Neil Tanner, the Nobel Prize, and heavy water

21 October 2015

Earlier this year, we celebrated Neil Tanner’s initiative in promoting access to Oxford by applicants from state schools.

But Neil was not only a remarkable tutor; he was also a very astute physicist with an impressive track record. Last week, Oxford Physics celebrated its contributions to the study of neutrinos at the Sudbury Neutrino Observatory, which were a crucial component in the experiment that led to the award of this year’s Nobel Prize in Physics to Arthur McDonald, representing the SNO collaboration.

One of the prime movers in the Oxford collaboration was Neil Tanner – the ‘Unforgettable Neil Tanner’ as the current chairman of physics described him. With his students and postdocs, Neil developed techniques to ensure that background levels in the huge vat of heavy water (D2O) that was the core of the experiment were minimised and that the purity was maintained, enabling the neutrino oscillation effect to be detected, and confirmed that neutrinos are not massless particles.

Robin Devenish has provided more details of the underlying physics here:

The 2015 Nobel prize in physics has been awarded to Takaaki Kajita, Super-Kamiokande Collaboration(University of Tokyo) and Arthur B. McDonald, Sudbury Neutrino Observatory Collaboration SNO  (Queen’s University, Canada) “for the discovery of neutrino oscillations, which shows that neutrinos have mass”

Neil Tanner became aware of the proposal for the SNO experiment through a Canadian colleague, David Sinclair, at the time a lecturer in the Oxford Particle Physics Department. After David returned to Canada to take up a post at Queen’s University, Neil took over as leader of the Oxford SNO group. He recruited a number of excellent students, including Robert Boardman (Hertford) and post-docs including Dave Wark (from Los Alamos, now a professor at Oxford). George Doucas – for many years a very effective lecturer in maths for physics at Hertford – was also a key member of the Oxford group.

Before these experiments were undertaken it was known that there were three types of neutrino – with very small masses or even massless (thus travelling at the speed of light). Electron neutrinos are produced copiously in the nuclear fusion reactions that provide the energy generated in a star. Using the measured total energy output from the Sun, it is possible to calculate the total flux of neutrinos expected from the ‘Standard Solar Model’. In the 1960s Ray Davis and John Bahcall measured this to be 1/3 of that expected – thus initiating the ‘Solar neutrino Problem’ – either our understanding of energy generation in stars was defective (unlikely) or something was happening to neutrinos between the Sun and the Earth.

If neutrinos had mass they would travel at a speed less than that of light. The crucial requirement for oscillations to be possible is that the different types of neutrinos have different masses. To test this idea one needs a detector sensitive to all types of neutrino. This is what the SNO experiment was designed to do.  The detector was filled with ‘heavy water’ (D2O, rather than H2O) with deuterium D (a chemical cousin of hydrogen H) composed of a proton and a neutron, in place of a single proton in hydrogen. With heavy water there are three possibilities:  (n + d  to e + p +p), in which the neutrino and its energy is absorbed, breaking up the deuteron and converting the neutron into an electron and a proton;  (n + d to n + p + n) in which the deuterium is broken up but the neutrino is unchanged; (n + e to n + e) so-called elastic n-e scattering. The key difference between these reactions is that the first one can only happen with an electron type neutrino whereas the second and third can occur with any type. Because the SNO experiment could measure all three processes, it was able to show that neutrinos were not being ‘lost’, but changing their type (‘oscillating’) on the journey between the Sun and the Earth.

The ‘Standard Model’ of particle physics has been incredibly successful, having accommodated all experimental challenges for more than twenty years. However, as it assumes that neutrinos are massless, it cannot be the complete theory of the fundamental constituents of matter. (Robin Devenish)