Direct observation of the superallowed -decay of Te

Cox, I.*; Grzywacz, R.*; King, T. T.*; Rykaczewski, K. P.*; 西尾 勝久   ; 他30名*

Cox, I.*; Grzywacz, R.*; King, T. T.*; Rykaczewski, K. P.*; Nishio, Katsuhisa; 30 of others*

The radioactivity of the particle is among the most compelling evidence for the existence of cluster structures in atomic nuclei. During the decay process, a preexisting particle tunnels through the potential barrier formed by the residual nucleus. The degree of preformation of the particle, a strongly bound system of two protons and two neutrons, is extracted from the data by dividing the -decay probability by the barrier penetrability for a given particle energy. The preformation probability changes rapidly near nuclear shell closures, which is direct evidence that clustering is connected to nuclear structure. Enhanced preformation was observed in the lightest -particle emitters, spherical tellurium and xenon isotopes decaying to magic isotopes of tin. Here we show the most extreme case of -particle preformation from the measurement of the decay of tellurium-104 (Te). With a half-life of 7.2 ns, Te is the fastest ground-state -emitting nucleus known so far. The deduced preformation demonstrates that the enhancement is greater for Te than for any other nucleus. One nuclear model that can explain our observation postulates that the particle can exist only in the low-nuclear-matter-density regions on the surface of the nucleus. The uniquely high preformation for Te is attributed to its relation to doubly magic tin-100 (Sn), creating conditions conducive to form an particle.