The piE5 is a low energy (10-120 MeV/c) pion and muon beam line, viewing the thick target at 175° with respect to the primary proton beam. The main properties of the beam line are given in the following table . The beam line and a layout of the experimental area are shown in Fig 1 and Fig 2 respectively.
Length 10.4 m
Solide angle 150 msr
Momentum acceptance (FWHM) 10 %
Momentum resolution (FWHM) 2 %
Spot size (FWHM) 15 mm horizontal
20 mm vertical
Angular divergence (FWHM) 450 mrad horizontal
120 mrad vertical
The second bending magnet AST is so designed that the beam can serve alternatively two experiments at the location U and Z as indicated in the layout, within a short switching time.
In the middle of the beam line, where the momentum restricting slit system is located, the aberrations are rather large and therefore the momentum resolution is poor (2% FWHM). The situation is better at the final focus since the beam line is built up symmetrically causing some abberations to vanish at the end.
There are three sets of horizontal and one set of vertical slits, which define the momentum and/or the acceptance in the two versions of optics which correspond to the two positions for experiments at locations U and Z.
Plots of the rates and phase space at the focus just behind the seocnd bending manet AST are shown if Fig 3 and Fig 4 respectively. In the Z version, pions will be injected after a third bending magnet into a 8.5 m long solenoid called the Pion to Muon converter (PMC) This is a device designed for the muon-electron conversion experiment (SINDRUM II) to generate a pion-free muon beam (pi-/µ- ratio < 1e-9). The flux of particles at the focus of the optical version U is reduced compared to the Z version by two factors : pion decay in a 2 m additional path length, and losses in the refocussing quadrupole doublet (~20%). Additional losses have to be taken into acount in case when a separator is used to reduce strongly the high electron and positron contamination at low momenta. Depending on the experimental setup, a transmission factor of 70 to 80% is typical. Measurements of the particle fluxes show that the rate are higher than predicted at momenta lower than 90 MeV/c, and lower at higher momenta. Aproximately independent of the momentum, the electron rates is 2 x 1.e9 mA¯¹ s¯¹. Since there is no vacuum window , but only an optional thin foil (3 mg/cm²) to hold back the radioactive gases, there will be "subsurface" muons with momenta down to 10 MeV/c, where the rate is 5 x 1.e6 mA¯¹ s¯¹.
Due to the large opening of the vacuum chamber and the relatively small bending radii, there is a high neutron background in the region of the second bending magnet. Estimated fluxes are : 150 neutrons mA¯¹ s¯¹ cm¯² at energies larger than 120 MeV, and low energetic neutrons,which produce a radiation level of 5 rem mA¯¹ h¯¹.
The coordinator for this beam line is D. Renker