A non-destructive method of analysing sealed plutonium-beryllium sources is under development, using a combination of γ spectrometric determination of the plutonium isotope composition and neutron counting. A passive neutron collar was designed with nine ~3He counters embedded in a polyethylene moderator surrounding the sources to be measured. The measurement cavity is lined with cadmium to shield the source against slow neutrons diffusing backwards from the moderator. The electronics consist of a home-made, nine channel chain of pulse amplifiers and discriminators as well as a commercial shift register. The determination of plutonium content seems to be possible by establishing the neutron yield of the sources on the basis of total neutron counting, by adopting specific (γ,n) reaction yields of individual plutonium (and americium) isotopes from the literature. The coincidence counts are due to self-multiplication, i.e. neutron induced fission of the odd plutonium isotopes.
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