Obituary: Professor P. B. Moon
Wednesday 19 October 1994
P. B. MOON made outstanding and original experimental contributions which stimulated the development of whole fields of research involving neutrons, gamma rays and novel methods of studying chemical reactions.
He was a master, in the Rutherford tradition, of adapting the simplest and most economic version of a technique to solve important physical problems.
Thus he managed, with very modest means, to initiate developments which were elaborated successfully elsewhere. He had a quick and ingenious mind, was modest and shy but highly respected by his friends and colleagues. He had a passionate interest not only in physics but also in people. His knowledge and memory of old students of the Department of Physics at Birmingham University is legendary.
Philip Burton Moon was born in 1907, attended Leyton County High School, and in 1925 entered Sidney Sussex College, Cambridge. He became a research student in the Cavendish Laboratory under Sir Ernest (later Lord) Rutherford. From 1928 to 1931 he worked with MLE (now Sir Mark) Oliphant on the collision of atoms with solids. Rutherford called him 'Oliphant's satellite' and that influence remained with him for decades.
Shortly after being appointed to a lectureship in physics at Imperial College, London, he was excited by the discovery of the neutron by James Chadwick in Cambridge and speculated about its omnipresence, on Earth and in the form of stars. The first of these he could put observational limits on, which he published. The second was too speculative and he, in his last years, regretted not having commented on it in print. He did, however, persuade GP Thomson to let him study thermal neutrons, after Enrico Fermi and associates in Rome had seen strong neutron activation in a number of elements. Working with a research student, JR Tillman, he conclusively showed in 1935 that fast neutrons were slowed down to thermal energies in hydrogeneous materials, such as wax, and showed that such neutrons were selectively absorbed by particular nuclei. Rowing on the Serpentine in Hyde Park played a role in this research. Hans Bethe, on being told of the results by Moon, replied, 'If true . . . the whole of quantum mechanics falls to the ground.' In due course theoretical physics adapted to these notions, Niels Bohr interpreted such nuclear reactions in terms of a compound nucleus and Bethe later wrote a much-cited review article in which he surmised that an analogous phenomenon, the nuclear scattering of gamma rays, would probably be unobservable. He did not reckon with Moon.
In 1937 Oliphant was appointed to the Poynting Chair of Physics at Birmingham University and Moon followed him there as a lecturer a year later. Thomson 'borrowed' Moon in the summer of 1939 for a quick test of the possibility of a nuclear chain reaction in one ton of (natural) uranium oxide mixed with water and/or paraffin wax. The result was negative - fortunately for the experimenters. During the war Moon worked in the 'Tube Alloys' project on microwaves, but later moved to the Los Alamos Laboratory in the United States as a member of the British scientific delegation associated with the Manhattan Project working on the atomic bomb.
In spare moments there he conceived the idea of a small project to apply the ideas of nuclear reactions to chemical reactions and at the same time separate himself from Oliphant's grand projects on return to Birmingham after the war. No doubt his love of cricket played a role in conceiving the idea of batting atoms or molecules with a fast rotor in order to accelerate them to controlled high speeds. He started this work in 1945 but was only able to devote himself to it for five years before a heavy burden descended on him.
In 1950 Oliphant left for Australia: Moon was appointed Head of Department and Poynting Professor. He dutifully and successfully carried out the unfinished task of supervising the completion of the 1000 MeV proton synchrotron accelerator, the first such machine to be started (in 1946) but, once again on a shoestring budget, and not surprisingly it was not the first one to be finished. The Cosmotron in Brookhaven, Mississippi, was started a year later, but, with ample money and manpower, was completed a year earlier.
The Birmingham synchrotron started operating in the summer of 1953, and Moon was able to leave the experimental studies to others. Moon and a research student, T. Bull, continued with the chemical work only until 1954. Moon resumed it in 1969, together with colleagues, when light and strong carbon-fibre reinforced materials allowed higher speeds and therefore energies of molecules to be reached and Dr (now Professor) John Simons in the Chemistry Department expressed great interest. The carbon-fibre rotor held, according to The Guinness Book of Records, the record speed of 2.2km per second until recently; but more importantly it kept Moon busy devising better methods of accelerating molecules to the last year of his life.
Developments in the United States of similar chemical studies by Dudley Herschbach and Yuan Lee were rewarded with the Nobel Prize in 1986.
In 1948 Moon came across a publication of a failed experimental attempt by a colleague from his Los Alamos days to observe resonances of nuclei excited by gamma rays. He realised immediately that his rapidly moving rotors were the ideal tool to compensate the recoil energy lost in these processes. The experiments were successful, but the absence of effects of the binding of nuclei in liquids or solids puzzled Moon; his repeated queries to eminent theoretical physicists were not answered imaginatively. Eventually this work led directly to the discovery elsewhere by Rudolf Mossbauer of nuclear resonances without recoil which allowed a variety of experiments in relativity, nuclear, condensed matter and chemical physics. Mossbauer was awarded a Nobel prize for this discovery in 1961.
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