Establishing a spike material for the analysis of Np-237

K. Hain, M. Martschini, P. Steier, A. Wiederin (Isotope Physics, University of Vienna)

A. Sakaguchi (University of Tsukuba, Japan)

A. Yokoyama (Kanazawa University, Japan)

J. Zheng (QST, Chiba, Japan)

Anthropogenic actinides like ^239,240Pu and more recently ²³⁶U have been successfully applied as tracers for various environmental processes. The chemical properties of Neptunium (Np) suggest that it would provide complementary information to the nuclides mentioned before. ²³⁷Np is the second most abundant anthropogenic actinide in the environment after ²³⁹Pu, with releases of 1500kg ²³⁷Np by nuclear weapons testing and 340kg from the Sellafield Nuclear Reprocessing Plant alone. Nevertheless, studies on environmental ²³⁷Np are very limited to date due to various analytical challenges, and it has consequently been called “the neglected actinide”.

The long half-life of 2.144 My leads to a low specific activity of ²³⁷Np in the environment so that the detection of ²³⁷Np by activity measurements  is unfeasible. Accelerator Mass Spectrometry (AMS) is able to fully suppress background from primordial ²³⁸U and therefore is well suited for the detection of ²³⁷Np. However, the lack of an isotopic yield tracer impedes the reliable quantification of the measurement results. Using non-isotopic tracers like ²⁴²Pu typically comes at the cost of large uncertainties and low reproducibility as different elements may lead to fractionation during sample preparation and the AMS measurement. Satisfying results can be obtained only when carefully optimizing the chemical preparation procedure for a specific sample matrix, e.g. sea water.

Our joint Austrian-Japanese research project aims to realize the full potential of ²³⁷Np by developing a suitable isotopic spike material for environmental samples. The isotopes ^236gNp (T_1/2=153000 y), ²³⁵Np, (T_1/2=396 d) are explored as candidates to serve this purpose in (A)MS measurements. ^236gNp would be the most convenient choice for AMS applications due to its long half-life and the absence of an abundant primordial atomic isobar. Unfortunately, the ²³⁸U(p,3n)²³⁶Np reaction to produce this isotope proposed in previous studies also co-produces a significant proportion of ²³⁷Np turning this material unusable as a spike for ultra-trace detection of ²³⁷Np. Based on promising preliminary calculations with the EMPIRE code, the ²³⁷Np co-production can be reduced significantly using the ²³²Th(⁷Li,xn)^239-xNp reactions, where x denominates the number of neutrons evaporated by the excited nucleus. The model calculations show x to be highly energy dependent, with the maximum ^236gNp production expected in the range of 35-40 MeV while built-up of ²³⁷Np would be suppressed by 2 orders of magnitude. It has to be noted that the excitation functions for the two nuclear reactions discussed before have not been determined experimentally before. The data obtained within the frame of our project will help to close this knowledge gap in nuclear physics.

For ²³⁵Np, the suppression of ²³⁷Np can be increased to 2.5 orders of magnitude by suitably adjusting the energy for the ²³²Th(⁷Li,xn)^239-xNp reactions according to the EMPIRE model calculations. To date, this isotope has not been considered as a potential yield tracer for mass spectrometric methods, as the primordial isobar ²³⁵U cannot be suppressed sufficiently by chemical means. The ILIAMS system at VERA turns ²³⁵Np into a viable candidate. Optimization of the sample preparation and the ILIAMS measurement procedure to maximize the laser-photodetachment for uranium while maintaining a high transmission for neptunium will be a top priority for the work in Vienna. This research should provide a viable alternative in case the co-production of ²³⁷Np remains problematic for ^236gNp, even with the new reaction.

While the irradiation and subsequent chemical purification will be carried out by the Japanese team, the VERA team will analyze the ratio of ²³⁶Np to ²³⁷Np as well as ²³⁶U and ²³⁶Pu, the decay products of ^236mNp (T_1/2= 22.5 h), a metastable excited state of ²³⁶Np, to assist in the development of the chemical separation procedure. As the branching ratio between the long-lived ground state ^236gNp and ^236mNp in the proposed nuclear reaction is not known, the decay products of the metastable ²³⁶Np have to be suppressed chemically and/or by the ILIAMS method to allow the quantitative determination of the produced amount of ²³⁶Np. Depending on the achieved purity of the spike material we will then decide either for ²³⁶Np or ²³⁵Np as spike material and demonstrate its use on a comprehensive set of environmental samples including Pacific Ocean water and peat bog samples.