The Harvard Cyclotron produced its first beam on June 3, 1949 and continued to operate until 2003 - a record breaking 54 years. This uniquely long operating life for a high energy cyclotron was due to the importance and variety of its applications and to its relatively low cost for operations and maintenance. These low costs came from clever design features and from its dedicated staff.

The initial research with the Harvard Cyclotron was for nuclear and particle physics. It concentrated on using proton beams of up to 160 Mev to study the interactions of protons with target protons, deuterons, helium and other nuclei. The cross sections for protons scattered by protons were measured at a number of different proton energies. In other experiments the angular distributions of the differential cross sections and polarizations were measured. Proton-deuteron scattering experiments gave information on the neutron-proton interaction. When a beryllium target was struck by high energy protons a beam of high energy neutrons was produced at approximately 90 Mev and these were scattered in turn by protons to study the neutron proton interaction. The Harvard Cyclotron proved to be an excellent facility for training graduate students. 30 Harvard Ph.D. theses, and as many from neighboring Universities and many physics papers were based on the Harvard Cyclotron.

While directing the design of the Harvard Cyclotron, Robert R. Wilson in 1946 wrote an important paper in which he pointed out that a high energy proton beam should be effective for cancer therapy since, near the end of its range, a proton beam produces a greater ionization density. By arranging for the cancer to be bombarded from different directions but with the ends of the ranges being at the cancer, the damage to the cancer would be maximized with minimum damage to the non-cancerous tissue. This pioneering paper led to research at the Cyclotron by doctors from Massachusetts hospitals in developing procedures for cancer therapy. Most of the Cyclotron's final years were devoted to cancer therapy. The Cyclotron continued to operate until a replacement accelerator, dedicated to cancer therapy, was in operation at the Massachusetts General Hospital.

It is fortunate that this history of the Harvard Cyclotron has been recorded while many of the participants were still alive to provide historical information.

Norman Ramsey was director of the Harvard Cyclotron Laboratory from about 1948 to 1950 and was responsible for the actual construction and first operation. He later was awarded the Nobel Prize in physics for his work on molecular beams.