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 with neutrons to produce , which then underwent beta decay to .
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 2.144 million years, with a half-life of 153,000 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 five meta states, with the most stable being (t1/2 22.5 hours).
The isotopes of neptunium range from to , though the intermediate isotope has not yet been observed. The primary decay mode for nuclei lighter than the most stable isotope, , is electron capture (and also alpha emission), and the primary mode after is beta emission. The primary decay products for nuclei lighter than 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.
List of isotopes
|-id=Neptunium-219 | | style="text-align:right;" | 93 | style="text-align:right" | 126 | 219.031602(99) | style="text-align:center" | 2018 | | α | 215Pa | 9/2−# | |-id=Neptunium-220 | | style="text-align:right;" | 93 | style="text-align:right" | 127 | 220.032716(33) | style="text-align:center" | 2019 | | α | 216Pa | 1−# | |-id=Neptunium-222 | | style="text-align:right;" | 93 | style="text-align:right" | 129 | 222.033575(41) | style="text-align:center" | 2020 | | α | 218Pa | 1−# | |-id=Neptunium-223 | | style="text-align:right;" | 93 | style="text-align:right" | 130 | 223.032913(89) | style="text-align:center" | 2017 | | α | 219Pa | (9/2−) | |-id=Neptunium-224 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 131 | rowspan=2|224.034388(31) | rowspan=2 style="text-align:center" | 2018 | rowspan=2| | α (83%) | 220m1Pa | rowspan=2|2−# | rowspan=2| |- | α (17%) | 220m2Pa |-id=Neptunium-225 | | style="text-align:right;" | 93 | style="text-align:right" | 132 | 225.033943(98) | style="text-align:center" | 1994 | | α | 221Pa | 9/2−# | |-id=Neptunium-226 | | style="text-align:right;" | 93 | style="text-align:right" | 133 | 226.03523(11) | style="text-align:center" | 1990 | | α | 222Pa
|-id=Neptunium-227 | | style="text-align:right;" | 93 | style="text-align:right" | 134 | 227.034975(83) | style="text-align:center" | 1990 | | α | 223Pa | 5/2+# | |-id=Neptunium-228 | rowspan=3| | rowspan=3 style="text-align:right;" | 93 | rowspan=3 style="text-align:right" | 135 | rowspan=3|228.03631(11)# | rowspan=3 style="text-align:center" | 1994 | rowspan=3| | EC (59%) | 228U | rowspan=3|4+# | rowspan=3| |- | α (41%) | 224Pa |- | β+, SF (0.0126%) | (various) |-id=Neptunium-229 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 136 | rowspan=2|229.03629(11) | rowspan=2 style="text-align:center" | 1968 | rowspan=2| | α (68%) | 225Pa | rowspan=2|5/2+# | rowspan=2| |- | β+ (32%) | 229U |-id=Neptunium-230 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 137 | rowspan=2|230.037828(59) | rowspan=2 style="text-align:center" | 1968 | rowspan=2| | β+ (>97%) | 230U | rowspan=2|4+# | rowspan=2| |- | α (<3%) | 226Pa |-id=Neptunium-231 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 138 | rowspan=2|231.038244(55) | rowspan=2 style="text-align:center" | 1950 | rowspan=2| | β+ (98%) | 231U | rowspan=2|5/2+# | rowspan=2| |- | α (2%) | 227Pa |-id=Neptunium-232 | | style="text-align:right;" | 93 | style="text-align:right" | 139 | 232.04011(11)# | style="text-align:center" | 1950 | | β+ | 232U | (5−) | |-id=Neptunium-233 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 140 | rowspan=2|233.040739(55) | rowspan=2 style="text-align:center" | 1950 | rowspan=2| | β+ | 233U | rowspan=2|5/2+# | rowspan=2| |- | α (0.0007%) | 229Pa |-id=Neptunium-234 | | style="text-align:right;" | 93 | style="text-align:right" | 141 | 234.0428932(90) | style="text-align:center" | 1949 | | β+ | 234U | (0+) | |-id=Neptunium-234m | rowspan=2 style="text-indent:1em" | | rowspan=2 colspan=3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | (2020)Only published in a conference abstract and not a refereed journal | rowspan=2|~ | IT | 234Np | rowspan=2|5+ | rowspan=2| |- | EC | 234U |-id=Neptunium-235 | rowspan=2| | rowspan=2 style="text-align:right;" | 93 | rowspan=2 style="text-align:right" | 142 | rowspan=2|235.0440615(15) | rowspan=2 style="text-align:center" | 1949 | rowspan=2| | EC | 235U | rowspan=2|5/2+ | rowspan=2| |- | α (0.00260%) | 231Pa |-id=Neptunium-236 | rowspan=3|Fissile nuclide | rowspan=3 style="text-align:right;" | 93 | rowspan=3 style="text-align:right" | 143 | rowspan=3|236.046568(54) | rowspan=3 style="text-align:center" | 1949 | rowspan=3| | EC (86.3%) | 236U | rowspan=3|(6−) | rowspan=3| |- | β− (13.5%) | 236Pu |- | α (0.16%) | 232Pa |-id=Neptunium-236m | rowspan=2 style="text-indent:1em" | Order of ground state and isomer is uncertain. | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 1955 | rowspan=2| | EC (50%) | 236U | rowspan=2|(1−) | rowspan=2| |- | β− (50%) | 236Pu |-id=Neptunium-237 | rowspan=3| | rowspan=3 style="text-align:right;" | 93 | rowspan=3 style="text-align:right" | 144 | rowspan=3|237.0481716(12) | rowspan=3 style="text-align:center" | 1948 | rowspan=3| | α | 233Pa | rowspan=3|5/2+ | rowspan=3|TraceProduced by neutron capture in uranium ore |- | SF (<2×10−10%) | (various) |- | CD (<4×10−12%) | 207Tl + 30Mg |-id=Neptunium-237m | style="text-indent:1em" | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1990 | | IT | 237Np | 13/2− | |-id=Neptunium-238 | | style="text-align:right;" | 93 | style="text-align:right" | 145 | 238.0509446(12) | style="text-align:center" | 1949 | | β− | 238Pu | 2+ | |-id=Neptunium-238m | style="text-indent:1em" | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1970 | | SF | (various)
|-id=Neptunium-239 | | style="text-align:right;" | 93 | style="text-align:right" | 146 | 239.0529375(14) | style="text-align:center" | 1940 | | β− | 239Pu | 5/2+ | Trace |-id=Neptunium-240 | | style="text-align:right;" | 93 | style="text-align:right" | 147 | 240.056164(18) | style="text-align:center" | 1953 | | β− | 240Pu | (5+) | TraceIntermediate decay product of 244Pu |-id=Neptunium-240m | rowspan=2 style="text-indent:1em" | | rowspan=2 colspan="3" style="text-indent:2em" | | rowspan=2 style="text-align:center" | 1960 | rowspan=2| | β− (99.88%) | 240Pu | rowspan=2|(1+) | rowspan=2|Trace |- | IT (0.12%) | 240Np |-id=Neptunium-241 | | style="text-align:right;" | 93 | style="text-align:right" | 148 | 241.058349(33) | style="text-align:center" | 1959 | | β− | 241Pu | (5/2+) | |-id=Neptunium-242 | | style="text-align:right;" | 93 | style="text-align:right" | 149 | 242.061738(87) | style="text-align:center" | 1979 | | β− | 242Pu | (1+) | |-id=Neptunium-242m | style="text-indent:1em" | | colspan="3" style="text-indent:2em" | | style="text-align:center" | 1981 | | β− | 242Pu | (6+) | |-id=Neptunium-243 | | style="text-align:right;" | 93 | style="text-align:right" | 150 | 243.064204(34)# | style="text-align:center" | 1979 | | β− | 243Pu | 5/2+# | |-id=Neptunium-244 | | style="text-align:right;" | 93 | style="text-align:right" | 151 | 244.06789(11)# | style="text-align:center" | 1987 | | β− | 244Pu | 7−# |
Actinides vs fission products
Notable isotopes
Neptunium-235 Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by: Electron capture: the decay energy is 0.125 MeV and the decay product is uranium-235. Alpha emission: the decay energy is 5.2 MeV and the decay product is protactinium-231.
Neptunium-236 Neptunium-236 has 143 neutrons and a half-life of 153,000 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.
Neptunium-236 is a fissile material; it has an estimated critical mass of , though precise experimental data is not available (as sufficient material for criticality is not).
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of ,Analysis of the Reuse of Uranium Recovered from the Reprocessing of Commercial LWR Spent Fuel, United States Department of Energy, Oak Ridge National Laboratory. 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 extensively 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.
Neptunium-237
decays via the neptunium series, which terminates with thallium-205, which is stable, unlike most other actinides, which decay to stable isotopes of lead. Until the discovery of its alpha decay (with an extremely long half-life), the series was thought to end with bismuth-209.
In 2002, 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. 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).
Inventory in spent nuclear fuel is the only neptunium isotope produced in significant quantity in the nuclear fuel cycle, both by successive neutron capture by uranium-235 (which fissions most but not all of the time) and uranium-236, or (n,2n) reactions where a fast neutron occasionally knocks a neutron loose from uranium-238 or isotopes of plutonium. Over the long term, also forms in spent nuclear fuel as the decay product of americium-241.
is considered to be one of the most mobile radionuclides at the site of the Yucca Mountain nuclear waste repository (Nevada) where oxidizing conditions prevail in the unsaturated zone of the volcanic tuff above the water table.
Raw material for production