Nuclear fusion research is rapidly expanding in the U.S. and across the globe. One of the important issues with respect to the magnetic confinement approach is how to accurately measure the actual power produced by a fusion reactor.
     Currently, magnetic confinement fusion devices such as tokamaks and stellarators rely completely on absolute neutron counting as a direct way of measuring fusion power.
     Fusion researchers say, “This technique presents several difficulties: the emission and the transport of neutrons from an extended source like the tokamak, and their interaction with reactor materials, require the use of complicated simulation codes, as well as long and costly calibration campaigns to validate the codes.”
    Researchers have developed a groundbreaking new method for measuring power in nuclear fusion reactors. They have discovered that gamma rays which are produced during the deuterium-tritium nuclear reaction, can serve as a highly accurate and alternative means of measuring the power output of new fusion reactors.
    The new method makes use of the gamma-ray-to-neutron branching ratio in the deuterium-tritium reaction, a measurement that was previously unidentified. The researchers have found a way to count the rare gamma rays emitted during a fusion reaction. By counting the gamma rays emitted during a fusion reaction, researchers can now obtain valuable information about fusion power, independent of traditional neutron counting techniques.
     The research team explained that “Absolute counting of deuterium-tritium gamma rays could provide the secondary neutron-independent technique required for the validation of scientific results and as a licensing tool for future power plants.” This novel method involves the precise measurement of two particular gamma rays with energies around thirteen million electron volts and seventeen million electron volts.
     Marica Rebai is a researcher at Consiglio Nazionale delle Ricerche (CNR-ISTP) and an author of the study. She said, “From this measurement, never before carried out with sufficient accuracy, it was possible to determine the energies and relative intensities with which the two gamma rays are emitted. This gamma ray emission process has a relative probability (called branching ratio) which is much lower than that of fourteen million electron volts neutron emission.”
     Andrea Dal Molin and Davide Rigamonti led another study on the same subject. They said that this work enabled them to find that one gamma ray is emitted for every forty-two thousand fourteen million electron volts produced.
     Dal Molin and Rigomonti added that “It paves the way for the use of absolute gamma-ray measurement as a new alternative and complementary method to neutron measurements for determining the power achieved in new fusion reactors based on the deuterium-tritium reaction, such as ITER and SPARC”.
     The International Thermonuclear Experimental Reactor (ITER) is a global collaborative effort focused on proving the viability of fusion power. It requires two independent methods for precisely measuring the power it generates.
     Marco Tardocchi is a research director at CNR-ISTP. He said that “Until now, the absence of a direct and alternative method to absolute neutron counting has been an obstacle to the independent validation of results obtained from ongoing experiments and the authorization of future commercial plants.”