"X-Particle"

Some scientific discoveries are made because they were theoretically predicted and diligently looked for. Such was the discovery of the planet Pluto whose existence and probable orbit were indicated by irregularities in the orbits of other planets. So, too, deuterium (heavy hydrogen) was identified because its discoverer already had intimations of its existence, and the positive electron was foreshadowed in the cogitations of at least one mathematician before its track turned up in the laboratory. In fact, some things are made use of even before they are discovered -- e.g., the little uncharged particle called the neutrino which atomic physicists need in their calculations but which has never yet come to light experimentally. Quite different are unexpected and sometimes unwelcome discoveries* which do not fit into any preconceived picture and further complicate the problems with which scientists struggle. Such a thing is a new particle of matter, which as yet has no name except "X-particle."

Physicists probing within the atom toward the ultimate constitution of matter already had their hands full with positive and negative electrons, neutrons (uncharged particles), protons (nuclei of hydrogen atoms), deuterons (nuclei of doubleweight hydrogen), tritons (nuclei of tripleweight hydrogen), alpha particles (nuclei of helium), and the theoretical but necessary neutrino (little neutron).

But now Carl David Anderson of California Institute of Technology and Jabez Curry Street of Harvard are positively convinced that the X-particle exists, and last week Dr. Street raised the unwelcome particle to a status where it cannot possibly be laughed off: he announced that he had approximately measured its mass.

Boyish, modest Dr. Anderson discovered the positive electron in 1932 and is now a Nobel Prizeman. In 1934, in an inconspicuous footnote, he hinted at the possible existence of a new particle. The dilemma which confronted him then was a choice between theory and observation. He was studying electrons which occur in cosmic rays. Such electrons are supposed to behave according to the Bethe-Heitler theory, which ascribes certain penetrating powers to electrons of certain velocities. Dr. Anderson's data showed that electrons did conform up to energies of 300,000,000 volts. Above that energy level there appeared to be an unorthodox particle of great penetrating power. If this was an electron, the Bethe-Heitler theory broke down at high energies; if the theory was valid the particle was something heavier than an electron.

By August of last year Anderson was more inclined to believe that a new particle really existed than that the Bethe-Heitler theory was at rault. Further help for the theory came from the researches of Caltech's H. Victor Neher at San Antonio, Tex., and Madras, India. Cosmic ray particles are pulled toward the earth's poles by its magnetic field. Particles of high energy resist this pull, and so predominate in the region of the Equator. The latitude difference between Madras (13DEG N.) and San Antonio (29DEG N.) furnished valuable data on electrons in the energy band between seven billion and 17 billion volts. These high energy electrons were found to conform to the Bethe-Heitler theory, rapidly losing their power to ionize (electrify) air as they pass through the atmosphere. Since the X-particle's extraordinary penetrating power was entirely out of line with these findings, it could not possibly be an electron.

Last May Dr. Anderson came out flatly for the new particle's existence. At the same time, Dr. Street of Harvard, who had been conducting independent researches of his own, also plumped for it. Fortheoretical reasons both Drs. Anderson and Street believe that the X-particle is not a part of the primary cosmic radiation, but arises from cosmic ray collisions in the upper air. An important question remained: What is the X-particle's mass? It appeared to be heavier than an electron but lighter than a proton. But this is a wide range, about as wide as between a pound and a ton.

In the current Physical Review Dr. Street undertakes to answer the mass question--approximately, not exactly. He took 1,000 photographs of cosmic ray activity in a "cloud chamber," an apparatus in which water vapor condenses in the path of ionizing particles as droplets of fog which can be photographed. Dr. Street rigged his apparatus so that the condensing value would operate not instantly when an ionizing particle passed through, but one second later. This allowed the fog tracks to spread a little, enabling him to get a better count of the droplets.

Out of his 1,000 pictures Dr. Street obtained two tracks which seemed significant. One was ruled out, however, as a proton. The other was obviously not a proton, yet its track was about six times as heavy as could be expected from an electron. It was clearly ticketed as an X-par-ticle. Counting the fog droplets as carefully as he could and taking into consideration the track's curvature as bent by a magnetic field, Dr. Street figured its mass at 130 times the mass of the electron--with a probable error of 25% either way.

*Among startling unlocked for discoveries were X-rays, natural radioactivity, artificial radioactivity. X-rays caused such a furor among laymen after their discovery by Roentgen that the New Jersey Legislature introduced a law forbidding their use in opera glasses, for fear that prurient individuals would be able to see through the garments of ladies.

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