Optimisation of the thickness of the moderator for positron annihilation process study in Ar gas Toshitaka Oka, Yosuke Sano, Yasushi Kino and Tsutomu Sekine Eur. Phys. J. D, 68 6 (2014) 156 Published online: 23 June 2014 DOI: 10.1140/epjd/e2014-40807-2