Valley of stability
Regions of instability within the valley at high atomic number also include radioactive decay by alpha radiation or spontaneous fission.
The shape of the valley is roughly an elongated paraboloid corresponding to the nuclide binding energies as a function of neutron and atomic numbers.
The region of proton and neutron combinations outside of the valley of stability is referred to as the sea of instability.
[4][5] Scientists have long searched for long-lived heavy isotopes outside of the valley of stability,[6][7][8] hypothesized by Glenn T. Seaborg in the late 1960s.
[9][10] These relatively stable nuclides are expected to have particular configurations of "magic" atomic and neutron numbers, and form a so-called island of stability.
[3] The concept of the valley of stability is a way of organizing all of the nuclides according to binding energy as a function of neutron and proton numbers.
[1] The valley of stability provides both a conceptual approach for how to organize the myriad stable and unstable nuclides into a coherent picture and an intuitive way to understand how and why sequences of radioactive decay occur.
The approximate symmetry of isospin treats these particles as identical, but in a different quantum state.
One consequence of these complications is that although deuterium, a bound state of a proton (p) and a neutron (n) is stable, exotic nuclides such as diproton or dineutron are unbound.
[citation needed] Stable nuclides require approximately equal numbers of protons and neutrons.
For this reason, the valley of stability does not follow the line Z = N for A larger than 40 (Z = 20 is the element calcium).
The line of beta stability follows a particular curve of neutron–proton ratio, corresponding to the most stable nuclides.
On one side of the valley of stability, this ratio is small, corresponding to an excess of protons over neutrons in the nuclides.
On the other side of the valley of stability, this ratio is large, corresponding to an excess of neutrons over protons in the nuclides.
On this side of the valley of stability, β− decay also serves to move nuclides toward a more stable neutron-proton ratio.
The binding energy expression gives a quantitative estimate for the neutron-proton ratio.
From this bottom, the average binding energy per nucleon slowly decreases with increasing atomic mass number.
The heavy nuclide 238U is not stable, but is slow to decay with a half-life of 4.5 billion years.
The figure at right shows the average binding energy per nucleon across the valley of stability for nuclides with mass number A = 125.
The α particle carries away two neutrons and two protons, leaving a lighter nuclide.
Drip lines are defined for protons, neutrons, and alpha particles, and these all play important roles in nuclear physics.
Colloquially speaking, the nucleon has 'leaked' or 'dripped' out of the nucleus, hence giving rise to the term "drip line".
Proton emitters can be produced via nuclear reactions, usually utilizing linear particle accelerators (linac).
Although prompt (i.e. not beta-delayed) proton emission was observed from an isomer in cobalt-53 as early as 1969, no other proton-emitting states were found until 1981, when the proton radioactive ground states of lutetium-151 and thulium-147 were observed at experiments at the GSI in West Germany.
[16] Research in the field flourished after this breakthrough, and to date more than 25 nuclides have been found to exhibit proton emission.
The study of proton emission has aided the understanding of nuclear deformation, masses and structure, and it is an example of quantum tunneling.
So when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells.
The fission processes that occur within nuclear reactors are accompanied by the release of neutrons that sustain the chain reaction.
Like all nuclides with a high atomic number, these uranium nuclei require many neutrons to bolster their stability, so they have a large neutron-proton ratio (N/Z).
In 1956, Reines and Cowan exploited the (anticipated) intense flux of antineutrinos from a nuclear reactor in the design of an experiment to detect and confirm the existence of these elusive particles.
