In a gas filled spectrograph, particle trajectories coalesce into that of
the mean charge state of the equilibrium charge state distribution. By
adjusting the pressure in the spectrograph, all particles of one charge state
distribution can be collected in the focal plane. The comparison between
operating a magnetic separator under high vacuum and gas filled is shown in
the two figures.

The gas filled spectrograph increases the efficienc
for detecting low yield ERs at near barrier energies.
The technique is particularly useful for reactions using low intensity
radioactive ion beams.
The layout of gas filled split-pole spectrograph is shown below.

A mylar window of 0.94 micron is installed at the entrance of the
spectrograph to separate the pressure in the target chamber and the magnet
chamber. The focal plane detector consists
of a position sensitive avalanche counter (PSAC) and a plastic scintillator.
The detector provides position, timing and energy signals.