  An on-line beam position monitoring and regular beam stability tests are of
utmost importance for the Quality Assurance (QA) of the patient treatment at
any particle therapy facility. The Gantry$\hspace{0.5 mm}2$ at the Paul
Scherrer Institute uses a strip ionization chamber for the on-line beam
position verification. The design of the strip chamber placed in the beam in
front of the patient allows for a small beam penumbra in order to achieve a
high-quality lateral beam delivery. The position error of 1 mm in a lateral
plane (plane perpendicular to the beam direction) can result in a dose
inhomogeneity of more than $5 \%$. Therefore the goal of Gantry 2 commissioning
was to reach a sub-millimeter level of the reconstruction accuracy in order to
bring a dose uncertainty to a level of $1 \%$. In fact, we observed that for
beams offered by Gantry 2 signal profiles in a lateral plane can be
reconstructed with a precision of 0.1 mm. This is a necessary criterion to
perform a reliable patient treatment. The front end electronics and the whole
data processing sequence have been optimized for minimizing the dead time in
between two consecutive spots to about 2 ms: the charge collection is performed
in about 1 ms, read-out takes place in about 100 $\mu$s while data verification
and logging are completed in less than 1 ms.

Medical Physics;Instrumentation and Detectors