Page 216 - Dynamics and Control of Nuclear Reactors

16.2 Sensor characteristics 215

A voltage is applied between the tube metal and the wire. The tube is filled with a
neutron-absorbing gas, typically BF 3 or has boron coated on the inside wall of the
cylinder. Neutron absorptions in boron produce charged particles that migrate under
the voltage difference and produce a measurable current. Ionization chambers can
monitor individual events (pulse mode) when the neutron flux is small. It can provide
a continuous current when the neutron flux is large.

16.2.1.2 Fission detectors
A fission detector is an ionization chamber containing fissile material coated on the
inner wall of a metallic sheath (see Fig. 16.2).
Incident neutrons cause fissions and result in release of charged particles. The
charged particles migrate due to the applied voltage and produce a measurable
current.

16.2.1.3 Self-powered neutron detectors
Self-powered detectors contain a neutron absorber (typically Rhodium, Vanadium or
Platinum) inside an insulated metallic sheath (see Fig. 16.3).
Neutron absorptions produce an isotope that decays by beta emission. The beta
particle (electron) migrates to the sheath, creating a potential difference between the
sheath and a small electrical current. This current (microamperes) is measured to pro-
vide the neutron flux at the sensor.
Self-powered detectors are small and require no external power supply. They are
used for in-core measurement of local neutron flux.

FIG. 16.2
A fission chamber.