Neutrons are invaluable tools for scientific research across various disciplines, including material science, chemistry, and biology. Traditionally, neutrons are generated using nuclear reactors and spallation sources, both of which have limitations. Nuclear reactors, while offering continuous neutron beams, produce significant nuclear waste and often have long pulse durations unsuitable for certain applications. Spallation sources generate high-flux neutron bursts, but require large-scale, expensive facilities. The demand for compact, cost-effective neutron sources has led to the development of Compact Accelerator-driven Neutron Sources (CANS). This research explores the feasibility of a laser-driven neutron source as a potential alternative, offering the advantages of compactness and cost-effectiveness while providing high-brightness, temporally-resolved neutron beams. The advancements in high-intensity laser technology and laser-driven ion acceleration techniques have paved the way for exploring laser-based neutron sources. This paper details the initial steps in developing a table-top laser-driven cold neutron source, focusing on the experimental verification of cold neutron production and flux estimation.

The paper extensively reviews existing neutron sources, highlighting the advantages and drawbacks of fission reactors and spallation sources. It discusses the limitations of existing methods, such as the high cost and size of spallation sources, and the nuclear waste associated with fission reactors. The review then shifts to the emerging field of laser-driven neutron sources, highlighting advancements in high-intensity lasers and ion acceleration techniques. Specifically, it mentions the Target Normal Sheath Acceleration (TNSA) mechanism and the pitcher-catcher technique, where high-energy ions generated by laser-matter interaction are used to drive neutron-producing reactions in a converter target. The importance of neutron moderation for achieving cold neutrons and its underlying principles are also discussed, along with the selection criteria for moderator materials. The relevance of cold neutrons for various applications is also discussed, demonstrating the need for a compact, efficient cold neutron source.

The experiment was conducted at the Institute of Laser Engineering (ILE), Osaka, using the LFEX laser system. A 1.2 ps, 300 J laser pulse was focused onto a 5 µm thick C₂D₄ target (the 'pitcher'), producing ions via TNSA. These ions, predominantly protons and deuterons, then impinged on a beryllium target (the 'catcher') generating fast neutrons through Be(d,n)¹⁰B and Be(p,n)⁸B reactions. The ion beam energy distribution was measured using a Thomson Parabola spectrometer, revealing proton energies up to ~20 MeV and deuteron energies up to ~5 MeV. A wing-shaped polyethylene pre-moderator was employed to thermalize some off-axis neutrons before reaching the main cryogenically cooled hydrogen moderator (~11 K). The main moderator cell had a hydrogen thickness of ~27 mm along the beam axis and was surrounded by copper reflectors. The fast neutrons (MeV range) were detected using an EJ-232Q plastic scintillator at 8.2 m and 15° off-axis. A time-of-flight (ToF) technique determined their energy distribution. Cold neutrons (≤ 25 meV) were detected using 3He proportional counters positioned on-axis at 3.28 m, shielded to reduce background radiation. Background subtraction was crucial for accurately measuring the cold neutron flux from the moderator. PHITS Monte-Carlo simulations provided insights into the neutron angular distribution and pulse duration for different neutron energies.

The experiment successfully generated a cold neutron flux of ~2 × 10³ n/cm²/pulse at a distance of 20 cm from the moderator exit. This represents the first successful demonstration of a cold neutron source driven by a laser. The measured cold neutron energy distribution showed a broadened peak extending down to the meV range. This peak was consistent with the expected distribution based on the moderator's cryogenic temperature. Monte-Carlo simulations accurately modeled the observed cold neutron spectrum. Analysis of the fast neutron signal using the plastic scintillator detector revealed a high flux in the MeV range (≤10⁹ n/sr/pulse). The pulse duration of the cold neutrons was simulated using PHITS and found to be in the range of 1-100 µs for energies below 1 eV, decreasing to below 100 ns for energies above 10 eV. The temperature of the hydrogen moderator cell was successfully maintained around 11K during the experiment using helium cooling. The experimental results showed a good agreement between the measured cold neutron flux and the results from Monte Carlo simulations, validating the experimental setup and methodology.

The successful generation of cold neutrons using a laser-driven source demonstrates a significant step towards the development of compact, cost-effective neutron sources. The relatively high cold neutron flux achieved in this proof-of-principle experiment highlights the potential of this approach for various applications. The measured pulse duration of the cold neutrons suggests that this source is suitable for time-resolved neutron scattering experiments. Furthermore, the use of a cryogenically cooled hydrogen moderator efficiently moderates the fast neutrons generated by the laser, increasing the cold neutron yield. The good agreement between the experimental data and Monte Carlo simulations strengthens the validity of the findings and suggests that further optimization of the system parameters is feasible. Future experiments can focus on optimizing the neutron yield and pulse duration by improving the moderator design and utilizing higher-intensity lasers.

This research demonstrated for the first time the feasibility of generating a cold neutron source using a laser-driven fast neutron source and a cryogenically cooled hydrogen moderator. The measured cold neutron flux and energy distribution confirmed the successful moderation of fast neutrons to cold neutron energies. This achievement paves the way for future development of compact, laser-based neutron sources for various applications in neutron science. Future research should focus on enhancing the neutron flux by improving the moderator design and employing higher-repetition-rate, higher-energy lasers. The use of different neutron-producing reactions and advanced moderator configurations to potentially achieve higher cold neutron fluxes and shorter pulse durations can also be explored.

The current experiment demonstrated a relatively low cold neutron flux compared to conventional neutron sources. Further improvements in the laser system, moderator design, and neutron detection efficiency are necessary to increase the flux and achieve practical applications. The limited size and thickness of the pre-moderator may have limited the thermalization of off-axis neutrons. The γ-ray saturation in the detectors at epithermal energies caused a mismatch between the experimental data and simulations. These limitations, however, are inherent to the early stage of development and can be addressed with further research and technological advances.