1954 to 49,383 curies in 1955. There were also considerable increases in shipments of cobalt 60 and cesium 137 for large irradiation units. A new 177-page loose-leaf radioisotope catalog was compiled, printed and distributed.

Twenty-nine new short half-life products, ranging from antimony 122 to yttrium 90, were offered for sale as a result of increased manipulator cell facilities and faster processing techniques. Most of these products were irradiated in the Low Intensity Testing Reactor (LITR) at Oak Ridge, Tenn., and users were supplied with higher specific activity material, assayed, and in solution form.

A remote manipulator cell for handling multikilocurie amounts of radioactivity was completed and placed in operation at Oak Ridge during this reporting period. Radiation sources containing more than 15,000 curies of cobalt 60 have been simultaneously handled in this cell. The addition of this facility has made it possible to assemble and fabricate large radiation sources without underwater operations. The largest single source assembled to date, 10,000 curies of cobalt 60, was prepared for General Electric Co. to use in studies of radiation damage.

PHYSICS

In physics research, during the first half of 1956 techniques and equipment were improved for working with high-energy accelerators in order to advance knowledge about fundamental nuclear particles; a predicted but elusive particle, the neutrino, was discovered; the properties of the newly identified antiproton were investigated; and lowenergy reasonances of neutrons released by plutonium fission were measured. Very pure isotopes of heavy elements were prepared for research, and fundamental studies were carried out in a number of promising fields.

High Energy Physics Research

Discovery of the free neutrino. Experimental evidence for the existence of a nuclear particle of vanishingly small mass and without electrical charge, the neutrino, was collected in this reporting period by a team of research investigators from Los Alamos Scientific Laboratory, operated for the Commission by the University of California. This is the second theoretically predicted nuclear particle the existence of which has been detected at Commission laboratories within recent months. The first, the antiproton, is reported on in the following section.

The Nobel laureates, Enrico Fermi and Wolfgang Pauli, predicted the existence of the free neutrino to account for (among other things)

the release of energy, otherwise unaccounted for, in a radioactive process known as beta decay. The neutrino was postulated to carry away part of the energy released in this process.

The neutrino interacts very weakly with material that is, it is extremely penetrating-and would pass through billions of miles of solid matter. Its detection consequently posed an extremely difficult scientific problem.

Frederick Reines and Clyde Cowan, Jr., who headed the research team, believed they first observed the neutrino in 1953 when, with the help of other scientists, they set up equipment near a production reactor at Hanford, Wash.22 They installed a rather novel liquid scintillation system as a sensitive detector. Although evidence obtained at that time indicated the neutrino's existence, the experiment was not entirely conclusive. Cosmic rays and other background radiation made it difficult to make sure of the signals of the neutrino.

A new and more complex detecting system was built and last fall was set up deep underground near one of the large production reactors of the Commission's Savannah River plant, operated by the E. I. duPont de Nemours & Co., many of whose personnel cooperated in the experiment. Several months of work enabled the scientists to conclude that they had checked each important characteristic of the neutrinos caught in their equipment.

The detector included a target containing more than 100 gallons of water in which cadmium salts had been dissolved. Cadmium is a strong absorber of neutrons. The target was "watched" by a scintillation system containing over 1,000 gallons of nuclear radiation. sensitive liquid, and 330 large photomultiplier tubes. Despite the huge size of the detector, and the billions of neutrinos produced within the reactor which passed through the detector each second, only a few neutrino captures were observed in the target each hour.

The discovery marks the first time that scientists have, knowingly, caused a direct reversal of the process of beta decay. In the process of beta decay, the nucleus of an atom emits a negative electron. In effect, a neutron in the atomic nucleus apparently loses this negative charge and becomes a proton. In the present experiment, stable protons in the target water, were made to absorb neutrinos, emit positive electrons, and become neutrons. The particle thus detected. by simultaneous detection and identification of the positron and neutron shows the expected properties of the neutrino as predicted by the theory of Fermi and Pauli.

The importance of the discovery is that it confirms theories which scientists believe will ultimately evolve into an understanding of the nature of the forces which hold together the atomic nucleus.

See p. 31, Fifteenth Semiannual Report (July-December 1953).

Antiproton research. A major research program carried out at the University of California Radiation Laboratory, Berkeley, with the bevatron during this report period was the investigation of the properties of the antiproton. This research followed the discovery of the antiproton in October 1955 by electronic techniques.23 It was expected that if an antiproton came into contact with a proton (or neutron) a mutual annihilation would result which would transform the mass of both particles into energy. The technique used to demonstrate the annihilation properties of the antiproton consisted of placing a stack of photographic plates coated with nuclear emulsion in the path of antiprotons produced by the bevatron. Any charged nuclear particle passing through nuclear photoplates produces an image which, after development, consists of a track-like series of small silver grains. This technique permitted observation of the fate of an antiproton going through the photoplates.

Shortly after the discovery of the antiproton, the same bevatron beam which had been shown to contain antiprotons was used to irradiate a stack of photoemulsion plates. Half the exposed plates was sent to Italy to be examined by a group of scientists at the University of Rome, the remaining half was studied at Berkeley. While no evidence of interactions was found in the California study, the Italian group reported discovering one "star" in the emulsion, the first antiproton interaction seen by man.

The depicted star was caused by an antiproton entering a nucleus of either a silver or bromine atom in the photoemulsion. The resulting explosion liberated seven heavy particles such as protons or alpha particles, and two mesons, probably pions (see photograph). The antiproton track, "L" in the photograph, extended 4 inches into the emulsion and the fact that it produced a large star indicated that it was a negative particle. The energy released may be calculated from an analysis of the star. Tracks (a) and (b) in the photograph are interpreted as those of pi mesons, and the other tracks as those of protons or alpha particles. Only charged particles are visible since neutrons escape direct detection.

Since this star was found, 30 antiprotons have been detected in nuclear photoplates in a large cooperative effort by physicists in Berkeley. In all cases antiprotons have been seen to annihilate with either a proton or a neutron since the energy released in the process has been shown to be greater than that corresponding to the mass of a single proton. Thus the existence of the antiproton predicted in 1930 by P. A. M. Dirac, an English Nobel laureate, has been conclusively proved.

23 See pp. 59-60, Nineteenth Semiannual Report to Congress (July-December 1955).

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Some interesting facts have been learned about the annihilation phenomenon. In the annihilation most of the energy is carried away in pi mesons, about 5 pi mesons on the average being emitted. It was also found that antiprotons have a much larger probability than expected of interacting with protons (or neutrons) when passing through matter. It is now thought that this must mean there are strong mutual forces between the antiproton and proton (or neutron), which cause annihilation to occur at larger relative distances apart than was first predicted.

Heavy-mesons and hyperons. The use of the cosmotron at Brookhaven National Laboratory, Upton, Long Island, N. Y. and the bevatron at University of California Radiation Laboratory, Berkeley, Calif., has made available for laboratory study new and important data on the interaction and the interrelation of fundamental nuclear particles. Many particles previously found only in cosmic radiation now can be produced in sufficient numbers in laboratories for quantitative investigation. The heavy mesons and hyperons 24 are as yet little understood. The heavy meson has less mass than a neutron or proton; the hyperon has a mass equal to that of a proton or neutron plus that of a meson. In seeking to understand the relationship of the mesons and hyperons to each other, and to the better known nuclear particles such as the proton and neutron, investigators have studied the simplest interactions, since these should be the easiest to understand. The production of new particles in the elementary collision of an accelerated proton with another free proton, or studies of the interaction of one of the newly produced particles with other protons represent such simple interactions.

The mechanical and electrical restrictions of such accelerators as Brookhaven's cosmotron make it difficult to use free protons as a target inside the machine since hydrogen, needed in the liquid state to achieve necessary target density, requires a temperature of 252.8 degrees below zero centigrade to become liquid. Brookhaven has successfully developed an efficient method for extracting from the cosmotron an intense proton beam and directing it on a liquid hydrogen target placed outside the accelerator. This makes it possible to study the important elementary acts of production of heavy mesons and hyperons in hydrogen. Extracting the beam also permits greater freedom in locating apparatus for the detection of the particles. Since the new particles are highly unstable, having lifetimes between onemillionth and one-millionth of one millionth of a second, freedom of placement is an important advantage in experimentation.

The heavy-meson beam facilities at University of California's bevatron have been greatly improved during the past year. As a re

See p. 35, Sixteenth Semiannual Report (January-June 1954).