Decay scheme

The decay scheme is usually plotted in a coordinate system, where the vertical axis is energy, increasing from bottom to top, and the horizontal axis is the proton number, increasing from left to right. The arrows indicate the emitted particles. For the gamma rays (vertical arrows), the gamma energies are given; for the beta decay (oblique arrow), the maximum beta energy. Since energy is conserved and since the particles emitted carry away energy, arrows can only go downward (vertically or at an angle) in a decay scheme.

Examples

Cobalt-60

Consider the cobalt isotope cobalt-60. ⁶⁰Co decays (with a half-life of 5.272 years) by emitting an electron (beta decay) into an excited state of ⁶⁰Ni, which then decays very quickly to the ground state of ⁶⁰Ni, via two gamma decays.

Nickel is to the right of cobalt, since its proton number (28) is higher by one than that of cobalt (27). In beta decay, the proton number increases by one. For a positron decay and also for an alpha decay (see below), the oblique arrow would go from right to left since in these cases, the proton number decreases.

Gold-198

A somewhat more complicated scheme is shown here.

¹⁹⁸Au can be produced by irradiating natural gold in a nuclear reactor. ¹⁹⁸Au decays via beta decay to one of two excited states or to the ground state of the mercury isotope ¹⁹⁸Hg. In the figure, mercury is to the right of gold, since the atomic number of gold is 79, that of mercury is 80. The excited states decay after very short times (2.5 and 23 ps) to the ground state.

Metastable

While excited nuclear states are usually very short lived, decaying almost immediately after a beta decay, some decay slowly and are called "metastable".

For example, the excited state of the technetium isotope ^99mTc (the "m" means it is metastable) decays to the ground state via gamma decay with a half-life of 6 hours.

Alpha decay

Polonium-210 discovered by Marie Curie, is the penultimate member of the uranium–radium decay series. It decays into a stable lead-isotope with a half-life of 138 days. In almost all cases, the decay is via the emission of an alpha particle of 5.305 MeV. Only in 0.001% of cases, the decay emits an alpha particle of 4.502 MeV, leading to an excited level of ²⁰⁶Pb, which then decays to the ground state via gamma radiation.