# Design and Diagnostics of High-Precision Accelerator Neutrino Beams
## 9 Hadron Dump Diagnostics  
Particles in the decay region need to be stopped into a dedicated structure (hadron dump-Figure 1). The hadron dump absorbs the beam stopping all non-decayed hadrons, or particles other than neutrinos, within the thermo-mechanical and radio-protection constraints. It should be noted that in some wide-band beams the hadron dump is also a \"proton dump\" since it is hit by a significant fraction of high-energy primary protons that cross the target without interacting. In this case, the dump requirements are much tighter in terms of particle fluence and irradiation than narrow-band beams. In narrow-band beams, a large fraction of muons come from the beam halo component having the right momenta to be channeled into the transport line of Section 7. This background is a challenge for diagnostics unless it can be separated by exploiting the peculiar transverse and momentum distributions. In general, the production of \(\delta\) rays from muons has also to be taken into account to estimate the correct muon flux. Muon monitors placed downstream of the hadron dump have been routinely used in many neutrino beams to provide a constraint on the \(\
u_{\mu}\) flux and the beam alignment. As noted above, they are sensitive to the amount of \(\
u_{\mu}\) produced by \(\pi^{+}\rightarrow\mu^{+}\
u_{\mu}\) because they monitor the muons collinear to the pions. Hence, these devices may constrain the leading contribution of \(\phi_{\
u_{\mu}}(E)\). They provide statistically compelling information even with a single proton pulse and deliver continuous monitoring of the beam conditions. The diagnostics, however, must stand huge rates due to punch-through of interactions in the hadron dump and, in particular, the penetrating component made of neutrons and muons. We will briefly recall the solutions implemented at the CERN-PS experiments, IHEP-Serpukhov, CNGS, FNAL-NuMI, and J-PARC. In most cases, the choice of the detector technology has fallen on gas ionization chambers, where the current generated by the large flux of particles can be readout without damaging the detector. More recently, in the J-PARC beam, silicon PIN diodes are being used and the beam designers are considering Electron Multiplier Tubes (EMT [143]), as well. At the CERN-PS, eight monitoring stations instrumented a 20 m concrete shielding positioned at the end of the decay volume. At the  
Figure 12: A Beam Current Transformed that can be adapted to measure the muon flux in nuSTORM and, in general, in a Neutrino Factory. Reproduced under CC-BY-SA license with permission from CERN. IHEP-Serpukhov neutrino beam, the iron shielding was sampled using 16 stations, again exploiting ionization chambers. In both cases, the decay volume was short enough to allow a full sampling of the transverse development of the muon beam. Ionization chambers were also used for the muon monitoring of the CNGS beam [28] where two stations composed of a cross-shaped array of \(N_{2}\) filled ionization chambers (Figure 13, left), were deployed downstream of the hadron dump with 67 m of rock in between (20 and 50 GeV/c threshold for muons). At the J-PARC beam, the muon monitoring system (MUMON) [144] is composed of a hybrid detector comprising \(7\times 7\) ionization chambers and \(7\times 7\), 1 cm\({}^{2}\), PIN photodiodes (Figure 13, right). The muon monitoring system of the NuMI beam [145] is composed of a hadron monitor, installed at the end of the decay pipe to measure the rates and profile of the surviving beam component and by three muon monitors arrays with a transverse extension of 2.2 m, placed at about 6, 18 and 36 m downstream of the hadron monitor. With this arrangement, the minimal threshold for muons is 5, 12 and 24 GeV for the three stations respectively. Each station consists of arrays of \(7\times 7\) or \(9\times 9\) He-based ionization chambers with a gap of 1 or 3 mm for the hadron and muon stations respectively.