1. n. [Formation Evaluation]
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 licensing requirements.