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.[2]
Twenty-five neptunium radioisotopes have been characterized, with the most stable being 237 Np with a half-life of 2.14 million years, 236 Np with a half-life of 154,000 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 (t1/2 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 (with a good deal of 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 bound isotope is 220Np.[3]
^( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
^# – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
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 (with a half-life of 23 million years) to thorium-232.
Beta emission: the decay energy is 0.48 MeV and the decay product is plutonium-236. This usually decays (half-life 2.8 years) to uranium-232, which usually decays (half-life 69 years) 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; it has an estimated critical mass of 6.79 kg (15.0 lb),[16] though precise experimental data is not available.[17]
236 Np is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237 Np ,[18] however, it is nearly impossible to separate in any significant quantities from its parent 237 Np .[19] 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.[17] 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.[20] Several alternative production routes for this isotope have been investigated, namely those that reduce isotopic separation from 237 Np or the isomer236m Np . The most favorable reactions to accumulate 236 Np were shown to be proton and deuteron irradiation of uranium-238.[20]
In 2002, 237 Np was shown to be capable of sustaining a chain reaction with fast neutrons, as in a nuclear weapon, with a critical mass of around 60 kg.[21] However, it has a low probability of fission on bombardment with thermal neutrons, which makes it unsuitable as a fuel for light water nuclear power plants (as opposed to fast reactor or accelerator-driven systems, for example).
When exposed to neutron bombardment 237 Np can capture a neutron, undergo beta decay, and become 238 Pu , this product being useful as a thermal energy source in a radioisotope thermoelectric generator (RTG or RITEG) for the production of electricity and heat. The first type of thermoelectric generator SNAP (Systems for Nuclear Auxiliary Power) was developed and used by NASA in the 1960's and during the Apollo missions to power the instruments left on the Moon surface by the astronauts. Thermoelectric generators were also embarked on board of deep space probes such as for the Pioneer 10 and 11 missions, the Voyager program, the Cassini–Huygens mission, and New Horizons. They also deliver electrical and thermal power to the Mars Science Laboratory (Curiosity rover) and Mars 2020 mission (Perseverance rover) both exploring the cold surface of Mars. Curiosity and Perseverance rovers are both equipped with the last version of multi-mission RTG, a more efficient and standardized system dubbed MMRTG.
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 (like Martian dust storms). Space probes and rovers also make use of the heat output of the generator to keep their instruments and internals warm.[22]
Shortage of 237 Np stockpiles
The long half-life (T½ ~ 88 years) of 238 Pu and the absence of γ-radiation that could interfere with the operation of on-board electronic components, or irradiate people, makes it the radionuclide of choice for electric thermogenerators.
237 Np is therefore a key radionuclide for the production of 238 Pu , which is essential for deep space probes requiring a reliable and long-lasting source of energy without maintenance.
Stockpiles of 238 Pu built up in the United States since the Manhattan Project, thanks to the Hanford nuclear complex (operating in Washington State from 1943 to 1977) and the development of atomic weapons, are now almost exhausted. The extraction and purification of sufficient new quantities of 237 Np from irradiated nuclear fuels is therefore necessary for the resumption of 238 Pu production in order to replenish the stocks needed for space exploration by robotic probes.
Neptunium-239
Neptunium-239 has 146 neutrons and a half-life of 2.356 days. It is produced via β− decay of the short-lived uranium-239, and undergoes another β− decay to plutonium-239. This is the primary route for making plutonium, as 239U can be made by neutron capture in uranium-238.[23]
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."[24][25]
^Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
^Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4. "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
^This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
^Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is eight quadrillion years.
^ abReed, B. C. (2017). "An examination of the potential fission-bomb weaponizability of nuclides other than 235U and 239Pu". American Journal of Physics. 85: 38–44. doi:10.1119/1.4966630.
^[Film Badge Dosimetry in Atmospheric Nuclear Tests, By Committee on Film Badge Dosimetry in Atmospheric Nuclear Tests, Commission on Engineering and Technical Systems, Division on Engineering and Physical Sciences, National Research Council. pg24-35]
