1—
Some Preliminaries
The naturally occurring radioactive isotopes emit nuclei of helium (a particles), fast electrons (b particles), and penetrating x rays (g rays). The energies of these particles and rays are usually given as multiples of the "electron-volt," the energy of motion an electron acquires in falling through a potential difference of one volt (two-thirds the voltage of a standard flashlight battery). An electron-volt is an absurdly small measure, a little over a million-millionth of the "erg," the basic energy unit of the physicist; and the erg itself amounts to less than the footfall of a fly. To fix ideas and notation, an alpha particle from the naturally occurring isotope polonium with atomic weight 210 (Po210 ) has an energy of 5.3 million electron-volts (5.3 MeV) and a velocity of 1.6 billion centimeters a second (1.6·109 cm/sec), about one-twentieth the velocity of light. The individual polonium a particle, although projected with tremendous speed, has a negligible mechanical effect on the surface that stops it. A great many of them, however, are noticeable.
A noticeable quantity is 37 billion, roughly the number of disintegrations occurring each second in a gram of radium; this rate of collapse, called a curie, is the basic measure of radioactivity. If each of these billions of disintegrations resulted in an alpha particle as energetic as polonium's, all together they would carry away
some 3.5·105 erg, or a little less than a hundredth of a calorie, each second. An ampere of alpha particles is the equivalent of almost a hundred million curies (108 Ci). A milliamp (mA) or even a microamp (µA) of alpha particles greatly exceeds the flux from available natural isotopes: 1µA = 80 Ci, 1 mA = 8·104 Ci; a microamp playing on a surface would heat it at the rate of almost one calorie a second, a milliamp at almost a thousand.
Since the nucleus occupies about as much of an atom as the earth does of the sphere whose radius is its distance from the sun, the chance that an alpha particle will strike a nucleus head on before it comes to rest is very small. But only in such collisions can the particle force its way into the nucleus; hence, if forced entry is the experimenter's goal, he would do well to furnish himself with a microamp rather than with a curie of projectiles. The µA has an even greater advantage than the preceding numbers indicate; the radiation from a natural source goes in all directions, whereas the outpouring from an artificial source may be tightened into a directed beam.
In 1919 Sir Ernest Rutherford unintentionally introduced alpha particles into nitrogen nuclei and, what was much more difficult, recognized what he had done. His initial objective was to study collisions between alpha particles and other light nuclei. As source he used a descendent of radium, RaC (Bi214 ), with a maximum strength of 0.08 Ci. That was strong enough to make the effect under investigation—knocking on, or driving forward, one nucleus by another—easy to detect. The knock-on particles were caught on a screen coated with a material that glowed where hit; and the source could be brought so close to the detector that as many as 40 or 50 flashes a minute could be spotted in the field of a microscope pointed at the screen. In this way, with hydrogen gas as target, Rutherford found swift knock-on protons, and, with oxygen and nitrogen, somewhat slower bumped ions, as he had expected.[1] A slight anomaly occurred in nitrogen, however. Despite every precaution against traces of hydrogen in the experimental space, protons even swifter than knock-on hydrogen ions persistently obtruded. It appeared to Rutherford that a few alpha
particles from RaC, making particularly intimate contact with nuclei of nitrogen, had driven out protons from the heart of the atom. He believed rightly that he had disintegrated the nucleus and wrongly that the process resembled a successful shot at marbles, in which both the impinging and the struck balls end up outside the target area.[2] He continued this work in collaboration with James Chadwick, an excellent experimenter trained at the Cavendish Laboratory, who had perfected his physics in Germany as a prisoner of war and who had returned to Cambridge, where he became in time the associate director of the laboratory. By enlarging the field of their microscope and viewing the disintegration protons at right angles to the direction from source to target, Rutherford and Chadwick showed that alpha particles could knock protons from most light elements and that the yield increased sharply with the energy of the bombarding particles.[3]
In these experiments perhaps ten alpha particles in a million made a collision that resulted in a detectable disintegration proton. Since the average source was 0.05 Ci and the solid angle at the microscope about 0.0002 steradians at most, the maximum number of countable protons was one a second, or, taking the efficiency of the screen into account, around one a minute per mCi. That sufficed to detect disintegration protons but not to give satisfactory quantitative measurements of their speeds, penetration, angular distribution, or yield as a function of the energy of bombardment. Until late in 1924, and probably for some time afterwards, Rutherford and Chadwick continued to suppose that they were engaged in a game of marbles, and tried to fit their meager quantitative results to irrelevant billiard-ball mechanics. But that year, in one of those triumphant syntheses that distinguished the Cavendish, one of its senior members, P.M.S. Blackett, succeeded in photographing in a Wilson cloud chamber (an invention of the laboratory) the trajectories of the particles participating in productive collisions of the type discovered by Rutherford. In Blackett's beautiful pictures—he obtained records
of eight disintegrations in 23,000 photographs presenting 400,000 tracks—no trace appeared of an alpha particle leaving the site of a productive collision (plate 2.1). The mental picture required that it continue on; the optical evidence indicated that it disappeared, swallowed by the target nitrogen nucleus. If so, the resultant nucleus, after expulsion of the swift proton, should be an isotope of oxygen.[4] In keeping with the convention used in the 1930s, we shall write the reaction as N14 (a ,p)O17 and refer to it as an (a ,p) transformation.
The problem of accounting for the energy in nuclear transformations became pressing and frustrating. Here another bit of Cavendish pioneering enriched and complicated the proceedings. Following up work he had done before the war as assistant to J.J. Thomson, Francis Aston perfected his system of separating the isotopes of the light elements (in very small quantities, to be sure!) by electric and magnetic fields. From the trajectories of the ions of the various isotopes he could calculate their masses; which, by 1930, he was reporting to a ten-thousandth of the mass of a proton (mp ). Now 0.001 mp is about 1 MeV: hence Aston's measurements of isotopic masses appeared to be just accurate enough to be used in working out the energy balance in nuclear transformations provoked by the input of a few million electron volts. The upshot: the numbers obtained by Aston, Blackett, and Rutherford and Chadwick did not agree, and prompted the dispiriting hypothesis that the normal internal states of nuclei of the same isotope differ energetically.[5]
If only the statistics were better! If only the sources were not so weak! If only the geometry of the experiments could be improved! Laments of this character enliven the pages of Rutherford and Chadwick.[6] To improve upon the work of nature appeared to
require a machine capable of producing at least a microamp of light positive ions and of accelerating them to a few million volts; if a nucleus propelled an alpha particle at 5 MeV, would it not be necessary to hurl it at 5 MeV to return it? Already before the war Rutherford had judged the creation of machines operating at the highest possible voltage to be "a matter of pressing importance" for the study of beta rays;[7] but for many years it proved impractical to make insulators that could hold much more than seven or eight hundred thousand volts (700 or 800 kV) or accelerating tubes that could withstand even half that much. Subsequent improvements in electrical equipment, x-ray tubes, and insulating materials, the closer ties between industry and academy forged during the war, and the newly important field of nuclear transformation gave new urgency to the matter.
Rutherford properly took the lead in promoting development of million-volt accelerators. In 1927, in addressing the Royal Society of London as its president, he challenged his audience to fulfill his long-time wish for "a copious supply" of projectiles more energetic than natural alpha and beta particles. His appeal received wide attention and many proposals for realizing it came forward. Progress was slow at the laboratory level. In 1930, while his associates struggled to make a source of a few hundred thousand volts, Rutherford asked big electrical industry for help in raising the "puny experiments in the laboratory" to nature's scale. "What we require [he said, at the opening of a new High Tension Laboratory at Metropolitan-Vickers Electrical Company] is an apparatus to give us a potential of the order of 10 million volts which can be safely accommodated in a reasonably sized room and operated by a few kilowatts of power. We require too an exhausted tube capable of withstanding this voltage. . . . I see no reason why such a requirement can not be made practical."[8]