Nuclear physicists using accelerator beams require specialty targets for their studies. During the 1940s, experiments with relatively low-voltage accelerators (2 million volts (MV) or less) looked at scattering particles such as protons or deuterons from elemental targets such as gold, magnesium or copper.
In these experiments, the beam particles lose energy to, or exchange nucleons between, the beam and the target nuclei; the energy levels of the target nuclei can be determined by measuring the energy of the outgoing particle. It soon became clear that high resolution work could only take place if the target could be made very thin (a few micrograms per square cm).
Producing these thin targets at Chalk River, for early researchers such as Harry Gove and Allan Bromley, became the job for Walt Woytowich. The process began by spreading a few drops of VYNS (polyvinyl chloride-acetate copolymer) on water. The thin copolymer would then be picked up and dried on aluminum target frames, typically several square cm in area. The desired element would then be vacuum deposited on the VYNS to produce a thin target.
Walt developed a vacuum chamber with appropriate manipulators to produce such excellent targets that Allan Bromley, after leaving Chalk River for Yale, attempted to get Walt to move there. The Canadian Nuclear Heritage Museum collection has not only Walt’s original vacuum chamber but also a copy of an October, 1960, letter from Bromley to Woytowich, who was still a bachelor, with the enticement that “I might add by way in inducement that we have a Swedish girl (ex-Swedish TV) living with us this year”. Despite this, Walt declined the move.
After Walt’s retirement in 1976, Joe Gallant, assisted by Wayne Perry, took on the role of target manufacturer for the nuclear physics group. Two important advances had happened since the start of work with accelerator beams. Initially targets were made using elemental samples, but experimenters now desired single-isotope targets. The often-rare and expensive isotopes necessary became increasingly available from the isotope-separators at Oak Ridge.
The second advance was the 1959 arrival of the world’s first tandem accelerator at Chalk River. With the tandem, not only was much higher voltage (up to 7 MV) available but, in addition, the positive, high-voltage terminal was located in the center of the accelerator. This introducing a great advance. Negative ions, for instance chlorine (charge state -1), injected from an ion source would be accelerated to the high-voltage terminal gaining 7 MeV of energy.
At the terminal, the beam would pass through a thin carbon-on-VYNS film and would emerge having lost not only the extra electron but several more. These, now positively charged ions, would be accelerated further along the accelerator to exit at higher energies. A +5 charge-state chlorine ion, for example, would gain five times the terminal voltage (7 MeV x 5 = 35 MeV) and would emerge with a total energy of 42 MeV.
The production of these thin carbon “stripper” targets became a large part of Joe’s work and he became a world-leading expert on them – not only on the first, EN, tandem but also for the later MP or Emperor tandem. Chalk River also became a commercial supplier for targets for other accelerator laboratories.
Joe Gallant’s retirement resulted in the transfer of the target-maker mantle to Peter Dymtrenko. Peter further refined the task of target making for Chalk River and outside laboratories and developed the stripper targets needed for the next stage of experiments at Chalk River, when beams for the tandem accelerator would be further stripped of electrons (typically chlorine to charge state +11). These ions were injected into the superconducting cyclotron of the TASCC facility to be accelerated to even higher energies.
Chalk River’s reputation for world-leading nuclear physics research in the era from 1945 to 1990 is often remembered by the names of the prominent physicists attached to publications. Walt Woytowich, Joe Gallant, Wayne Perry and Peter Dymtrenko were some of the excellent technicians who developed the needed tools for that research.
