The development in the mid-1960s in Chalk River of the lithium-drifted germanium detectors by George Ewan and Alistair Tavendale led to an enormous leap in the studies of gamma rays emitted from nuclei. The improved resolution led to the discovery of many new gamma rays and greatly improved the accuracy with which their energies could be measured.
Chalk River researchers had participated in a similar leap in nuclear spectroscopy capabilities in the mid-1950s, a leap now mostly forgotten. That instrument was an iron-free beta spectrometer called the π√2 or “pi-root 2” spectrometer.
Leading the role in the design and operation of this spectrometer were R.L. (Bob) Graham and Graham Lee-Whiting. Bob was the physicist interested in the nuclear spectroscopy and Graham had the mathematical skills to design the instrument.
The instrument was housed in a wooden, non-magnetic building behind the main nuclear physics laboratory. It was based on three pairs of iron-free coaxial coils, the largest being 4 m in diameter. The stringent tolerances on dimensions of the aluminum coils were met by an outside contractor, Canadair, and construction was completed in late 1958.
For the next decade, the π√2 was the premier spectrometer in the world for accurate electron and gamma ray (via conversion electrons) energy measurements. No accurate measurements could however be made at the start or end of the normal work days. The movements of buses in the parking lots several hundred meters away altered the magnetic fields at the spectrometer sufficiently to cause problems.
The Chalk River instrument was copied at a number of other laboratories. Versions, at different scales, were built at The Institute for Nuclear Study (Tokyo), the California Institute of Technology (Caltech), Idaho National Laboratory, Oak Ridge National Laboratory, the National Research Council of Canada (Ottawa) and the National Physical Laboratory (U.K.).
Among the final experiments with the spectrometer at Chalk River, before it was retired in the late 1980s, was a careful measurement of the beta spectrum for the rest mass of the neutrino. There had been a prediction of the existence of a “heavy neutrino” in the decay of tritium. The spectrometer proved this not to be the case.
Unfortunately, no parts of this historic spectrometer were salvaged when it was dismantled around 1990. However, the Society for the Preservation of Canada’s Nuclear Heritage Inc. does have a number of photographs and copies of Lee-Whiting’s original calculations to show you on your next visit to the Canadian Nuclear Heritage Museum in Deep River. Please contact us at info@nuclearheritage.com to book a visit.
Bob Graham making final detector adjustments