# Design and Diagnostics of High-Precision Accelerator Neutrino Beams
## 8 Decay Section
### The Decay Volume of Monitored Neutrino Beams  
The core device of a monitored neutrino beam is the instrumentation of the decay volume for the identification of the leptons (see Figure 10). In the following, we will detail this instrumentation referring mainly to NP06/ENUBET, which has developed the most advanced design, to date. In ENUBET, the vacuum requirement in the decay volume is slightly tighter (\(\sim\)0.1 mbar) than standard neutrino beams, but can still be handled by root pumps. As mentioned above, monitored beams are neutrino beams equipped with radiation-hard instrumentation in the lateral walls of the decay volume to identify large-angle leptons produced by the decay of the secondaries. For ENUBET, the instrumentation identifies large-angle positrons from \(K^{+}\to e^{+}\pi^{0}\
u_{e}\) (\(K_{G3}\)) and muons from \(K^{+}\to\mu^{+}\
u_{\mu}\) (\(K_{\mu 2}\)), as shown in Figure 10. In this case, background reduction is essential. An interesting ancestor of ENUBET was the K2K [132] beamline, where a Cherenkov counter was positioned inside the tunnel in dedicated low-intensity runs to estimate the kaon contamination and, in general, the composition of the hadrons. Similarly, during the first attempts to build a \"tagged neutrino beam\" in USSR (see Section 11) physicists installed trackers to measure the trajectory and the time-of-flight of the secondaries [133; 134]. Other options resorted to transforming the tunnel itself in a Cherenkov radiator to identify the muons but the extremely harsh environment and the additional material in the tunnel makes the use of Cherenkov tagging quite challenging [135]. A completely different approach for the instrumentation of the channel was proposed in 2010 [136]: here, the authors gave up the possibility to intercept the core of the hadron beam and designed a detector to track only large-angle particles. This \"beam scratching\" approach was perfected in 2015, where a proposal to instrument just the walls of the tunnel was put forward [12]. Unlike earlier attempts, the proposal of Ref. [12] reaps a decade-long R&D for detectors capable of sustaining high particle rate in a harsh radiation environment. The current ENUBET design is based on cost-effective detectors to instrument a significant fraction (\(>\)50%) of the decay volume. Since ENUBET is tuned to maximise the kaon decays instead of the pion decays, the tunnel is unusually short (40 m), which further reduces the cost of the instrumentation. The ENUBET positron detectors are a set of modules made of iron interleaved by plastic scintillator slabs, whose light is read by wavelength shifter fibers. The fibers transport the light on the outer part of the tunnel radius where the photosensors (Silicon Photomutpliers, SiPMs) are shielded against the expected neutron flux (10\({}^{9}\)-10\({}^{10}\) n/cm\({}^{2}\)). Each module samples 4.3 \(X_{0}\) and therefore monitors the development of electromagnetic and hadronic showers, together with the trajectory of the minimum-ionizing-particles (mips). \(e/\gamma\) separation is achieved by a photon veto made of scintillator tiles located in the inner radius of the cylinder. Using three radial layers of modules, ENUBET achieved a signal-to-noise of 2 for positrons from \(K_{e3}\) and 6 for muons from \(K_{\mu 2}\). These results were the outcome of a full GEANT4 & FLUKA simulation and were validated by a beam test campaign carried on at CERN's East Area between 2017 and 2018 [137]. Two other paths explored by the ENUBET detector R&D turned out to be of significant interest for applied physics. Firstly, in 2017 we developed a segmented calorimeter made of \"Ultra-Compact-Modules\" (UCMs [138]) where the SiPMs are embedded in the bulk of the calorimeter and the light is read by fibers punching both the iron and the scintillator tiles, perpendicularly to the tile plane (the so-called shashlik readout).