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 238U with neutrons to produce 239U, which then underwent beta decay to 239Np. 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 with a half-life of 2.14 million years, Neptunium-236| with a half-life of 154,000 years, and 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 4 meta states, with the most stable being . The isotopes of neptunium range from to, though the intermediate isotope has not yet been observed. The primary decay mode before the most stable isotope,, is electron capture, and the primary mode after is beta emission. The primary decay products before are isotopes of uranium and protactinium, and the primary products after are isotopes of plutonium. Uranium-237 and neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following nuclear fallout from a nuclear detonation, with 239Np dominating "the spectrum for several days".
Electron capture: the decay energy is 0.125 MeV and the decay product is uranium-235
This isotope of neptunium has a weight of 235.044 063 3 u.
Neptunium-236
Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods:
Electron capture: the decay energy is 0.93 MeV and the decay product is uranium-236. This usually decays to thorium-232.
Beta emission: the decay energy is 0.48 MeV and the decay product is plutonium-236. This usually decays to uranium-232, which usually decays to thorium-228, which decays in a few years to lead-208.
Alpha emission: the decay energy is 5.007 MeV and the decay product is protactinium-232. This decays with a half-life of 1.3 days to uranium-232.
This particular isotope of neptunium has a mass of 236.04657 u. It is a fissile material with a critical mass of 6.79 kg. is produced in small quantities via the and capture reactions of, however, it is nearly impossible to separate in any significant quantities from its parent. It is for this reason that despite its low critical mass and high neutron cross section, it has not been researched as a nuclear fuel in weapons or reactors. Nevertheless, 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 or the isomer. The most favorable reactions to accumulate were shown to be proton and deuteron irradiation of uranium-238.
When exposed to neutron bombardment can capture a neutron, undergo beta decay, and become, this product being useful as an thermal energy source in a radio-isotope thermoelectric generator for the production of electricity and heat in deep space probes and, of recent note, the Mars Science Laboratory. These applications are economically practical where photovoltaic power sources are weak or inconsistent due to probes being too far from the sun or rovers facing climate events that may obstruct sunlight for long periods. Space probes and rovers also make use of the heat output of the generator to keep their instruments and internals warm.
