Phase transition of NpO2 at 25 K

We have measured the Np-237 Mossbauer spectra of NpO2 whose specimens have been stored for 12 years before and after annealing at 773, 973, 1373 K, Th e Mossbauer spectrum of NpO2 after annealing consists of a symmetrical abso rption line, while the line shape of the sample before annealing is broad a nd asymmetric below 25 K. The change of line shape is attributed to the pha se transition at 25 K which might be enhanced by the existence of the self- irradiation-induced defects. Irradiation-induced damage can be recovered by annealing above 973 K. (C) 2000 Published by Elsevier Science B.V. All rig hts reserved.