Detecting Neutrons
Neutrons are electrically neutral and have in most scattering experiments extremely small energies of typically 25 meV. One can register them only by induced nuclear reactions. If a certain atomic nucleus captures a neutron, it decays into charged fragments with high kinetic energies (for a chemical reaction one would speak about an exothermal reaction.) Such energetic charged particles ionize the counting gas along their trajectories within the detector, a kind of modern Geiger-counter. The ionized gas atoms release electrons, which are collected by high voltage applied to thin wires or metal stripes. The counting gas can be for example Helium-3 (3He), which has a high probability for absorbing neutrons.
The simplest design of such a neutron detector is the depicted counting tube, which is similar to the Geiger-Mueller counter. When a neutron reacts with a 3He atomic nucleus, it decays into a proton (1H) and a triton (3H, "super heavy proton"). These charged particles ionize the counting gas, which is composed of 3He + 1% CO2. The released negatively charged electrons move towards the central wire, which is at a high positive voltage of 1000 V.
On their way to the central wire, the moving electrons induce an electric signal that can be amplified and registered by further electronics. This kind of counter determines the position of the incident neutron with an accuracy of 5 to 50 mm, depending on the size of the counter. A higher precision of the position can be achieved by installing several wires in a detector housing and a more sophisticate readout. A typical position resolution of such a neutron detector (a "multi-wire proportional chamber") is about 2 to 3 mm.
The position resolution as well as the time resolution of these counter tubes is not sufficient for the new experiments, which are planned to be performed at the European Spallation Neutron Source (ESS). Therefore, at the Helmholtz Centre Berlin we develop currently (2002) a new type of neutron detector. In this detector the neutrons induce nuclear reactions not in a 3He gas, but in a solid metallic material called converter (containing specific nuclei or isotopes, e.g. 157Gd or 6Li). The charged particles emitted in nuclear reactions will also be detected by ionization of the counting gas. The use of solid converters has the advantage that the neutrons are captured in a only very small volume, which allows position resolution of 0.1 to 0.5 mm and much higher counting rates. These detectors will enable us to study very small objects and fine structures with great accuracy.
Texts: Th. Wilpert , Ch. Schulz
