It is now more than 90 years since the neutron was discovered. Sir James Chadwick, who was later to play an important role in the Canadian nuclear program, is regarded as the discoverer of the particle and was awarded the 1935 Nobel prize in physics for his work.
Les Cook, Director of the Chemistry and Metallurgy Division at Chalk River from 1945-1956, writes in his book Birthpangs of CANDU, “If anyone can be regarded the father of Chalk River, of NRX and CANDU, it is Sir James Chadwick. His share in their creation was never widely known, and has long been forgotten with the demise of those who knew.”
In the late 1940s, Chalk River with its world-leading neutron flux from NRX was the ideal laboratory to study the neutron. The man ready and eager to study the properties of the neutron was John Robson. John, a graduate of Cambridge, came to Chalk River just after the end of WW II. He set out to measure the half-life of the “free” neutron. (A free neutron being one not bound by nuclear forces in a nucleus.)
The measurement of the half-life proved to be a very elusive and John’s initial result gave a half-life of between 9 and 25 minutes. A competing group at Oak Ridge set limits of between 10 and 30 minutes. Robson’s final results were for a half-life of 12.8 +/- 2 minutes. This value stood as the accepted value for a long time. However, the subject does not end there.
Measurements being conducted at Oak Ridge in the 2020s now point to a half-life close to 890 s (15 minutes). However, two different techniques, one with and error of +/- 0.8 s and the other +/- 2.2 s differ by an unexplainable 9.2 s. Furthermore, a group at John Hopkins and one from Durham University in the U.K. have come up with a value 780 +/-130 s based on NASA data of neutrons originating on Venus or Mercury. Theories now abound that the neutron may be decaying to dark matter via some yet unknown and possibly variable decay path. Dark matter research is the current hot topic in physics including at SNOLab in Sudbury. We must stand by for further developments.
John Robson’s research at Chalk River did not end with the half-life measurements. In the 1970s, while he was Chair of the Physics Department at McGill, he returned to Chalk River to develop a highly successful ultra cold neutron source at NRU. A man of many talents.
While we must admit to having few free neutrons to display at the Canadian Nuclear Heritage Museum, we do have a huge number of bound neutrons in a large number of fascinating artifacts. Watch the billboards near the Museum for opening times and Open Houses or send an email to infor@nuclearheritage.com to arrange a tour.