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.

The project aimed to produce a sufficiently pure neptunium isotope as "spike" for mass spectrometry. This material is added to environmental samples to enable the quantitative measurement of the anthropogenic Neptunium isotope Np-237 (half-life: 2,140,000 years), which has been released by nuclear waste reprocessing plants and nuclear weapons testing, for example. Apart from the need to monitor such emissions, Np-237 has also been proposed as a tracer for studying water mass transport in the field of environmental science. Following an iterative development phase, the primary candidate, Np-236g (t1/2 = 154,000 years), was successfully produced by irradiating thorium (Th) foils with a lithium (Li) beam at sufficiently high energies by our Japanese partners, which was then radiochemically purified. The production efficiency of mass 236 and the Np-237 suppression were verified using different established detection methods, including gamma spectrometry and accelerator mass spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA). However, this project required the development of new approaches to characterise mass 236. This was particularly important, as even small impurities of Uranium(U)-236 and Plutonium(Pu)-236, which are co-produced in the irradiation process, would lead to an incorrect concentration of the spike material and, consequently, a false measurement of environmental Np-237. For this purpose, we combined AMS with the unique ion-laser-interaction mass spectrometry (ILIAMS), using interactions with both, lasers and reactive gases, as well as element-specific fluoride anion formation analysis. Successful separation of Np from U and Pu, as well as Pu from Am, represents the first isobar suppression in this high mass range in mass spectrometric measurements, opening up new possibilities for emission source identification. However, the first experimental data on the formation probability of Np isotopes for the studied Th-Li reaction, obtained within this project, showed that the maximum production rate of Np-236g is more than ten times lower than expected from theoretical nuclear model calculations. While this is an important contribution to international databases, it means that the best achievable purity of Np-236 with respect to co-produced Np-237 is also reduced correspondingly. Although the maximum 236/237 ratios achieved for the pilot spike were only around 9, we demonstrated that this material can be used to analyse a range of environmental Np-237 concentrations if the spike amount is chosen carefully. The accuracy of the Np-237 results obtained for different reference materials (IAEA-381, 385) and artificial seawater samples, when normalised to the Np-236g spike, is considerably improved compared to the previously used normalisation methods. The Np-236g spike was therefore used to determine the Np-237 concentration in selected environmental samples, e.g. Pacific Ocean / Mediterranean Sea water. The quality of the spike material might be further improved in the future by applying purification methods using resonant laser excitation.