Establishing a spike material for the analysis of 237Np

The long-lived neptunium isotope 237Np, which has a half-live of T1/2=2,140,000 years and belongs to the chemical group of the actinides, ranks 3rd in environmental abundance among the anthropogenic isotopes. It was globally spread by nuclear weapons testings (around 1.8 t). The analysis of Irish Sea water indicates also high emissions from the Sellafield nuclear waste reprocessing plant. Its chemical properties suggest that it might outperform the more established nuclides, e.g. 129I or 137Cs, as an environmental tracer because it is considered as very mobile in many environmental conditions. Due to its long half-life, the detection of environmental 237Np concentrations by decay counting requires large sample masses especially for water samples. In contrast to other mass-spectrometric (MS) techniques, whose sensitivity is limited by background due to the naturally occuring uranium isotope 238U, Accelerator Mass Spectrometry (AMS) can measure 237Np largely background-free. Nevertheless, 237Np is not yet applied to environmental studies owing to the lack of a suitable second Np isotope to normalize the chemical and instrumental yield and to obtain the concentration in the sample material in this way. The present approach of using a non-isotopic plutonium (Pu) spike is not satisfactory as large uncertainties for MS measurements are obtained. The aim of the proposed project is to produce a suitable isotopic Np spike material for MS which is sufficiently pure with respect to mass 237. The two sufficiently long-lived candidates 236gNp (T1/2=154,000 a) and 235Np (T1/2=1.1 a), can be produced in the same irradiation experiment either by bombarding a thorium (Th) target with a lithium (Li) beam or by bombarding an uranium (U) target with protons (p+) at sufficiently high beam energies. These experiments will be performed by our Japanese partner facilities after successful development of a radiochemical purification method for Np from the target material. The efficiency of the production processes and the suppression of 237Np by the proposed nuclear reactions will be verified by a number of analytical methods including AMS measurements at the Vienna Environmental Research Accelerator (VERA). In particular for the Li-Th reaction there is no experimental data on the formation probability for the different Np isotopes available. A suppression method for isobaric background, especially for the primordial nuclide 235U, which has the same mass as the potential spike nuclides, i.e. 235Np or 236Np, must be developed for the analysis of the spike material by MS measurements. The new method of Ion-Laser-InterAction-Mass-Spectrometry (ILIAMS) available at VERA seems well suited for this purpose. Its ability to suppress isobaric U, Pu background using the element selective detachment of electrons from negatively charged ions by a laser will be explored. With a successful application of ILIAMS for isobar suppression of actinides, VERA would take the world lead in AMS development, as even high-energy AMS facilities are not able to suppress isobaric background in this high mass region. This will allow for first showroom applications on relevant environmental samples within the scope of this project, contributing to climate research and radio-ecology.