Artificial Radioactivity Produced by Deuton Bombardment

Abstract

Although the problem of transmuting chemical elements into each other is much older than a satisfactory definition of the very concept of chemical element, it is well known that the first and most important step towards its solution was made only nineteen years ago by the late Lord Rutherford, who started the method of the nuclear bombardments. He showed on a few examples that, when the nucleus of a light element is struck by a fast α-particle, some disintegration process of the struck nucleus occurs, as a consequence of which the α-particle remains captured inside the nucleus and a different particle, in many cases a proton, is emitted in its place. What remains at the end of the process is a nucleus different from the original one; different in general both in electric charge and in atomic weight. The nucleus that remains as disintegration product coincides sometimes with one of the stable nuclei, known from the isotopic analysis; very often, however, this is not the case. The product nucleus is then different from all "natural" nuclei; the reason being that the product nucleus is not stable. It disintegrates further, with a mean life characteristic of the nucleus, by emission of an electric charge (positive or negative), until it finally reaches a stable form. The emission of electrons that follows with a lag in time the first practically instantaneous disintegration, is the so-called artificial radioactivity, and was discovered by Joliot and Irene Curie at the end of the year 1933. These authors obtained the first cases of artificial radioactivity by bombarding boron, magnesium, and aluminium with α-particles from a polonium source. They produced thus three radioactive isotopes of nitrogen, silicon and phosphorus, and succeeded also in separating chemically the activity from the bulk of the unmodified atoms of the bombarded substance. The neutron bombardment Immediately after these discoveries, it appeared that α-particles very likely did not represent the only type of bombarding projectiles for producing

Aritificial Radioactivity.

“…Yost, Ridenour, and Shinohara (97) showed that the radioelement was N13. V Lauritsen and Crane (23) found that the same element was formed in the bombardment of carbon by protons, as had Cockcroft, Gilbert, and Walton (17,18,19) and Henderson, Livingston, and Lawrence (56). Hafstad and Tuve (50) could not at first find evidence of such activity, but the discrepancy has been cleared up, and Hafstad and Tuve (51) have now made the best measurements of the voltage excitation function for this resonance reaction.…”

“…Crane and Lauritsen (22,24), Cockcroft, Gilbert, and Walton (17,18), and Henderson, Livingston, and Lawrence (56) have found a positron activity of about 10.1 minutes half-life in carbon bombarded by deutons. Yost, Ridenour, and Shinohara (97) showed that the radioelement was N13.…”

Enrico Fermi’s Discovery of Neutron-Induced Artificial Radioactivity:The Influence of His Theory of Beta Decay

“…38 Further, Ernest O. Lawrence and his coworkers at the University of California in Berkeley, and H. Richard Crane (1907Crane ( -2007 and Charles Lauritsen (1892-1968) at the California Institute of Technology in Pasadena, bombarded light nuclei with deuterons (and also protons), producing neutrons and radioactive nuclei that decayed with the emission of positrons. 39 In our view, therefore, this asymmetric experimental situation provided a strong motivation for Fermi, just a few days after he presented Wick's paper to the Accademia dei Lincei on March 4, 1934, to produce an intense Rn-Be neutron source and to construct the experimental apparatus he needed to bombard elements with Rn-Be neutrons, as we discussed earlier. 40 In other words, Fermi's theory of beta decay constituted a crucial precondition for his discovery of neutron-induced artificial radioactivity a few weeks after Wick had applied it to the case of positron decay.…”

Enrico Fermi’s Discovery of Neutron-Induced Artificial Radioactivity: Neutrons and Neutron Sources

“…Walton at the Cavendish Laboratory in Cambridge bombarded carbon with 600-keV (kilo-electronvolt) protons and detected an element that decayed exponentially with a half-life of 10.5 minutes, which they assumed was 7 N 13 that was produced according to the reaction 6 C 12 + 1 H 1 → 7 N 13 , but in a different excited state than the isotope that Joliot and Curie had found with a half-life of 3 minutes 15 seconds. 50 51 The production of artificial radioactivity by neutron bombardment was hindered primarily by the difficulty in obtaining intense neutron sources, ones comparable in intensity to polonium alpha-particle sources. Thus, as Joliot noted in July 1933, neutrons "are ideal projectiles" because they are uncharged, but "great efforts are required to artificially produce strong beams of neutrons."…”