Since their discovery over 90 years ago, neutrons have become one of the premier tools in the study of the structure and dynamics of matter and materials. The main nuclear processes to generate a large number of free neutrons are fusion, fission, and spallation, which have been well established for using neutrons in broad areas of physics, material science, engineering, life sciences, and elsewhere. The vast majority of experiments that use neutrons as a probe require a directional, well-collimated beam of neutrons. Over the years, methods have been developed to deliver such neutron beams sufficiently, but it is still much desired to improve the efficiency of neutron sources. With the advent of high-powered lasers, laser-driven neutron sources suggest an attractive possibility. Laser photons can be converted to neutrons by accelerating particles (electrons, protons, and deuterons) and then either utilize hard x rays from, for example, electron acceleration to create photoneutrons or nuclear reactions, such as deuteron break-up. The maturity of such processes in recent years might have reached a state where such neutron sources are becoming useful and beneficial to the neutron community. In the present report, the current state-of-the-art of a laser-driven neutron source and its future development for neutron applications are presented, and existing sources are described. The basic physical principles of laser-driven neutron production and the current state-of-the-art of production techniques are outlined. The potential developments and the role of such sources in the landscape of neutron sources in the future are critically commented on.

Abstract We present calculations concerning the surface gravitational redshift in neutron star at finite temperature using general relativity. The method is presented explicitly in detail. Numerical applications are shown and discussed. Confrontation with experiment and other calculations is performed with some success. In addition to use more complex cold or hot equation of state for the nuclear matter than the free neutron one to describe massive neutron stars, this work shows that we can also take into account the temperature.

Neutron star physics in the multi-messenger era

This article presents a comprehensive set of experimental and computational methods aimed at determining heat transfer regimes in superfluid helium-4, evaluating thermal resistance at various temperatures, and developing a scalable model for analyzing the performance of an ultracold neutron (UCN) converter. The results obtained constitute an essential part of the scientific foundation required for the development and optimization of modern UCN sources, which operate on the principle of neutron thermalization through inelastic interactions with the medium. This study is fundamental in nature and is oriented toward addressing strategically important problems in neutron physics, such as refining the neutron lifetime and searching for the neutron electric dipole moment. These challenges are crucial for advancing our understanding of nucleosynthesis and the formation of matter in the early Universe. The advancement of heat transfer models in cryogenic environments, particularly in superfluid helium, plays a key role in ensuring the stability and efficiency of UCN sources. The paper discusses approaches to the metrological verification of thermal parameters and provides recommendations for applying the results in future engineering and design developments.

To enable monitoring of neutron physics parameters during the operation of liquid lead-bismuth targets, a fiber based neutron detector system for neutron flux measurement was developed. The system comprises a scintillator probe, quartz optical fiber, silicon photomultiplier tube (SiPM) module, FPGA-based data acquisition card, and associated software. Given the high neutron flux and broad energy range in spallation targets, stringent requirements are imposed on the detector design. This study evaluates neutron scintillation materials based on 6LiF/ZnS,6LiCaAlF6, and Gd2O2S:Tb to fabricate neutron probes tailored to the neutron field characteristics of spallation targets. By characterizing the detector's temperature response, a temperature compensation coefficient of 21.3 mV/° was determined to enhance detector stability. Through optimization of the detector threshold and integral value settings, the false count rate can be reduced below 1.3 × 10-3. The fiber neutron detector developed based on this was subjected to validation experiments at the white neutron beam. The detector demonstrates good linearity and consistency at a neutron flux of 6.35 × 105 ∼ 8.58 × 106n/cm2/s. The switching of the cadmium absorber sheet at the collimator exit will verify the detector's response to fast and thermal neutrons and provide a foundation for future measurements of higher neutron flux in high-power spallation targets.

Quantitative Geometrical Thermodynamics (QGT) exploits the entropic Lagrangian-Hamiltonian canonical equations of state as applied to entities obeying the holographic principle and exhibiting Shannon information, the creation of which measures the (validly defined) "entropic purpose" of the system. QGT provides a physical description for what we might consider the true "atoms" of physical science and has also recently enabled a number of significant advances: accounting ab initio for the chirality of DNA and the stability of Buckminsterfullerene; the size of the alpha particle (and other nuclear entities) and the lifetime of the free neutron; and the shape, structure, and stability of the Milky Way galaxy. All these entities, ranging in size over more than 38 orders of magnitude, can each be considered to be an "atom"; in particular, the size of the alpha is calculated from QGT by assuming that the alpha is a "unitary entity" (that is, than which exists no simpler). The surprising conclusion is that clearly compound entities may also be physically treated as unitary ("uncuttable") according to a principle of scale relativity, where a characteristic size for such an entity must be specified. Since QGT is entropic, and is therefore described using a logarithmic metric (involving hyperbolic space), it is not surprising that the length scale must be specified in order to account for unitary properties and for an entity to be appropriately considered an "atom". The contribution to physics made by QGT is reviewed in the context of the related work of others.

As a representative Generation IV sodium-cooled fast reactor (Gen-IV SFR), neutron physics characteristics studies of the China Experimental Fast Reactor (CEFR) core are crucial for its safety case. In this study, a three-dimensional core model of the CEFR was developed using the Monte Carlo-based MCNP5 code, with its reliability validated through five neutronics benchmark experiments. Based on this model, the fundamental neutronics characteristics of minor actinide (MA) transmutation in the sodium-cooled fast reactor were investigated. The results demonstrate that as the minor actinide (MA) loading fraction in the core increases from 0% to 8%, the effective multiplication factor (Keff) exhibits a significantly nonlinear decrease, accompanied by a corresponding reduction in neutron flux, necessitating increased fuel enrichment to maintain core criticality. Opposite impacts on reactivity are observed for different MA nuclides: 237Np, 241Am, 243Am and mixed MA reduce Keff, whereas 244Cm and particularly 245Cm significantly enhance Keff. The reactivity change rate sharply decreased from −1242.5 to −312.7 pcm/wt%, clearly demonstrating saturation effects in MA neutron absorption. Crucially, reactivity remained deeply negative across all operational scenarios, with safety requirements being satisfied even at maximum MA loading levels, confirming the inherent safety of the proposed approach.

Abstract Uncertainty quantification (UQ) in well log prediction remains a significant challenge within geoscientific research, as conventional methodologies predominantly yield single-point estimates without addressing the associated predictive uncertainty. The absence of uncertainty assessment may result in suboptimal decisions during the exploration phases and reservoir characterization. This study addresses this limitation by applying the Conformalized Quantile Regression (CQR) technique to predict missing AC (P-Sonic) log curves, while simultaneously quantifying the uncertainty of these predictions. The analysis utilises nine LAS files from the Volve Field, with exploratory data analysis revealing substantial missingness, particularly in AC (P-sonic) logs. Pearson correlation analysis identified strong relationships between AC (P-Sonic), density (DEN), and neutron (NEU) logs. The conformal prediction framework partitions the dataset into training, validation, and test subsets, enabling the derivation of statistically valid prediction intervals. The CQR approach achieved AC (P-sonic) log predictions with 95% coverage. This work demonstrates the value of uncertainty quantification in well log prediction by providing accurate predictions accompanied by reliable prediction intervals, thereby supporting robust and informed decision-making in exploration and reservoir characterization. The methodology developed herein offers a comprehensive tool for risk management, enabling geoscientists to proactively identify and mitigate potential adverse outcomes associated with well-log data utilisation.

Abstract The precise determination of the free neutron lifetime is of great significance in modern precision physics. This key observable is linked to the mixing of up and down quarks via the Cabibbo-Kobayashi-Maskawa matrix element $$V_{ud}$$ V ud , and the abundance of primordial elements after the Big-Bang Nucleosynthesis. However, the two leading measurement techniques for the neutron lifetime currently yield incompatible results, a discrepancy referred to as the neutron lifetime puzzle. To address the systematic uncertainties arising from neutron interactions with material walls, the $$\tau $$ τ SPECT experiment employs a fully magnetic trap for ultra-cold neutrons (UCNs). UCNs velocities are extremely low-energy neutrons with typical velocities below $$8\,\text {m/s}$$ 8 m/s , which can be manipulated using magnetic fields, gravity, and suitable material guides, whose surface can reflect them at any angle of incidence. To precisely study and characterize UCN behavior during production, guidance, storage, and detection in $$\tau $$ τ SPECT, we have developed a dedicated simulation framework. This framework is built upon the externally developed UCN Monte Carlo software package and is enhanced with two companion tools: one for flexible and parametrizable upstream configuration of such that the simulation’s input settings can be adjusted to reproduce the experimental observations. The second package is used for analyzing, visualizing, and animating simulation data. The simulation results align well with experimental data obtained with $$\tau $$ τ SPECT at the Paul Scherrer Institute and serve as a powerful resource for identifying systematic uncertainties and guiding future improvements to the current experimental setup.

The synthesis of free neutrons into free protons and deuterium, tritium, helium-3 and helium-4 nuclei has been studied under the coronal conditions of the Cygnus X-2 accretion disks. By studying the temporal evolution of the mass fraction abundances of neutrons and of the hydrogen and helium isotopes, it was possible to estimate the average lifetime of free neutrons, as well as the final relative distributions of protons with respect to their isotopes, and of helium-4 with respect to helium-3. Similarly, the mean, maximum and minimum values of the mass fraction abundances of each species were obtained, together with the absolute distributions for each one. Simulations were performed for 10 different temperatures, which allowed conclusions to be drawn about how temperature influences the production and consumption of the nuclei studied.

It is necessary to create a reserve of the maximum service life of fuel and structural materials to increase the service life of fuel rods in a nuclear reactor. This paper considers various methods of reducing the radial unevenness of fuel element burnup by redistributing fissionable nuclei (profiling) using the example of the unit cell of the VVER-1200 reactor in the infinite breeding medium. The fuel pellet is divided into a different number of concentric layers to be profiled, the enrichment of the layers varis while maintaining the total number of fissile nuclei in the fuel pellet at the beginning of the fuel campaign. In the Serpent 2 PC, neutron physics calculations of the corresponding models are performed, after that the characteristics obtained for them are compared: the infinite multiplication factor, the average rate of reactivity loss, the proportion of delayed neutrons at the beginning of the campaign, radial burnup profile and energy release distributions (at different time points), and changes in reactivity effects after profiling. The result of this work is a selected optimal method to reduce burnup on the fuel pellet periphery by increasing enrichment in the central part of the fuel pellet and, as a result, a more uniform burnup profile in the whole of the tablet. The research shows that there is no change in the neutron-physical characteristics as a result of the transformations carried out. A possible problem when using this profiling method may be an increase in the peripheral layer temperature as a result of a significant increase in energy release in it during the fuel campaign.

Monte Carlo (MC) simulations are considered the gold standard for calculating radiation dose in complex radiation fields. However, these simulations often require substantial computational resources. Based on our team's existing graphics processing unit (GPU) modules for photons and electrons/positrons, this research developed neutron GPU physics modules including elastic scattering, inelastic scattering, radiative capture, and fission. These were integrated into the Neudep (GPU-based NEUtron-photon-electron/positron coupled Dose Estimation Program). This program enables coupled multi-particle transport of neutrons, photons, and electrons/positrons across broad energy ranges and incorporates comprehensive physics for all particle interactions. During neutron interactions, photons and secondary neutrons are produced. These photons undergo various physical processes: the photoelectric effect, Compton scattering, and pair production, generating photoelectrons, Compton electrons, and recoil electron-positron pairs, respectively. The associated electron interactions include bremsstrahlung, ionisation, and multiple scattering. Bremsstrahlung, in particular, gives rise to secondary photons. Additionally, positron annihilation results in the production of secondary photons. All these secondary particles are stored in a memory stack and are transported only after the primary neutron transport process is completed. The Neudep program was validated for accuracy and tested for computational efficiency using both a homogeneous Water Phantom and the Chinese adult male voxel model (CRAM). The results indicate that the energy deposition discrepancies between Neudep and the reference MC code are less than 2%, with neutron incident energies of 3 MeV showing deviations of less than 0.5%. Organ dose differences generally remain within 5%. While maintaining computational accuracy, the Neudep program efficiently simulates 1 million neutrons in just 2 s. Additionally, the transport time for 10 million neutrons through a complex human model can be reduced to under 1 min. Neudep can reduce computation times by 78-5000 times compared to traditional central processing unit-based MC software. This tool demonstrates tremendous potential for rapid and accurate dose calculations.

Cross sections of the 6Li(n, t)4He reaction were measured in the fast neutron energy range from 3.3 to 5.3 MeV using a gridded ionization chamber (GIC) and well–calibrated experimental setup at the EG–5 Van de Graaff accelerator of the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research (FLNP, JINR). Lithium fluoride (6LiF) samples with varying thicknesses and krypton–CO2 gas mixtures with different pressures were used to optimize the detection of both alpha particles and tritons. Neutron fluxes were monitored using two high–purity (99.999%) 238U3O8 samples placed inside the GIC, complemented by an externally calibrated 3He long counter. The measured 6Li(n, t)4He cross–section data were compared with existing results of measurements and evaluations from EXFOR and ENDF nuclear data libraries, and the results showed a good agreement in the measured neutron energy range. These new measurements provide reliable cross–section data that contribute to the refinement of evaluated nuclear data files and support applications in nuclear physics, tritium production, and reactor design.

Nucleon structure functions, as measured in lepton-nucleon scattering, have historically provided a critical observable in the study of partonic dynamics within the nucleon. However, at very large parton momenta, it is both experimentally and theoretically challenging to extract parton distributions due to the probable onset of nonperturbative contributions and the unavailability of high-precision data at critical kinematics. Extraction of the neutron structure and the d quark distribution have been further challenging because of the necessity of applying nuclear corrections when utilizing scattering data from a deuteron target to extract the free neutron structure. However, a program of experiments has been carried out recently at the energy-upgraded Jefferson Lab electron accelerator aimed at significantly reducing the nuclear correction uncertainties on the d quark distribution function at large partonic momentum. This allows leveraging the vast body of deuterium data covering a large kinematic range to be utilized for d quark parton distribution function extraction. In this Letter, we present new data from experiment E12-10-002, carried out in Jefferson Lab Experimental Hall C, on the deuteron to proton cross section ratio at large Bjorken x. These results significantly improve the precision of existing data and provide a first look at the expected impact on quark distributions extracted from parton distribution function fits.

The lifetime of free neutrons measured in the laboratory has a longstanding disparity of ≈9 s. A space-based technique has recently been proposed to independently measure the neutron lifetime using interactions between the galactic cosmic rays and a low atmosphere planetary body. This technique has not produced competitive results yet due to constraints of nonoptimized data that contain large systematic errors. We use data from the neutron spectrometer on-board NASA's Lunar Prospector, and study two large systematics in the measurement of neutron lifetime: the lunar subsurface temperature and the lunar surface composition. We use the HeCd and HeSn neutron spectrometer data when the spacecraft was in a highly elliptical orbit during the orbit insertion period. We report the neutron lifetime using four different models that each have different choices of surface temperature and composition. The 5∘ [Prettyman ., ] and 2∘ [Wilson ., ] rebinned maps result in 777.6±11.7 s and 739.6±10.8 s, respectively. For the 20∘ map (Prettyman ., 2006), constant equatorial and a latitude-dependent temperature model result in 738.6±10.8 s and 767.3±11.2 s, respectively. Increasing the complexities of the models accounting for the systematic effects increase the measured lifetime. However, the reported measurements are not competitive with the laboratory results due to large unaccounted systematics resulting from nonoptimized measurements and modeling assumptions. This work serves as a study of systematic uncertainties for future neutron lifetime measurements using the space-based technique. We estimate the effect on the lifetime from the choice of temperature model to be to be 28.7±15.5 s, and choice of compositional map (for 20∘ and 5∘ maps) to be 10.3±12.2 s. Published by the American Physical Society 2025

Abstract We explore the decay of free neutrons into exotic long-lived particles, whose decays could be detected in the next-generation free neutron experiments. We show that such a possibility is viable as long as the exotic particle is highly mass-degenerate with the neutron, avoiding exclusion by large-volume detectors. We estimate the number of observable events and identify the most promising final states from both theoretical and experimental perspectives. Our analysis highlights the unique capability of the HIBEAM-NNBAR experiment at the European Spallation Source to probe this unexplored region of parameter space, opening a new avenue for exploring physics beyond the Standard Model. We estimate that several events per year could be observed in the NNBAR experiment.

In nuclear physics experiments involving neutron measurements, spectroscopic information such as neutron energy, width, and intensity can be measured using the time-of-flight (TOF) method. Analyzing TOF spectra is often challenging because neutrons suffer severe scattering from various materials in the surrounding experimental apparatus. Therefore, it is inevitable to evaluate scattered neutrons with the experimental setup and decompose TOF lines into direct and out-scattered components. We demonstrate the utility of the large-area neutron detector MAGRO for counting low-energy neutrons down to 300keV in the β-delayed neutron-decay study of rare isotopes. We performed Monte Carlo simulations using GEANT4 to investigate the detector's responses to incident radiation. In the simulations, physics models for optical physics and high-precision neutron physics were invoked. The optical physics parameters were adjusted to reproduce experimental data for light attenuation and propagation time, which were measured with the 90Sr β source. The simulation codes were further validated with in-beam tests at HIMAC (Heavy Ion Medical Accelerator in Chiba) of QST (National Institutes for Quantum Science and Technology) and RCNP (Research Center for Nuclear Physics) of Osaka University for intrinsic efficiency and TOF measurements using 16C and 17N beams. The neutron efficiency was calculated for a single MAGRO detector from 200keV to 7MeV, and it is shown that the calculated efficiency curve follows the measured data below 2MeV. TOF spectra were reproduced in the simulations with the details of the experimental setup for β-n-γ coincidence, providing a unique way to identify out-scattered neutron events in the measurements.

Traditionally, when conducting neutron physics calculations in cases where the moderator is a steam + water system, the effect of vaporization, for example, in boiling reactors of the BWR type, is taken into account homogeneously by reducing the density of water in accordance with the proportion of steam. The paper attempts to investigate the effect of the heterogenic structure of water + vapor bubbles on the neutron-physical characteristics of the fuel assemblies of the BWR reactor. To create the model, data from the OECD/NEA Burnup Credit Criticality Benchmark Phase IIIB are used. The research is carried out using the SERPENT-2 software package which allows randomly scattering spheres of various radii filled with various materials in different areas of the reactor core. The calculations are performed using the JEFF-3.1.1 library. The dependence of the observed effect on the radii of steam bubbles and on the proportion of steam in the coolant is investigated. It is found that the differences for homogeneous and heterogeneous fuel assembly models can be up to 0.3% in the value of Kinf, which is significantly higher than the accuracy with which calculations are performed (~0.01%). It is shown that α decreases with increasing vapor bubble size when compared with the homogeneous model. Thus, a change in the neutron spectrum is justified, which affects the change in the multiplication coefficient. A calculating study of the void effect revealed that discrepancies reach 8% when considering models with different vapor contents and identical vapor bubble sizes. This value is important for the correct interpretation of the reactivity power effect in fuel optimization problems. In particular, it is relevant to justify the introduction of new fuels.

A calculated estimate of the neutron-physical characteristics of a light-water reactor core with a changing neutron spectrum is presented. The software tool DESNA-7 neutron physics module, designed for three-dimensional modelling of the core in a two-group approximation, is used. The calculation of the neutron-physical cross sections is carried out in the software tool SAPHIR-95.1. Fuel assembly models with guide channels for the placement of absorbing elements or mobile displacers is developed forming two cores with the same thermal power and different ways of regulating reactivity: boron and spectral (with partial boron). To compensate the excess reactivity partially, gadolinium fuel rods in the form of Gd2O3 with a concentration of 5% by weight are used. Uranium oxide with natural content of U5 is used as the material for the displacers. The two methods of displacer extraction are considered: linear displacement during the campaign and complete extraction of displacers (stationary from the beginning of the campaign) on day 326. They do not lead to a significant change in the critical concentration of boric acid. It is shown that the use of spectral regulation makes it possible to reduce the concentration of boric acid in the coolant by 35 percent. It is shown that the reactivity coefficients in terms of temperature of fuel and coolant, and density of coolant increase modulo during the campaign, maintaining their sign. The value of the reactivity coefficient for the concentration of boric acid in the first reactor circuit, depending on the water-uranium ratio, decreases modulo. The maximum value of the axial power peaking factor in the core with spectral regulation is 1.6. The possibility of using absorbing rods placed in the fuel assembly guide channels as emergency protection devices is considered.

Abstract Neutron physics is one of the oldest branches of the experimental nuclear physics, but the investigation of the spontaneous neutron emission from the ground state along the neutron dripline is still at its beginning, in spite of the crucial importance for nuclear astrophysics. The proton dripline is much better investigated and a systematics of spontaneous proton half lives corected by the centrifugal barrier (monopole transitions) is given by the Geiger–Nuttall law log 10 T ∼ χ , where χ ∼ ZQ −1/2 is the Coulomb parameter characterizing the outgoing Coulomb–Hankel wave in terms of the daughter charge Z and Q-value. Our purpose is to propose a similar simple systematics of spontaneous neutron half lives, but in terms of the nuclear reduced radius ρ = κR ∼ A 1/3 Q 1/2, characterizing the ‘neutral’ outgoing spherical Hankel wave. It turns out that the half life in emission of neutral particles is governed by the scaling law T ∼ ρ −2 ∼ A −2/3 Q −1 for monopole transitions. We evidence the important role of the angular momentum carried by the emitted neutron. The influence of the neutron wave function generated by a Woods–Saxon nuclear mean field is also analyzed.