During this building’s first decade, it housed a powerful particle accelerator on its subbasement level. This instrument did not live up to expectations, but it laid the foundations for a successful particle physics program.
The McLennan Building, which you are standing in, was opened in 1967. It was built with a 35 MeV linear accelerator (linac) buried two floors beneath its front courtyard. This barrier was one of many measures meant to shield the intense (though short-lived) radiation that it generated. Thick concrete walls encase the experiment rooms and beamlines. Massive blocks mounted on hydraulic lifts sealed off the machine and target rooms before it was turned on.
Built by the English firm Vickers-Armstrong, it was hoped that its acquisition would place the University of Toronto among an elite group of institutions able to perform cutting-edge nuclear physics. Unfortunately, the machine did not come close to achieving its promised power. Located beneath a busy downtown campus, there was little hope of upgrading it to keep pace with the advancing field. The project generated relatively little research before the machine was shuttered in 1978 and subsequently sold.
The legacy of the U of T Linac lives on in the thick-walled labyrinth of beamlines, experiment rooms, and workshop infrastructure in the McLennan building’s subbasement. This space is currently being renovated for the Department of Astronomy.
The Department of Physics Linac machine shortly after its installation.
Polycarbonate Damage Track (PDT) Detectors (c. 1971)
These detector plates represent one of the few artifacts remaining from the U of T linac.
These detector plates were made for experiments into the electron-induced fission of magnesium-24. When excited to high energies by the electron beam of the linac, magnesium-24 can fission into various isotopes, including into two stable carbon-12 atoms. This experiment was designed by U of T Professor Ted Litherland, based on research that took place during his time at Chalk River Laboratories
The aluminum frames house a very thin polycarbonate film that is aluminized on one side. When exposed to fission products of the beam striking a target magnesium film, the relatively massive ions pit the surface of the film to a characteristic depth. The detector plates are floated on an etching solution of sodium hydroxide. After an etching period corresponding to the depth of the track, a visible hole is formed in the aluminum surface.
Venting the Linac
If you were to enter the McLennan physics building from the St. George Street main entrance, you would pass infrastructure from the old linear accelerator
A sculptural air vent, meant to look like a radiation symbol when viewed from above, provided an inlet for air circulated through the subbasement. This ventilation was needed to clear short-lived radiation that was produced when the linac was operated. This cast concrete feature was devised by Stephen Irwin (1939 – 2019) of the Shore & Moffat architectural firm, which designed the McLennan building.
This model of the linac’s air intake was made by Erich Weidenhammer in 2019.