A detector to monitor online the protontherapy beam at the Istituto Nazionale di Fisica Nucleare (INFN), Laboratori Nazionali del Sud (LNS) has been built and characterized. The detector is made of two ionization chambers: each chamber has the anode splitted into 256 0.5mm strips with vertical and horizontal orientation. The chambers are part of the beam line: signals can be processed online at a speed up to 100Hz and results are promptly available. Thus the beam geometry can be controlled continuously during patient treatment, and in case of deviation from the required conditions, the treatment can be directly concluded.