Hugh Carmichael, F.R.S.C.
George Craig Laurence
1905-1987
TRANSACTIONS OF THE ROYAL SOCIETY OF CANADA / SERIES V / VOLUME IV / 1989
GEORGE CRAIG LAURENCE, physicist, born in Charlottetown, P.E.I., died in Deep River, Ontario on 7 November, 1987 in his eighty-third year. He was the first person in the world, after the discovery of nuclear fission in January 1939, to induce fission by neutrons in a very large quantity of uranium surrounded by carbon, to investigate the possibility that the fission chain reaction needed for a useful release of nuclear energy could be produced with these materials. He went on to play major roles in the development and control of the Canadian system for the production of electrical power from uranium surrounded by heavy water.
Many years before the fission process was discovered Laurence was involved with uranium. His first research experiment, suggested by Professor G.H. Henderson, F.R.S., after Laurence had obtained his B.SC. at Dalhousie University in 1925, was to measure the ranges in air of the alpha particles emitted by Uranium I and Uranium II - the uranium isotopes now called U-238 and U-234. His success with this difficult experiment won him an 1851 Exhibition Scholarship which enabled him to work for a PH.D. under Rutherford at the Cavendish Laboratory, Cambridge, from 1927 until 1930.
Laurence's Dalhousie work was referred to by Rutherford in 1930 in the book, Radiations from Radioactive Substances: “Recently the ranges for U-I and U-II have been measured by Laurence by a new method. A thin film of uranium was placed on the cylindrical surface of an expansion vessel . . . . A large number of ray tracks were photographed and the ranges were measured. In terms of air at 15ºC and 760mm., the range of the particles for U-I was found to be 2.73 cm, for U-II 3.28 cm. These ranges . . . . are believed to be correct within 1 per cent.”
Even though Laurence's thin film of uranium covered all of the surface available, the rate of emission of alpha particles was so small that, on the average, a track was seen only once in eighty expansions. So, to avoid a prohibitive photographic cost, Laurence had to hold the camera shutter release in one hand, while with the other hand he slowly turned a crank to produce successive expansions. Whenever he saw the track of a particle he quickly snapped the camera and read and recorded the temperature and the pressure of the air in the vessel for that particular photograph. Eventually, after over 120,000 expansions, he had about 1600 photographs, of which 900 were suitable for measurement.
Unknown at this time was the fact that about 0.7 percent of the atoms of uranium are those of the isotope U-235, which is the key material for the production of atomic energy. This isotope also emits alpha particles; so it is fascinating that in 1926-27, Laurence must actually have seen and photographed the tracks of about 37 of the alpha particles emitted by U-235.
In Cambridge, Laurence compared magnetically the velocities of the alpha particles from four well-known alpha emitters to within 0.05 percent. His thesis for the coveted PH.D. (Cantab) was not completed until after he had returned to Canada in 1930. A letter dated 7 February, 1931, from H.W. Phear says, “I have today taken your PH.D. degree for you and have sent the certificate to your address in Halifax.” Before going to Cambridge, Laurence had met Elfreda Elizabeth Blois who was also a student at Dalhousie. He married Elfreda on 2 January, 1931. She continues to reside in their Deep River home which contains many boxes filled with Laurence's paperwork.
For twelve years, from September 1930 until December 1942, Laurence worked in Ottawa with the National Research Council (NRC). He established and headed a laboratory to standardize the measurement of X-rays and radium gamma rays in terms of roentgen. His mathematical studies of the difficult problems of measurements facing the radiologist are still being quoted in textbooks of radiation dosimetry. He also wrote, with Edith H. Quimby, a ten-chapter technical bulletin for the Radiological Society of North America, The Measurement of Dose in Roentgen Therapy, which was approved at their annual meeting in December 1939.
By this time, Canada was at war, and Laurence and his staff were busy introducing radiological inspection for manufacturers of military equipment. Nevertheless, Laurence kept abreast of the spate of publications following the discovery of fission and he soon realized that a nuclear fission chain reaction might occur if carbon was used to slow down neutrons between separate lumps of a very large quantity of uranium. He also knew that ample uranium oxide was available in Canada and he borrowed one tonne from Gilbert LaBine of the Eldorado Company, in several hundred small bags. By March 1940, he had designed a beautiful miniature reactor in a strong wooden container of roughly spherical shape, nine feet in diameter, lined with a neutron reflector of paraffin wax. It had a horizontal tube to support a source of neutrons at the centre to drive the reactor. The tube also allowed the neutron flux to be measured along a radius. By September, the reactor had been built, in secret in a locked room. The objective was to measure the neutron flux, first when the device was filled with ten tonnes of carbon powder (in larger bags) and, next when several hundred bags of uranium oxide formed a uniform lattice throughout the carbon. It could then be found, after elaborate calculation, whether a similar reactor of unlimited size would be self-sustaining.
At this time, Dr. J.D. Cockcroft visited Ottawa. He was so impressed with Laurence's work that he persuaded Imperical Chemical Industries of Britain to send a contribution of $5,000. In December 1940, Laurence was asked to visit the United States where he met L.J. Briggs, Chairman of the Uranium Committee, and other scientists, including Fermi. An exchange of relevant secret reports lasting for nearly two years resulted.
The first U.S. lattice, using seven tonnes of uranium oxide in a eight-foot graphite cube, was built in July 1941. In November 1941, Fermi reported that oxide of greater purity that than ordered from Eldorado in the spring of 1941 was needed for a self-sustaining uranium graphite reactor. Laurence, with summer help form professor B.W. Sargent of Queen's University, got the same result in the summer of 1942. He had had an initial lead time of one year, but progress was slow because of his other duties. On 2 December, 1942 the first self-sustaining nuclear reactor in the world operated in the United States. Coincidentally, in December 1942, Canada's reactor expert, with his wife and their two daughters, drove to Montreal in a snowstorm to begin work as the senior Canadian scientist in the newly established Montreal Laboratory of the NRC.
The Montreal Laboratory had been established to enable Canada and Britain, in collaboration with the United States, to help the was effort by developing the heavy water uranium system as an alternative to the graphite uranium system. By late 1942, the U.S. had decided to use the latter system because the huge amount of graphite needed could, unlike heavy water, be obtained without delay. In January 1943, after their army had been placed in charge of their atomic bomb project, the U.S. collaboration ceased. Collaboration was not restored until Cockcroft became director at Montreal at the end of April 1944, with the mandate to build NRX, a high-power, heavy-water, uranium reactor. This was the origin of the Chalk River Nuclear Laboratories.
A low-power reactor, ZEEP, the first self-sustaining reactor outside the United States, was started up at Chalk River on 5 September, 1945. On 22 July, 1947, the NRX reactor, with the highest neutron flux and by far the most advanced design in the world, began operation.
Under Cockcroft until 1946, and then under Dr. W.B. Lewis, Laurence directed the groups developing and often fabricating the new instruments needed for ZEEP, NRX and environmental monitoring. Then, from 1949, he directed the branches involved in the design of NRU and also a branch charged with planning the Canadian approach to nuclear power, formed early in 1954. By this time, in April 1952, the Crown Corporation, Atomic Energy of Canada Limited (AECL), had been formed.
In December 1952, the NRX reactor suffered a very serious accidental power surge due to operator error and malfunction of the shutoff rods. The centre of the reactor was destroyed and the whole building was contaminated with radioactivity. Cleanup and repair took fourteen months. Laurence was chairman of the investigating committee. The accident had a profound effect on Laurence, which lasted for the rest of his life. As a result, he then devoted himself almost entirely to ensuring the safety of nuclear power in Canada. First, he got his team of chemists, physicists, and engineers designing the NRU reactor to keep improving the control system whenever new ideas occurred or new instruments became available. The reconstructed NRX reactor continually benefited from this work. Next, in 1956, while still with AECL, Laurence became chairman of the Reactor Safety Advisory Committee, newly created by Dr. C.J. Mackenzie, President of the Atomic Energy Control Board (AECB). Finally, in 1961 he succeeded Mackenzie as President of the AECB until his mandatory retirement at age sixty-five in 1970. In these positions he was instrumental in establishing the unique Canadian system for the regulation of reactor safety. In addition to nuclear reactors, the AECB had control of all equipment and materials producing nuclear radiation in Canada. Also, the AECB could arrange for the funding of major nuclear installations in the universities. Thus, Laurence had become a very influential person. Of himself Laurence said, “I feel I was of most help in setting the procedures and practices for safety control in nuclear power stations.” He spent much of his last year writing for the Deep River Science Association an excellent lecture entitled, “Safety of Nuclear Power in Canada.” (Printed in Diversa, Vol. III, No. 3 [1989], pp. 16-26.) Because of illness he had to sit and read it, in contrast with his customary habit of striding vigorously on the platform while the audience listened to a well-rehearsed extemporaneous delivery.
Laurence was appointed a Member of the Order of the British Empire for his work during the war. In 1966, the Canadian Association of Physicists awarded him its Medal for Achievement in Physics. In 1975, the Canadian Nuclear Association presented him with the W.B. Lewis Medal. In 1988, in San Diego, at the annual meeting of the American Nuclear Society, the president of the AECL Research Company accepted, on behalf of the Laurence family, a special plaque awarded posthumously to Dr. G. C. Laurence. It was later given to his wife by the President at a ceremony on top of the NRU reactor.
An appreciation by Fred Boyd in the Bulletin of the Canadian Nuclear Society says: “Although he claimed not to be religious, his manner and life reflected all that most consider noble and good, which, with his scientific excellence, combined to make him a very special person.”