Physics monitor decay (positron emission) in massive gamma rays were first observed from stars having violent surface activity. the direction of the centre of our More galactic However because these objects are Galaxy in the early 1970s, in the antimatter relatively common in the Galaxy, the vicinity of a "Great Annihilator". radioactive materials, and so the ew maps of gamma rays from resulting positrons, would be NNASA's Compton Gamma Ray distributed throughout the Galaxy, Observatory show evidence of a including the Earth, which is not the previously unknown and unexpected case. A century and its half cloud of antimatter, in the form of Another way positrons might be positrons, extending 3,000 light years created is when matter falls into a hile 1997 is being widely above the centre of our Galaxy. black hole. As matter is sucked into Wcelebrated as the hundredth The classic aroma of positrons is the gravitational pit, its temperature anniversary of the discovery of the the 511 keV gamma radiation increases until it becomes hot electron by J.J. Thomson at produced when positrons and enough to create pairs of positrons Cambridge (see page 6), the 50th electrons annihilate. Such radiation and electrons. This flow may be anniversary of the 1947 discoveries was first observed from the direction intermittent, changing abruptly as the of the pion by Cecil Powell at Bristol of the centre of our Galaxy in the black hole sucks in large fragments and 'V-particles' by George early 1970s, and the new maps were from nearby stars, while the number Rochester and Clifford Butler of expected to show a large cloud of of positrons created by radioactive Patrick Blackett's group in antimatter near the galactic centre decay would be steady over long Manchester draw less attention. and along the plane of the Galaxy, periods of time. J.J. Thomson went on to win the caused by the explosions of young A third possibility is that within the 1906 Nobel Physics Prize. His massive stars. The maps show that last million years this region was the Cambridge contemporaries included gamma ray activity, but also show a site of a massive galactic fireball Owen Richardson (1928 Physics mysterious second cloud of caused by the merger of two neutron Nobel), F.W. Aston (1922 Chemistry antimatter well off the galactic plane. stars. Such events are widely Nobel), C.T.R. Wilson (1927 Physics On NASA's Compton Gamma Ray believed to be responsible for the Nobel), G.P. Thomson (his son, 1937 Observatory, launched in April 1991, enigmatic gamma-ray bursts which Physics Nobel) and Ernest one instrument, the Oriented have baffled astronomers for over Rutherford (1908 Chemistry Nobel). Scintillation Spectrometer twenty years and have recently been After this initial proliferation of talent, Experiment (OSSE), is sensitive to studied by the Compton Gamma Ray it was under Ernest Rutherford, who 511 keV gamma rays. Observatory. took over from Thomson at the The centre of our disc-like Galaxy, Because the Universe appears to Cavendish in 1919, that the about 25,000 light-years away in the contain more matter than antimatter, Laboratory went on to attain the direction of Sagittarius, is normally however, once the positrons are apogee of its fame. obscured by intervening interstellar created it is only a matter of time After Rutherford's death in 1937 the dust and gas. However, this material before they are annihilated. 511 keV Cavendish came under the direction is transparent to gamma rays. of W.L. Bragg, who firmly set the Positrons, and antimatter in laboratory on a new course. This general, are thought to be relatively veered away from nuclear physics rare in the Universe. Positrons could but eventually led to epic discoveries arise through natural radioactive in molecular biology. As products of the Cavendish Laboratory, it was Powell at Bristol An antimatter 'fountain' discovered by NASA's and Blackett at Manchester who Compton Gamma Ray Observatory spews out inherited the valuable Thomson- positrons from the centre of our Galaxy. The Rutherford legacy, and thus the 1947 contours show the fountain superimposed on discoveries of the pion at Bristol and the usual distribution of positrons around the Galactic centre. the V-particles at Manchester can CERN Courier, June 1997 1 Physics monitor Particle century - fifty years ago, in 1947, half a century after the discovery of the electron, Don Perkins, working at London's Imperial College, was the first to observe a clear example of what appeared to be the nuclear capture of a meson, producing a nuclear disintegration. On 17 April, Perkins gave a presentation at CERN on one hundred years of elementary particles. One half of this particle century is spanned by his own contributions. More than anything else, the 1947 discovered electron-positron pair discoveries made physicists realize production, a key prediction of that the subnuclear world was more Dirac's ideas. complex than had been suspected by Cloud chambers played a major looking at everyday nuclei. The role in cosmic ray studies in the discovery of the pion and its following years, leading to the discov­ subsequent decay highlighted the ery of the 'mesotron' in 1937, origi­ role of the muon (discovered by nally identified as the nuclear force Anderson and Neddermeyer in carrier postulated by Hideki Yukawa 1936), while the Vs were the first in 1935. However, several difficulties examples of 'strange' particles soon arose with this hypothesis, even containing a third type of quark. though pictures of its decay to an The following article by Owen Lock, electron, as postulated by Yukawa to formerly of Bristol, Manchester, explain beta-decay, were observed in Birmingham and CERN, recalls the cloud chamber pictures in 1940. In pion discovery. Another article later particular, the mesotron appeared to this year will cover the discovery of have a very weak nuclear interaction V-particles. with matter, conclusively demonstrated by the counter experiments of Marcello Conversi, Ettore Pancini and Oreste Piccioni in Rome from 1943-1947. A possible explanation of these Half a century ago - difficulties had been put forward in trace their parentage back to 1897. Japan in 1942 and 1943 by Yasutaka Blackett was awarded the Nobel the pion pioneers Tanikawa and by Shoichi Sakata and Physics Prize in 1948 and Powell in Takeshi Inoue, who suggested a two- 1950. These discoveries were to be hile the classic discoveries of meson hypothesis with a Yukawa- the last major particle physics WThomson and Rutherford type meson decaying to a weakly revelations of a war-torn Europe. The opened successive doors to interacting mesotron. Because of the next European milestone - the subatomic and nuclear physics, war their ideas were not published in discovery of the neutral current in particle physics may be said to have English until 1946 and 1947, the 1973 - had to await the establishment started with the discovery of the journals in question not reaching the of CERN. positron in cosmic rays by Carl USA until the end of 1947. Sighting the pion and unravelling its Anderson at Pasadena in 1932, Unaware of the Japanese work, decay liberated physics from more verifying Paul Dirac's almost Robert Marshak had put forward a than a decade of dilemma, and the simultaneous prediction of its similar hypothesis in June 1947, at a pion looked full of promise. Perhaps existence. conference of American theoreticians this new particle held the key to the Anderson used a cloud chamber, on Shelter Island (off Long Island), mysterious forces which held the expanded at random, in a high and which he published later that nucleus together. However this hope, magnetic field. At the same time, year with Hans Bethe. None of the cherished since the time of Yukawa, Patrick Blackett at Cambridge was scientists at the conference knew was not to be fulfilled, and the joined by an inventive young Italian, that such two-meson decay events significance of the pion as a particle Giuseppe Occhialini, sent by a had already been observed some has diminished as our understanding master of counter coincidence weeks earlier by Cecil Powell and his of nuclear forces in terms of a deeper techniques, Bruno Rossi, then in collaborators in Bristol, using the layer, quarks, has advanced. If the Florence, to learn about cloud then little known photographic pion does play a special role, it is chambers. Very soon Blackett and emulsion technique, but which in because it is the lightest strongly Occhialini had built a counter- Powell's hands became a powerful interacting particle. controlled chamber with which they research tool. 2 CERN Courier, June 1997 .