µE1 beam line

The µE1 is an experimental area for an intense medium energy polarized muon beam, with very low pion and electron contamination. As in the case of the piE1 beam line, pions from the thick target TE are extracted at an angle of 10° in the forward direction using a triplet of half quadrupoles. The beam line consists of three sections

• a pion collection section
• a long superconducting solenoid
• the muon extraction

The main characteristics of the beam line are listed in the following table, and a layout of the experimental area and of the magnetic components are shown in Fig 1 and Fig 2

Table 1 : Characteristics of the µE1 beam line

Pions produced at target TE are momentum selected by a bending magnet and then focused by a quadrupole doublet onto the entrance of a superconducting solenoid of 8 m length, 12 cm inner free diameter and 5 T field strength. There muons are collected from pions decaying in flight.

• The length of the solenoid was chosen in relation to the pion decay length l(pi) [m] = 0.055 × P(pi) [MeV/c].
• About half of the pions decay in the solenoid at P(pi) = 220 MeV/c. In this case a broad spectrum of muons is available at the solenoid exit, ranging from 120 to 240 MeV/c.

The second part of the beam line allows selection of a central muon momentum different from that of the injected pions, preferably at the low momentum edge corresponding to muons decaying backward in the center of mass system. In this case the contamination from electrons originating at the pion production target is very low (e+/µ+ ~5%), and the muon beam polarization reaches a high value around 75%. Two different modes of operation are available :

• Mode A is achromatic with low momentum resolution for experiments aiming at the highest possible flux.
• Mode B is chromatic with good momentum resolution for experiments with thin targets.

Rates for negatively charged muons and spot sizes are given in the above table for two of the most commonly used momenta. The rate for positive muons is about three to four times higher than that for negative muons.