Fission chambers (Figure 14) with a coating of U-235 are suitable for both source, intermediate ranges and power range. In the source range it is used in pulse counting mode, and in the intermediate range it is used with both, pulse counting mode and Campbelling mode. The structure of a fission chamber is quite similar to that of a boron coated chamber, only the deposit on the electrode is different which has two consequences:

  • The cross section of uranium (580x10-24 cm2) is smaller than that of boron (3900x10-24 cm2) and, therefore, the neutron reaction per unit time is smaller in a given fluence.

  • The energy developed in the gas by the fission fragments is much greater than that of the boron reaction (80 MeV instead of 2.8 MeV) and the fission chamber is less influenced by disturbing gamma radiation.

Special features of the utilization of fission chambers is their application to the wide range monitoring system, where a single detector is effectively used with a combination of pulse counting mode and Campbelling mode to monitor the reactor operation for the range of about 10 decades, from start-up to power. Miniature size fission chambers are also available for in-core use. In case of wide range monitoring systems, regenerative fission chambers with a mixed coating U-235 and U-238 are currently applied in power reactors, and can also be used in research reactors.

Fission chambers used in the pulse mode: The typical sensitivity of fission chambers in pulse mode is between 0.01 cps/nv to 4 cps/nv, the maximum count rate such a chamber can perform depends on various chamber parameters such as size, counting gas etc. A very sensitive chamber (1 cps/nv) delivers pulses of 80 ns width and the counting rate may reach 106 cps. A low sensitive chamber (0.01 cps/nv) may measure neutrons up to 108 cps. The chamber can withstand gamma radiation up to 104 Gy/h.

Fission chambers used in the current mode: The saturation characteristics of a fission chamber using the current mode is similar to those of a UIC. The sensitivity to neutrons is comparable to a UIC and depends on the quantity of uranium deposited at the electrode, typical values are between 10-14 A/nv and 10-12 A/nv. The maximum neutron flux a fission chamber can measure in the current mode without loss of linearity is lower the more sensitive the chamber is. The usual maximum current produced by a fission chamber is in the order of 1 mA. Therefore, a very sensitive fission chamber with 6x10-13 A/nv can measure a maximum fluence of about 2x10-9 nv. A typical value of its gamma sensitivity as 10-9 A/Gy/h.

Fission chambers in the Campbelling mode: For high fluence rates, pulse detectors are limited by the pile-up phenomenon of pulses. One pulse has no time to disappear before the new pulse appears and superposes itself on the preceding one. The signal delivered by the detector is fluctuating. The electronics provided to process the pulses is not able to distinguish two successive pulses and counting loss is observed. It is, however possible to increase the operational range of the detector by using this pile-up phenomenon. The signal fluctuation is a measurement of the neutron fluence rate, this measurement is increasingly accurate the greater the energy deposit is caused by a neutron event. The important point is that the difference between the neutron signal and the gamma signal is as large as possible. Due to the high energy deposited by a neutron compared to a gamma interaction in the counting gas the fission chamber is the only detector capable of using fluctuations to create a neutron proportional signal. The Campbell theorem states that the mean square voltage of the signal fluctuation is proportional to the neutron fluence rate. To obtain this proportionality the signal, therefore, has to be squared. Unlike pulses or currents which give the required value directly, the neutron fluence rate is obtained by calculation.