  By means of a time-of-flight technique, we measured the longitudinal profile
of prompt $\gamma$-rays emitted by 73 MeV/u $^{13}$C ions irradiating a PMMA
target. This technique allowed us to minimize the shielding against neutrons
and scattered $\gamma$-rays, and to correlate prompt gamma emission to the ion
path. This correlation, together with a high counting rate, paves the way
toward real-time monitoring of the longitudinal dose profile during ion therapy
treatments. Moreover, the time correlation between the prompt gamma detection
and the transverse position of the incident ions measured by a beam monitor can
provide real-time 3D control of the irradiation.
