[Valley of Stability]
The progress that has been made in our understanding of the subatomic world over the past decades has allowed us to classify 287 types of ‘natural’ (i.e. stable or quasi-stable) nuclei and over 2,500 radioactive nuclei. Some of these radioisotopes have such short half-lives that they barely have the time to acquire orbiting electrons and be classified as atoms. Experts often arrange all these nuclei on a map of number of constituent protons versus the number of constituent neutrons. This nucleus graph is an important part of the nuclear physics Bible.
Just as geographical maps indicate the relative heights of different points, the nuclear map provides information about the degree of stability the nucleus possesses. This degree of stability is measured by the binding energy of the nucleons within the nucleus.
The average mass of one nucleon in a nucleus – which can be directly linked to this binding energy – can be compared to ‘altitude levels’ on an geographic map. The stable or quasi-stable nuclei can then all be found in a region at the heart of the map – an area known as the bottom of the stability valley. The valley stretches from light nuclei, which have approximately equal numbers of protons and neutrons, to heavy nuclei, which have a slight neutron excess.
Located at the far end of the valley, bismuth 209 is the heaviest stable nucleus with its 83 protons and 126 neutrons. Beyond that point, we find unstable nuclei, heavier structures which decay to form lighter nuclei. These decay processes can be very slow, however, and unstable nuclei can persist for millennia in the Earth’s crust. Thorium 232, uranium 235 and uranium 238 are examples of radioisotopes that have not yet completely disappeared. Uranium 238 is, in fact, the heaviest natural nucleus with 92 protons and 136 neutrons.
The average binding energy per nucleon is a measure of the stability of the nucleus. This value is a function both of its size and composition, and is zero for the nucleus of hydrogen, which consists of one solitary proton. The lightest nuclei tend to have the weakest binding energies – with the exception of helium 4 (also known as the alpha particle), whose binding energy of 7 MeV per nucleon is far greater than any of its neighbours deuterium, tritium, helium 3 and lithium. This peculiar stability explains the emission of alpha particles by heavy nuclei.
The binding energy per nucleon begins to increase with the size of the nucleus, reaching a maximum value of 8.8 million electronvolts (MeV) for nickel 62: the most stable of nuclei. It then gradually decreases, reaching 7.6 MeV for uranium.
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