The πE1 beam line supplies high intensity pion (π) and muon (μ) beams with momenta ranging from 10 to 500 MeV/c. The particles from the thick target E (cible épaisse) are extracted in the forward direction at an angle of 10°. In order to obtain a large angular acceptance, half quadrupoles with pole tip radii of 20 cm are used as the first focusing elements of the beam line. There are no vacuum windows in the system, so that surface- or subsurface muons can be extracted.
Two modes of operations are available:
- Mode A provides high flux with low momentum resolution. Its momentum is limited by the focusing strength of the first quadrupoles to values lower than 280 MeV/c.
- Mode B is a low acceptance, high momentum resolution version up to momenta of 500 MeV/c.
The characteristics of the beam line are summarized in the table. The layout of the experimental hall is shown in Fig 1 and the magnetic elements in Fig 2.
| Mode | A | B |
|---|---|---|
| Length [m] | 16 | 16 |
| Max. momentum [MeV/c] | 280 | 500 |
| Solid angle [msr] | 32 | 13 |
| Momentum acceptance (FWHM) | 7.8% | 8.0% |
| Momentum resolution (FWHM) | 0.8% | 0.26% |
| π(+) stop density [g¯¹ mA¯¹ s¯¹] | 2.5×107 | 1.0×107 |
The muon rates around 28 MeV/c are shown in Fig 3, while the pion and electron rates are shown in Fig 4. Although these rates have not been measured, mode B provides pions down to a momentum of 100 MeV/c with a three times lower rate compared to mode A. The beam profiles of pions are shown in Fig 5. Without an electrostatic separator, the positron contamination amounts to Re/Rµ ~ 50. The use of a short crossed-field separator (Leff = 0.7 m, E = 150 kV/13 cm, transmission ~80%) and a subsequent quadrupole doublet reduces this contamination to a level of a few percent. There are three slit systems in the beam line. The slits control either the beam intensity (FS53), by reducing the angular acceptance of the beam, or the momentum band acceptance (FSH51 and FSH52) and, hence, the momentum resolution of the transported beam. There is a manual in pdf format with more details. The slit values (in mm) can be set directly by using the SetPoint program.
