An ultrasound reactor (30) having a reactor body (1), a reactor tube (9) disposed within the reactor body and a magnetostrictor transducer (11) comprising at least one annular element (28a-d) concentric with, and mounted to an external wall of said tube and a process for ultrasonic treatment of a reaction material are disclosed. The transducer is mounted on the reactor tube in such a way so as to transmit ultrasound radiation to the interior of the tube and to induce an upwardly flowing cavitation stream whose longitudinal axis substantially coincides with a longitudinal axis of said tube, causing physical changes to the reaction material.