The Black Sea Experiment: The Use of Helicopter-Borne Neutron Detectors to Detect Nuclear Warheads in the USSR-US Black Sea Experiment

The Soviet Navy used a helicopter/ship system called “Sovietnik” to detect a cruise-missile warhead in the joint USSR-US experiment on the Black Sea, which took place on 5 July 1989. The system consists of a ship-based helicopter, equipped with a neutron detector and processing equipment, and associated ship-based equipment. The system was developed at the I.V. Kurchatov Institute of Atomic Energy. Its operation is based on detecting the neutrons emitted from the spontaneous fission of the plutonium-240 contained as an impurity in weapon plutonium. The Sovietnik neutron detector is based on helium-3 counters in a moderator. It is designed to detect the neutron flux from a single nuclear warhead at distances up to 100–150 meters. At these distances, the flux does not exceed 10 percent of the natural background and is comparable with its variations. Therefore when developing the Sovietnik system, great attention was given to factors affecting the value and behavior of the background. The neutron detector, together with instrumentation for recording am preliminary processing and analysis of initial information, is located on the helicopter, which, carries out the measurement while flying slowly past the ship in question. The presence of fissile material on the ship is considered to be established if the measured neutron-radiation signal in the ship zone is higher than a “threshold” defined as a level that exceeds expected background fluctuations