Abstract : This thesis presents the precision measurement of the neutron polarized structure g1n performed by the E154 collaboration at the Stanford Linear Accelerator Center, USA, in autumn 1995, using a 48.3 GeV polarized electron beam scattered off a polarized Helium 3 target. The scattered electrons were detected using two spectrometer arms, covering the deep inelastic scattering range : 0.014 < x < 0.7 and 1 GeV2 < Q2 < 17 GeV2 at an average value of Q2 = 5 GeV2. Two electromagnetic calorimeters have been designed by the LPC in Clermont-Ferrand and the SphN-CEA in Saclay to measure the scattered electron energy and to reject the contaminating hadron background, using a cellular automaton and a neural network, widely described in this thesis. The analysis performed in Clermont-Fd and presented in this document led us to the integral on the measured region : integral (g1n(x)dx, xmin=0.0135, xmax=0.7) = -0.03 +- 0.003 STAT +- 0.004 SYST +- 0.001 EVOL at Q2 = 5 GeV2 where our data have been evolved to Q2 = 5 GeV2 using the next-to-leading order DGLAP evolution equations and a world parametrization of the polarized parton distributions. The Ellis and Jaffe sum rule is clearly violated. Using different low x extrapolations, our integral is compatible with the Bjorken sum rule. The quark contribution to the nucleon spin is DeltaSigma = 29 ± 6 % in the MS bar scheme and DeltaSigma = 37 ± 7 % in the AB scheme, at Q2 = 5 GeV2. The gluon contribution seems to be positive and within the range : 0 < DeltaG <2.