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We report the results of an experimental study on the radiation hardness of 4H-SiC diodes used as alpha-particle detectors with 1 MeV neutrons up to a fluence of 8times1015 n/cm2. As the irradiation level approaches the range 1015 n/cm2 , the material behaves as intrinsic due to a very high compensation effect and the diodes are still able to detect with a reasonable good Charge Collection Efficiency (CCE=80%). For fluences >1015 n/cm2 CCE decreases monotonically to ap20% at the highest fluence. Heavily irradiated SiC diodes have been studied by means of Photo Induced Current Transient Spectroscopy (PICTS) technique in order to characterize the electronic levels associated with the irradiation-induced defects. The dominant features of the PICTS spectra occur between 400-700 K; in this temperature range the deep levels associated with the induced defects play the main role in degradation of the CCE. Enthalpy, capture cross-section and concentration of such deep levels were calculated and we found that two deep levels (Et=1.18 eV and Et=1.50 eV) are responsible for the decrease in CCE. They have been associated to an elementary defect involving a carbon vacancy and to a defect complex involving a carbon and a silicon vacancy, respectively