Nuclear transition based on dominant chemical states: verification of the process of electron cross linking in the (229m-Th) isotope

The static interaction between nucleus and atom system is observed as the hyperfine interaction. Dynamic interactions, however, have not been observed yet clearly although some theoretical predictions have been reported. In the ultraheavy nuclei, the interaction is expected to be observable via the Electronic-Bridge (EB) Mechanism. In this paper, recent experimental progress toward the verification of the EB mechanism of 229mTh is reported. Since the excitation energy of 229mTh is about 3.5eV which is lower than the lowest atomic ionization energy, the internal conversion does not occur. Therefore 229Th is suitable for observing the EB mechanism. In order to detect 229mTh measurements of photons and α are planned. To detect photons of 350nm wavelength expected at the deexcitation of the isomer transition (γ) and 500nm photons at the EB mechanism, a special low background photon detecting system is being manufactured. Research and development of a high efficiency wavelength-selective photon counter system, which makes it possible to observe time dependence feature, is also under way. For the measurement of α, silicon detectors are used at present. Experiments of generating 229mTh sources by 228Ra(n, γ)229Ra reaction with reactor neutrons, 230Th(γ, n)229Th and 232Th(γ, p2n)229Ac reactions with bremsstrahlung irradiation and 232Th(p, p3n)229Th reaction with AVF cyclotron protons and their results of α measurements are reported. (S. Funahashi)