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In the discrete setting, a weight function : + is a positive function defined on a discrete set, which is typically finite or countable. The weight function w ( a ) := 1 {\displaystyle w(a):=1} corresponds to the unweighted situation in which all elements have equal weight.
Calcium-48 is very neutron-rich for such a relatively light element, but like calcium-40, it is stabilized by being doubly magic. As an exception, although oxygen-28 has 8 protons and 20 neutrons, it is unbound with respect to four-neutron decay and appears to lack closed neutron shells, so it is not regarded as doubly magic.
The valuation can be interpreted as the order of the leading-order term. [b] The third property then corresponds to the order of a sum being the order of the larger term, [c] unless the two terms have the same order, in which case they may cancel and the sum may have larger order.
The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellar gas has become enriched with heavy elements as time passed. As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier.
In nuclear science, nuclei of heavy metals such as chromium, iron, or zinc are sometimes fired at other heavy metal targets to produce superheavy elements; [173] heavy metals are also employed as spallation targets for the production of neutrons [174] or isotopes of non-primordial elements such as astatine (using lead, bismuth, thorium, or ...
The need for a physical description was already inspired by the relative abundances of the chemical elements in the solar system. Those abundances, when plotted on a graph as a function of the atomic number of the element, have a jagged sawtooth shape that varies by factors of tens of millions (see history of nucleosynthesis theory). [4]
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Pair production often refers specifically to a photon creating an electron–positron pair near a nucleus. As energy must be conserved, for pair production to occur, the incoming energy of the photon must be above a threshold of at least the total rest mass energy of the two particles created.