Fig. 3-10 Newly found nuclides The numbers of protons in the stable nuclei (black) and unstable nuclei (yellow) found to date have been plotted against the number of neutrons. The upper side of the black region is the proton-excess region and the lower side is the neutron-excess region.

This picture, 52KB

Fig. 3-11 The experimental apparatus used to find new nuclides This apparatus has been installed in the tandem accelerator at JAERI. It consists of a gas-jet transport apparatus, an on-line isotope separator and an on-line short-lived nuclear spectrometer. It is designed so that a short-lived nuclide can be detected shortly after it is produced.

When the numbers of protons in the atomic nuclei of the known nuclides are plotted against the numbers of neutrons, the nuclides are found to exist in a narrow band (stable region) as shown in Fig. 3-10. The study of nuclides existing apart from the stable region provides us with new knowledge regarding the limitation, structure and properties of atomic nuclei in the limit of the region. Although such nuclides are produced by nuclear reactions with large accelerators, their measurement is made difficult by their short half-lives. Using a tandem accelerator equipped with an experimental apparatus as shown in Fig. 3-11, we are developing studies on new nuclides that exist apart from the stable region. For this purpose, a target material is irradiated with a charged particle beam from the accelerator in the gas-jet transport apparatus. The products of the nuclear reaction are recoiled into the helium or argon carrier gas, where they are trapped with a sodium chloride aerosol and carried to the on-line mass separator, to be separated by mass and finally measured with the on-line nuclear spectrometer for short-lived nuclides. In the neutron-excess region, three new nuclides have been found using this technique: terbium -166 (166Tb, half-life 21 s), gadolinium-165 (165Gd, 10.3 s), and samarium-161 (161Sm, approximately 4 s). They are produced by the proton fission of uranium, and they are being watched with interest for their role in the production of synthetic heavy elements. In the neutron-deficient actinide region, we have found americium-236 (236Am, 4.4 min), which is produced by irradiating uranium with 6Li particles. It decays mainly by electron capture (EC). A characteristic process of the decay is predicted, which results in the EC delayed nuclear fission from an excited state of the daughter nuclide.

Reference S. Ichikawa et al., Search for Unknown Isotopes Using the JAERI-ISOL, Nucl. Instrum. Methods Phys. Res., Sect. B 125, 1 (1997).

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