'Acta Physica Polonica A ', vol: 137, pages: 164-166 (2020) Acta Physica Polonica A, 137 (2) Proceedings of 15th International Workshop on Slow Positron Beam Techniques & Applications (SLOPOS-15) Acta Physica Polonica A Acta Physica Polonica A, Polish Academy of Sciences. Institute of Physics, 2020, Proc. of the 15th Int. Workshop on Slow Positron Beam Techniques and Applications, Prague, September 2–6, 2019, 137, pp.164-166. ⟨10.12693/APhysPolA.137.164⟩ Acta Physica Polonica A, 2020, Proc. of the 15th Int. Workshop on Slow Positron Beam Techniques and Applications, Prague, September 2–6, 2019, 137, pp.164-166. ⟨10.12693/APhysPolA.137.164⟩ Acta Physica Polonica A, Polish Academy of Sciences. Institute of Physics, 2020, 137 (2), pp.164-166. ⟨10.12693/APhysPolA.137.164⟩