Enhancement of β⁺-Decay Rate of ²²Na in Metal Pd at Low Temperature

Temperature dependence of nuclear decays in metallic environments is a controversial issue. We measured the temperature dependence of the β⁺-decay half-life of ²²Na implanted into the metal host of palladium. It is found that the β⁺-decay half-life of ²²Na in the metal Pd cooled to T=15K is shorter by 0.46(14)% than that at room temperature. The result is consistent in sign with, but clearly smaller than, the estimated one by the Debye model.