AMS measurements of ⁵⁵Fe in steel - an example of a simple analysis with a big machine

1 Introduction

The characterization of steel regarding its content of the short-lived 55Fe (T_1/2 = 2.756 years [1]) is considered important in processes of nuclear decommissioning. In steel, 55Fe is mainly produced in a neutron capture reaction on 54Fe. We present a pilot study using Accelerator Mass Spectrometry (AMS) at the DREAMS (DREsden AMS) facility showing that 55Fe determination in steel samples with a weight of some mg is possible without any radiochemical separation [2]. The minimalist requirements in sample preparation together with reduced measurement times allow for very short turnaround times and low costs.

2 Experimental Methods & Results

We used a few mg of fine to coarse grained steel chips from a nuclear reactor vessel without any chemical processing, which were directly pressed in the AMS sample holder. For validation the same material was chemically processed and prepared for liquid scintillation counting (LSC) and AMS, respectively. AMS uses negative ions of Fe created in a Cs sputter ion source. The use of negative ions is an advantage: There is no isobaric background from the stable 55Mn as this element does not form negative atomic ions. The counting of 55Fe events is performed relative to the measurement of ion currents of the stable isotopes 54Fe or 56Fe.
Three instances of quality control were carried out: (1) AMS measurements in three aliquots of the steel chips without further pretreatment, (2) an AMS-internal comparison of the steel chips and chemically treated AMS samples, and (3) a comparison of AMS data and LSC measurements. For these different comparisons results overlap within 3% to 20% on levels of 10² Bq/g Fe to 10⁴ Bq/g Fe. At those levels, the actual AMS measurement of a single sample can be carried out within a few minutes. In this pilot study, the sensitivity of the AMS determination reached 3 Bq/g Fe and was mainly limited by the short measurement time.

3 Outlook

Collaboration and further development of 55Fe and other radionuclide (e.g. 36Cl, 41Ca [4]) measurements within trans-national access are supported by the Horizon 2020 program RADIATE [3]. A dedicated AMS facility is planned at HZDR, for which allocation of beam times can be handled more flexibly than at the larger multi-user facility DREAMS, leading to further reduction of turnaround times. Additionally, an isobar suppression technique using the interaction of lasers with the ion beam is presently developed and may open options for AMS analysis of 59Ni, 93Zr and other isotopes also of interest for nuclear decommissioning.

References:

[1] Pommé, S., Stroh, H., Van Ammel, R.: The 55Fe half-life measured with a pressurised proportional counter. Appl Rad Isot (2019) 148:27-34.
[2] Merchel, S., Rugel, G., Lachner, J., Wallner, A., Walther, D., Ziegenrücker, R.: Evaluation of a sensitive, cheap, and fast detection method for 55Fe in steel. J Radioanal Nucl Chem (2021) submitted.
[3] https://www.ionbeamcenters.eu/radiate/radiate-transnational-access/
[4] Hampe, D. Gleisberg, B., Akhmadaliev, S., Rugel, G., Merchel, S.: Determination of 41Ca with LSC and AMS: method development, modifications and applications. J Radioanal Nucl Chem (2013) 296:617-624.