According to Wikipedia, Sakharov in his early scientific career investigated cosmic rays. It offers no other details. It claims that it was for this work that he was awarded his doctorate.
However according to Spartacus Educational, he joined the Lebedev Institute in 1945 to work on his doctorate under the supervision of Igor Tamm and defended his thesis on nuclear physics two years later. There is no mention of cosmic rays here.
Of course it is likely that Sakharov worked on a number of projects during this period. What research did he specifically carry out in investigating these rays?