An encapsulated radioactive material that emits neutrons for use in neutron porosity measurements. The most common source relies on alpha-beryllium reactions in a ²⁴¹Am-Be mixture. Beryllium releases a neutron of approximately 4 MeV when struck by an alpha particle. The americium is the source of alpha particles. ²⁵³Californium fission is an intense source of 2. 3 MeV neutrons but is used only in special applications due to its short half-life of 2. 65 years and special licencing requirements.