πE3 beam line

The πE3 beam line delivers pions and muons in the momentum range from 10 to 250 MeV/c. It is designed to match the optical characteristics of the low energy pion spectrometer LEPS, and it is the only beam line with a vertical bending plane, the experimental area being 6 m above the floor level. The channel views the thick target E (cible épaisse) at an angle of 90°, images more than half of the 6 cm long target, and is therefore also an excellent beam line for surface and cloud muons. 

A layout of experimental area and of the magnetic components are shown in Fig 1 and Fig 2 

Two optical modes of operations are available :

• A large acceptance achromatic mode
• A high resolution chromatic mode

There are two pairs of slits (in the bending and non-bending plane) allowing a reduction of either the beam intensity or the momentum acceptance.

This mode is optimized for high flux experiments with stopped pions or muons. Its characteristics are given in the table 1 (at the same target position as for the chromatic mode, LEPS pivot point in Fig 2). Usually this mode is used for experiments with surface muons. The measured intensities of positive muons around 28 MeV/c at focus point are given in Fig 3 and the pions flux in Fig 4 top By the use of a crossed field mass separator and a quadrupole doublet the high positron contamination can be reduced to a level of 10 to 20 % depending on the experimental setup. The transmition of this device is around 70 %. 

It is also possible to tune the beam to a mode with a dispersive focus at the position of the first slit, in order to reduce the momentum band transported. Otherwise in this mode the slits are used to reduce the intensity 

Table 1 : Characteristics of the achromatic mode

This mode has been specially designed in combination with the Low Energy Pion Spectrometer LEPS. The corresponding characteristics are given in table 2. Note that the dispersion plane is vertical.

In this mode there is a dispersive focus at the position of the second slit, where the accepted momentum band can be defined. The pion rates are given in Fig 4 bottom and the corresponding phase space characteristics are shown in Fig 5 .

Table 2 : Characteristics of the chromatic mode