The “reactor era”, which started in 1942, introduced the world of nuclear spectroscopy to a huge expansion in the number of radioactive isotopes that could be studied. Reactors became the source of abundant supplies of previously-unavailable fission products. In addition, the reactors provided sufficient neutron fluxes to allow activation of isotopes of many stable elements.
The availability of these new isotopes was soon followed by the development of a wide range of gamma ray detectors. Most of these were based on scintillation materials that produced small flashes of light when struck by gamma rays. When coupled to photomultiplier tubes, these scintillations resulted in electrical pulses proportional to the energy of the emitted gamma rays.
A great leap in the quality of scintillation materials occurred in 1948 when Robert Hofstadter at Princeton University developed the thallium-activated, sodium iodide NaI(Tl) scintillator. These relatively-inexpensive detectors were soon acquired by many university laboratories and were the main tools used by a generation of aspiring nuclear physicists.
The energy resolution of the NaI(Tl) detectors, while an improvement on previous scintillation detectors, still left a lot to be desired. Many of the peaks produced in the observed gamma ray spectra contained multiple transitions and much effort was spent by the spectroscopists attempting to determine the correct number of transitions, their intensities and energies by “fitting” the peak shapes with various calculated combinations.
George Ewan at Chalk River, and Alistair Tavendale, an Australian post-doctoral fellow at Chalk River, introduced the next great leap in gamma ray spectroscopy in 1963. With the help of the Counters Group under Dick Fowler at Chalk River, Ewan and Tavendale ushered in the era of the lithium-drifted germanium (Ge(Li)) detector. Their work was celebrated in 1967 when the American Nuclear Society Radiation Industrial Award was awarded to Ewan and Tavendale.
The Ge(Li) detectors provided a huge improvement in the energy resolution available in gamma ray spectroscopy. Where previously a spectrum might exhibit a broad peak which corresponded to 3-5 gamma rays, the spectrum from a Ge(Li) detector showed each of the peaks separately and the accuracy of energy and intensity determination was greatly improved.
Another Chalk River development that went hand-in-hand with the Ge(Li) detector was the improvement in electronics. In the late 1950s, F.S. (Fred) Goulding led the development of a 100-channel kicksorter at Chalk River which greatly simplified data collection. Fred moved to the Lawrence Berkeley National Laboratory and, based largely on his designs, by 1963 the kicksorter design evolved to units that had up to 4096 channels. Over the next few years, the gamma spectra of essentially all the radioactive isotopes known were determined much more accurately.
