Design and Construction of a 1m×1m Thermal Neutron Imager Operating in Ionization Mode with Pad Readout

We have developed a 3 He based thermal neutron imager with an active area of 1m×1m, for the small-angle neutron scattering beamline QUOKKA at the Australian Nuclear Science and Technology Organisation (ANSTO). The detector operates in ionization mode without any electron multiplication in the gas. The electrons created by the neutron- 3 He interaction are collected by discrete anode pads, each connected to an input channel of an ASIC directly mounted on the back of the anode pad plane. These custom ASICs and their readout boards can process up to 25k /s per channel. Combining the nearly 40,000 parallel channels on this detector, a total count rate of 10 8 cps can be achieved.Design and construction details of this detector, as well as some early test results are described.