A particle beam system for delivering energetic particles or bunches of energetic particles to a target comprising • a particle beam source for delivering energetic particles or bunches of energetic particles; • beam directing means for directing the energetic particles or bunches of energetic particles in a beam direction pointing to said target; • a timing controller configured for providing a timing reference signal representing a time structure for delivering said energetic particles or bunches of energetic particles; • a first gamma detector configured for detecting prompt gammas emitted from said target; • a second gamma detector configured for detecting prompt gammas emitted from said target and located with respect to said first detector such that the first and the second detector are detecting prompt gammas emitted from different angles with respect to said beam direction; • a data acquisition system for measuring prompt gammas in synchrony with the said timing reference signal so as to obtain a timing profile indicating a number of prompt gammas measured as a function of a time elapsed since the delivery of a particle or a bunch of particles, said first and second detector are coupled to said data acquisition system so as to acquire a first timing profile from the first detector and to acquire a second timing profile from the second detector; characterized in that said particle beam system further comprises • a data analyser comprising an algorithm configured for ∘ determining from said first timing profile a first time width ΔT1; ∘ determining from said second timing profile a second time width ΔT2; ∘ determining a photon travel shift ΔDP defined as a distance travelled by a photon in the time interval equal to the difference between the first and the second time width: ΔDP = ΔT 2 − ΔT 1 * c whereby c is equal to the speed of light; ∘ determining a particle beam penetration depth by correlating said photon travel shift ΔDP with the difference in detector location between the first and second detector. • a particle beam source for delivering energetic particles or bunches of energetic particles; • beam directing means for directing the energetic particles or bunches of energetic particles in a beam direction pointing to said target; • a timing controller configured for providing a timing reference signal representing a time structure for delivering said energetic particles or bunches of energetic particles; • a first gamma detector configured for detecting prompt gammas emitted from said target; • a second gamma detector configured for detecting prompt gammas emitted from said target and located with respect to said first detector such that the first and the second detector are detecting prompt gammas emitted from different angles with respect to said beam direction; • a data acquisition system for measuring prompt gammas in synchrony with the said timing reference signal so as to obtain a timing profile indicating a number of prompt gammas measured as a function of a time elapsed since the delivery of a particle or a bunch of particles, said first and second detector are coupled to said data acquisition system so as to acquire a first timing profile from the first detector and to acquire a second timing profile from the second detector; • a data analyser comprising an algorithm configured for ∘ determining from said first timing profile a first time width ΔT1; ∘ determining from said second timing profile a second time width ΔT2; ∘ determining a photon travel shift ΔDP defined as a distance travelled by a photon in the time interval equal to the difference between the first and the second time width: ΔDP = ΔT 2 − ΔT 1 * c whereby c is equal to the speed of light; ∘ determining a particle beam penetration depth by correlating said photon travel shift ΔDP with the difference in detector location between the first and second detector. ∘ determining from said first timing profile a first time width ΔT1; ∘ determining from said second timing profile a second time width ΔT2; ∘ determining a photon travel shift ΔDP defined as a distance travelled by a photon in the time interval equal to the difference between the first and the second time width: ΔDP = ΔT 2 − ΔT 1 * c whereby c is equal to the speed of light; ∘ determining a particle beam penetration depth by correlating said photon travel shift ΔDP with the difference in detector location between the first and second detector.