Upon his return to Copenhagen in 1950, Bohr began working with Ben Mottelson to compare the theoretical work with experimental data. In three papers, that were published in 1952 and 1953, Bohr and Mottelson demonstrated close agreement between theory and experiment; for example, showing that the energy levels of certain nuclei could be described by a rotation spectrum.[21][22][23] They were thereby able to reconcile the shell model with Rainwater's concept.[19] This work stimulated many new theoretical and experimental studies.[17] Bohr, Mottelson and Rainwater were jointly awarded the 1975 Nobel Prize in Physics "for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection".[1] Because his father had been awarded the prize in 1922, Bohr became one of only four pairs of fathers and sons to win the Nobel Prize in Physics.[24] He married Marietta Soffer in 1950. They had two sons, Vilhelm and Tomas, and a daughter, Margrethe.[25]