Self-powered detectors24 operate on the well — publicized principle of the nuclear battery. The incident — neutron flux activates a central electrode, which emits betas that are collected by a surrounding electrode. This type detector is usually designed for in-core neutron-flux sensing. It is discussed in Chap. 3, Sec. 3-3.3.

2-4.3 Activation Detectors

Neutron flux at a given position in a reactor can be measured by exposing a material object25 to the flux, removing it from the flux, and determining the activity that has been induced by exposure to the flux. From the exposure time and the known properties of the exposed material, the incident-neutron flux can be determined. This method can be used in in-core neutron-flux mapping. The exposed material can be in the form of wire, foil, ribbon, etc. Even liquids and gases26 can be used.

2-4.4 Solid-State and Scintillation Detectors

Solid-state and scintillation detectors2 7 30 can only be used where the neutron — and gamma-flux levels are low, for example, in radiation-monitoring systems. Solid-state de­tectors convert directly to an electrical signal, while scintillation detectors require an intermediate photoelectric stage. There is a wide variety of types. Most, however, are not applicable at neutron fluences above 101 5 neutrons/ cm2 and integrated gamma exposures of 1012 rad.

2-5 INSTALLATION

Once a nuclear radiation sensor has been selected, installation arrangements must be made (see also Chap. 10).