Novel detectors for the age of fusion
Researchers at the Tor Vergata University in Rome, Italy, are using detectors made from synthetic diamond in nuclear fusion experiments. The goal is to eventually produce a fusion power plant that is cheap, clean, and uses a virtually inexhaustible fuel.
Synthetic single-crystal diamond-based detectors offer high radiation hardness, band-gap (leading to low-noise measurements and high temperature operation), carrier mobility and breakdown voltage (allowing fast response times), and a low atomic number, resulting in a good discrimination capability between neutrons and gamma rays. Conventional silicon detectors cannot be used as they do not allow neutron spectroscopy and are sensitive to gamma-ray radiation.
Furthermore, they are not as radiation hard, so their signal degrades over time. ‘Neutron detection is probably the most important diagnostic for fusion energy, since in every single reaction one neutron is emitted [with] most of the energy produced by the reaction itself,’ notes Professor Marco Marinelli at Tor Vergata.
‘The results obtained so far, in terms of stability, reproducibility, efficiency and resolution, indicate that diamond-based detectors can be successfully used for neutron monitoring in large-scale experimental fusion reactors’.
Fabrication and testing
The detectors are produced in a p-type intrinsic/metal configuration by homoepitaxial microwave chemical vapour deposition (CVD) of detector grade diamond onto single-crystal synthetic diamond substrates.
The electric field is across the intrinsic CVD layer, ruling out substrate contribution. ‘Such a device detects fast neutrons by means of a nuclear reaction between the carbon-12 atoms of the detector and the incident neutrons. This produces beryllium-9 ions and alpha particles, which are easily detected by the device,’ says Marinelli.
To extend detection capability to thermal neutrons, a lithium fluoride-6 layer is deposited onto the sample surface. It reacts with thermal neutrons, producing 2.73MeV tritium and 2.06MeV alpha particles. ‘The configuration we have adopted permits simultaneous detection of both thermal and fast neutrons,’ notes Marinelli.
A charge collection efficiency of 100%, as well as an energy resolution in the range of 0.5-1.5%, are routinely obtained. In a test on a one-megawatt fission reactor at TRIGA in Frascati, Italy, the detector was positioned 80cm above the core mid-plane, where the neturon flux was 2.2x109 neutrons/cm2, resulting in a count rate of around 150,000 counts per second. The device showed good stability and reproducibility over the whole reactor power range.
The researchers are also involved in international research projects, both at the Joint European Torus (JET), located at Culham, UK, and at ITER, the latest fusion experiment being built in Cadarache, France.
Another of the team’s long-term interests is the properties of CVD diamond, both polycrystalline and single crystal, as well as its potential technological applications.
Professor Enrico Milani of Tor Vergata says, ‘There could be room for improvement using other device geometries and detector matrices. And of course, diamond quality improvements are always possible’. He believes the added value market for the detector could be very high. Marinelli agrees, ‘Our purpose is to produce diamond-based devices such as gas sensors, radiotherapy dosimeters and UV detectors’. They are looking for industry partners.