Study of neptunium measurement by alpha liquid scintillation with rejection of β–γ emitters

The measurement of very low alpha activities by means of alpha liquid scintillation with rejection of β–γ emitters (PERALS spectrometer) is receiving much attention because of its fast sample preparation and its good sensitivity. Among the actinide series, there is no real investigation for the measurement of neptunium although this element gathers interest for waste management or the control of illicit nuclear activities. Therefore, it became necessary to study the conditions corresponding to the optimal extraction of neptunium. For that purpose, we studied the recovery in terms of pH, HCl, HNO 3, H 2SO 4 concentration, valence state (IV, V and VI) for several extractive scintillator cocktails commercialized for the PERALS spectrometry, like ALPHAEX, THOREX, URAEX and POLEX (these cocktails contain, as extractant, di(2-ethyl-hexyl)-phosphoric acid (HDEHP), 1-nonyldecylamine, tri- n-octylamine (TNOA) and tri-octyl-phosphine oxide (TOPO), respectively). We showed that TOPO is the best extractant, giving a D value equal to 370. In order to get quantitative extraction, pentavalent neptunium is first oxidized by silver (II) oxide to the hexavalent state, then extracted by TOPO in 0.5–0.75 M nitric acid. We incorporated this procedure with success in a general procedure for the determination of actinides (Th, U, Np, Pu, Am and Cm) in a single solution. Some applications to biological samples are also reported.