# Current Status and Future Developments of Micromegas Detectors for Physics and Applications
## 7 Micromegas for Neutron Detection
### Micromegas for Neutron Time-of-Flight Measurements
The first 2D beam profile detector used a CAST-like detector with a bulk Micromegas and segmented anode. Until recently, the mesh-anode ensemble had to be realized in a PCB structure due to its multi-layer character. Such a massive in-beam detector resulted in a large scattered neutron background for possible down-stream experiments. This was changed since the breakthrough for Microbulk detectors with the possibility to segment not only the anode but also the mesh of a Microbulk detector. The absence of an in-beam PCB in such a detector resulted in the first prototype of a very low-mass neutron profiler [100]. The typical dimensions of a reaction chamber with an integrated detector are \(30\times 30\times 30\) cm\({}^{3}\), without the external acquisition cards. The mesh-anode ensemble both consisted of 1-mm wide and 6-cm long strips with a micropattern of of 60-um diameter holes with a pitch of 100 um. Improvements to this Microbulk detector consist of the use of Laser Direct Imaging (LDI) instead of photolithography, thus, allowing for a reduced interstrip region and a more uniform gain among the strips. The uniformity of the electric field in the drift region can also be improved by implementing a field degrader. The in-beam material balance and, therefore, the amount of background from neutron interactions, for a Microbulk detector is very low. This transparency for neutrons allows stacking several detectors in a single reaction chamber and, in this way, significantly decreasing the necessary beam time for the required counting statistics. The data acquisition system has to cope with different time scales that depend on several parameters. The orders of magnitude of the three time ranges, also illustrated in Figure 21, are given in the list below:  
* the repetition rate of the neutron bursts ranging from 0.8 Hz (n_TOF) to 800 Hz (GELINA) or 800 kHz (NFS);
* the time window (up to 100 ms in the case of n_TOF) following each burst in which neutron-induced reactions will be detected for which the time-of-flight (and therefore the kinetic energy of the neutron) is measured; and
* the subsequent detection times of the charged particles in the different strips, typically space by several ns, following a neutron-induced reaction. A data acquisition system based on the AGET ASIC chip [101] has been used recently for the multichannel transparent strip detector [100]. This system offers the possibility of  
Figure 21: Time structure of the events. Each subsequent panel depicts an expanded view of a part of the previous one. In the upper panel (**a**), the pulsed neutron source provides neutrons during a limited time window \(\Delta T\). The neutron production pulses are repeated with a frequency f and indicated by a trigger \(T_{0}\) providing the start for the time of flight of reaction events in a single pulse, which are visualized in panel (**b**). Each reaction event will trigger an ionization track detected by a cluster of several adjacent x- and y-strips as shown in panel (**c**). The coarse structure in panel (**b**) provides the incident neutron energy by its time of flight, while the fine structure of the timing of the strip events in panel (**c**), combined with the amplitude information, provides information on the particle track. Figure 22: The principle of the low-mass Microbulk Micromegas detector as a neutron detector shown in the left panel. The neutron beam has a typical diameter of several cm. The drift electrode is coated with a thin layer of material creating charged particles by neutron-induced reactions. The ionization track of the resulting charged particle is read out at the mesh/anode. In the right panel, the strip signals from a single neutron interaction as measured in Ref. [100] are shown.