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Near the end of his period at Toronto, Bert accepted an offer to join Donald Hurst's neutron physics group at the Chalk River Nuclear Laboratories of NRC's Atomic Energy Project (later to become, in 1952, Atomic Energy of Canada Limited, AECL), and he moved to Chalk River in 1950. Don Hurst effectively charged Bert with finding something interesting to do with neutron beams. He certainly did. Over the next dozen years, he played the dominant role in laying the foundation for the field of neutron inelastic scattering or, as he often preferred to call it, neutron spectroscopy, as later recognized by the Nobel Committee. Soon after arriving at Chalk River, Bert concluded that it might be feasible to measure the energy changes experienced by neutrons scattered by elementary excitations (phonons) using the National Research Experimental (NRX) reactor at Chalk River, then the most powerful research reactor in the world. If so, this would allow the dispersion curves for the phonon excitations to be determined, which in turn would yield information about the forces between atoms in samples and represent a major breakthrough in the fundamental understanding of condensed-matter systems. Realizing that the signal was going to be very weak, Bert and his scientific and technical collaborators set about developing/inventing techniques and instrumentation, including the improvement of all the components (monochromator crystals, detectors, filters, collimators, shielding, etc.) to make such measurements possible. Almost from the outset, Bert, always one to keep things simple, decided to employ single-crystal diffraction for neutron energy selection rather than the time-of-flight techniques being pursued by most of his competitors. This path led ultimately to two of his renowned achievements, the Triple-Axis Crystal Spectrometer (TACS) and the method of Constant-Q (constant momentum transfer), which have now been in widespread use for over four decades. The project was seriously delayed by the shutdown of the NRX reactor for over a year and a half in 1952-1954 but Bert benefited from a ten-month visit to Brookhaven National Laboratory during this period.
The Triple Axis Neutron Spectrometer (circa 1957) invented by Bertram N. Brockhouse. The monochromator shielding drum is still in use in the NRU reactor in the E-3 spectrometer. The crystal table, analyser and detector assembly to the right of the drum, are now in the Canadian Museum of Science and Technology in Ottawa.
By the time NRX resumed operation, a primitive TACS was functioning and large metal (Al and Pb) monochromator crystals, which were much more efficient than the NaCl crystals used earlier, had been grown. Rapid progress was then made. At the January 1955 APS meeting, Bert was able to present preliminary but substantial results for the inelastic scattering by liquid and solid Pb, H2O, D2 O, V and several paramagnetic materials. Later in 1955, he and Alec Stewart published the first convincing observation of a phonon dispersion curve, in a single crystal of Al, an unequivocal demonstration that short-wavelength phonon excitations existed in a metal. In 1955, following up on a proposal by Placzek and Van Hove in 1954, Bert also made the first determination of the frequency distribution of the normal modes in a material ( V) from the incoherent one-phonon scattering. In addition to the TACS, Bert and his co-workers developed, used and continually improved other instruments, in particular a Be/ Pb Filter-Chopper spectrometer and a Rotating-Crystal spectrometer. By 1957, the TACS was fully developed, with all angles variable so that one could operate in the highly advantageous mode of fixed scattered energy. The Constant-Q method was invented in 1958, and by 1959 it was possible to program the spectrometer to scan in energy at pre-selected momentum transfers, normally along high-symmetry directions. By then Bert also had a new TACS installed at the recently commissioned and much more powerful National Research Universal (NRU) reactor, the world's best research reactor for several years.
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