Background study of absorbed dose in biological experiments at the Modane Underground Laboratory

Aiming to explore how biological systems respond to ultra-low background environments, we report here our background studies for biological experiments in the Modane Underground Laboratory. We find that the minimum radioactive background for biology experiments is limited by the potassium content of the biological sample itself, coming from its nutritive medium, which we find in our experimental set-up to be 26 nGy/hr. Compared to our reference radiation environment in Clermont-Ferrand, biological experiments can be conducted in the Modane laboratory with a radiation background 8.2 times lower than the reference above-ground level. As the radiation background may be further reduced by using different nutritive media, we also provide measurements of the potassium concentration by gamma spectroscopy of yeast extract (63.3 ± 1.2) mg/g and tryptone (2.5 ± 0.2) mg/g in order to guide media selection in future experiments. (authors)