A method is proposed for the numerical and analytical calculations of the adjoint neutron spectrum in the resonance energy region, for narrow resonances, in order to reduce the numerical computations involved. The adjoint neutron spectrum of infinite cylindrical fuel cell is calculated according to a very simple analytical method with reasonable level of accuracy. The results obtained with this analytical method were compared to a reference solution obtained with a numerical solution of the neutron balance adjoint equations. The adjoint neutron spectrum is useful to determine the neutron resonance absorption, so that multigroup adjoint cross sections used by the adjoint diffusion equation can be obtained.

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