(Per the American Catholic History Research Center and University Archives of The Catholic University of America, "The following years are public domain: 1946-1963, inclusive, and 1972." (See here for details on permission) Link: http://libraries.cua.edu/achrcua/TreasureChest.html)
Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.