The central wire of a self-powered neutron detector is made from a material that absorbs a
neutron and undergoes radioactive decay by emitting an electron (beta decay). Typical materials
used for the central wire are cobalt, cadmium, rhodium, and vanadium. A good insulating
material is placed between the central wire and the detector casing. Each time a neutron interacts
with the central wire it transforms one of the wires atoms into a radioactive nucleus. The
nucleus eventually decays by the emission of an electron. Because of the emission of these
electrons, the wire becomes more and more positively charged. The positive potential of the wire
causes a current to flow in resistor, R. A millivoltmeter measures the voltage drop across the
The electron current from beta decay can also be measured directly with an
There are two distinct advantages of the self-powered neutron detector: (a) very little
instrumentation is required--only a millivoltmeter or an electrometer, and (b) the emitter material
has a much greater lifetime than boron or U235 lining (used in wide range fission chambers).
One disadvantage of the self-powered neutron detector is that the emitter material decays with
a characteristic half-life. In the case of rhodium and vanadium, which are two of the most useful
materials, the half-lives are 1 minute and 3.8 minutes, respectively. This means that the detector
cannot respond immediately to a change in neutron flux, but takes as long as 3.8 minutes to reach
63% of steady-state value. This disadvantage is overcome by using cobalt or cadmium emitters
which emit their electrons within 10-14 seconds after neutron capture. Self-powered neutron
detectors which use cobalt or cadmium are called prompt self-powered neutron detectors.
Wide Range Fission Chamber
Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar
in construction to that of an ionization chamber, except that the coating material is highly
enriched U235. The neutrons interact with the U235, causing fission. One of the two fission
fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall.
One advantage of using U235 coating rather than boron is that the fission fragment has a much
higher energy level than the alpha particle from a boron reaction. Neutron-induced fission
fragments produce many more ionizations in the chamber per interaction than do the neutron-
induced alpha particles. This allows the fission chambers to operate in higher gamma fields than
an uncompensated ion chamber with boron lining. Fission chambers are often used as current
indicating devices and pulse devices simultaneously. They are especially useful as pulse
chambers, due to the very large pulse size difference between neutrons and gamma rays.
Because of the fission chambers dual use, it is often used in "wide range" channels in nuclear
instrumentation systems. Fission chambers are also capable of operating over the source and
intermediate ranges of neutron levels.