The α-particle consists of 2 protons and 2 neutrons and is a relatively large particle. For a nucleus to be capable of releasing so large a particle, the nucleus must be relatively large itself. With few exceptions, nuclei which decay by α emission have a Z # >83

α-EMISSION: DECAY OF Rn222

N/P RATIOS FOR DECAY OF Rn222

ALPHA DECAY CHAINS

Many alpha-emitters decay from parent to daughter to granddaughter in chains that may have as many as 12 steps (refer to diagram above). Each represents a small increase in the n/p ratio so it takes multiple steps to achieve stability.

A typical alpha-decay scheme is displayed below:

In the alpha decay scheme, there are three possible routes of decay from parent to ground state of the daughter; the only mode of decay is alpha.
1. In Route 1, the decay is directly to the ground state. There is no excited or metastable state formed and no gamma rays are released.
2. In Route 2, there is decay by alpha to a metastable state of the daughter, followed by emission of a gamma ray and transition to the ground state.
3. In Route 3, there is alpha decay to a highly excited state of the daughter, followed by gamma ray emission either directly to the ground state or indirectly to the ground state through sequential emission of two separate gamma rays. Regardless of the route taken, the energy expended in transition from parent to daughter is a fixed amount.

ENERGY BALANCE

Although the total energy change for the decay of Rn-222 to Po-218 is 5.58 MeV, the energy of the alpha particle is only 5.48 MeV. The difference of 0.10 MeV is the recoil energy imparted to the newly formed Po²¹⁸ nucleus. Assuming the validity of nonrelativistic mechanics, this recoil energy can be calculated:

Mass_alpha x Energy_alpha = Mass_atom x Energy_alpha

for Po²¹⁸, E_recoil is (4) (5.48) / 218= 0.101 MeV

α-particles from a specific nuclear transformation are monoenergetic.

ALPHA RECOIL REACTIONS