Determination of the effective centres of the NPL long counters.

At the National Physical Laboratory, the standard instruments for measuring the neutron fluence of accelerator based neutron fields are long counters. This work follows on from an earlier project to derive improved knowledge of the response functions of the instruments using Monte Carlo calculations and measurements. In the present work new values for the effective centre as a function of energy have been calculated and validated experimentally. The efficiency values from the earlier work have been revisited in the light of new measurements and calculations. A sensitivity analysis has been performed to investigate the uncertainties in the calculated values. A final set of effective centre and efficiencies has been produced with realistic uncertainties for both the De Pangher and McTaggart type long counters.