A new prototype of a single-crystal diamond detector has been tested at JET for measurements of DD and DT neutrons. The detector was installed in the JET torus hall on a collimated line of sight, which is shared with other neutron diagnostics, the MPRu neutron spectrometer and the NE213 scintillator. The results of the first measurements in the JET campaign C31 are presented in this paper. The measurements exploited for the first time the possibility of performing good resolution spectroscopy measurements of 2.5 and 14MeV neutrons using fast charge preamplifier combined to a digital acquisition system. The prototype detector has been developed for operation at very high cont rate applications, such as the one expected from high power DT plasmas. A distinction should be made in the neutron detection between neutrons of energy below and above 6MeV. The 2.5MeV neutrons from DD reactions, which are measured using elastic scattering on carbon nuclei, provide small amplitude signals and require a dedicated preamplification stage. This is the main component of the neutron energy spectrum in JET deuterium plasmas. Neutron spectrometry of 14MeV DT neutrons is possible via the 12C(n,a)9Be. Preliminary measurements of the 2.5MeV neutron emission and of the 14MeV triton burn-up neutrons, which represents about 1/100 of the neutrons in JET deuterium plasmas, are presented.