Neutron drip line

The nuclear drip line is the boundary delimiting the zone in which atomic nuclei lose stability due to the transmutation of neutrons, causing an isotope of one element to mutate into an element with one more proton.

An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus. One can think of moving up and/or to the right across the table of nuclides by adding one type of nucleon to a given nucleus. However, adding nucleons one at a time to a given nucleus will eventually lead to a newly formed nucleus that immediately decays by emitting a proton (or neutron). Colloquially speaking, the nucleon has 'leaked' or 'dripped' out of the nucleus, hence giving rise to the term "drip line".

Drip lines are defined for protons, neutrons, and alpha particles, and these all play important roles in nuclear physics. The nucleon drip lines are at the extreme of the proton-to-neutron ratio: at p:n ratios at or beyond the driplines, no stable nuclei can exist. The location of the neutron drip line is not well known for most of the nuclear chart, whereas the proton and alpha driplines have been measured for a wide range of elements.

Nuclear stability is limited to those combinations of protons and neutrons described by the chart of the nuclides, also called the valley of stability. The boundaries of this valley are the neutron drip line on the neutron rich side, and the proton drip line on the proton-rich side. Between those two lines, when a nucleus has a reasonable balance of protons and neutrons, the total nuclear mass is limited by alpha decay, or the alpha drip line, which connects the proton and neutron drip lines. The alpha drip line is somewhat more difficult to visualize as it also branches down through the center of the chart. These limits exist because of particle decay, whereby an exothermic nuclear transition can occur by the emission of one or more nucleons (not to be confused with particle decay in particle physics). To understand the concept, one only needs to apply the principle of conservation of energy to nuclear binding energy.

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