Four other types of radiation detectors are the self-powered neutron detector, wide range fission chamber, flux wire, and photographic film.

EO 2.9

DESCRIBE how the following detect neutrons:

a. Self-powered neutron detector

b. Wide range fission chamber

c. Flux wire

EO 2.10

DESCRIBE how a photographic film is used to measure the following:

a. Total radiation dose

b. Neutron dose

Self-Powered Neutron Detector

In very large reactor plants, the need exists to monitor neutron flux in various portions of the core on a continuous basis. This allows for quick detection of instability in any section of the core. This need brought about the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation. Figure 29 illustrates a simplified drawing of a self-powered neutron detector.

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Figure 29 Self-Powered Neutron Detector

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 wire's 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 resistor. The electron current from beta decay can also be measured directly with an electrometer.

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.