Feasibility of neutron activation methods for measurement of sodium and aluminium in green liquor

Abstract

The feasibility of neutron activation methods including prompt gamma-ray neutron activation analysis (PGNAA), neutron activation analysis (NAA), and continuous neutron activation analysis (CNAA) has been investigated for both the off-line and on-line continuous measurement of sodium and aluminum in green liquor. It is found that in the measurement of sodium and aluminium dissolved in green liquor at the 11% and 5.5% levels, respectively, one must also know the chlorine content to determine the average neutron flux for any method involving neutron activation. Chlorine may be present in green liquor in amounts up to 0.5% which would significantly (as much as 10%) affect the average thermal neutron flux in a sample. It was found that PGNAA is quite sensitive to chlorine and somewhat to sodium, but very insensitive to aluminum. However, NAA and CNAA are quite sensitive to aluminum, then sodium, but very insensitive to chlorine. The final proposed approaches for discrete samples and continuous samples are to use NAA and CNAA, respectively, with time lags incorporated between activation and detection to reduce the effect of the otherwise overly intense aluminum spectrum on the less intense sodium spectrum and use of a 3He detector to monitor the average neutron flux (a function primarily of chlorine content) in the samples. Extensive experimental and benchmarked Monte Carlo simulations are used to evaluate the feasibility of this approach. It appears that the approach is viable and will allow accurate (1% relative standard deviation for both sodium and aluminum) discrete sample or on-line continuous analysis of both sodium and aluminum in green liquor in reasonable times (10–30 min) with a relatively small (15 μg) 252Cf source. Prototype devices are planned for future development.