The nitrogen split interstitial defect introduced by high-energy particle irradiation in n-type GaN has been investigated by very high (up to 324 GHz) frequency electron paramagnetic resonance (EPR) and Q-band electron nuclear double resonance (ENDOR) spectroscopy. The increased resolution of the EPR spectra at 324 GHz has allowed us to determine the g-tensor anisotropy, which is not resolved at X or Q band. The good agreement of the principal values g xx = 1.9966, g yy = 2.0016, and g zz =

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... 36 with the theoretically predicted g tensor confirm the (N-N) N 0 defect model. The hyperfine interactions of this defect have been studied by Q-band ENDOR. We observed well-resolved ENDOR lines with distant Ga atoms from which the quadrupole coupling constants and the electrical field gradients were determined and discussed with the help of theoretical values. The observation of ENDOR spectra of the central N and Ga atoms predicted in the 20-90-MHz range required the use of field-frequency ENDOR due to the large linewidth of the ENDOR lines. Our results confirm the importance of the nitrogen split interstitial in particle irradiated GaN similar to the case of diamond and silicon carbide in which the stable configuration at room temperature of the carbon interstitials is also the split interstitial configuration.