Abstract: A chlorinated hydrocarbon sensor is substituted as a portion of a cone perometer. The sensor includes a titanium casing in which a cylindrical sleeve of beryllium alpha target material is fixed. A rod-shaped americium alpha particle source is disposed within the casing and is moved by an electromagnetic relay into an interacting state in which the beryllium sleeve encases the americium alpha particle source. The interfacing beryllium and americium emit high energy neutrons used to detect the chlorinated hydrocarbons. The emission takes place only when an electromagnetic relay is energized so that cessation of energy will cause a cessation of neutron generation. The rod-shaped alpha particle source will then be withdrawn from the cylindrical beryllium sleeve by a retracting spring. The generated neutrons interact with hydrogen and chlorine in the soil surrounding the sensor to produce characteristic gamma radiation that is detected and measured by a sodium iodide scintillation crystal.