In nature, the "coldest" nucleus could be defined as the most energy-depleted. Which is the same way of saying - highest binding energy per nucleon. The dividing line, or the interface, between positive energy and negative energy will be cold, relative to its surroundings.
Nickel-62 is the coldest nucleus in nature - an isotope having 28 protons and 34 neutrons - with the highest binding energy per nucleon (8.8 MeV). It is often stated (wrongly) that iron-56 is the "most stable nucleus", but actually 56Fe only has the lowest mass per nucleon (not binding energy per nucleon). This misconception probably originated from astrophysics, since those guys pay less attention to the little picture than to the big picture. OKAY - so what? Did I mention that the dividing line between positive energy and negative energy will probably be cold; and consequently a good choice for gateway into an energy "sink" will likewise be very cold relative to surroundings. This gives two prime choices, and one of them, nickel, has proton affinity - which the other lacks ... and in fact iron becomes embrittled on proton exposure whereas nickel absorbs. Jones
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