More powerful versions include synchrotrons and cyclotrons used in nuclear research. Most fast neutron therapy beams are produced by reactors, cyclotrons (d+Be) and linear accelerators. Most installed proton therapy systems utilise isochronous cyclotrons. They may involve nuclear reactors, cyclotrons, and certain devices used in cancer therapy. Over 1200 cyclotrons are used in nuclear medicine worldwide for the production of radionuclides. They typically do not prohibit other nuclear technologies such as cyclotrons used in particle physics. Isochronous cyclotrons are capable of producing much greater beam current than synchrocyclotrons. Examples of isochronous cyclotrons abound; in fact almost all modern cyclotrons use azimuthally-varying fields. The name was derived from "California university cyclotrons". As a result, isochronous cyclotrons became more popular in the research field. Lawrence's response was to press on with the creation of still larger cyclotrons. One of the world's largest cyclotrons is at the RIKEN laboratory in Japan. Lawrence went on to build a series of ever larger and more expensive cyclotrons. He then commenced work on his doctorate, assisting Marshall Holloway with the cyclotrons. In June 1945, McMillan's thoughts began to return to cyclotrons.