2. High Energy Physics By Ernest O. Lawrence Director of Radiation Laboratory, University of California (where electromagnetic process was developed for Manhattan District) The total energy of a heavy atom like lead or uranium is about two hundred billion electron volts, and therefore studies of energy transformations over a vast range are of importance for a deeper understanding of atomic phenomena. In an ordinary nuclear reaction involving only a few nuclear particles, such as the transmutation of one element into its neighbor, the energy changes involved are only a few million electron volts. In cosmic-ray investigations, fragmentary evidence of transformations of very high energies have been obtained, but the discovery of atomic fission marked the real beginning of high energy physics in the laboratory. The whole range of nuclear energy is marked by a few mileposts. There is the mass-defect energy of the particles in the nucleus-about ten million electron volts; there is the energy of the mesotron-perhaps about 100 million electron volts; there is the energy of the particles themselves (neutrons and protons)-one billion electron volts; and finally there is the total energy of a heavy nucleus-some 200 billion electron volts. Although much remains to be done in the region of mass-defect energies, it is of great interest to proceed to experimental investigations in the laboratory in the higher ranges, for there are indications already that the mesotron plays an important role in nuclear structure and that the protons and the neutrons themselves are by no means elementary particles. Just as the recent extensive investigations of nuclear phenomena depended on the development of instruments for accelerating particles to several million electron volts, so now the prerequisite to the attack of the energy range of mesotrons and protons is the development of great accelerators for the energy range above 100 million electron volts. Indeed, if mesotrons are more frequently produced in pairs when nuclei are bombarded, the bombarding particles would have to be above 200 million electron volts and, in order to have a certain margin in excess of such a limiting value, one would like about 300 million electron volts for experimental purposes in the laboratory. Of course, such particles are available in cosmic rays, but they are either not present in sufficient abundance or not controllable with sufficient convenience for the kind of broad experimental attack which would lead to rapid progress in our understanding of this domain of nature. Accordingly, at the present time, in many laboratories over the world the development of great electron and ion accelerators is under way. It is clearly impractical to reach the region of several hundred million volts by producing such high voltages directly; rather, the full development of methods of multiple acceleration is required. The Betatron For the acceleration of electrons to perhaps a quarter billion volts the betatron is in many respects an ideal device. The 100-million-volt machine of the General Electric Company has proved to be an extremely convenient and effective research tool, and the great installation now being built at the University of Illinois will likewise undoubtedly be very effective in an energy range several times higher. The Synchrotron The synchrotron, which was conceived independently here, in England, and in Russia, offers an attractive approach to even higher electron energies. Not only would it presumably cost less than the betatron in the region of a few hundred million volts, but also it does not suffer from a practical limitation on producible electron energies introduced by radiation losses as the electrons are accelerated. A 300million-volt synchrotron installation is now under construction at the University of California, and machines are also planned for several other laboratories in this country, notably the Massachusetts Institute of Technology, Cornell University, the General Electric Company, and the University of Michigan. The University of Michigan equipment has certain novel features which may be advantageous in the design of installations to reach the domain of billions of volts. The F.M. Cyclotron The conventional cyclotron which has been widely used for the acceleration of ions, especially protons, deuterons and alpha particles, to tens of millions of electron volts suffers from a very practical limitation in the direction of higher energies. For as the particles gain energy their masses and periods of rotation in the magnetic field correspondingly increase, tending to destroy the synchronism of the rotation with the alternating electric field. Despite this practical limitation, before the war it seemed entirely feasible to build a great cyclotron (184" pole face) that would produce 50-million-volt protons, 100-million-volt deuterons and 200-million-volt alpha particles-possibly just high enough to reach the mesotrons in the nucleus. This could be done by the simple expedient of applying such high accelerating voltages (about two million volts) that the ions would receive their accelerations in a relatively few turns before getting out of phase with the accelerating voltage. But to put much more than two million volts on the dees and therefore to obtain higher energies in this way was clearly impractical and some other approach to this problem was indicated. Of course, it was recognized that, by suitably modulating the frequency of the accelerating voltage, the rotating ions could in principle be kept in step with the oscillations as they were accelerated to high energies, but before the war this approach to the problem did not seem very practicable. However, recently McMillan in this country and Veksler in Russia pointed out the phase stability of the ions accelerated in a frequency modulated cyclotron and, as the extent of the phase stability became clear, it became likewise evident that a frequency modulated cyclotron offered a promising practical approach to the problem of accelerating protons to hundreds of millions of electron volts. With the 184" cyclotron in mind, experiments were accordingly undertaken at the University of California to test the theory of the F.M. cyclotron in all its aspects, both practical and theoretical. Gratifying success was immediately achieved, as the introduction of frequency modulation on the 37′′ cyclotron gave results immediately in accord with the theoretical expectations. To be specific, the 37'' cyclotron adapted for frequency modulation produced with ease and reliability several microamperes of 16-million-electron-volt protons, the highest energy protons thus far produced in the laboratory. Accordingly, the 184" cyclotron originally designed along conventional lines has been redesigned as a frequency modulation cyclotron and, in view of the success with the 37" installation, there are excellent prospects that the 184" equipment will be producing in the fall of this year deuterons of energies near 200 million electron volts and alpha particles of twice this energy. Later on, it is planned to modify the apparatus to accelerate protons to perhaps 350 million electron volts. Thus we may look forward soon to reaching well into the domain of the mesotron. The Linear Accelerator The method of linear multiple acceleration of particles is also under extensive development. At a number of laboratories, notably Stanford University, Purdue University, the California Institute of Technology, the General Electric Company, and the Massachusetts Institute of Technology, electron accelerators using microwaves are being developed, while at the University of California the effort is in the direction of the acceleration of protons by means of 200-megacycle waves. Although it is too early to be sure, there are good prospects that for protons of a few hundred million electron volts the linear accelerator will be competitive with the cyclotron and for the region of one billion electron volts the linear accelerator may be less costly. At any rate, since presumably the cost of a linear accelerator should rise rather linearly with output voltage while the cost of a F.M. cyclotron would increase with a higher power of the voltage, at some point the linear device should be more economical. Of course, both devices have their advantages and disadvantages, quite apart from economic considerations, and therefore both should be developed to their maximum usefulness in the laboratory. The Future Now that the prospects are so good that we shall soon be investigating nuclear phenomena with accelerated particles of energies some ten times greater than heretofore available in the laboratory, it is tempting to speculate on the new knowledge that lies ahead. But, of course, such speculation in any detail is no more than a guessing game and hardly a profitable endeavor. However, that interesting and important discoveries will come there can be no doubt-and so we may look forward to the years ahead in keen anticipation. 3. Practical Applications of Radiation Chemistry By James Franck Former Director of Chemistry Division, Metallurgical Lab oratory, University of Chicago Professor of Chemistry, University of Chicago Milton Burton Former Section Chief, Chemistry Division, Metallurgical Professor of Chemistry, University of Notre Dame Atomic energy piles and the radioactive products from such piles produce high-velocity particles (alpha and beta particles, fast neutrons, and fission recoils) and gamma rays which can interact with matter and produce a variety of chemical changes. In many cases these changes influence the physical and chemical properties of materials. Therefore, great care was necessary in choice of materials of construction of the piles and of the chemical substances used for the separation of the various radioactive and fissionable products. Consequently, studies of problems in radiation chemistry were, from the beginning, an integral part of the plutonium projects. During the progress of these projects, with the field of photochemistry often serving as a guide to the solution of problems, many of the major features of radiation chemistry became better understood. Light has the advantage, when used for chemical purposes, that its action is often quite specific in the production of only one or a few substances, but its very specificity has the disadvantage that only certain substances which absorb the light can be affected. Furthermore, the light-radiation fields are, for the most part, relatively weak; thus, in general only small quantities of chemical substances can be treated. There is the further disadvantage that light of a type required to produce a chemical change in a substance must be strongly absorbed to produce that change; as a consequence the light is absorbed practically only in the surface layer, and therefore large volumes of material cannot be conveniently or adequately treated in this way. The high-energy radiations now available for radiation chemistry are very penetrating, thus permitting treatment of large quantities of material. All substances treated by such radiations can be pro |