“…For calibration of an ESR instrument, a suitable reference material has to be employed, e.g., a powder containing Mn 2+ ions in lime (CaO) (Negut & Cutrubinis, 2017). The Mn 2+ ion has effective spin S = 5/2, nuclear spin I = 5/2 and its ESR spectrum consists of a hyperfine sextet (Figure S1) (De Biasi & Grillo, 2014).…”