Gamma Spectrometric Determination of Plutonium : Isotopic Composition and Age

The isotopic composition of plutonium can either be measured using mass- or gamma spectrometry. Mass spectrometry offers many orders of magnitude higher sensitivity as compared to that of gamma spectrometry. However, it can not easily be used in field applications. Chemical separation is also a prerequisite for MS. Gamma spectrometric analysis can be performed on site, but has its limitations when it comes to sensitivity due to low photon yields of all plutonium isotopes. This report describes a method for determination of the isotopic composition and the age of plutonium using gamma spectrometry. Results are compared with ICP-MS (Inductively Coupled Plasma Mass Spectrometry) measurements and good agreement is obtained between the methods and reference values. The gamma spectrometric method offers acceptable accuracy and precision when performing early risk assessments at an incident site. The possibility to estimate the amount of plutonium using gamma spectrometry was investigated by using Monte Carlo simulations. For a source of PuO2 (p=11.4 g/mL) differences in efficiency are possible to observe for masses up to approx. 100 g, i.e. the maximum mass that can be determined with respect to mass is approx. 100 g. For a metallic plutonium sphere (p=19.8 g/mL) this mass will be approx. 1 g.