Small samples, nearly no chemistry and a big accelerator: Beryllium‑7 measurements as low as 1 mBq

Introduction
Beryllium-7 (T_1/2 = 53.22 d), mainly measured via gamma-spectrometry, is used as a (natural) radiotracer for educational and scientific purposes. For samples with lower activities (<0.1 Bq) and especially for natural samples containing both ⁷Be and the longer-lived ¹⁰Be (T_1/2 = 1.387 Ma), accelerator mass spectrometry (AMS) is the method-of-choice. Here, we demonstrate that ⁷Be- and ¹⁰Be-AMS can be performed at the Dresden AMS facility (DREAMS) [1,2] on the same chemically prepared BeO from rain water samples collected in Germany.

Results
Detection limits for ⁷Be are as low as 0.6 mBq, which is one-to-two orders of magnitude better than “standard/ordinary” and “sophisticated” decay counting (e.g. in an underground laboratory). Validation measurements by gamma-counting of two larger rainwater samples were in excellent agreement with our AMS results. Uncertainties are usually 6-7% for small samples.
Sample sizes as small as tens of milliliters of rain water can be chemically processed to BeO within a few hours without the need for more expensive, time-consuming and labor-intense methods like ion exchange. Basic steps are: Acidification (of utmost importance), filtration, ⁹Be carrier addition, hydroxide precipitation, washing, drying, ignition, and mixing with Nb. Isobar (⁷Li) suppression by chemistry and AMS is sufficient.

Conclusion and outlook
Both the detection limit and uncertainty can be improved by more precise decay counting measurements of the calibration material (high ⁷Be activities from p-activated Li), the removal of so-called “dummy” steps currently required by the AMS machine software, and better tuning conditions. Our study qualifies AMS at DREAMS for being an ultrasensitive, cheap, and fast detection method for ⁷Be allowing high sample throughput.
Our ⁷Be and ¹⁰Be data clearly showed the very first rain (<5 min) collected being enriched in particulate matter (Fig. 1). Hence, AMS analyzing small samples can be used for time evolution studies of rain. The low detection limit and the high sample throughput will also enable future studies of small timescale phenomena where high-precision measurements of small sample volumes are needed. Further information is given by Tiessen et al. [3].

Figure 1: ⁷Be concentrations of rainwater water samples from Dresden (Drs) and Hannover (Hann). Drs 05_05, Drs 05_06, Drs 2, and Drs 3 were collected at the start of rainfall containing a larger amount of dust. Drs 5 was also from the start of rainfall but after long rain the night before, likely depleting the air of particulate matter. *Both Hannover samples are depleted in ⁷Be due to long collection times and partially missing acidification (Hann only).

Acknowledgements
Parts of this research were carried out at the Ion Beam Centre (IBC) at the Helmholtz-Zentrum Dresden-Rossendorf e. V., a member of the Helmholtz Association. We appreciate support of Dominik Güttler, René Ziegenrücker and the DREAMS operator team during AMS-measurements, of Gyürky György (Hungarian Academy of Sciences) for providing ⁷Be for the calibration material, and of BMBF (05K16MG1) and DAAD-RISE Professional (HZDRPH-456) for funding. It was a pleasure to discuss ⁷Be-AMS with Andrew Smith (ANSTO).

References
[1] S. Akhmadaliev et al., Nucl. Instr. Meth. B 294 (2013) 5-10.
[2] G. Rugel et al., Nucl. Instr. Meth. B 370 (2016) 94-100.
[3] C. Tiessen et al., Accelerator mass spectrometry (AMS) for beryllium-7 measurements in smallest rainwater samples, JRNCh, 2018, doi: 10.1007/s10967-018-6371-6.