Bremsstrahlung emission in α-decay and tunnelling motion of α particles

In α-γ coincidence measurements with Si and Ge detectors, bremsstrahlung photons associated with the α-decay of ²¹⁰Po and ²⁴⁴Cm were measured. Emission probabilities of the bremsstrahlung deduced for ²¹⁰Po were 10^-11 ∼ 10^-12 keV^-1 sr^-1 decay^-1 for 100 ≤ E_γ ≤ 600 keV, and upper limits of the same order of magnitude were obtained for ²⁴⁴Cm. They are much smaller than those predicted by a Coulomb acceleration model and are compared with a quasi-classical calculation in which the bremsstrahlung emission in tunnelling motion of α particles is taken into account. It is shown that the data can be interpreted as a consequence of destructive interference of radiative amplitudes outside the Coulomb barrier with those in tunnelling.