EPJ D Highlight - Probing neptunium’s atomic structure with laser spectroscopy

The mass separator used in the experiments. Green: ion trajectory; blue and violet, laser beams. Credit: EPJ, Kaja et al.

A new technique developed by researchers in Germany can measure ionisation states of this element more precisely than before, with implications for its detection and remediation in radioactive waste.

The radioactive element neptunium is one of the principal components of nuclear waste. Mass spectrometry can be used to probe its complex atomic structure, which is of value both for its intrinsic interest and for determining the isotope composition of neptunium waste. Magdalena Kaja at Johannes Gutenberg University, Mainz, Germany and her co-workers have now demonstrated a novel method of laser spectroscopy that can analyse the ionisation potential of neptunium more precisely than earlier methods. This work is now published in the journal EPJ D.

Neptunium, an actinide metal, sits next to uranium in the Periodic Table with an atomic number of 93; almost self-evidently, its name derives from the planet beyond Uranus in the Solar System, Neptune. It has no fewer than 25 known isotopes. Most of these are very short-lived, but the most stable, neptunium-237 (237Np) has a half-life of over 2 million years. It is largely this isotope that makes it so dangerous as a nuclear contaminant.

The samples of neptunium isotopes that are available for this type of analysis are tiny: they generally comprise only a few atoms of an isotope. “Multistep resonance ionisation using a laser source has been proven to be the most useful technique for this, providing high sensitivity, specificity and precision”, explains Kaja. The state-of-the-art apparatus that she and her colleagues used incorporates a solid-state titanium: sapphire laser system, a refined laser ion source and a high transmission mass separator.

The researchers used this technique to measure the first ionisation energy of neptunium: that is, the energy needed to remove a first electron from its outermost electron shell, forming a positive ion. The value they determined, 6.265608(19) eV, agrees well with the values reported in the literature but is more than ten times as precise as any of them. “We now aim to extend our investigations to rare neptunium isotopes”, adds Kaja. The techniques can also be used to detect and analyse traces of neptunium in radioactive contaminants.

This was our first experience of publishing with EPJ Web of Conferences. We contacted the publisher in the middle of September, just one month prior to the Conference, but everything went through smoothly. We have had published MNPS Proceedings with different publishers in the past, and would like to tell that the EPJ Web of Conferences team was probably the best, very quick, helpful and interactive. Typically, we were getting responses from EPJ Web of Conferences team within less than an hour and have had help at every production stage.
We are very thankful to Solange Guenot, Web of Conferences Publishing Editor, and Isabelle Houlbert, Web of Conferences Production Editor, for their support. These ladies are top-level professionals, who made a great contribution to the success of this issue. We are fully satisfied with the publication of the Conference Proceedings and are looking forward to further cooperation. The publication was very fast, easy and of high quality. My colleagues and I strongly recommend EPJ Web of Conferences to anyone, who is interested in quick high-quality publication of conference proceedings.

On behalf of the Organizing and Program Committees and Editorial Team of MNPS-2019, Dr. Alexey B. Nadykto, Moscow State Technological University “STANKIN”, Moscow, Russia. EPJ Web of Conferences vol. 224 (2019)

ISSN: 2100-014X (Electronic Edition)

© EDP Sciences