COSMIC RAYS

CARL ANDERSON once remarked to me that, if we can find how to measure something that couldn't be measured before, or how to measure it much more accurately, we are almost sure to find something interesting. The story of the positron and of the ensuing stream of amazing discoveries that followed is an illustration of those rare, happy instances in which several essential factors came together under just the right circumstances to bear great fruit. By the late 1920s, cosmic rays had developed into a very active field of research that had uncovered many intriguing and rather puzzling facts which resisted satisfactory explanation in terms of the particles, radiations, and physical interactions then recognized. The most characteristic property of the rays near sea level was their great penetrating power. By analogy with X rays, whose penetrating power was known to increase as the voltage across the X-ray tube is increased, the sea-level cosmic rays would appear to correspond to X-ray tube voltages of hundreds of millions of volts. Yet, above a few thousand feet altitude, the intensity of cosmic radiation, as measured by the rate of production of ions in the air, increased rapidly with height, indicating the presence at high altitiudes of a highly absorbable (lower "voltage") component. Both of the above features showed a regular variation with latitude (specifically with geomagnetic latitude) that signaled the presence of charged particles among the primary rays outside the earth's atmosphere. The problem of untangling all the known effects and placing them into a coherent pattern, in terms of incoming primary rays interacting with the atmosphere to produce various secondary effects, was difficult because of the complexity of the phenomena and the relative coarseness of the observing tools of the time. These were mainly ionization chambers, which measured only the total ionization produced, irrespective of the nature or energies of the particles or radiations present. At about the same time, the right basic tool for the problem became ripe for exploitation: the cloud chamber within a strong magnetic field (the "magnet cloud chamber"). The cloud chamber itself is a well-known device that renders visible the tracks of charged particles moving through it (by condensation of a supersaturated vapor into droplets upon the ion trails left by the particles along their paths). It had long been a valuable tool in the study of the alpha, beta, and gamma rays of radioactivity and the nuclear disintegrations these rays sometimes induce in their passage through matter. The addition of a magnetic field by Skobeltsyn in 1929, in his study of gamma rays emitted by radioactive substances, provided the means for measuring the sign of charge and the momentum of charged particles. Anderson at Caltech, and others elsewhere, soon adopted this technique. (The product of the magnetic field strength B, and In this 1949 photograph Robert Leighton looks for tracks in a ''jalling cloud chamber, " designed to take full advantage of the magnetic field. While the particles passed through, the chamber remained enclosed by the magnet, and then dropped into view during the fraction of a second that it took the droplets to form tracks. The instrument was used to study the disintegration products of the muon.