Excess of neutrons

In nuclear physics, neutron excess is called the difference between the number of neutrons N and the number of protons Z in an atomic nucleus :

{\ displaystyle NUe = NZ}

Since A applies to the mass number , the neutron excess is also defined as: ${\ displaystyle A = N + Z \ Leftrightarrow N = AZ}$

{\ displaystyle \ Rightarrow NUe = A-2Z}

The neutron excess of stable nuclei is up to 15 Exceptions greater than zero and increases with increasing mass number A .

Occasionally the respective deviation from the bisector of the nuclide map, namely the number , is also referred to as the neutron surplus - the term " relative neutron surplus" is better here . ${\ displaystyle N = Z \ Leftrightarrow {\ frac {N} {Z}} = 1}$ ${\ displaystyle {\ frac {NUe} {Z}} = {\ frac {N} {Z}} - 1 = {\ frac {A} {Z}} - 2}$

Nuclide map with radioactive types of decay:
black = stable,
pink = β - decay due to excess neutrons,
blue = EC or β + decay due to excess protons,
yellow = alpha decay

Effect on the stability of atomic nuclei

The picture (a nuclide map ) shows how the ratio of the number of neutrons to the number of protons affects the stability of an atomic nucleus:

The stable, i.e. non-radioactive nuclides are shown as black fields. They range from hydrogen ( ¹ H) at the bottom left to lead ( ²⁰⁸ Pb) well before the end at the top right. The locations of these nuclides form a slightly curved “banana” with several gaps for certain proton or neutron numbers. For example, there are no stable nuclei with a proton number of Z = 43 ( technetium ) or Z = 61 ( promethium ).
To the right of this - in the violet area - one finds the nuclides with a relatively high neutron excess. They are radioactive , the excess is mostly broken down by β ^- decay .
To the left - in the blue area - there is a shortage of neutrons (instead of a neutron shortage , this can also be called a proton excess ). These nuclides are also radioactive, they are subject to β ⁺ decay or electron capture .

If the nucleus contains more than 82 protons, it is always unstable.

Red : Relative excess of neutrons in stable nuclides . It increases approximately linearly with the ordinal number. Green : Relative surplus of neutrons of heavier, radioactive, “relatively stable” nuclides. Here the relative neutron surplus drops again somewhat.

Stretched representation of the range from A / Z - 2 = 0.0 to 0.7

Effect on nuclear fission

The mass dependence of the relative neutron excess explains why fission products are usually beta-minus emitters . The high neutron surplus of a nucleus such as U-235 is found in its fragments (the fission fragments ) after the nuclear fission ; therefore they contain too many neutrons for their nuclear mass. The excess is gradually reduced through three processes:

direct emission of prompt neutrons within ^10-14 seconds after decay;
delayed neutron emission of the still neutron-rich fission products in milliseconds to seconds;
Beta-minus decays, i.e. conversion of neutrons into protons.

Extreme values

Hassium-278 and Darmstadtium-282 have the largest absolute neutron surplus of isotopes produced so far with 62.

Individual proof

^ Karl Heinrich Lieser: Nuclear and Radiochemistry . 2nd, revised edition, Wiley-VCH 2001, ISBN 3-527-30317-0 , page 9

[1] Karl Heinrich Lieser: Nuclear and Radiochemistry . 2nd, revised edition, Wiley-VCH 2001, ISBN 3-527-30317-0 , page 9