A riser control device, particularly designed to be used in connection with spool or horizontal production trees for wells in sub-sea oil and gas installations. The resonator chamber of each of the thermo-acoustic engines is of a different length to generate a different frequency whose amplitude is proportional to the neutron activity at the axial and radial position of the thermo-acoustic engine. The slide sole is fastened to the upper part of the housing with slip joints between pivot-shaped projections on the upper part and counter-elements formed on the slide sole. The self-testing unit generates is used to determine the gain, frequency characteristics, selectivity, noise floor, and distortion behavior of the receiver, transmitter and LO generator. Target structures may be blood vessels or hair follicles.