However, due to the small size of plasma induced on the metal target, further reducing the plate spacing would result in lower transmission of emission. The laser-induced plasma on the glass matrix is much larger, making it possible for the plate spacing to be relatively small compared to the size of plasma. As the plate spacing continues to decrease, the dynamics and enhancement of uranium are still unclear, especially in the mixture of high-Z and low-Z elements.

In this study, spatially resolved spectral images were developed to analyse the spatial height and expansion process of several particles in laser-induced plasma. To study the dynamics of plasma and shockwaves, an optical diagnostic system including fast photography, optical emission spectroscopy, and shadowgraphy was developed. A FO-LIBS system coupled with a pair of parallel plates was used to create plasma. It was found that the U ions have a different expansion process compared to Ca ions, and spatial confinement could achieve two enhancements at different times with a specific plate spacing. The LOD of uranium in this system reached 95 ppm.

2 Experimental setup

2.1 FO-LIBS system

The experimental system is shown in Fig. 1. The system consists of hardware devices such as lasers, spectrometers, time schedule controllers, and optical devices, such as beam splitters, lenses, and optical fibers. Two adjustable flat plates were placed on the surface of the sample to achieve spatial confinement. A laser beam generated by Nd:YAG laser#1(Lapa-80, Beijing Beamtech, 1064 nm, FWHM 8 ns) was coupled into a multimode fiber (5 m in length and 1 mm in core diameter) through lens L1. The laser emitted out of the fiber was focused onto the surface of the sample by L2 and L3 to generate plasma. The focal spot was adjusted to about 300 μm, and the energy output was set to 30 mJ per pulse. The calculated power density was 3.48 GW cm⁻².

Fig. 1 Schematic of the experimental system.

The emission from plasma was reflected by the dichroic mirror R1 and coupled into a fiber (2 m in length and 0.2 mm in core diameter) with high ultraviolet transmission efficiency, and then received by the Echelle spectrometer (ARYELLE-Butterfly, LTB, spectral range 270–690 nm, and >12 500 resolution) coupled with an Intensified Charge Coupled Device (DH334T, Andor) to collect spectra. A Czerny–Turner spectrometer (Shamrock SR-750, Andor, UK) was used to collect the lateral image and the corresponding spatially resolved spectrum of plasma using mirror R2 and a camera lens (AF-S 105mm f/2.8G, Nikon, Japan). Additionally, a shadowgraphy system was developed to observe the dynamics of shockwaves produced by plasma under spatial constraints. The laser emitted by Nd:YAG laser#2 (DAWA-100, Beijing Beamtech, FWHM 532 nm, 8 ns) penetrated the spatial confinement region. Through the 4-f imaging system composed of double lenses L5 and L6, the image was finally captured by an SLR camera (EOS 700D, Canon, Japan). The timing control of the system was implemented using a Digital Delay Generator (DG645, Stanford Research Systems).

2.2 Samples

This study used the national standard materials GBW04101 and GBW04102 as uranium-containing substances. The main components of these materials are shown in Table 1. They were crushed, ground, and sieved until they could all pass through 200 mesh sieves thoroughly. Two samples were mixed in a stainless-steel drum at different proportions for 2–3 hours to form multiple samples with different concentrations of uranium. To simulate the vitrification of uranium-containing nuclear waste, the mixed powders were solidified during a hot-pressing sintering process at 1100 °C and 30 MPa. The concentrations of uranium predicted from mixing proportions and measured by ICP-OPS/MS in the final samples are shown in Table 2.

Table 1 Content information of uranium standard samples

Number	U (wt %)	SiO2 (wt %)	Al2O2 (wt %)	Fe2O3 (wt %)	CaO (wt %)	MgO (wt %)	CO2 (wt %)
GBW04101	3.29	81.31	6.29	1.74	0.806	0.312	Residue
GBW04102	0.0679	89.75	3.10	0.38	0.38	0.159	Residue

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Table 2 Mixing ratio and uranium content of mixed samples

Number	Proportion of GBW04101 (%)	Proportion of GBW04102 (%)	Predicted concentration of U (wt %)	Concentration of U measured by ICP (wt %)
1	100	0	3.29	3.51
2	70	30	2.32	2.35
3	30	70	1.03	1.01
4	20	80	0.712	0.705
5	0	100	0.0679	0.0658

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3 Results and discussion

3.1 The evolution of the typical uranium spectrum

During the expansion process of laser-induced plasma, the height of emission shows different dynamics due to the effects of fluid dynamics and electrostatic forces.²⁰ Research on the kinetic energy distributions (KEDs) of particles typically employs Langmuir probes,²¹ Faraday cups (FCs)²² and time-of-flight mass spectrometry (TOFMS).²³ Spatially resolved spectroscopy was developed to obtain the height information of emissions from different particles, and its optical system does not interfere with the plasma. In 2005, Siegel et al. analyzed the highly positional differences in the evolution of Bi atoms and ions through spatially resolved spectroscopy.²⁴ The kinetic differences among particles in the plasma are typically studied in vacuum chambers, as the lower pressure of the atmosphere makes the expansion of the plasma more pronounced. This section analyzes the dynamics of the typical spectral lines of high-Z and low-Z elements under low power density and atmospheric pressure.

The parameters for spatially resolved spectroscopy is shown in Table 3. Eight spectral lines including Ca II 393.36, Ca I 430.25, U II 424.17, U I 428.88, Al II 429.39, Al I 439.40 and Fe I 404.581 were selected following the principle of similar upper energy levels. Their information is shown in Table 4. Fig. 2(a) shows spatially resolved spectroscopy images at a delay of 2000 ns and several typical lines of uranium, while Fig. 2(b and c) shows the position of the maximum intensity and front edge varying with delay. The position of the upper edge was calculated using a threshold of 5% of the maximum intensity, with each data point representing values obtained after computing 20 spectra and filtering out outlier points using the 3σ criterion.

Table 3 ICCD detection parameters for spatially resolved spectroscopy

Spectral part (nm)	Delay (μs)	Gain	Gate width (μs)
405.5	300–1000	3500	200
	1000–2000	3500	300
430	300–1000	3800	200
	1000–2000	3800	300
393	300–1000	3000	200
	1000–2000	3000	300

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Table 4 Spectral lines of Ca, U, Al, and Fe observed from the spectrum

Element	Charge state	Spectral lines	Energy of the upper energy level (cm^−1)	Energy of the upper energy level (eV)
Ca	II	393.36	25 414	2.19
	I	430.25	38 551	3.32
U	I	428.88	29 558	2.55
	II	424.17	28 154	2.43
Al	I	394.40	25 347	2.18
	II	429.39	134 919	11.63
Fe	I	404.58	36 686	3.16

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Fig. 2 Spatially resolved spectroscopy (a) and position of (b) maximum intensity and (c) front edge varying with delay.

In Fig. 2, the ions of Ca, Al and U expanded faster than their corresponding atoms, which is probably attributed to the ‘space charge effect’. In this study, the 8 ns pulse width of the laser is significantly longer than the electron-ion thermalization time (10⁻¹⁰ s).²⁵ After the laser energy is converted into electron kinetic energy by inverse bremsstrahlung, it is further transferred to atoms and ions through collisions. After the laser interaction, the temperatures of the species can be considered approximately equal. During the expansion of the plasma, electrons expand further due to their lighter mass and higher velocities,^26–28 forming a charge separation electric field. This electric field exerts an electrostatic force on the charged particles, accelerating their expansion.²⁹

At the same time, it was observed that, as primary ionized ions, uranium ions moved more slowly than calcium ions. At 2000 ns, the absolute expansion distance of calcium ions exceeds that of uranium ions by 22%. This is because uranium ions have a greater particle mass, resulting in lower acceleration under the influence of the electric field. However, the ∼1/6 charge-to-mass ratio does not cause a 1/6 difference in the expansion distance of ions. In terms of additional expansion distance relative to the atoms, calcium ions exceeded uranium ions by 31% at 2000 ns.

In time-of-flight studies of plasmas excited by nanosecond-pulse-width lasers, it has been observed that as the plasma evolves in a vacuum, the charge-to-mass ratio is directly proportional to velocity after 10–20 μs.^30–32 Possible explanations for the differences observed in this experiment include: (1) atmospheric conditions constrained the expansion of the plasma, approaching its limit at about 1.4 mm. As a result, the velocities of individual particles decrease. (2) Interaction between the electron layer and ions expanding to the edge reduced the strength of the electric field due to the space charge effect, thereby diminishing the acceleration. By comparing the evolution size of the plasma in Fig. 4(b), it is suggested that this interaction may commence around 1.2 μs.

Although the spatial evolution of U and other low Z elements showed no significant difference under the atmospheric environment, due to the characteristics of high Z elements, their spectral lines are much weaker, especially due to the low power density of the FO-LIBS system. The difficulty during detection is the identification and enhancement of uranium spectral lines.

3.2 Evolution of plasma under spatial confinement

Spatial confinement utilizes reflected shockwaves from confinement walls to compress and enhance the plasma. The shape and plate spacing of confinement walls have been investigated. Normally the compress effect of shockwaves (reducing of plasma width) is related to the enhancement of plasma luminescence. In our previous study, the compression degree of the plasma width first increases with the widening of the plate spacing and then decreases after reaching the maximum at 4–5 mm plate spacing. However, the lower plate spacing may hinder the transmission of emission. There are a few studies on the interaction between shockwaves and plasma under low plate spacing. Their sample, plasma width, and plate spacing parameters are listed in Table 5. In this study, due to the larger plasma size of the plasma induced on the glass matrix, relatively smaller plate spacing became possible. However, the process of its enhancement is still unclear.

Table 5 Information of several studies on spatial confinement

Target	Maximum width of plasma per mm	Plate spacing of confinement walls per mm	Reference
Cu	0.8	4–11	33
Cu	4	8–14	34
Cu	1.7	3	35
Stainless steel	1.6	2–5	18

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To determine the enhancement process and effects of shockwaves, fast photography was employed to study the temporal evolution of plasma under spatial confinement. Images of plasma emission were collected parallel to the sample surface direction and the confinement wall direction. Delay is defined as the time interval between the laser and the rising edge of the ICCD trigger signal. For ICCD detection parameters, the gain and integration time increased gradually, while the delay increased until 1000 ns. After 1000 ns, plasma evolution entered a stable period. ICCD parameters were fixed to compare the enhancement of intensity (gain: 200 and gate width: 300 ns). Each image represents the accumulation of 10 laser ablations, and the results in this paper are the average of 20 images.

Fig. 3 shows the emission evolution image of plasma under different conditions. Without spatial confinement, the plasma expanded from a flat cylindrical shape and mainly expanded upwards after 700 ns. Under the influence of two types of confinement walls, the plasma experienced the effects of reflected shockwaves at different evolution stages. The shockwaves not only decreased the width and increased the strength of the plasma (as evident in the intensity values in Fig. 3), but also impacted the later-stage evolution of it. For instance, at 4500 ns and 5500 ns under the confinement of 2.5 mm, the plasma continued to develop upwards after compression and then detach from the surface.

Fig. 3 The evolution of plasma without confinement, and under 2.5 mm plate spacing and 3.5 mm plate spacing.

The width and height of plasma were extracted from the images to further analyze the spatial confinement effects using a threshold of 20% of the maximum intensity. From the changes in width, the initial time of shockwaves interacting with the plasma was more precisely determined as 1200 ns for 2.5 mm and 3000 ns for 3.5 mm. In comparison to the plasma without confinement, the plasma widths were compressed by approximately 46% under the two spatial confinements, while there were no significant differences in height. The increase in height after 4000 ns under 2.5 mm confinement in Fig. 4 was mainly attributed to the plasma having already detached from the target surface. In comparison to the research on fiber-coupled LIBS on metal targets under spatial confinement,¹⁸ despite a higher threshold setup for size calculation, LIP on uranium glass exhibited distinct characteristics: (1) larger plasma width and (2) more width compression under spatial confinement.

Fig. 4 The evolution of width (a) and height (b) extracted from the plasma image.

The maximum intensity of plasma and the enhancement factor under spatial confinement are shown in Fig. 5. The maximum enhancements in intensity under the two confinement conditions were approximately 2.6 and 1.8 times, respectively. Comparison between Fig. 4 and 5 revealed that the enhancement under confinement coincides with the time when the plasma width was compressed, indicating that changes in plasma dimensions can characterize the effects of spatial confinement. Additionally, the time point of maximum enhancement under 3.5 mm confinement aligned with that of the maximum compression (5 μs). Under 2.5 mm confinement, two peaks of enhancement were observed. The enhancement at 3.5 μs corresponded to the time point of maximum width compression as shown in Fig. 4, while a stronger enhancement occurred at 2 μs.

Fig. 5 Intensity and enhancement of plasma without confinement and under 2.5 mm plate spacing and 3.5 mm plate spacing.

To determine the cause of the two enhancements under 2.5 mm confinement, a shadowgraphy system was developed to observe the dynamics of shockwaves (Fig. 6). Under the 2.5 mm confinement, the shockwave was observed to reach approximately 2.5 mm within 2.5 μs. Therefore, the secondary enhancement under 2.5 mm confinement was not a result of secondary rebound. The evolution of the width of plasma at different thresholds and the shockwave front with a plate spacing of 2.5 mm is shown in Fig. 7.

Fig. 6 The shadowgraphy result of shockwaves.

Fig. 7 The evolution of the width of plasma at different thresholds and the shockwave front with a plate spacing of 2.5 mm.

An explanation for the two peaks of enhancement is as follows: under the 2.5 mm confinement, when the shockwave started to interact with the plasma (500–800 ns), the whole plasma was compressed. After the core part of the plasma came into contact with the front of the shockwave (1300 ns), the first peak of enhancement occurred when the core part was compressed to its minimum (2000 ns). Although the front of the shockwave converged at 2500 ns and the emission decreased, the whole plasma continues to be compressed by the rear part of the shockwave, reaching its minimum at 3500 ns, where the second peak of enhancement occurred. The difference in time between the two peaks of enhancement is because the plasma has not yet entered a stagnation period at 1300 ns. With higher emission and density, the core part of the plasma was more affected by the shockwave than the outer part. The first peak is also related to the declining period of emission during 1300–2500 ns. For a larger plate spacing (3.5 mm), the plasma was already in a stagnation period at the beginning of the enhancement effect (3500 ns). With a small density difference between the core and outer part, the width compression between them was consistent over time, resulting in only one peak of enhancement.

3.3 Enhancement of the spectrum and calibration of uranium

Based on Section 3.2, the enhancement times for spatially constrained regions of 2.5 mm and 3.5 mm were approximately 1.2–4.2 μs and 3–6 μs, respectively. Integral spectra were collected during the enhancement time to observe the enhancement effect of spatial constraints on the spectrum of uranium. Plasma temperature and electron number density were calculated using the Saha–Boltzmann method. The spectral line information used for calculating plasma parameters is presented in Table 6.

Table 6 List of the relevant parameters of selected emission lines for plasma temperature calculation

Species	Wavelength (nm)	Transition	A ij /Akl (s^−1)	E i /Ek (eV)
Ca I	422.673	3p^64s^2 → 3p^64s4p	2.18 × 10^8	2.932
Ca I	430.253	3p^64s4p → 3p^64p^2	1.36 × 10^8	4.779
Ca I	430.774	3p^64s4p → 3p^64p^2	1.99 × 10^8	7.763
Ca II	396.847	3p^64s → 3p^64p	1.40 × 10^8	3.123

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In a plasma with local thermodynamic equilibrium (LTE), the intensity of atomic or ionic transition I_ij^0,+ can be expressed as

where the superscripts 0 and + represent the atomic and ionic transitions, respectively; F is a constant related to the experimental conditions; n_S^0,+ is the number density of the corresponding emitter; U_S^0,+(T) is the partition function as a function of plasma temperature T; λ_ij and A_ij are the wavelength and transition probability of the current transition; g_i and E_i are the degeneracy and energy of the upper energy level; c and k_B are the speed of light and the Boltzmann constant. By taking the logarithm of both sides, the atomic transition and the ionic transition can be separated as³⁶where the transition i → j represents atomic emission lines, while the transition k → l represents ionic emission lines. The Saha ionization equation is introduced to establish the same intercept in eqn (2), given aswhere χ_S and Δχ are the ionization and the ionization potential defect of the current chemical element; Φ is a constant containing F. In this context, one can deduce the plasma temperature from the slope by selecting a series of stable and interference-free ionic and atomic lines to spread a broad abscissas range. The necessary spectral parameters like A_ij, g_i, and E_i can be found in the NIST database, as listed in Table 7. The result conforms to the local thermodynamic equilibrium criterion, which is

n_e ≥1.6 × 10¹²T_e^1/2(ΔE_ij)³ (4)

Table 7 Average plasma temperature, electron number density and enhancement of uranium spectral lines' intensity under different parameters

Plate spacing	Integral time (μs)	n e (cm^−3)	T e (K)	Enhancement of intensity
				U II 409.01	U II 424.16	U II 428.88	Ca II 396.847	Ca I 422.673
FE	1.2–4.2	2.87 × 10^16	8.23 × 10^3	3.24	4.01	3.68	3.17	2.73
2.5 mm	1.2–4.2	3.44 × 10^16	8.50 × 10^3
FE	3–6	2.77 × 10^16	7.07 × 10^3	2.21	2.19	1.71	2.46	1.83
3.5 mm	3–6	2.89 × 10^16	7.31 × 10^3

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Table 7 shows the plasma temperature, electron number density, and enhancement of uranium spectral lines under different plate spacings and integration times. The spatial confinement primarily enhances the electron number density, particularly pronounced at 2.5 mm, aligning with the mechanism of spatial confinement. Fig. 8 presents the spectrum without confinement and under 2.5 mm and 3.5 mm spatial confinement. Similar to the enhancement of plasma emission discussed in Section 3.2, a larger enhancement was observed under 2.5 mm confinement than 3.5 mm confinement. For the spectral lines U II 409.01 nm, U II 424.16 nm, and U II 428.88 nm, the net intensities were amplified by factors of 3 to 4 under 2.5 mm confinement. However, the enhancement factors for signal-to-noise ratios were 4.05, 3.16, and 1.91, respectively. This discrepancy arises from the spatial confinement amplifying numerous weak spectral lines, resulting in lower calculated background noise. The enhancement factors of Ca lines were similar to those of U lines, while ionic lines showed larger enhancement than atomic lines, which fits the calculated T_e. The difference in dynamics in section 3.1 did not influence the enhancement factors, which is because the difference in dynamics is inconspicuous under long integration times.

Fig. 8 The comparison of spectra without confinement and under 2.5 mm plate spacing and 3.5 mm plate spacing.

The typical enhancement time under spatial confinement occurs during the late stages of plasma evolution and comparisons of the enhancement effects of spatial confinement are often conducted with a short integral time. However, without confinement, the emission intensity was typically very weak after 2 μs, which means that the enhancement factors calculated may not provide guidance for the calibration process. In calibration, long integration times (about 10 μs) are typically employed to obtain a stable and strong spectrum. In this study, an integration time of 10 μs was adopted, covering almost the entire emission time.

The comparison of line intensities and signal-to-noise ratios for U II 409.01 nm and U I 358.48 nm under different detection delays between 2.5 mm confinement and without confinement is shown in Fig. 9. 20 spectra were collected for each parameter, and each spectrum was the accumulation of 20 shots, and outliers were filtered out using the 3σ criterion. The intensity showed enhancement from 500 to 2000 ns for the integration time, which exceeds the duration of spatial confinement enhancement. As the delay increased, Bremsstrahlung radiation gradually decreased, leading to a more pronounced enhancement in the signal-to-noise ratio. Under a long integration time, the spatial confinement enhances the signal-to-noise ratio by a factor of 1.32 and 1.13 for U II 409.01 nm and U I 358.48 nm compared to the maximum without confinement.

Fig. 9 The intensity and SNR vary through delay without confinement and under 2.5 mm plate spacing.

Based on the enhancement by the 2.5 mm confinement, the calibration curves of uranium were established for a detection delay of 1300 ns using U II 409.01 nm, U II 367.01 nm, and U I 358.48 nm spectral lines. The spectra of samples and calibration curves are shown in Fig. 10, while the calibration curve parameters and LOD (limit of detection) are presented in Table 8. Noise is defined as the standard deviation of the background near the lines. The spectral range used for calculating the noise value of U II 409.01 nm, U II 367.01 nm, and U I 358.48 nm is 408.7–408.9 nm, 367.2–367.4 nm and 358.3–358.4 nm, respectively. The LOD was calculated using 3σ/b, and the lowest LOD reached 95 ppm, which is close to other results from traditional LIBS systems, which directly focus the laser on the surface of the sample.

Fig. 10 (a) The spectrum from calibration samples and (b) calibration curves based on U II 409.01 nm, U II 367.01 nm and U I 358.48 nm.

Table 8 Calibration curves established based on uranium spectral lines and their parameter

Spectral lines	R ^2	RMSEC (wt %)	Slope of the model (counts per wt %)	Noise (counts)	LOD (ppm)
U II 409.01 nm	0.9958	0.0801	7323	39.38	161
U II 367.01 nm	0.9956	0.0829	5001	39.27	236
U I 358.48 nm	0.9970	0.0677	9201	29.26	95

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Table 9 lists several research studies including the system setup, sample, lines and LOD. By using the enhancement method, the lowest LOD in this study reached a level similar to those of other studies, despite using a FO-LIBS system. This study supports the on-site application of LIBS detection of uranium.

Table 9 Results of other research studies

Year	System setup	Sample	Spectral line	LOD
2009 (ref. 6)	Focus directly, 13 mJ, Ar	Soil	U II 409.01	2600 ppm
2011 (ref. 2)	Focus directly, 10 mJ, air	Glass matrix	U I 358.49	150 ppm
2012 (ref. 37)	Focus directly, 10 mJ, air	Ore	U I 356.66	158 ppm
2016 (ref. 5)	Focus directly, 10 mJ, CO2	Soil	U II 409.01	272 ppm
2022 (ref. 38)	Focus directly, 160 mJ, air	Ore	U II 304.41	38 ppm
This study	Focus after 5m fiber, 30 mJ, air	Glass matrix	U I 358.48	95 ppm
			U II 409.01	161 ppm
			U II 367.01	236 ppm

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4 Conclusion

In this research, a simulated uranium-containing glass waste material was prepared through hot press sintering of ore samples. The evolution of uranium spectral lines in height was investigated by spatially resolved spectroscopy. The enhancement mechanism of spatial confinement with different plate spacings in the glass matrix was investigated using an optical diagnostic system including fast photography, optical emission spectroscopy, and shadowgraphy. Two enhancements were found in the plate spacing close to the width of plasma, which may contribute to the following: (1) the difference in density between the core and outer part of plasma causes a difference in compression time and (2) the plasma is still in the expansion period when the shockwaves interact. A related smaller spatially constrained cavity shows a stronger enhancement in the spectrum with a 3–4 times enhancement in the intensity of uranium spectral lines, and a 2–4 times enhancement in the signal-to-noise ratio, with a significant improvement in electron number density. After optimization of plate spacing and detection delay, a calibration curve was established, achieving a uranium LOD of 95 ppm. This study supports the application of the FO-LIBS system in the storage and management of uranium-containing nuclear waste materials.

Data availability

Data for this article, including the image of plasma (.sif), spatially resolved spectroscopy used to investigate the evolution of different elements' dynamics (.sif), integrated spectroscopy used to compare the enhancement under spatial confinement (.mat), shadowgraphy used to observe the shockwave (.JPG), and spectroscopy used to optimize the detecting delay and calibration (.ary) are available at the Open Science Framework at https://osf.io/dysvx/?view_only=bfdcdb501b0549a2845ffd088628273a.

Author contributions

Xinyu Guo: methodology, investigation, formal analysis, project administration, writing – original draft, and writing – review & editing. Jian Wu: conceptualization, writing – review & editing, and supervision. Jinghui Li: investigation and data curation. Mingxin Shi: investigation and project administration. Xinxin Zhu: investigation and data curation. Ying Zhou: investigation and data curation. Di Wu: investigation and data curation. Ziyuan Song: investigation and data curation. Sijun Huang: data curation and writing – original draft. Xingwen Li: project administration and supervision.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported in part by the National Key Research and Development Plan of China (No. 2021YFB3703200) and in part by the CNNC Science Fund for Talented Young Scholars.

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