Neutron nuclear physics

Participating countries and international organizations:

Albania, Armenia, Azerbaijan, Belarus, Botswana, Bulgaria, CERN, China, Croatia, Czech Republic, Cuba, Egypt, Finland, France, Georgia, Germany, Hungary, IAEA, India, Italy, Japan, Kazakhstan, Moldova, Mongolia, North Macedonia, Poland, Republic of Korea, Romania, Russia, Serbia, Slovakia, Slovenia, South Africa, Switzerland, Thailand, Turkey, USA, Uzbekistan, Vietnam.

The problem under study and the main purpose of the research:

 Nuclear physics research with neutrons is traditionally one of the priority areas developed at JINR. These studies are carried out within the framework of the scientific theme “Investigations of neutron nuclear interactions and properties of the neutron”
(03-4-1128-2017/2023). The integrated use of the FLNP basic facilities (IREN pulsed source of resonance neutrons, IBR-2 pulsed reactor, EG-5 electrostatic generator, as well as TANGRA facility) makes it possible to conduct nuclear physics research in a wide range of neutron energies from cold neutrons to ~20 MeV, and the use of external neutron sources, such as the n_TOF neutron time-of-flight facility at CERN, allows expanding the energy range to several hundreds of MeV.

The research and development activities within the framework of the theme are aimed at implementing the tasks formulated in the proposals for the JINR Seven-Year Development Plan 2024–2030 in the field of “Nuclear Physics”. The physics investigations can be grouped into three research areas:

1. Study of violations of fundamental symmetries in the interactions of neutrons with nuclei, obtaining nuclear data.

2. Study of fundamental properties of the neutron, physics of ultracold and very cold neutrons.

3. Applied and methodological research.

The scientific program of the theme “Neutron Nuclear Physics” will be implemented within the framework of three projects: two scientific ones (“Investigations of neutron nuclear interactions and properties of the neutron” and “TANGRA”) and one scientific and technical project (“Modernization of the EG-5 accelerator and its experimental infrastructure”). Work on the development of the concept of a UCN source at a pulsed reactor is planned to be singled out as a separate activity.

Brief annotation and scientific rationale:

Information about neutron-nuclear interactions is extremely important for both fundamental and applied physics. The fact that the neutron has no electric charge makes it a unique probe for studying nuclear forces. Due to electrical neutrality, the high penetrating power of neutron radiation makes it promising for studying the structure of matter at both the nuclear and molecular levels. Neutrons are also widely used for applied purposes: in inspection systems, non-destructive elemental analysis facilities, in instruments for studying the immediate environment of boreholes (logging), as well as in the creation of neutron and gamma radiation detectors used on board orbital and descent spacecraft for analysis of soil and atmosphere of celestial bodies. Information about neutron-nuclear reactions is also necessary for the design of promising nuclear power facilities, as well as for modeling various devices and objects that interact with neutron radiation in one way or another. An indicator of the relevance of studying the characteristics of neutron-nuclear interactions can be the fact that the list of the most requested nuclear data for the most part consists of queries directly related to neutron-nuclear reactions.

The TANGRA (TAgged Neutrons and Gamma Rays) project is aimed at studying neutron-nuclear reactions using the tagged neutron method, finding new ways to use neutron methods in fundamental and applied research, improving existing and creating new approaches to processing the results of nuclear physics experiments. One of the tasks to be solved within the framework of the project is the interpretation of existing experimental data on the reactions of interaction of fast neutrons with atomic nuclei, their systematization and validation. The priority area of work is the acquisition of nuclear data.

Expected results upon completion of the project:

1. Performing experiments to study the angular distributions of scattered neutrons.

2. Experimental study of (n,γ) and (n’,γ)-correlations.

3. Theoretical description of the studied reactions.

4. Conducting experiments to study the reaction (n,2n).

5. Conclusion on the applicability of the tagged neutron method for elemental analysis of soils. In case of a positive result, the creation of prototypes of stationary and mobile facilities, as well as methodological recommendations for their use for agricultural and environmental monitoring.

The results obtained during the implementation of this project will be valuable for both fundamental and applied science. The obtained experimental data on the yields and angular distributions of γ-rays can be used to increase the accuracy of Monte Carlo simulations of various physics facilities. Another planned application of the obtained experimental results is fast elemental analysis. Optimized model parameters can be used to theoretically describe previously unstudied reactions. The developed prototypes of facilities for elemental analysis of soils can become the basis for creating devices useful for intensifying agriculture and monitoring the state of the environment.

Expected results of the project in the current year:

1. Measurements of total and differential cross sections of characteristic gamma lines for various elements using high-resolution detectors of -rays.

2. Development of an experimental setup for studying (n’γ) correlations and performing test measurements.

3. Implementation of a part of the experimental program for the development and verification of a methodology for measuring carbon concentrations in soil.

4. Simulation of a facility for measuring carbon concentration in soil to develop a method for deep profiling of macroelement concentrations.

Brief annotation and scientific rationale:

The project is aimed at modernizing the main systems of the electrostatic charged particle accelerator EG-5, developing ion-beam and complementary methods for studying the elemental composition and physical properties of near-surface layers of solids.

Goals of the project: to provide technical feasibility for the implementation of the scientific program of the JINR Topical Plan in studying reactions with fast quasi-monoenergetic neutrons; development of nuclear physics methods for studying the elemental composition; solution of problems of neutron-radiation materials science; implementation of practical applications of neutron physics; ensuring technical feasibility for the implementation of the unique options of the microbeam spectrometer.

Objectives of the project. The main technical task of the project is to restore the energy range of accelerated particles of
900 keV – 4.1 MeV and increase the ion beam current to 100–250 μA while maintaining the energy stability of the ion beam at a level no worse than 15 eV, ensuring the spatial stability of the ion beam, sufficient to implement the option of the microbeam spectrometer / nuclear microprobe.

The main organizational task is the formation and development of human resources potential to ensure the full implementation of the project for at least 3 seven-year periods.

The objectives of the project also include the upgrade of the experimental infrastructure of the accelerator complex, in particular, the development of new methods for studying the physical properties of the surface of materials that can complement and improve the quality of the obtained scientific results, the intensification of international scientific and technical cooperation, the organization of user policy, the formation of an interlaboratory accelerator center on the basis of FLNP JINR to solve a wide range of unique scientific and technological problems.

The main criteria for the successful implementation of the project are providing a neutron flux sufficient to conduct nuclear physics experiments with fast neutrons, and an energy stability of the ion beam sufficient to create a microbeam spectrometer/nuclear microprobe.

Expected results upon completion of the project:

As a result of the implementation of the project, the technical parameters of the accelerator will be restored (energy of accelerated particles of 4.1 MeV at a maximum current of at least 100 μA), which will make it possible to conduct studies of reactions with fast neutrons at JINR, as well as provide technical conditions for the installation of a microbeam spectrometer. A neutron generator based on a solid-state lithium target with a moderator will be added to the existing neutron generator with a gas target, and the chamber for irradiating samples with ion beams will be modified.

A new specialized laboratory will be created for the preparation of objects of study, which will be equipped with complementary methods for studying the optical and electronic properties of the surface, such as ellipsometry, optical and electron microscopy, methods for studying electrical properties at direct and alternating current (voltammetry, impedancemetry).

In addition to modernization and expansion of the instrumental base of the accelerator complex, the formation of personnel potential for the next 20-30 years will be carried out. The available methods of elemental analysis will be supplemented by methods of analysis based on prompt gamma rays from inelastic neutron scattering and neutron activation analysis.

Modernization of EG-5 at JINR, where there are highly qualified specialists, good detecting equipment and valuable developments in the field of neutron investigations of atomic nuclei, will make it possible in the short term to conduct a number of new, unique experiments on obtaining the energy spectra and angular distributions of charged particles from (n, α) and (n, p) / (α, n) and (p, n) reactions and integral and differential cross sections of the latter in the neutron energy range up to ~6 MeV, on processes of fission of atomic nuclei by fast neutrons, activation analysis, experiments in the field of neutron materials science and etc.

Expected results of the project in the current year:
– certification and commissioning of the EG-5 accelerator and its experimental halls in test mode;

– replacement of the worn high-voltage accelerating tube and ion source that has lost its performance characteristics.

– reaching an ion beam current of over 100mkA;

– improvement and adjustment of the parameters of the ion optics of the EG-5 accelerator (reconfiguration from the focussator mode to the condenser);

– modernization and automation of the gas cylinder economy, adaptation of the technological scheme;

– modernization of the vacuum system;

– automation of all accelerator service systems;

– launch of a laboratory for engineering and sample research using complementary ion-beam methods;

– formation of the personnel potential of the group;

– implementation of technical projects, in particular, a project with JSC “Micron” “Carrying out preparatory work, including the manufacture of an ion beam scanning system in a raster, as well as test electron beam processing and testing using impedance spectroscopy of semiconductor wafers with a diameter of 150 mm in an amount of up to 20 pcs.”, a technical project with the State Corporation “ROSATOM” “Investigation of the dependence of the sensitivity of the UDKN-04R device on neutron energy”, projects with Angstrom JSC, etc.

Brief annotation and scientific rationale:

Nuclear processes and structural changes in materials induced by slow, resonance and fast neutrons and accelerated charged particles are traditionally in the focus of research attention at FLNP JINR. The interaction of neutrons with atomic nuclei is of interest for both fundamental and applied research.

The integrated use of the FLNP basic facilities (IREN pulsed source of resonance neutrons, IBR-2 pulsed reactor, EG-5 electrostatic generator) makes it possible to conduct nuclear physics research in a wide range of neutron energies from cold neutrons to ~20 MeV, and the use of external neutron sources, such as the n_TOF neutron time-of-flight facility at CERN, allows expanding the energy range to several hundreds of MeV. Fundamental research carried out at the FLNP Department of Nuclear Physics includes studies on the violation of space and time symmetry, the mechanism of nuclear reactions, the structure of atomic nuclei, fission processes induced by neutrons, neutron-induced reactions with the emission of light particles, the properties of the neutron as an elementary particle, the properties of ultracold and very cold neutrons, quantum mechanical effects involving neutrons.

Also, in FLNP, a variety of research programs has been developed for applied investigations, such as obtaining nuclear data and information on the radiation resistance of materials for nuclear technologies, power engineering and transmutation, radiation mutagenesis on fast neutrons, neutron activation analysis using thermal and epithermal neutrons, neutron activation analysis using prompt gamma-rays, elemental analysis using neutron resonances, elemental analysis using fast neutrons, analysis of the elemental composition of thin films, investigation of the radiation resistance of materials to the effects of accelerated charged particles on electrostatic accelerator beams, development of radiation-resistant nanostructured materials using accelerated ion beams.

Expected results upon completion of the project:

1. Refinement of characteristics of known resonances and detection of previously unknown ones. Measurement of reaction cross sections and product correlations in the resonance region with an accuracy sufficient to study P- and T-odd effects.

2. Performing experiments to study TRI and ROT effects in fission, measuring the mass-energy and angular distributions of fragments, prompt neutrons and gamma-rays; searching for rare and exotic fission modes, using both IBR-2 and third-party sources.

3. Conducting experimental and theoretical studies of neutron-nuclear reactions in a wide range of energies of incident particles.

4. Investigation of the pattern of nonstationary neutron diffraction by surface acoustic waves. Verification of the validity of generally accepted laws of neutron optics in the case of large accelerations.

5. Development of models for calculating the transport of UCN and CN in the material of nanodiamond reflectors and the extension of their applicability to the range of thermal neutrons.

6. Study of the structure of graphites after their intercalation and measurement of cross sections for cold neutron scattering by intercalated graphites.

7. Obtaining data for nuclear power engineering and astrophysics: measurement of integral and differential neutron cross sections, angular correlations in the energy range from cold neutrons to hundreds of MeV.

8. Study of radiation resistance of various materials, including those promising for use as neutron reflectors and moderators. Development and study of radiation resistance of electronic components, including those operating on new physical principles.

9. Development of energy and electronics devices using powder nanotechnology and ion beams.

10. Obtaining new data and monitoring the environmental situation in certain regions of the JINR Member States with the help of NAA.

11. Study of the influence of neutron irradiation on the properties of biological objects and tissues.

12. Investigation of layered structures, including high-temperature superconductors using RBS, ERD and PIXE techniques.

13. Performing elemental analysis of various objects of cultural heritage.

Expected methodological results:

1. Development of the method of neutron spin interferometry with UCN.

2. Determination of optimal technologies for the synthesis and modification of substances for use as UCN and CN reflectors.

3. Development of methods for cleaning water and soil, and assessing the quality of food products.

4. Study of the processes of accumulation of nanoparticles in the organs of animals and plants, assessment of their impact on the health of living objects under study.

5. Development of a technique for non-destructive elemental analysis using prompt gamma-rays. Improvement of existing methods of activation analysis using thermal and resonance neutrons.

6. Performing work on the manufacturing of electronics and ionizing radiation sensors based on new physical principles.

The fundamental results obtained during the implementation of the project will be of great importance for understanding the mechanisms of neutron-nuclear reactions and the development of theoretical ideas about these processes. The study of P- and
T-odd effects will provide information on the contribution of the weak interaction to nuclear forces and can serve as an alternative method for determining the mixing coefficient Vud of a CKM matrix. Obtaining new information about ROT and TRI effects, as well as exotic fission modes, will make it possible to clarify the features of one of the stages of this process – the scission of a fissile nucleus into fragments. The data obtained during the implementation of the neutron-optical part of the project will be needed to create new neutron moderators and reflectors. In addition, they will allow significant progress in the development of neutron microscopy methods and studies of the magnetic structure of various objects. The implementation of the applied research program of the project will be of great social importance and contribute to the progress of environmental, materials science, archaeological, and nanotechnological investigations, as well as promising developments in the field of modern electronics and energy. The techniques of elemental and structural analysis being created and modernized will be in demand in many branches of human activity.

Expected results of the project in the current year:

1. Preparation for measurements of P- and T-violation effects at the IREN facility. Carrying out test experiments to study the angular distributions of gamma-rays from resonance capture.

2. Modernization of the ENGRIN facility and measurement of prompt fission neutron multiplicity and mass-energy distributions of ²³⁵U(n_res,f) reaction products with higher accuracy.

3. Development and construction of a prototype setup for measuring angular correlations in the vicinity of p-wave resonances in medium-heavy and heavy nuclei on beamline 4 of the IREN facility: calculation and creation of shielding, determination of beam parameters, test measurements.

4. Reconstruction of a large liquid scintillation detector on a 60-m flight path of the IREN facility.

5. Determination of the elemental composition of a number of archaeological samples using neutron resonance analysis at the IREN facility.

6. Purchase of materials and equipment for the development and construction of a facility for studying the ROT effect on beamline 1 of the IBR-2 reactor. Performing test measurements of the angular distributions of prompt neutrons and
gamma-rays emitted during the fission of uranium nuclei.

7. Obtaining the results of an experiment to search for the true and pseudo-quaternary fission of ²⁵²Cf. Start of a project to modernize the instrument using more advanced Timepix3 detectors.

8. Development of a compact scintillation detector using a micropixel avalanche photodiode.

9. Measurements of cross sections for ⁶Li(n,α) and ¹⁴⁸Sm(n,α) reactions with fast neutrons.

10. Measurement of the forward-backward asymmetry coefficient in the reaction (n,p) for ³⁵Cl in the resonance energy region.

11. Carrying out measurements of cross sections of (n,p) and (n,α) reactions using gas samples (nitrogen, neon).

12. Assessment of atmospheric precipitation of heavy metals in the JINR Member States.

13. Development of new methods of remediation of soils and wastewater.

14. New results on the study of the effect of metal nanoparticles and neutron radiation on living organisms.

15. Study of the possibility of setting up a new experiment with UCN to test the weak equivalence principle with an accuracy
of 10^–4.

16. Measurement of the Goos-Hänchen shift in the experiment on the total reflection of a neutron wave from a resonance structure (provided that the beamtime is available at a modern high-resolution neutron reflectometer).

17. Completion of studies of hydrogen-containing impurities in fluorinated detonation nanodiamonds (FDND).

18. Carrying out studies of the radiation resistance of FDND oxide, metal and high-entropy compounds.

19. Measurement of neutron scattering cross sections of DND powder depending on its density, as well as of fluorinated intercalated graphite.

20. Completion of development of the concept of a UCN source on the basis of a pulsed reactor.

21. Obtaining mutants for breeding drought-resistant and saline soils of rice and wheat varieties.

22. Development of homogeneous electronics and adsorption energy devices for critical and promising construction technologies. 

Brief annotation and scientific rationale:

Since the discovery of ultracold neutrons (UCN), a number of intense UCN sources have appeared in the world, and several more of them are under construction. There is no UCN source in Dubna, which is largely due to the features of the IBR-2M reactor. Its average power of 2 MW is relatively low for creating a steady-state UCN source, while the repetition rate of 5 Hz is too high to accumulate neutrons produced in each individual pulse. However, the pulsed flux of thermal neutrons from the reactor is very high, since the interval between pulses is hundreds of times longer than their duration.

A specific feature of the future UCN source at JINR is the pulsed filling of the trap, when neutrons arrive in it only during the pulse, while the rest of the time the trap remains isolated. The practical implementation of this idea is hindered by the fact that, due to the presence of biological shielding, the trap is far from the moderator in which UCNs are generated, and has to be connected to it by a transport neutron guide. In this case, the spread of transport flight times can significantly exceed the intervals between pulses, which makes the very idea of accumulation meaningless. To solve this problem, it was proposed to use a special device — a temporary lens that changes the energy of neutrons in a dosed manner as they arrive at this lens. Such a device makes it possible to restore the pulsed structure of the neutron beam immediately before entering the trap.

Recently, the idea of pulsed filling of a UCN trap has been the subject of intense discussion in the literature. Alternative approaches to time focusing of neutrons and methods for slowing down faster, so-called very cold neutrons (VCN) to energies characteristic of UCN have emerged. There have appeared theoretical works devoted to aspects of the formation of a neutron pulse by a time lens, as well as to the features of the time structure of a neutron beam when using a flipper moderator with a strong magnetic field. As a result, a significant number of ideas and proposals have emerged that can form the basis of a project for a new UCN source.

The aim of the work within the framework of “Activities” is to formulate the concept of a UCN source in a pulsed reactor on the basis of an analysis of both existing and some new ideas regarding the transport of UCN, the evolution of the duration of neutron bunches and the formation of the optimal time structure of bunches at the entrance to the trap. This can be either the IBR-2M reactor available at FLNP or the NEPTUN reactor currently being designed. It is expected that the final UCN spectrum at the entrance to the trap will be formed by slowing down the VCN.

Expected results upon completion of the activity:

Development of a conceptual design for an ultracold neutron (UCN) source at a pulsed reactor.

Expected results of the activity in the current year:

1.  Development of a design for a neutron flipper-moderator with an energy transfer of the order of µeV.

2. Selection of a fast shutter design that ensures pulsed filling of the UCN trap and minimally affects the density of neutrons stored in the trap.

3. Analysis of possible variants of a UCN converter-moderator, which would provide the highest UCN flux density at the desired pulse duration.

Collaboration