The fusion power generated by DD or DT plasmas in magnetic confinement devices is determined by measuring the total neutron yield. At JET, the primary diagnostic for measuring the time-resolved neutron emission is a set of fission chambers, whereas the timeintegrated neutron yield is measured by means of the activation technique. The latter involves the irradiation of a sample of known mass by the neutron flux in specific locations close to the JET vacuum vessel. The induced radioactivity is then counted by a detector which is absolutely calibrated both in efficiency and energy. From the knowledge of the gamma emission probability, the half life, the detection efficiency and the timing of the irradiation, the total initial activity induced in the sample A0 can be determined. The initial activity is related to the total neutron yield Yn. The use of suitable (n,n'), (n,2n), (n,p), (n,a) nuclear reactions with different energy thresholds allows the measurement of both DD/DT yields and plasma neutron spectra. Particularly for the evaluation of spectra, a novel kind of samples consisting of multi elements mixtures ("mixed sample") have been developed, prepared and deployed in JET. The preliminary results of the measurements confirm the potential of the multielement activation technique for cross calibration of other neutron diagnostic systems and even rough neutron spectrum measurements.