发光体材料6.ppt

Neutron Emissions and Measurements Lesson: CRS 1-6 Lesson Objectives List methods for neutron production Name isotopic sources of neutrons Cite reactions for portable sources of neutrons Describe a large research facility for production of neutrons. Overview of Presentation Methods for Neutron Production Isotopic Neutron Sources Portable Neutron Sources Large-Scale Research Facilities Methods for Neutron Production Spontaneous fission of fissionable material Relatively high energy charged particle interactions with materials, which includes: Accelerators Cosmic rays Decay of radioactive materials High energy photon interactions with some materials Chain reactions Fusion reactions Spontaneous Fission Naturally Occurring Heavy Elements 235U, 238U, and 232Th all occur in nature and may occasionally spontaneously fission. However, they normally decay by alpha emission, as shown previously on decay chains for naturally occurring radioactive materials. Artificially Produced Heavy Elements 239Pu, 233U, and 252Cf are three examples of artificially produced heavy elements that spontaneously fission. They also normally decay by emission of an alpha. 252Cf undergoes spontaneous fission approximately one time for every 30 alpha decays and is widely used as a neutron source. Only a few micrograms of this material are needed for excellent sources of neutrons. Charged Particle Production Accelerators and Cyclotrons Protons are normally used as projectiles on many different target materials to produce neutrons. 2H and 3H are reacted to produce neutrons with small portable devices. Electrons are also be used as projectiles on targets to produce neutrons, such as at the ORELA facility at ORNL. Cosmic Rays High energy cosmic rays interacting in the upper atmosphere, and on earth, are the source of a few neutrons. Radioactive Decay that Produces Charged Particles Heavy, high Z elements that decay by emitting alpha particles can be used as projectiles on a variety of targ