Since 2014, JINR has been developing the TANGRA interlaboratory project (TAgged Neutrons and Gаmma RAys) with facilities based on the tagged neutron method (TNM). Within the framework of the project, specialists are currently developing a prototype of a mobile device for carbon analysis in soil. Experts are going to use this tool at carbon test sites – special territories for the development and testing of technologies for measuring emissions and absorption of greenhouse gases.

The tagged neutron method was developed in the 60s. However, specialists did not widely use it due to the fact that they could obtain neutrons only at stationary equipment installed at accelerators. In the 2000s, TNM received a new impetus. At that time, experts created the first compact portable neutron generator that produced tagged neutrons. For example, an accelerator of the Laboratory of Neutron Physics JINR, which was used for the first experiments to study TNM, occupies a tower with a height of 6 floors. As for a portable neutron generator, it has a size of 30 cm and a weight of 8 kg.

What are neutrons tagged with?

Two hydrogen isotopes collide in the portable neutron generator. Deuterons accelerated up to an energy of about 100 keV collide with a tritium target and produce neutrons with an energy of 14 MeV. A deuterium-tritium reaction produces not only a neutron, but also an alpha particle (a nucleus of the helium atom 4He) that travels in the opposite direction to the neutron, almost 180°.

“If researchers manage to register the α-particle, then they can determine the direction that the neutron produced together with it has taken. This procedure is called neutron tagging. In a substance, a tagged neutron induces inelastic scattering reactions, in which a nucleus stops being excited emitting gamma quanta. The energy spectrum of the γ-quanta of each element is unique and serves as some kind of fingerprints, allowing identifying one element or another,” a chief researcher at the Laboratory of High Energy Physics JINR, a participant of the TANGRA collaboration Mikhail Sapozhnikov explained the essence of the method.

Fig. 1. General scheme of the tagged neutron method

Scintillation gamma detectors, devices that emit light when exposed to ionizing radiation, register gamma quanta. Specialists determine relative concentrations of elements in the substance by the ratio between peak intensities in the gamma-quantum spectrum.

TNM makes it possible to locate an object inside a solid in 3D. For example, specialists can find the exact location of a diamond in a piece of kimberlite, and even determine its size. The most remarkable property of TNM is that it is possible to obtain information about the third spatial coordinate along the neutron trajectory direction. To do this, specialists determine the time of flight that passes between the moment when an α-particle reaches the alpha-detector and the moment when a gamma quantum from an object of inspection reaches the corresponding gamma detector. Knowing the time of flight, it is possible to calculate the distance to the point where the γ-quantum is emitted since the neutron velocity is the same and equals 5 cm/ns,” Mikhail Sapozhnikov explained.

“Ordinary neutron sources emit them in all directions as an ordinary light bulb emits photons. Using the tagged neutron method, specialists irradiate the object of inspection with a set of narrow neutron beams, sort of analogue of laser pointers,” the scientist added.

TNM compares favourably with many other methods of elemental analysis in that it is remote, non-destructive, and does not require any sample preparation such as, for example, cleaning, drying, or grinding of the sample.

Fundamental research

Yuri Kopach, Deputy Director for Scientific Work of the Laboratory of Neutron Physics JINR and Head of the TANGRA project, said that solutions for fundamental tasks of the project help to conduct applied research with high accuracy. The fundamental part of TANGRA firstly includes measurements of an inelastic neutron scattering reaction with fast neutrons on various nuclei. The inelastic neutron scattering occurs when a neutron, usually with an energy of several MeV, hits a nucleus and makes it excited. Afterwards, the nucleus emits the neutron again but already with a lower energy. In such a reaction, the neutron loses some of its energy, giving it to the nucleus. In its turn, the excited nucleus emits gamma quanta or other particles.

“For effective implementation of the method, it is necessary to know well the energies of the characteristic gamma quanta for each element, as well as the chances of their emission. The assessment of these chances is one of the tasks of the project “Development of a method for position-sensitive neutron-gamma element analysis”. This project was among the winners of the competition 2023 “Conducting fundamental scientific research and exploratory scientific studies by individual scientific groups” by the Russian Science Foundation. The second direction of fundamental research within the TANGRA project is the measurement of angular correlations between neutrons and gamma quanta. The latter are emitted nonisotropically, i.e. unevenly in space, and they have some angular dependence on the direction of the incoming neutron,” Yuri Kopach explained.

Carbon test sites, fast neutrons

The task of carbon analysis in soil belongs to the currently implemented practical applications of TNM. It is being solved together with Diamant LLC (Dubna), a participant of the TANGRA collaboration that actively use TNM for different purposes. JINR together with this commercial organization will develop a prototype of a mobile setup to analyse soil in the field.

TNM currently seems to be almost the only reliable and simple way to determine the mass concentration of chemical elements over large arears. While using stationary methods of chemical soil analysis, it is necessary to take samples in many places and average their results. “Nevertheless, the obtained data is unreliable, because it is impossible to pass the entire field using this way. However, our setup will hopefully be able to analyse the whole field and provide complete data on it. He added that specialists already use a prototype of a mobile setup on fast, but not tagged neutrons. “We are going, in fact, in the same direction,” the scientist explained.

Experiments by the TANFRA project on the determination of carbon in soil, conducted in the laboratory, have shown encouraging results. “In the laboratory, we can determine the percentage of carbon content in the soil. However, we need to test this method on the prototype of the mobile setup to see how it will work in the field,” Yuri Kopach said. He explained that specialists would install a geo-positioning system, since the greatest interest of soil scientists and agronomists is the study of carbon concentration over a large territory, albeit not with such accuracy as it can be determined on a small area. The main goal here is to quickly take measurements over a large area. It is planned that soil scientists will join the research in the field, who will indicate territories for the search for carbon, and possibly other chemical elements.

In 2021, Diamant LLC already carried out pilot measurements in the field at one of the first in Russia carbon test sites in the Kaluga Region. Researchers made sure that the setup could operate while moving in the field. They also assess the values of carbon concentration measurement accuracy.

Based on the measurements of the carbon content in the soil in the laboratory using TNM within TANGRA, the Physics of Elementary Particles and Atomic Nuclei, Letters journal published an article “Determination of carbon concentration in soil using the tagged neutron method” authored by V. Yu. Aleksakhin, E. A. Razinkov, Yu. N. Rogov, A. B. Sadovsky, M. G. Sapozhnikov, I. D. Dashkov, D. N. Grozdanov, Yu. N. Kopatch, V. R. Skoy, N. A. Fedorov. The article covers the capabilities of a prototype of the setup for determining the elemental composition of soil by the tagged neutron method, presents the developed calibration procedure for the setup and measurements of soil samples and calibration ones that were taken using not only TNM but also chemical analysis. The latter can be considered as a reference method to determine chemical composition in a sample. However, it is necessary to prepare the sample beforehand, grind it almost into powder, to use it. For mass concentration of carbon, researchers obtained values of concentration measurement accuracy with the help of a new method.

Fig. 2. Energy spectrum of γ-quanta of soil sample. The points show experimental data. Different colours indicate the contributions from the energy spectra of individual elements. The blue line shows the contribution from oxygen, the green one shows the contribution from silicon, and the red one shows the contribution from carbon. The dark-blue line shows the total contribution from all elements

Analysis of rocks, dangerous cargoes, and other applications of TNM

In addition to obtain measurements to solve the problem of global warming, specialists could use TNM for various industrial applications. Using this method, it is possible to determine the content of 24 elements of the Periodic Table, such as Na, Mg, C, N, O, F, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Zr, Pb, Sn, Bi. It is these elements that give pronounced peaks formed by the energy spectra of γ-quanta.

TNM is turned out to be in great demand in the metallurgical, coal, and cement industries. In all these areas, specialists collected samples and carried out a subsequent chemical analysis of them to control raw materials on the conveyor. It took at least several hours. Conveyor TNM analysers allow experts to get information on the elemental composition of raw materials on the conveyor every minute without any sampling.

In the USA, the method was used even to carry out an elemental analysis of animals and humans in vivo. Thus, experts at the Brookhaven National Laboratory irradiated 20 volunteers at eight points of the body, measuring the concentration of nitrogen, carbon, and other elements in the human body. Specialists analyse the body composition to assess the physical development of a person, their adaptation to the environment, as well as to the conditions of professional and sports activities, such as extreme sports, work under conditions of hypogravity, hypoxia, insufficient insolation, etc. In clinical medicine, the study of body composition is associated with the diagnosis and evaluation of the effectiveness of the treatment of certain diseases, such as obesity and osteoporosis.