Measurement of the gamma-ray to neutron branching ratio in the deuterium-tritium reaction

Magnetic confinement fusion devices are technologies for achieving controlled nuclear fusion reactions, using magnetic fields to confine hot plasmas. These devices could contribute to the ongoing transition to more sustainable energy production methods.

The optimal operation of these devices relies on the ability to accurately measure fusion power. Existing devices achieve this only through absolute neutron counting, a technique that measures the total number of neutrons produced during a plasma discharge, which provides information on the speed and efficiency of reactions.

Researchers from the National Council of the Rich (CNR-ISTP), the University of Milan-Bicocca and other institutes around the world, under the coordination of Dr. Marco Tardocchi, have recently identified an additional measurement that could offer valuable insights into fusion energy.

This measure, described in an article published in Physical Exam Lettersis the branching ratio of gamma rays to neutrons in the deuterium-tritium reaction.

“Our project started with ITER, the next-generation International Thermonuclear Experimental Reactor, which aims to demonstrate the feasibility of producing electrical energy from nuclear fusion,” Andrea Dal Molin, first author of the paper, told Phys.org.

“One of the main technical challenges of ITER is that fusion power is measured using two independent methods. The first, absolute neutron counting, is a well-established technique, already adopted in current fusion experiments such as the Joint European Torus (JET). The second method has not yet been identified.”

Dal Molin and his colleagues set out to explore the possibility of using the rare gamma rays emitted during the deuterium-tritium reaction to measure fusion power. The method they proposed to obtain this measurement involves counting the two gamma rays emitted by the electromagnetic channel of the deuterium-tritium nuclear fusion reaction, in which a ⁵The nucleus produced in an excited state decays to lower levels.

“This reaction channel is much less likely (2.4×10^-5) than the nuclear reaction channel producing a neutron and an alpha particle,” explains Davide Rigamonti, co-author of the paper. “This results in a large neutron flux that is an unwanted source of background noise for gamma-ray measurements. The use of an efficient neutron attenuator was one of the main requirements that enabled this measurement.”

Dal Molin, Rigamonti and their colleagues spent considerable effort trying to identify precisely the relative energies and intensities of the two previously unknown gamma rays emitted. They then used the absolute measurement of the number of neutrons produced by the JET magnetic confinement device to determine the gamma-ray-to-neutron branching ratio.

“The precise determination of the gamma-ray to neutron branching ratio for the deuterium-tritium reaction paves the way for the use of absolute gamma-ray counting as a secondary, neutron-independent technique for measuring fusion power in next-generation fusion experiments,” said Dal Molin. “Our work thus provides an essential tool to directly validate the results and improve the accuracy of the measurements.”

This recent study by Dal Molin, Rigamonti and their colleagues opens up interesting possibilities for the precise measurement of fusion power in future nuclear fusion experiments. Notably, the method used by this team to measure the gamma-ray-to-neutron branching ratio could also be adapted and applied to other fusion reactions that do not produce neutrons, such as proton-boron or deuterium-helium-3 reactions.

“Fusion energy research is currently experiencing a significant resurgence of interest from both the public and private sectors,” Rigamonti added. “New tokamaks capable of operating with a deuterium-tritium mixture, such as ITER, are under development. Our project is to provide fusion energy measurements on the next magnetic confinement reactors using the gamma-ray method.”

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