Boron-10 lined ionisation chambers with enhanced neutron sensitivity

The paper describes the design, development and testing of a 10B lined ionisation chamber with baffle electrode configuration for reactor control and safety instrumentation. The detector consists of 35 nos. of natural boron coated annular aluminium discs and spacers stacked on the inner and outer diameters of the cylindrical HT and signal electrodes. This novel design of increasing the boron coated surface area without altering the detector dimensions resulted in an improvement in neutron sensitivity by a factor of 3.4 over the standard ion chamber with cylindrical electrodes and similar dimensions. Because of the increased electrode spacing and the non uniform electric field distribution between the baffles the chamber required marginally higher saturation voltage as compared to standard cylindrical ion chamber. The chamber also showed a slightly lower collection efficiency of 82% and a marginally higher non linearity of 11% in the signal response at various reactor power levels at the Thermal Column of Apsara reactor. The corresponding values for the collection efficiency and non linearity for the cylindrical ion chamber was 100% and less than 1 %. Efforts are in progress to reduce the electrode spacing and optimize the design to obtain better V/I characteristics and higher charge collection efficiency. (author)