Proof-of-principle experiment for laser-driven cold neutron source

The study introduces a compact, laser-driven approach to neutron production as an alternative and complement to reactor- and spallation-based sources. Conventional neutron sources provide powerful capabilities but face challenges including cost, availability, long pulse durations (reactors), and nuclear waste concerns. Compact Accelerator-driven Neutron Sources (CANS) address some of these issues but still require sizable infrastructure. Advances in high-intensity, short-pulse lasers and ion acceleration (e.g., Target Normal Sheath Acceleration) enable generation of multi-MeV light ions that can drive neutron-producing reactions in converter (catcher) targets. These laser-driven fast neutrons (>100 keV) can then be moderated to epithermal, thermal (25 meV), and cold (≤25 meV) energies to expand their use in applications such as scattering, imaging, and spectroscopy. The research question is whether a table-top, laser-driven neutron source can feasibly produce measurable cold neutron fluxes with useful temporal characteristics by coupling a laser-driven fast neutron source to a compact, cryogenic hydrogen moderator. The purpose is to demonstrate proof-of-principle, quantify flux and spectral characteristics, and assess beam duration, thereby establishing a pathway for compact neutron science platforms.

Background highlights include: (1) Production of neutrons via fission (reactors) and spallation (accelerators), with spallation offering shorter pulses but at high facility cost. (2) Emergence of CANS for wider accessibility. (3) Development of laser-driven ion beams since CPA, enabling Target Normal Sheath Acceleration and other mechanisms to produce multi-MeV protons/deuterons. (4) Laser-driven pitcher-catcher neutron generation using reactions such as Be(d,n), Be(p,n), Li(p,n), and C2D4-based targets, delivering short, bright fast-neutron bursts. (5) Moderation principles: selection of materials based on moderating power and moderating ratio; hydrogenous materials (H2, CH4) are efficient, with compact moderators favored to minimize pulse broadening. (6) Application domains spanning epithermal to cold neutrons for DINS, NRA, BNCT, small-angle scattering, refractometry, and imaging. Prior works have demonstrated laser-based fast neutron production and epithermal sources; this work advances to cold-neutron generation using a cryogenic H2 moderator.

Experimental setup at ILE (Osaka) used the LFEX laser delivering ~300 J in ~1.2 ps (FWHM), focused with an f/10 off-axis parabola onto 5 µm deuterated carbon (C2D4) pitcher targets, achieving peak intensities >5 × 10^18 W/cm^2. Laser-driven ions, characterized by a Thomson Parabola spectrometer, indicated TNSA acceleration with deuterons up to ~5 MeV and protons up to ~20 MeV. The ion beam impinged on a cm-thick 9Be catcher to generate fast neutrons predominantly via 9Be(d,n)10B and 9Be(p,n)8B reactions (the deuteron-induced channel having a low threshold and favorable angular distribution for the measured energies). An off-axis wing-shaped polyethylene pre-moderator (10 × 6 × 5 mm^3) attached to the catcher produced a thermal component at room temperature while aiming to minimize pulse length. The main moderator consisted of a cryogenic H2 cell: ~27 mm H2 thickness along the beam axis, surrounded by 2 mm copper reflectors, cooled via helium lines and monitored by superconducting temperature sensors. The H2 was condensed to liquid and cooled to ~11 K (noting the interior may be slightly warmer than the housing sensors). PHITS (v3.08) Monte Carlo with JENDL-4 data was used to model neutron transport, moderation, divergence (showing isotropy below 25 meV), spectra, and time profiles; temperature-dependent cross sections (tmp cards) were employed. Diagnostics used time-of-flight (ToF): a plastic scintillator EJ-232Q coupled to a Hamamatsu R2083 PMT at 8.2 m and 15° for MeV neutrons, and 3He proportional counters on-axis at 3.28 m for cold neutrons. Detectors were shielded with lead and plastic; the 3He tubes had additional cadmium layers to suppress low-energy scattered backgrounds, leaving an on-axis opening. The prompt gamma signal established ToF t0. Detector efficiencies were accounted for; 3He tube efficiency was evaluated using η = 1 − exp(−0.00482 P d), with P ≈ 10 bar and d in mm (wavelength-dependent). Fast-neutron spectra were derived from the scintillator ToF by correcting for distance, transmission, and detector response. Cold-neutron ToF spectra were obtained from 3He pulse trains with background subtraction (no-moderator shots) and normalized to the incident fast-neutron flux measured by the scintillator. Additional PHITS simulations estimated pulse durations versus energy for a ~3 cm H2 moderator similar to the experiment.

• Fast-neutron production: Repeated shots yielded similar MeV neutron spectra with peak flux up to ≤ 1 × 10^9 n/sr/pulse as measured by ToF at 8.2 m, 15°.
• Moderation characteristics: PHITS predicted isotropic divergence for cold neutrons (≤25 meV) from the H2 moderator. The moderated spectrum showed a broadened cold peak extending down to ~0.8 meV and a smaller thermal component (the latter mainly from the wing pre-moderator and partial moderation).
• Cold-neutron detection: 3He ToF data (background-subtracted) agreed well with Monte Carlo simulations at low energies; a mismatch in the epithermal region was attributed to early gamma-induced detector saturation lasting several microseconds.
• Flux at the nearest practical location: After accounting for detector efficiency and normalizing to the incident fast-neutron flux, the measured cold-neutron (≤25 meV) flux at ~20 cm from the moderator exit surface was ~2 × 10^7 n/cm^2/pulse. The thermal-neutron flux at the same location was ~5 × 10^3 n/cm^2/pulse.
• Moderator conditions: The H2 cell operated around ~11 K; a small temperature rise was observed upon shot arrival and returned to steady state within minutes.
• Temporal profile (simulated): For a ~3 cm H2 moderator, the FWHM pulse duration at energies <1 eV was ~1–100 µs, decreasing below ~100 ns for E ≥ 10 eV. These durations enable usable energy resolution at short sample distances.
• Pre-moderator contribution: Simulations indicated the thin polyethylene wing mainly contributed up to the thermal range; its limited thickness prevented deeper moderation to cold energies by itself.

The experiment demonstrates the feasibility of coupling a laser-driven fast-neutron source to a compact cryogenic hydrogen moderator to generate cold neutrons with measurable flux and favorable time structure. The findings directly address the research goal by showing a detectable cold-neutron spectrum consistent with low-temperature H2 moderation and by quantifying fluxes at a practical distance (20 cm). The short pulse durations predicted and the potential for compact sample-to-moderator distances suggest applicability to time-of-flight neutron scattering and imaging. The laser-driven approach also offers access to softer fast-neutron spectra via near-threshold reactions (e.g., 7Li(p,n)7Be), which could further reduce moderator size requirements and shorten pulse duration. System performance can be enhanced by moderator optimization, addition of reflectors and neutron guides, and increasing the incident fast-neutron flux via improved laser-driven ion sources. Together, these results position laser-driven neutron sources as promising, compact alternatives for neutron science in smaller laboratory settings.

A proof-of-principle laser-driven cold neutron source was realized by directing laser-accelerated light ions onto a Be converter to produce fast neutrons and moderating them in a compact, cryogenic H2 cell. The experiment provided the first laser-based demonstration of cold neutron generation, measured fluxes at a practical distance, and established agreement with Monte Carlo predictions for spectral features and isotropy. With advancements in high-repetition-rate, high-energy lasers and further moderator and beamline optimizations (e.g., reflectors, guides, near-threshold reactions), such systems are poised to support scattering and imaging experiments in compact facilities. The approach can also serve as a pre-moderator stage for ultra-cold neutron (UCN) production using additional SD2, opening avenues for precision measurements such as neutron lifetime studies with a compact light source.

• Detector effects: Early gamma flash caused temporary saturation in the 3He detection, leading to mismatch in the epithermal region of the spectrum for several microseconds.
• Pre-moderator thickness: The wing-shaped polyethylene pre-moderator was intentionally thin, limiting moderation to near-thermal energies and contributing minimally to the cold peak.
• Temperature monitoring: Moderator temperature was measured on the housing; the actual hydrogen temperature inside the cell may have been higher, introducing uncertainty in the exact cold-spectrum temperature.
• Geometry and distances: Measurements were reported at a nearest practical distance (~20 cm) from the moderator exit; fluxes at the immediate exit surface were inferred or limited by detector placement and shielding constraints.
• Current flux levels: While sufficient for proof-of-principle, higher fluxes and optimized transport (e.g., reflectors, guides) are needed for many applications and are planned for future iterations.