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 of²²Na implanted into the metal host of palladium. It is found that the β⁺ -decay half-life of²²Na in the metal Pd cooled to T = 15 K 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.