In fusion projects it is necessary to use materials resistant to high temperatures and radiation damage. Carbon-based materials, especially carbon nanotubes and graphene, are promising in this respect. Scientists from the NCBJ Reactor Research Department participated in the research on the resistance of graphene detectors to high neutron fluxes.

Thermonuclear reactors, such as the ITER (International Thermonuclear Experimental Reactor) research device currently under construction in Cadarache in France, or its successor – DEMO (Demonstration Power Plant), use a strong magnetic field to trap plasma, in which the synthesis reactions of light atomic nuclei take place. In order for the synthesis reaction to proceed efficiently, the plasma must be heated to a temperature of tens of millions of degrees Celsius. In order to ensure stable operation of the device, precise diagnostics of the magnetic field is necessary. Due to the conditions affecting the electronics inside the reactor, such as high temperature (several hundred °C) or strong neutron radiation, most of the commercially available semiconductor magnetic field sensors are unable to operate in such systems. For this reason, research is being carried out on metal detectors based on chromium or bismuth. Unfortunately, detectors based on them have low sensitivity and a large cross-section for interaction with neutrons.

An interesting alternative seems to be the detectors made in the technology of quasi-free epitaxial graphene on silicon carbide. Graphene layers can be formed into very sensitive Hall effect sensors: if the conductor through which the electric current flows is in a magnetic field, a potential difference appears in it – the so-called Hall voltage that can be used to measure a magnetic field. The resistance of graphene to radiation has already been investigated. The tests were carried out using both ion, proton and electron beams, and no significant changes in the properties of the irradiated samples were detected. Theoretical predictions suggest that graphene responds similarly to neutron radiation, but this has never been directly confirmed experimentally before.

A paper published in the Applied Surface Science journal investigates the effect of fast neutrons on a graphene-based detector system. „The Institute of Microelectronics and Photonics (IMiF) operating in the Łukasiewicz Research Network created a structure consisting of graphene on the surface of 4H-SiC (0001) silicon carbide saturated with hydrogen atoms. The whole Was covered with dielectric alumina passivation, which is the environmental protection of the active layer of the detector” – says PhD. Eng. Tymoteusz Ciuk, head of the works at Łukasiewicz – IMiF. The system prepared in this way Was then irradiated with fast neutrons inside the core of the MARIA reactor at NCBJ.

„The unique fast neutron irradiation installation installed in the core of the MARIA reactor allows us to conduct research on materials or components intended for use in fusion systems, in which fast neutrons are also generated” – says PhD. Eng. Rafał Prokopowicz, head of the NCBJ Reactor Research Department, co-author of the work. „In the case of research on detection structures made of graphene, the samples were irradiated for over 120 hours with fast neutrons with a fluency of 10¹⁷ cm^–2 to reflect the conditions to which electronics in thermonuclear installations are exposed” – adds Maciej Ziemba from the Reactor Research Department. „To ensure the safety of the research, component tests were performed when the activity of the samples Was no longer hazardous, that is, several months after irradiation."

Both before and after the irradiation of the samples, their structure and electrical properties were thoroughly examined at the Institute of Physics of the Poznań University of Technology. Raman spectroscopy, Hall effect studies as well as large-scale modeling using the density functional theory (DFT) were used for this. In addition, scientists from the Poznań University of Technology carried out the characterization of irradiated structures after annealing at a temperature of 100 to 350°C to investigate the effect of temperature, combined with the effect of fast neutrons, on electrical properties. „The tests revealed, for example, that due to radiation, the material exhibits a dependence of electrical properties on temperature, which Was not present before the samples were placed in the neutron stream” – explains PhD. Eng. Semir El-Ahmar, head of research at the Poznań University of Technology. „Moreover, neutron radiation reduces the density of charge carriers in the structure under study. It turns out, however, that the hydrogen layer is responsible for this, so irradiation has only a moderate effect on the structure and properties of graphene."

Based on the characterization of the properties of the examined structures before and after irradiation, the resistance of graphene to neutron radiation Was assessed as very good. The radiation damage density Was 7 orders of magnitude lower than the value of the neutron flux, which means a relatively low cross-section of graphene on the interaction with fast neutrons. „Although there was damage to the structure caused by radiation, compared to metal-based detectors, the sensitivity of the graphene system to the magnetic field remains several orders of magnitude higher,” concluded PhD. El-Ahmar. „Additionally, it turned out that a large part of the damage Was not related to the graphene layers themselves, but to the hydrogen layer, which in turn, at temperatures above 200°C, which will prevail in installations such as DEMO, it even shows a certain self-repair potential. Therefore, graphene magnetic field detectors may be promising structures for use in fusion reactors."

Further research will be carried out on the use of graphene as a base for magnetic field detection in fusion installations. Scientists are considering the use of a different type of substrate – e. g. 6H-SiC (0001), on which the formed structure may be more resistant to neutron radiation. It is also considered to replace the hydrogen layer with a buffer layer of carbon atoms.

The full results of the research are available in: Semir El-Ahmar, Maciej J. Szary, Tymoteusz Ciuk, Rafał Prokopowicz, Artur Dobrowolski, Jakub Jagiełło, Maciej Ziemba, Graphene on SiC as a promising platform for magnetic field detection under neutron irradiation, Applied Surface Science, Volume 590, 2022, 152992, ISSN 0169-4332, https://doi.org/10.1016/j.apsusc. 2022.152992