Nishina and Kimura considered that this isotope would be produced by the (n, 2n) reaction, just as in the case of producing 231Th from 232Th, shown in Eq. The difference, which corresponds to the net activity of exposed uranium oxide, showed a half-life of 6.5 d.Īn uranium isotope having a half-life of 6.5 d was unknown at that time. The activity of thus-obtained uranium oxide was measured with a Lauritsen electroscope, and compared with the activity of a non-irradiated uranium oxide sample of the same weight in order to subtract the growing β-activity due to the disintegration products of uranium. After exposure, the exposed uranium oxide was again purified so as to eliminate any possible elements produced by fission as well as by its own disintegration. Fast neutrons were produced by bombarding lithium with 3 MeV deuterons in a cyclotron. A few grams of it was exposed to fast neutrons for more than 50 h. Uranium oxide (U 3O 8) was carefully purified and freed from its disintegration products in advance. The discovery of a new uranium isotope, 237U 9) The formation of 231Th from 232Th was surmised to be due to the loss of a neutron, and the reaction processes were considered to be as follows: (In the original paper, “ 228Th” and “ 231Th” were denoted as “radiothorium (RdTh)” and “uranium Y (UY)”, respectively, according to the conventional nomenclature and symbols used at that time.) 231Th is the precursor of 231Pa in the actinium series. The 24.5 h half-life of another one coincided with that of 231Th, a member of the natural actinium series. 6) by slow neutron irradiation of thorium. One showed a half-life of 26 m, and was identified with 233Th, which was also observed by L. It was revealed that two periods of β-activities were produced. The exposed sample was chemically purified for thorium, and the activity of the thorium fraction was measured with a Lauritsen electroscope. The exposure duration ranged from 3 h to 15 h. Thorium nitrate, carefully freed from any disintegration products except for 228Th, was exposed to fast neutrons that were produced by bombarding lithium with 3 MeV deuterons in a cyclotron. In this paper, several main studies by Nishina and Kimura are reviewed along with some explanatory commentaries.Īrtificial production of 231Th from thorium 5) The work of Nishina and Kimura was remarked and evaluated rather by foreign scientists. For these reasons, only a few Japanese physicists and chemists acquainted themselves with these prominent studies.
This was carried out in a rather strained period of 1938–1940, shortly before the breakout of the Pacific War, and the papers were submitted to foreign journals and published in them.
#URANIUM NUCLEAR FISSION EQUATION TRIAL#
(Fig.1b) 1b) (Department of Chemistry, the University of Tokyo), and obtained several remarkable results, including the discovery of a new radioactive isotope of uranium, 237U, the discovery of symmetric nuclear fission and a trial to discover the missing element of atomic number 93. In the physical and chemical fields, he carried out fast-neutron bombardment experiments on thorium or uranium in cooperation with Kenjiro Kimura (Fig. He also started studies on the biological effects of radiations produced by the cyclotron. Nishina prepared such radioisotopes as 11C, 13N, 24Na and 32P with his cyclotron, and applied them to biological tracer studies, obtaining many interesting results. Nishina’s 27 inch cyclotron constructed on the campus of the Institute of Physical and Chemical Research, Tokyo in 1937. Thus, Japan became the second cyclotron-possessing country in the world after the United States. The construction of the cyclotron started in 1935 and was completed in 1937 (Fig. He intended to construct a 27 inch cyclotron on the campus of the Institute of Physical and Chemical Research (RIKEN) in Tokyo. (Fig.1a) 1a) had the opinion that a cyclotron is essentially necessary for Japan to develop experimental nuclear physics as well as to promote the production and application of radioisotopes. These important inventions and discoveries led to the rapid development of a new field of nuclear physical and radiochemical studies on artificial nuclear transformation.Īt that time, Yoshio Nishina (Fig. Joliot-Curie (1934) 4) were successively reported. Urey (1932) 3) and the discovery of artificial radioisotopes by J.F. Chadwick (1932), 2) the discovery of deuterium by H.C. Lawrence (1931), 1) the discovery of neutron by J. At the beginning of the 1930’s, epoch-making inventions or discoveries including the invention of the cyclotron by E.O.
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