In the 1958 Mossbauer experiment, what unusual property of gamma rays was observed?

In the 1958 Mossbauer experiment, recoil-free emission of gamma rays was observed.

Rudolf Mössbauer studied gamma rays emitted by iridium-191 at very low temperatures. Normally, an emitting nucleus recoils, slightly reducing the energy of the emitted photon. Mössbauer found that when nuclei were bound in a solid lattice, some gamma-ray emissions occurred without detectable recoil by the individual nucleus.

The entire crystal lattice could absorb the tiny momentum, making the energy of the gamma ray extremely sharp. This is known as the Mössbauer effect.

The discovery enabled Mössbauer spectroscopy, which measures minute changes in gamma-ray energy caused by chemical bonding, magnetic fields, and gravitational effects. It also helped confirm the gravitational redshift predicted by general relativity in the Pound–Rebka experiment.