Nuclear Disintegration and Inner-Shell Atomic Processes in Heavy Elements

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Higher-order interactions between electrons in an atom and its nucleus can affect the nuclear transition and disintegration. Such effects involve γ-ray emission via electronic bridge mechanism (EBM), nuclear excitation by electron transition (NEET), and inverse internal conversion. The goal of this project is to establish nuclear-atomic spectroscopy. We took up the following sub-themes to reach the goal :1) Development of experimental techniques for nuclear-atomic spectroscopy.2) Investigation of interactions between atomic processes and low energy nuclear transitions in ^<235>U, ^<229>Th, and so on.3) Accumulation of fundamental data on inner-shell atomic processes caused by nuclear transitions.The nucleus ^<229m>Th is the best candidate to study EBM, because the extremely low excitation energy of 3.5eV cannot be released by emission of an internal conversion electron. In the period of this fund, therefore, we focused on the decay properties of ^<229m>Th. The following experiments were carried out :a) Construction of a measuring system composed of a broad-type Ge detector (supplied by the funds) and a Si charged-particle detector for α-γ and X-γ coincidence measurements.b) Preparation of electronics and software for multi-parameter data taking.c) Measurements of α-γ coincidence spectra for ^<229>Th and ^<233>U.d) Measurements of photons from ^<229m>Th prepared by milking pure ^<233>U.We tried to extract the decay of ^<229m>Th from the time dependence of the intensity of photons measured with a cooled photomultiplier. At present, there is no evidence of having detected the decay photons from ^<229m>Th.e) Observation of the α-decay of ^<229m>Th.We observed a decay component with a short half-life of about 14h in the α spectrum of ^<229>Th produced by ^<230>Th (γ, n)^<229>Th. This component may be attributed to the decay of ^<229m>Th. We are planing similar measurements for different chemical forms of Th.