RADIATION TYPESRadiation DetectorsNeutronNeutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atomnucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of analpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but theyare also produced from the decay of radioactive elements. Because of its size and lack of charge,the neutron is fairly difficult to stop, and has a relatively high penetrating power.Neutrons may collide with nuclei causing one of the following reactions: inelastic scattering,elastic scattering, radiative capture, or fission.Inelastic scattering causes some of the neutron’s kinetic energy to be transferred to the targetnucleus in the form of kinetic energy and some internal energy. This transfer of energy slowsthe neutron, but leaves the nucleus in an excited state. The excitation energy is emitted as agamma ray photon. The interaction between the neutron and the nucleus is best described by thecompound nucleus mode; the neutron is captured, then re-emitted from the nucleus along witha gamma ray photon. This re-emission is considered the threshold phenomenon. The neutronthreshold energy varies from infinity for hydrogen, (inelastic scatter cannot occur) to about 6MeV for oxygen, to less than 1 MeV for uranium.Elastic scattering is the most likely interaction between fast neutrons and low atomic massnumber absorbers. The interaction is sometimes referred to as the "billiard ball effect." Theneutron shares its kinetic energy with the target nucleus without exciting the nucleus.Radiative capture (n, g) takes place when a neutron is absorbed to produce an excited nucleus.The excited nucleus regains stability by emitting a gamma ray.The fission process for uranium (U235 or U238) is a nuclear reaction whereby a neutron is absorbedby the uranium nucleus to form the intermediate (compound) uranium nucleus (U236 or U239). Thecompound nucleus fissions into two nuclei (fission fragments) with the simultaneous emissionof one to several neutrons. The fission fragments produced have a combined kinetic energy ofabout 168 MeV for U235 and 200 MeV for U238, which is dissipated, causing ionization. Thefission reaction can occur with either fast or thermal neutrons.The distance that a fast neutron will travel, between its introduction into the slowing-downmedium (moderator) and thermalization, is dependent on the number of collisions and thedistance between collisions. Though the actual path of the neutron slowing down is tortuousbecause of collisions, the average straight-line distance can be determined; this distance is calledthe fast diffusion length or slowing-down length. The distance traveled, once thermalized, untilthe neutron is absorbed, is called the thermal diffusion length.IC-06 Page 8 Rev. 0
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business