Drift of Electrons and Performance of BF3 Filled Neutron Proportional Counters

(BF3)-B-10 gas has regained interest among neutron detector developing laboratories as a promising alternative to globally scarce He-3. BF3 is an only direct replacement for use in proportional counter with coaxial geometry. Lower absorption cross section of B-10 and constraint on fill pressure limits the efficiency of these detectors. Electronegative nature of gas and impurities, affects the pulse shape at high fill gas pressure due to loss of electrons in drift region. Effect of electron attachment and position of primary charge cloud determines the gas gain and pulse broadening. Detailed study of gas behavior is essential to facilitate its efficient use in neutron detectors. Our in-house facility for generation-distillation of BF3 gas and development of neutron detectors has enabled us to carry out the detailed characterization of BF3 filled neutron detectors. Detectors are filled with the gas of best possible purity level by processing with repeated triple point distillations. Further detector is subjected to localized fine neutron beam tests. Position of neutron beam within the drift region is varied to investigate the pulse shape. Optimization of the detector design using these parameters, results in improvement of detection efficiency.