Isotopes of neptunium

Neptunium (₉₃Np) is usually considered an artificial element, although trace quantities are found in nature, so a standard atomic weight cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first isotope to be synthesized and identified was ²³⁹Np in 1940, produced by bombarding ²³⁸
U with neutrons to produce ²³⁹
U, which then underwent beta decay to ²³⁹
Np.

Trace quantities are found in nature from neutron capture reactions by uranium atoms, a fact not discovered until 1951.

Twenty-five neptunium radioisotopes have been characterized, with the most stable being ²³⁷
Np with a half-life of 2.144 million years, ²³⁶
Np with a half-life of 153,000 years, and ²³⁵
Np with a half-life of 396.1 days. All of the remaining radioactive isotopes have half-lives that are less than 4.5 days, and the majority of these have half-lives that are less than 50 minutes. This element also has five meta states, with the most stable being ^236m
Np (t_1/2 22.5 hours).

The isotopes of neptunium range from ²¹⁹
Np to ²⁴⁴
Np, though the intermediate isotope ²²¹
Np has not yet been observed. The primary decay mode before the most stable isotope, ²³⁷
Np, is electron capture (and also alpha emission), and the primary mode after is beta emission. The primary decay products before ²³⁷
Np are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Neptunium is the heaviest element for which the location of the proton drip line is known; the lightest proton-bound isotope is ²²⁰Np.

Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by:

Neptunium-236 has 143 neutrons and a half-life of 153,000 years. It can decay by the following methods:

Neptunium-236 is a fissile material; it has an estimated critical mass of 6.79 kg (15.0 lb), though precise experimental data is not available (as sufficient material for criticality is not).

²³⁶
Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of ²³⁷
Np, however, it is nearly impossible to separate in any significant quantities from its parent ²³⁷
Np. It is for this reason that despite its low critical mass and high neutron cross section, it has not been researched extensively as a nuclear fuel in weapons or reactors. Nevertheless, ²³⁶
Np has been considered for use in mass spectrometry and as a radioactive tracer, because it decays predominantly by beta emission with a long half-life. Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from ²³⁷
Np or the isomer ^236m
Np. The most favorable reactions to accumulate ²³⁶
Np were shown to be proton and deuteron irradiation of uranium-238.