Using the data plotted in Fig. 9.3(in image), find the total applied voltage necessary for a PM tube with a six-stage multiplier using GaP(Cs) dynodes to achieve an electron gain factor of 2.8 x 10⁶. 

50 40 GaP(Cs) 30 20 Cs, Sb 10- BeO MgO 100 200 300 400 500 600 700 800 Primary electron energy-eV Figure 9.3 Variation of the secondary emission yield with primary electron energy for standard dynode materials (lower three curves) and an NEA material [GaP(Cs)]. (From Krall et al.) Secondary emission ratio (6)