The particles produced in the target then enter a system of magnetic horns (c.f. Figure 7 on page 14), which will focus positive particles with a mean energy of 35 GeV and defocus the negative particles.

The interest lies in making the beam of pions and kaons, parents of the neutrinos, as parallel as possible. The first horn will cause excessive deflection of particles that have energies of less than 35 GeV and insufficiently deflect those with energies of over 35 GeV. To correct this a second horn known as the reflector, will be set up some 40 metres from the first. The combined focussing effect of the two horns will ensure that a maximum number of pions and kaons will be directed towards Gran Sasso.

2.2.3 The decay tunnel

Pions and kaons are not stable particles. For example, every 300 metres, some 15% of pions at 35 GeV decay. The higher the energy of the pion, the longer the mean decay length. In more than 99% of cases, the decay of pions produces a n_m neutrino and a muon.

To avoid any loss of pions and kaons through interactions with air, an evacuated decay tunnel is envisaged for the CNGS project. This tunnel contains a steel pipe similar to a large water main or oil pipe. The pipe, which will be one kilometre long and have a diameter of 2.45 metres, will be sealed into the rock. These dimensions are the result of a compromise between the cost of the tunnel and the number of neutrinos present in the CNGS beam. A simple vacuum pump will allow more than 99% of the air present in the pipe to be extracted within a week.

2.2.4 The hadron stop

Located at the end of the decay tunnel, the hadron stop is intended to absorb all protons not interacting in the target or the horns, together with all the pions and kaons that have not decayed before reaching this point.

The quantity of energy to be absorbed by the hadron stop is relatively high, so its construction (consisting of 3 metres of graphite followed by 15 metres of iron) must make for good heat dispersion. A closed-circuit water cooling system is also provided.

In addition to the neutrinos, muons are another group of particles that are hard to absorb in a beam stopper; they are particles which do not undergo strong nuclear interactions and only rarely react with atomic nuclei. These muons will therefore be absorbed further on in the molasse rock proper, behind the hadron stop. Within a kilometre, all the muons will have disappeared.

2.2.5 The muon detector stations

As these muons are the "sister" products of neutrinos with the same pion and kaon parents, the most practical way of checking the position, angle and intensity of a neutrino beam is to measure the trajectory of the muons. To do so, two detection stations are planned for the CNGS project: one directly behind the hadron stop and the second separated from the first by 67 metres of molasse. These sets of detectors can be used to measure the key parameters of the muon beam, hence also indirectly those of the neutrino beam towards the Gran Sasso.

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