A method and apparatus for detecting slow neutrons by monitoring Lyman alpha radiation produced by the 3He(n,tp) nuclear reaction induced by neutrons incident on a gas cell containing 3He or a mixture of 3He and other atoms and/or molecules. Such a method and/or apparatus includes the use of, for example, liquid 3He and 4He mixtures as a scintillation counter for the sensitive detection of neutrons using Lyman alpha radiation produced by the 3He(n,tp) reaction. The radiation can be detected with high efficiency with an appropriate photo-detector, or alternatively, it can be converted to radiation at longer wavelength by absorption in scintillation materials, with the radiation channeled to a photodetector. Because of the simplicity of the system and the fact that the radiation production mechanisms can be measured and/or calculated independently, the method and/or apparatus also has the potential for service as a calculable absolute detector.