The effect of neutron irradiation on the photoacoustic spectra of Er<sub>2</sub>O<sub>3</sub> and Nd<sub>2</sub>O<sub>3</sub> in powder form has been studied. The irradiation shows a quenching effect on the photoacoustic signal which regains its intensity after a lapse of some time. It is believed that the irradiation changes the oxidation state of the rare-earth ions from M<sup>3+</sup> to M<sup>2+</sup>. The half-life of the recovery process Er<sup>2+</sup> to Er<sup>3+</sup> in the case of irradiated Er<sub>2</sub>O<sub>3</sub> is found to be 46 h.
V. N. Rai, S. N. Thakur, and D. K. Rai, "Photoacoustic Studies on Neutron-Irradiated Rare Earth Oxide Powers," Appl. Spectrosc. 40, 1211-1214 (1986)