Article The First Two Decades of Neutron Scattering at the Chalk River Laboratories Thomas M. Holden Northern Stress Technologies, 208, Pine Point Rd., Deep River, ON K0J 1P0, Canada; [email protected] Abstract: The early advances in neutron scattering at the Chalk River Laboratories of Atomic Energy of Canada are recorded. From initial nuclear physics measurements at the National Research Experimental (NRX) reactor came the realization that, with the flux available and improvements in monochromator technology, direct measurements of the normal modes of vibrations of solids and the structure and dynamics of liquids would be feasible. With further flux increases at the National Research Universal (NRU) reactor, the development of the triple-axis crystal spectrometer, and the invention of the constant-Q technique, the fields of lattice dynamics and magnetism and their interpretation in terms of the long-range forces between atoms and exchange interactions between spins took a major step forward. Experiments were performed over a seven-year period on simple metals such as potassium, complex metals such as lead, transition metals, semiconductors, and alkali halides. These were analyzed in terms of the atomic forces and demonstrated the long-range nature of the forces. The first measurements of spin wave excitations, in magnetite and in the 3D metal alloy CoFe, also came in this period. The first numerical estimates of the superfluid fraction of liquid helium II came from extensive measurements of the phonon–roton and multiphonon parts of the inelastic scattering. After the first two decades, neutron experiments continued at Chalk River until the shut-down of the NRU reactor in 2018 and the disbanding of the neutron effort in 2019, seventy years after the first experiments. Keywords: neutron scattering; triple-axis spectrometer; lattice dynamics; phonons; correlation Citation: Holden, T.M. The First Two functions; liquids; superfluid helium; magnetism; magnons; exchange interactions Decades of Neutron Scattering at the Chalk River Laboratories. Quantum Beam Sci. 2021, 5, 3. https://doi.org/ 10.3390/qubs5010003 1. Introduction The building of the National Research Experimental Reactor, NRX, in 1947 set the Received: 15 December 2020 stage for nuclear research at Chalk River. A comprehensive account of the early history of Accepted: 11 January 2021 the Chalk River Laboratories was given in “Canada enters the Nuclear Age” [1], which Published: 18 January 2021 was written by many prominent Chalk River scientists. Briefly, the Laboratories grew out of the University of Montreal Laboratory where the original Norwegian heavy water Publisher’s Note: MDPI stays neutral had been transferred from Cambridge, England in 1942 and which had been tasked to with regard to jurisdictional claims in develop a heavy water reactor. By August 1944, the site at Chalk River had been identified, published maps and institutional affil- and construction had begun. The first low power reactor went critical in September 1945, iations. and the NRX reactor went critical in July 1947. In common with reactors elsewhere, NRX provided space for in-reactor irradiations as well as beams of neutrons and γ-rays external to the biological shielding for experiments. The earliest beam experiments were aimed at understanding nuclear structure, fission processes, and nuclear energy levels, as well as Copyright: © 2021 by the author. atomic structure. A photograph, Figure1, of the main floor of the NRX reactor taken from Licensee MDPI, Basel, Switzerland. the Nobel Prize Lecture of B.N. Brockhouse [2] shows experiments arranged around the This article is an open access article NRX reactor. distributed under the terms and According to G.C. Hanna in [1] p. 114, “W.B. Lewis (then director of Chalk River) . conditions of the Creative Commons left the scientific direction of pure research to the scientists”. That is, within broad guide- Attribution (CC BY) license (https:// lines, it appears that the staff scientists were given permission to carry out the research creativecommons.org/licenses/by/ that seemed most likely to them to lead to advances in science and technology. This aspect 4.0/). Quantum Beam Sci. 2021, 5, 3. https://doi.org/10.3390/qubs5010003 https://www.mdpi.com/journal/qubs Quantum Beam Sci. 2021, 5, 3 2 of 75 of Lewis’s philosophy and the consequent emphasis on basic research in the early days of the laboratory is emphasized in his biography [3] and contrasts radically with modern “top–down” approaches. However, the Lewis outlook characterized a great deal of the scientific Chalk River research until the 1980s. In common with laboratories elsewhere, Quantum Beam Sci. 2021, 5, x FOR PEERthere REVIEW were experts in many fields who now seem peripheral to the nuclear business2 butof 81 could be called upon for advice when needed. FigureFigure 1. 1.The The mainmain floorfloor ofof thethe National Research Ex Experimentalperimental (NRX) (NRX) reactor reactor at at the the Chalk Chalk River River Laboratory about 1950. The powder spectrometer constructed by Donald Hurst and colleagues is Laboratory about 1950. The powder spectrometer constructed by Donald Hurst and colleagues is visible near the center of the photograph. Most of the other equipment is concerned with nuclear visible near the center of the photograph. Most of the other equipment is concerned with nuclear physics or with physics of the neutron itself. For reasons of restricted space around the reactor, physicseach heavily or with shielded physics apparatus of the neutron is located itself. at For the reasons end of a of long restricted tube. This space figure around is taken the reactor, from Figure each heavily1 of ref. shielded [2] and reproduced apparatus is by located courtesy at the of At endomic of a Energy long tube. of Canada This figure Limited. is taken Copyright from Figure by the 1 of ref.American [2] and reproducedPhysical Society. by courtesy of Atomic Energy of Canada Limited. Copyright by the American Physical Society. According to G.C. Hanna in [1] p.114, “W.B. Lewis (then director of Chalk River)… left Accordingthe scientific to A.D.B.direction Woods of pure in a privateresearch communication, to the scientists”. “There That is is, no within doubt inbroad my mindguidelines, that Don it appears Hurst, thenthat the director staff ofscientists the Reactor were Researchgiven permission and Development to carry out Division the re- wassearch the that inspiration seemed behindmost likely the neutron to them scattering to lead to program advances at in Chalk science River”. and technology. In the late 1940s,This aspect Hurst of worked Lewis’s with philosophy N.Z. Alcock and andthe cons J. A.equent Spiers onemphasis neutron on scattering basic research from gasesin the andearly on days nuclear of the physics laboratory problems. is emphasized To augment in his the biography work on neutron[3] and contrasts scattering, radically Hurst hired B.N. Brockhouse in 1950 and D.G. Henshaw in 1951. The first monochromatic with modern “top–down” approaches. However, the Lewis outlook characterized a great neutron beams were extracted from NRX by diffraction from natural crystals, and shortly deal of the scientific Chalk River research until the 1980s. In common with laboratories afterwards, a second axis was added to create a diffractometer. While the existence of lattice elsewhere, there were experts in many fields who now seem peripheral to the nuclear vibrations had been inferred from low temperature heat capacity measurements, from business but could be called upon for advice when needed. thermal diffuse scattering of X-rays from solids, and from velocity of sound measurements, According to A.D.B. Woods in a private communication, “There is no doubt in my no definitive measurement of the energy–momentum relation for lattice vibrations existed. mind that Don Hurst, then director of the Reactor Research and Development Division Measurements of the wavelength dependence of the transmission of neutrons through was the inspiration behind the neutron scattering program at Chalk River”. In the late various light and heavy solids by Hurst and Brockhouse may have been the catalyst [2] 1940s, Hurst worked with N.Z. Alcock and J. A. Spiers on neutron scattering from gases for the realization that a spectroscopic method might be developed if the neutron flux and on nuclear physics problems. To augment the work on neutron scattering, Hurst was sufficiently high, and at that time, NRX did have the highest thermal neutron flux inhired the world.B.N. Brockhouse The goal of in measuring 1950 and the D.G. details Henshaw of the vibrationalin 1951. The spectrum first monochromatic of solids was formulatedneutron beams in 1950 were in extracted meetings betweenfrom NRX D.G. by diffraction Hurst, B.N. from Brockhouse, natural crystals, G. H Goldschmidt, and shortly andafterwards, N.K. Pope, a second which wasaxis mentionedwas added in to several create places a diffractometer. in the literature. While To the reach existence this goal, of lattice vibrations had been inferred from low temperature heat capacity measurements, from thermal diffuse scattering of X-rays from solids, and from velocity of sound meas- urements, no definitive measurement of the energy–momentum relation for lattice vi- brations existed. Measurements of the wavelength dependence of the transmission of neutrons through various light and heavy solids by Hurst and Brockhouse may have been the catalyst [2] for the realization that a spectroscopic method might be developed if the neutron flux was sufficiently high, and at that time, NRX did have the highest ther- Quantum Beam Sci. 2021, 5, x FOR PEER REVIEW 3 of 81 mal neutron flux in the world. The goal of measuring the details of the vibrational spec- Quantum Beam Sci. 2021, 5, 3 3 of 75 trum of solids was formulated in 1950 in meetings between D.G. Hurst, B.N. Brockhouse, G. H Goldschmidt, and N.K.