Study of Photo-Nuclear Interaction of muons in rock with the MACRO experiment

We present first results about the measurement of the characteristics of charged hadrons production by atmospheric muons in the rock above MACRO. Selection criteria which allow to discriminate hadron cascades from e.m. showers generated by muons are described. A comparison between the measured rate with that expected from a Monte Carlo simulation which treats the process as dominated by photo-nuclear interaction is presented. These data can be used to validate such models aiming to the evaluation of hadron background from cosmic muons in different experimental environments.