Simultaneous PW-scale laser driven MeV X-ray and neutron beam characterization for dual radiography capability

Abstract: Laser-driven, high-brilliance secondary sources (electrons, ions, neutrons, X-rays) open new perspectives for compact material probing and imaging of high-speed events. A key advantage is their ability to perform multiplexed probing, as these sources are generated simultaneously in a single shot using a single laser beam. Here, we report the first quantitative measurements of photon spectra (0.1--100 MeV) and angular distributions in the petawatt interaction regime, using an ultra-intense ($&gt;10^{21}\,\rm W/cm<sup>2$), ultra-short (24~fs) laser pulse. These results are complemented by the characterization of simultaneously produced MeV neutrons. We demonstrate that these neutrons, once moderated, can enable in-depth material identification via resonance transmission analysis. This work highlights the potential of compact, ultrashort-pulse PW lasers for dual neutron and X-ray radiography of dense materials.

• The paper establishes a quantitative basis for simultaneous MeV X-ray and neutron generation via PW-class laser-solid interactions.
• It employs advanced diagnostics and Monte Carlo modeling to characterize spectral, angular, and efficiency attributes across varied target regimes.
• The study highlights dual radiography potential, offering a compact, single-shot solution for high-resolution, element-specific inspection in HEDP and nuclear applications.

Simultaneous PW-scale Laser-Driven MeV X-Ray and Neutron Beam Characterization for Dual Radiography

Introduction and Motivation

The paper "Simultaneous PW-scale laser driven MeV X-ray and neutron beam characterization for dual radiography capability" (2604.15365) establishes the quantitative basis for simultaneous, highly brilliant MeV X-ray and neutron sources from petawatt (PW)-class ultra-intense laser-solid interactions. The principal objective is dual-modality high-resolution radiography of dense materials, leveraging a singular ultra-short, high-intensity laser pulse. The work addresses the lack of comprehensive characterization—spanning spectral, angular, and efficiency attributes—of simultaneous MeV X-ray and neutron generation using solid targets under PW laser irradiation. The implications for compact, multiplexed, single-shot radiographic diagnostics in high-energy-density physics and nuclear inspection are substantial.

Experimental Platform and Diagnostics

A petawatt-class, 24 fs, 0.8 $\mu$m, $\sim$15 J laser at ELI-NP was focused to $4.5\ \mu$m FWHM using $f/3.7$ off-axis parabola optics, yielding intensities $>3\times10^{21}$ W/cm$^2$ (Figure 1).

Figure 1: Schematic of the experimental geometry, illustrating diagnostics relevant to secondary X-ray, proton, and neutron characterization.

Laser pulse temporal contrast was rigorously measured to characterize pre-expansion dynamics and ensure control over target ionization and hydrodynamic expansion (Figure 2).

Figure 2: PW laser focal spot and pulse contrast demonstrate tight focusing and controlled pre-pulse conditions for clean solid-target interactions.

A suite of diagnostics enabled simultaneous collection of electron, ion (proton), X-ray, and neutron signals. Magnetic electron spectrometers, RCF stacks, scintillator-based X-ray spectrometers, and dedicated neutron activation assemblies (SPAC) were systematically deployed.

Fast Electron Generation and Transport

The primary driver of secondary X-ray and neutron emission is the generation of relativistic electron populations through direct laser acceleration mechanisms at the solid surface. MeV-range electron spectra were reconstructed and benchmarked against simulations for varying target thicknesses and materials (Figure 3).

Figure 3: Measured and simulated fast electron spectra reveal target material and thickness dependence, with distinct hot-electron temperature scaling and conversion efficiency maximization at few-micron thicknesses.

The data show dual interaction regimes: thin targets (sub-micron) facilitate overall pre-expansion and higher effective electron temperatures via strong refluxing, while thick targets suppress refluxing and shift electron cooling toward Bremsstrahlung-dominated regimes.

Proton Acceleration and Beam Imaging

RCF spectrometry and imaging revealed distinct acceleration mechanisms—TNSA dominates for micron-thick metals, while CSA is evidenced in nanometric, expanded foils. The maximum proton energy reached $\sim$31 MeV with Al targets in the TNSA regime (Figure 4).

Figure 4: Proton cutoff energy as a function of target thickness, indicating distinct acceleration regimes for different materials and expansion states.

Proton beam profiles exhibit strong divergence for thick targets and pronounced collimation for ultrathin (SiN) targets, correlating with acceleration physics (Figure 5).

Figure 5: Spatial beam profiles for 30 nm SiN (collimated CSA) and 2 $\mu$m Al (divergent TNSA), highlighting contrast in angular distribution arising from interaction regimes.

MeV X-ray Generation: Spectral, Efficiency, and Imaging Characterization

The study achieves robust, absolute photon spectral unfolding from 0.1–100 MeV using a stacked scintillator array combined with advanced stochastic unfolding algorithms. X-ray emission is predominantly attributed to Bremsstrahlung from relativistic electrons traversing the high-Z target. The unfolded photon spectra (Figure 6) confirm the Bremsstrahlung shape, with exponential cutoff tied to electron energy and material Z.

Figure 6: Unfolded X-ray spectra for various target materials and thicknesses; power-law with exponential cutoff, $Z^2$ scaling maxima for Au and W.

Comparison with target-thickness-dependent Bremsstrahlung modeling demonstrates that an "extended cold target" refluxing model best reproduces the observed spectral slopes and cutoffs (Figure 7).

Figure 7: Bremsstrahlung calculations overlayed with experimental spectra; energy-dependent electron refluxing accurately reconstructs the measured spectrum, especially the high-energy cutoff region.

Laser-to-photon conversion efficiency is maximized for thick, high-Z (Au, W) targets, strongly favoring radiographic flux in such configurations (Figure 8).

Figure 8: Laser-to-photon conversion per steradian as a function of target thickness and material, with dramatic enhancement for Au at large thickness.

X-ray imaging performance was quantified via direct radiography with single-shot exposure of IQI and step-wedge phantoms. Agreement between measured and simulated transmission through calibration phantoms using unfolded spectra validates both the spectral and imaging performance (Figure 9).

Figure 9: Comparison of measured and simulated X-ray transmission through an Al wedge, confirming spectral unfolding accuracy for radiographic applications.

Source spot size, as derived from MTF analysis, is sub-mm for thicker foils, indicating lateral expansion of electron populations and the effect of target pre-expansion. Nonetheless, spatial resolutions match requirements for sub-mm tomographic applications (Figure 10).

Figure 10: Effective X-ray source size as a function of thickness and material; smaller for thick, high-Z targets due to suppression of electron lateral reflux and larger emission zones in thin foils.

Spatial mapping with RPL dosimetry reveals angular anisotropy in X-ray emission correlated with laser and target geometry (Figure 11).

Figure 11: X-ray spatial dose distributions inside the chamber confirm emission mostly in the forward and target-surface directions, consistent with electron dynamics and laser incidence.

Neutron Production: Yields, Spectra, and Diagnostic Quantification

Neutron production employed a classic pitcher-catcher geometry with LiF converters. Activation analysis and Geant4/MCNP6-coupled simulations yield absolute fast-neutron flux and spectrum (Figure 12).

Figure 12: Simulated neutron energy spectra at multiple angles for proton+LiF reactions, revealing anisotropy and forward-peaked high-energy neutron production.

Experimental measurements systematically exceed simulations, especially for high-threshold reactions, suggesting a flatter proton spectral tail or contributions from additional nuclear channels.

Enabling Dual Radiography: Moderation and Multi-Spectral Probing

Theoretical and simulation studies demonstrate that MeV neutrons can be effectively moderated (HDPE) to the epithermal regime required for NRTA (Figure 13). Monte Carlo transport through composite samples confirms the ability to resolve both matrix and embedded isotope-specific resonances using moderated laser-driven neutrons (Figure 14).

Figure 13: Moderation of laser-driven neutrons as a function of polyethylene geometry, showing a significant increase in epithermal fraction relevant for resonance analysis.

Figure 14: Simulated neutron flux after passage through composite samples (concrete, Fe, Cs); isotope-specific resonance dips are visible, enabling unique elemental discrimination with a multiplexed dual-probe approach.

Implications, Limitations, and Outlook

PW-class laser-driven simultaneous MeV X-ray and neutron sources enable dual-modality, high-flux, single-shot radiography with strong practical advantages: compactness, micro-focus imaging geometry, flexible time-resolution via ultrashort pulses, and the prospect of dynamic, multiplexed, and element-specific inspection regimes.

Experimental determinations and validated models delineate parameter regimes where conversion efficiency, source size, and spectral brightness reach optimal values for industrial and scientific imaging. The high Z-dependence of X-ray yield, effective moderation to epithermal neutrons, and preserved energy resolution after moderation are critical for non-destructive elemental inspection, especially in nuclear waste, high-value manufacturing, and security applications.

Advanced Monte Carlo and PIC modeling frameworks establish a predictable basis for system optimization and broader adoption. Potential improvements include increased repetition rate operation, advanced target engineering (e.g., double-layer, foam-augmented structures), and integration with novel imaging architectures. The tractability and accessibility of compact, high-brilliance sources challenge the dominance of large national accelerator facilities for MeV radiography and neutron resonance interrogation.

Conclusion

The simultaneous characterization of PW-laser-driven MeV X-ray and neutron beams conclusively establishes the dual-probe capabilities of ultra-intense laser facilities for advanced radiography. The combination of systematic experimental measurements, quantitative spectral unfolding, advanced modeling, and practical imaging demonstrations validates such platforms for high-resolution, element-specific, and dynamic interrogation of dense objects. The path is clear for further technological integration, higher flux operation, and translation to industrial and security environments.

(2604.15365)