He spent a postdoctoral year at the Cavendish Laboratory, where he worked under the supervision of Ernest Rutherford, Mark Oliphant, and others.
After performing some calculations, Blewett concluded that the radiation would indeed be significant and would make it difficult to build machines for higher energy.
When the Blewetts arrived at BNL, security authorities of the Atomic Energy Commission barred them from going to work for six months because of a false guilt-by-association concern raised against John P.
He led a group of BNL physicists who introduced the alternating-gradient or “strong-focusing” method,[1] which was essential for the Cosmotron to become in 1952 the world's first billion-volt particle accelerator.
The Blewetts went to Bergen, Norway, where they contributed to the synchrotron's initial design and helped in the move to Geneva, Switzerland, where lab construction began in late 1953.
[1] In early 1954 John and Hildred returned to work at BNL, where construction of the Alternating Gradient Synchrotron (AGS) was beginning.
In retirement John Blewett participated in creating the proposal for BNL to build the National Synchrotron Light Source.