Energy resolution $\sigma(^{55}\rm{Fe})$ as a function of ion backflow (IBF) in a 4-GEM stack (S-LP-LP-S) in Ne-CO$_2$-N$_2$ (90-10-5) The gas gain is kept at 2000 in all measurements by adjusting the voltages on GEM\,3 and GEM\,4 at a fixed ratio of 0.8 or 0.95. Figure from  .