Isotopes of nitrogen

Nitrogen-13

Nitrogen-13 (¹³N) has a half-life of a little under ten minutes. It is produced in the atmosphere when gamma rays (for example from lightning) knock neutrons out of nitrogen-14.

¹³N decays to ¹³C, emitting a positron. The positron quickly annihilates with an electron, producing two gamma rays of about 511 keV. After a lightning bolt, this gamma radiation dies down with a half-life of 10 minutes, but these low-energy gamma rays go on average only about 90 metres through the air, so they may only be detected for a minute or so as the "cloud" of ¹³N and ¹⁵O floats by, carried by the wind.

Nitrogen-13 plays a significant role in the CNO cycle, which is the dominant source of energy in main sequence stars more massive than 1.5 times the mass of the Sun.

Nitrogen-13 is used in positron emission tomography in the form of ¹³N-labelled ammonia, for example for myocardial perfusion imaging. It can be produced with a medical cyclotron, using a target of pure water with a trace amount of ethanol. The reactants are oxygen-16 (present as H₂O) and a proton, and the products are nitrogen-13 and an alpha particle (helium-4):

¹H + ¹⁶O → ¹³N + ⁴He

In this endothermic reaction, the proton must be accelerated to have a total energy greater than 5.66 MeV. The presence of ethanol allows the formation of ammonia as nitrogen-13 is produced. Other routes of producing ¹³N-labelled ammonia exist, some of which facilitate co-generation of other light radionuclides for diagnostic imaging.

Nitrogen-14

Nitrogen-14 makes up the clear majority of natural nitrogen, about 99.62%, and is responsible for the Earth's stable atmosphere.

Nitrogen-14 is one of the very few stable nuclides with both an odd number of protons and of neutrons (seven each) and is the only one to make up a majority of its element. Unpaired protons or neutrons contribute a half-integer nuclear spin, which in this case is a spin 1/2 orbital, giving the nucleus a total magnetic spin of one (as the spins prefer to align).

The original source of nitrogen-14 and nitrogen-15 in the Universe is believed to be stellar nucleosynthesis, where they are produced as part of the CNO cycle.

Nitrogen-14 is the source of naturally occurring, radioactive, carbon-14. Some kinds of cosmic radiation cause a nuclear reaction with nitrogen-14 in the upper atmosphere of the Earth, creating carbon-14, which decays back to nitrogen-14 with a half-life of 5700 years.

Nitrogen-15

Nitrogen-15 is a rare stable isotope of nitrogen, comprising about 0.38%. Nitrogen-15 presents one of the lowest thermal neutron capture cross sections of all isotopes.

Nitrogen-15 is frequently used in NMR (Nitrogen-15 NMR spectroscopy). Unlike the more abundant nitrogen-14, which has an integer nuclear spin and thus a quadrupole moment, ¹⁵N has a fractional nuclear spin of one-half, which offers advantages for NMR such as narrower line width. As most nitrogen NMR studies look at a single nitrogen atom in an organic molecule, isotopic labeling is feasible.

Nitrogen-15 tracing is a technique used to study the nitrogen cycle.

Nitrogen-16

The radioisotope ¹⁶N is the dominant radioactivity source in the coolant water of nuclear reactors cooled by water during normal operation. It is produced from ¹⁶O (in water) via an (n,p) reaction, in which the ¹⁶O atom captures a neutron and expels a proton. It has a short half-life of 7.13 seconds, but its decay back to ¹⁶O produces high-energy gamma radiation (6.13 MeV principal line). Because of this, access to the primary coolant piping in a pressurised water reactor must be restricted during reactor power operation. It is a sensitive and immediate indicator of leaks from the primary coolant system to the secondary steam cycle and is the primary means of detection for such leaks.

Isotopic signatures