<P> Quantum mechanics, however, provides a ready explanation, via the mechanism of quantum tunnelling . The quantum tunnelling theory of alpha decay, independently developed by George Gamow and Ronald Wilfred Gurney and Edward Condon in 1928, was hailed as a very striking confirmation of quantum theory . Essentially, the alpha particle escapes from the nucleus by quantum tunnelling its way out . Gurney and Condon made the following observation in their paper on it: </P> <P> It has hitherto been necessary to postulate some special arbitrary' instability' of the nucleus; but in the following note it is pointed out that disintegration is a natural consequence of the laws of quantum mechanics without any special hypothesis...Much has been written of the explosive violence with which the α - particle is hurled from its place in the nucleus . But from the process pictured above, one would rather say that the α - particle almost slips away unnoticed . </P> <P> The theory supposes that the alpha particle can be considered an independent particle within a nucleus that is in constant motion, but held within the nucleus by nuclear forces . At each collision with the potential barrier of the nuclear force, there is a small non-zero probability that it will tunnel its way out . An alpha particle with a speed of 1.5 × 10 m / s within a nuclear diameter of approximately 10 m will collide with the barrier more than 10 times per second . However if the probability of escape at each collision is very small, the half - life of the radioisotope will be very long, since it is the time required for the total probability of escape to reach 50% . As an extreme example, the half - life of the isotope bismuth - 209 is 1.9 x 10 years . </P> <P> Working out the details of the theory leads to an equation relating the half - life of a radioisotope to the decay energy of its alpha particles, a theoretical derivation of the empirical Geiger--Nuttall law . </P>

During alpha decay an atom ejects two protons and two neutrons