2024:07 Alpha spectrometry of radioactive aerosols from the European Spallation Source

SSM perspective

Background

In Lund, the construction of the European Spallation Source (ESS) research facility is underway and the facility is also undergoing a stepwise licensing procedure in accordance with relevant legal requirements. At ESS, neutrons will be produced by spallation when high-energy protons from a linear accelerator hit a rotating tungsten beam target. The neutrons released from the radiation target will be used for various scientific experiments.

The Radiation Safety Authority (SSM) announced in 2021 funds for research to develop measurement methods for the specific radionuclides that may occur in the event of a release in an event beyond expected operation from the research facility European Spallation Source (ESS) in Lund. The purpose of the call was to enable further development of useful measurement methods and especially with regard to the specific radionuclides that have been shown to dominate the dose contribution to the public.

In the report Underlag till beredskapsplanering kring ESS (SSM 2018:22) it appears that ¹⁴⁸Gd, ¹⁸⁷W, ¹⁷²Hf, ¹⁸²Ta and ¹⁷⁸nHf are the radionuclides that contribute the most to the effective dose to the public in the event that considered as dimensioning for emergency planning around the facility. In the dimensioning event described in the Swedish Radiation Safety Authority’s report SSM2018:02, and which is the basis for placement of the ESS facility in threat category II, the inhalation dose is attributed to more than 50% of the effective dose during the first 7 days after the accident. More than half of the total effective dose during this period originates from ¹⁴⁸Gd (half-life=71.1 y; Eα=3183 keV). Even during normal operation, small amounts of ¹⁴⁸Gd and other radioactive nuclides may be released into the environment via stacks connected to the ESS facility’s ventilation system. For correct dose estimates, techniques to measure ¹⁴⁸Gd in ambient samples are needed. Since ¹⁴⁸Gd is expected to appear in aerosol form and since the inhalation dose is of the greatest importance, collection of airborne aerosols in air is suitable.

The present report aims to describe the basic requirements for being able to quantify ¹⁴⁸Gd in aerosol samples collected from ambient air of the ESS on filters using special air samplers, both during normal operation and at an accident scenario. The focus is on evaluating the possibility of direct alpha spectrometry, where the alpha radiation from the filter sample is measured without pre-treatment.

Results

The analysis of the filters shows that most of the alpha activity from the radon progenies on the filters has vanished after a few days. An estimate of the thickest suitable aerosol layer is presented, as well as suggestions for suitable airflows and collection times that may enable direct alpha spectrometry of aerosol samples collected on filters for environmental monitoring purposes and as emergency preparedness in case of accidental releases. The energy resolution will not only depend on the thickness of the aerosol layer, but also on the size of the aerosol particles and the extent to which they penetrate the filter. Best energy resolution is obtained for thin layers (< 200 µg cm-2 ), small particles (<<1 µm in diameter) and non-existent penetration in the filter. Low collection flows are therefore necessary. ><1 µm in diameter) and non-existent penetration in the filter. Low collection flows are therefore necessary.

Relevance

Knowledge of measurement methods and their application for the specific radionuclides produced at ESS is of vital interest as it increases the possibility of producing better data in the event of events beyond expected operation. This knowledge can also be useful for the operator and other similar activities in the world. This research is also a contribution to strengthening the national competence in radiation safety, which is in line with the needs identified in the government mission “The basis for a long-term competence supply in the field of radiation safety” (SSM2017-134-23). SSM intends to use the results as much as possible within permit review and supervision and disseminate the information to authorities that regulate similar activities.

Need for further research

Since the dosimetry of radioactive aerosols in man is closely linked to the size of the aerosol particles, it is recommended to further investigate the possibility of direct alpha spectrometry after collection with systems that fractionate the aerosols by size (impactor systems). The interferences in alpha spectra from other types of radiation needs to be further investigated (e.g. coincidences between alpha and other types of ionizing radiation, or the influence of other types of ionizing radiation registered in the alpha detector). Further estimates of the probable size of ¹⁴⁸Gd-containing aerosols generated at different conditions at the ESS are also needed to evaluate the potential of direct alpha spectrometry.