The activation technique, based on the irradiation of suitable materials placed close to the plasma, is used in large magnetic confinement devices for the measurement of the neutron yield and is being developed to provide the neutron spectra for both DD and DT operation. In previous experiments at JET, stacks of separate elemental foils were used. Nowadays High Purity Germanium (HPGe) detectors, with high efficiency and resolution, can be numerically characterized very well in terms of efficiency and energy calibration. This allows the application of samples with mixed elemental compositions and shapes. The technique presented in this paper was implemented on the JET neutron activation diagnostic.