From first beams to first experiments
The alternating-gradient electron synchrotron, Nina, at the Daresbury Nuclear Physics Laboratory, UK, produced its first high-energy beams on 2 December 1966. The following day, the design energy of 4 GeV was reached and 4.5 GeV was achieved on 5 December. One of Nina's most important design features is that it should be able to accelerate currents in excess of 1 μA and possibly up to 10 μA.
The decision on the site [now the home of Fermilab] for the proposed USA 200 GeV accelerator was announced on 16 December. From more than 200 sites, 85 were investigated by the National Academy of Sciences. From a short list of six reached in April 1966, the US Atomic Energy Commission selected Weston, a suburb of Chicago, not far from the Argonne National Laboratory.
On 30 December 1966, the world's first proton synchrotron to contribute to particle physics – the Cosmotron at Brookhaven National Laboratory, USA – was closed down. The machine began operation in 1952 and was capable of a maximum energy of 3 GeV. With the advent of the 33 GeV machine at Brookhaven in 1960, high-energy physics experiments went more and more to the bigger machine.
Experiments began at the end of November 1966, at the Stanford Linear Accelerator Centre, USA, using the 20 GeV linear accelerator, six months ahead of schedule. Electron beams with an energy of 10 GeV were achieved for the first time in May 1966 and since then the energy has been increased to give beams in the energy range 10 to 18 GeV.
Gravity matters
The preliminary results of a highly refined experiment carried out by Stanford University physicists F C Witteborn and W M Fairbank were announced at the end of December. They have succeeded in measuring the effect of gravity on electrons, the lightest of all of the particles.
The difficulty of the experiment does not lie in the lightness of the particle but in the fact that the gravitational force on an electron due to the field of the whole Earth is equalled by the electromagnetic force due to another electron at a distance of about 5 m. Thus, it is necessary to shield the electron as far as possible from all electromagnetic fields, including those from the material in the experiment itself.
Electrons emerged from a cathode at the bottom of a 5 cm-high, vertical copper tube, which shielded the particles from external fields. A large superconducting magnet formed a "magnetic bottle" in the centre of the tube. A few electrons were moving slowly enough for the time they took to spiral up to the top of the tube to be measured with an accuracy of about 2%. The gravitational force could then be determined from these flight times and the electric field required to prevent the electrons from accelerating.
The experimenters now hope to look at the effect of gravity on positrons (anti-electrons). If this proves feasible, it will test the idea, beloved of science fiction writers, that antimatter may fall up, instead of down, in a gravitational field.
• From articles on pp12–13.
Compiler's Note
The tendency of antimatter to annihilate with matter makes it immensely difficult to study. The gravitational effects of matter on antimatter have still not been conclusively measured, not to mention those of antimatter on antimatter. All effects may be attractive with the same strength as that of matter on matter, but some hypotheses, such as attempts to explain the accelerating expansion of the universe, postulate otherwise.
Meanwhile, science fiction rules OK!
Seven new mesons at the PS
The "missing-mass spectrometer" experiment was concluded at the CERN proton synchrotron in January. About 25 scientists and engineers from 10 European countries, the USA and the USSR participated at various stages in a team led by B Maglic.
Over a period of about a year, the experiment identified seven new heavy mesons.
The basic interaction investigated was π– + p → p + X–. Negative pions from the synchrotron, directed onto a hydrogen target, could produce recoil protons and negative mesons of different masses (X–). Measurements of the momentum and direction of an incoming pion and the recoil proton indicated the meson mass as "missing mass/energy". The novelty of the spectrometer lay in measuring the momentum of recoil protons without the conventional use of a large magnet. The proton detectors were mounted on a turntable that could be moved through angles known with respect to the direction of the incoming pion beam. For each meson that can be produced, there is an angle at which a high percentage of the recoil protons will emerge, the so-called Jacobian peaks.
The squares of the measured meson masses lie neatly on a straight line and it has been pointed out (for example, by Prof. Dalitz at the Berkeley Conference last year) that this fits well with the model of an underlying quark–antiquark system.
• From article on pp31–32.