Neutrons: illusive, yet powerful particles

Euro Fusion November 8th 2017

A prototype of the VERDI detector, prior to initial testing.

In commercial fusion power plants, the energy deposited by neutrons into a blanket surrounding a plasma will provide power to the national grid. JET has shown that deuterium and tritium create a promising fuel for fusion, producing neutrons travelling at around 40,000 km/s. It is essential that we are able to predict neutron intensities reliably in order to optimise a future fusion power plant

WHAT MAKES NEUTRONS INTERESTING?

In a fusion device neutrons are both instrumental and problematic. High-energy neutrons released during fusion reactions may escape from the plasma and deposit energy in the blanket. Converting this energy into heat will be the most promising way to power the national grid. The measured number of neutrons produced in fusion reactions provides information about the power output, as well as the rate of fuel consumption. In addition, neutrons are used to breed the tritium necessary to fuel a self-sufficient reactor.

See also the article „A blanket to fuel fusion“ by Paul Barron in this edition.

Picture 1: An illustration of nuclear reactions inside a tokamak device. Bottom left: inside the plasma, deuterium and tritium fuse together in a high temperature plasma releasing a neutron and an alpha particle. Bottom right: Neutrons cause activation by way of interactions with atoms, such as iron. This results in transmutation to other elements, such as manganese and the release of characteristic gamma rays using radioactive decay processes.

HOW NEUTRONS WILL HELP TO PREDICT FUSION PERFORMANCE

When neutrons collide with materials they may impart some of their energy, transforming and exciting the atoms within the material. This process is known as activation, and is illustrated in Picture 1. Activated materials then undergo radioactive decay. Activation is also a tool that may be used to indirectly measure the neutron production rate in fusion. The energy spectrum of neutrons is used to gather important plasma parameters such as the temperature. In the blanket of a fusion power plant, the spectrum information may be used to predict the rate of tritium production.

HOW CAN WE MEASURE WHAT WE CANNOT SEE?

The diameter of the neutron is on the femtometre scale (10-15 metres). For comparison, the diameter of a strand of human hair is on the micrometre scale (10-6 metres), thus a billion times larger than the diameter of a neutron. It is difficult even to comprehend something this small, but the nature of neutrons makes them even more challenging to measure. Conventional radiation detectors rely on the collection of chargeto provide information about the radiation detected. Unlike protons and electrons, neutrons have no charge; and unlike alpha, beta and gamma radiation, neutrons do not induce a charge in materials. To measure neutrons we must instead use an indirect process and look at the way in which they interact with other materials, a field known as neutronics (introduced in the Fusion in Europe March 2016 edition). Neutron activated materials emit gamma rays with characteristic energy“ fingerprints” thus providing the identification of the atom excited during activation. By detecting these gamma rays, along with simulations predicting how the atom may have been excited by neutron interactions, it is possible to deduce the neutron spectrum.

VERDI – THE COLLABORATIVE DETECTOR

The high temperatures, intense magnetic fields and high neutron fluences of a fusion environment present significant challenges when it comes to neutron detection. The UK, Greece and Italy have collaboratively developed the VERDI detector ( Picture 2). It will transport material samples inside the fusion device blanket, where they will be bombarded by neutrons. The samples will then be removed and placed in front of a conventional radiation detector. Carbon has a high melting point (over 3000°C), so by encapsulating material samples in a carbon shield the VERDI detector can be placed much closer to the plasma core than existing neutron detectors. Detecting neutrons is just one of the many challenges that must be overcome in nuclear fusion. It takes multiple experts from several nations to come together to solve even comparably small tasks in the huge quest for fusion energy.