Normally 87Rb can decay to 87Sr because Q(β-)= 282.2keV in a half life of 4.97*10^10 years. But do you know 87Sr can also decay back to 87Rb by EC(Electron Capture) in a very short half life 2.815 hours by a little chance of 0.3%? Of course, the reverse decay can not happen naturally, because Q(β+)= -Q(β-) = -282.2keV < 0, the only condition is that 87Sr should be excited to level of 388keV with spin-parity 1/2-. This example is so unique! I am wondering if this bi-direction decay phenomenon could apply to any other isotopes?