This is the thirty-seventh progress report of the Laboratory for Nuclear Science at the Massachusetts Institute of Technology. Progress during the period of June 1, 1955 through August 31, 1955 is reported on: (1) Chemistry of the fission elements group, (2) Nuclear chemistry (inorganic) group, (3) Nuclear chemistry (organic) group, (4) Cosmic ray group, (5) Elementary particle scattering group, (6) neutron physics group, (7) ONR generator group, (8) Radioactivity group, (9) Cyclotron group, (10) Synchrotron group, (11) Theoretical group, (12) Personnel listing.

Infrared spectra of solid NaXO2(Ac)3, with X=U, Np, Pu, and Am, have been observed. From the symmetric and asymmetric stretching frequencies of the O-X-O groups, approximate X-O force constants have been calculated and were found to decrease in the order kU—O>kNp—O>kPu—O> kAm—O, the respective values being about 0.705, 0.698, 0.675, and 0.612 megadyne/cm. From the cell constants for NaXO2(Ac)3 it is apparent that the X-O bond distance decreases in the same order—RU—O>RNp—O>RPu—O>RAm—O. Thus, a decrease in bond distance appears to be accompanied by a decrease in force constant, probably because the bond, though shortened by contraction of the electron shells of the metal, is weakened by interaction with the extra valence shell electrons.