The Transport System for Low-Energy Muons
After extracting the low-energy muons from the moderator region they
are separated from the fast muons by a 90 degree deflection.
After passing the mirror they are focused to the sample region. This is done
by an electrostatic transport system consisting of the
with the moderator, an electrostatic mirror, three 'Einzel-lenses', and
a conical shaped lens in front of the sample .
The picture below shows an 'Einzel-lens' used in our
experiment. The first and the third lens are cooled with liquid nitrogen
in order to reduce exposure of thermal radiation to the moderator and
the sample (the sample can be mounted on a cryostat which is not shown here).
The first lens focuses the low-energy muons onto the electrostatic
mirror which deflects them by 90 degree. The fast muons with energies
above a few tens of keV are only slightly influenced by the mirror and
are monitored by a MCP detector.
The second lens creates an intermediate focus at the position of
which gives the start signal for µSR measurements.
The last 'Einzel-lens' focuses the low-energy muon beam to the sample
region, where the conical shaped lens defines the final focus to reduce
the beam spot size to 10 mm FWHM. The sample is mounted electrically insulated
and can be set to a high voltage of up to +-12 kV in order to accelerate or
decelerate the low-energy muon beam before implantation.