# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 7 Micromegas for Neutron Detection
### Micromegas for Neutron Time-of-Flight Measurements  
Neutron-induced reactions play a key role in several domains covering nuclear technology, stellar nucleosynthesis, and the study of the properties of the nucleus-like level densities and gamma strength functions. For those applications, accurate and high-resolution neutron capture and neutron fission reactions as a function of neutron kinetic energy are necessary. This information, also known as nuclear data, also forms an important ingredient for modeling nuclear reactions. The way to address the neutron kinetic energy in neutron-induced reactions is to use the neutron time-of-flight method at a pulsed white neutron source, like GELINA at JRC-Geel [90], NFS at GANIL [91], and n_TOF at CERN [92; 93]. Most of the experience with Micromegas detectors has been acquired at the n_TOF facility, thus, providing neutrons in the meV to GeV range. Figure 20: The first TPC of the PandaX-III experiment (**left**). Principle of a Microbulk detector (**top**). Picture of a PandaX-III Microbulk module (**bottom**) and zoom of the surface (**bottom right**). The Micromegas neutron detectors used at neutron time-of-flight facilities thus far can be roughly divided in two main families: neutron reaction detectors and beam-profile detectors. Typical neutron converters are based on the neutron-induced reactions \({}^{6}\)Li(n,\(\alpha\))\({}^{3}\)H, \({}^{10}\)B(n,\(\alpha\))\({}^{7}\)Li, \({}^{235}\)U(n,f) and \({}^{238}\)U(n,f), which have well-known cross sections and are considered standard [94] over large energy ranges. Higher energetic neutrons can also be converted by elastic reactions with hydrogen or helium atoms. These elements can be added to the gas mixture, either as pure helium or hydrogen bound in organic molecules, such as methane (CH\({}_{4}\)) or isobutane (iC\({}_{4}\)H\({}_{10}\)). The organic gases are added at a small percentage!10% to keep the mixture non-flammable. The straightforward adaptation of the gain of a Micromegas detector allows for the adequate adjustment to the energy deposits resulting from the produced particles following the reaction. The neutron-reaction detectors are intended to measure the reaction products as a function of the time of flight for the cross section measurements. In practice, such a detector is implemented as a single-pad Micromegas detector, which means that the detector is not segmented. Data acquisition requires a single acquisition channel. The detector covers the full neutron beam with a diameter typically ranging from 1 to 10 cm. Such detectors can be used to measure the incident neutron flux as a function of neutron energy when a well-known or even standard (n,f) or (n,\(\alpha\)) reaction is used for the neutron conversion. An example of a flux measurement with Micromegas, in combination with other detectors, can be found in Ref. [95]. For fission measurements, a reference sample can be part of the stack of detectors, resulting in very accurate fission ratios, as for example shown in Ref. [96]. The second application of these detectors is the measurement of the energy-dependence of the spatial neutron-beam profile. The profile is not only important for beam diagnostics purposes but also to obtain the energy dependence of the fraction of the incident neutron beam. Since, for many neutron reaction experiments and, in particular for neutron capture, the used in-beam targets are smaller than the beam size, the beam interception factor is a crucial quantity needed to construct the normalization over a large energy range. For the purpose of beam profiling, a detector can be implemented either by pixelizing the anode or by using perpendicular X- and Y-strips and detecting coincidences. Both techniques have been used at n_TOF for beam profiling measurements [97; 98; 99]. The strip implementation was eventually preferred since a higher spatial resolution could be obtained due to smaller possible widths of the strips, typically on the order of 1 mm.