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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]
Nuclide [n 1] |
Z | N |
Isotopic mass (
Da)
[4] [n 2] [n 3] |
Half-life |
Decay mode [n 4] |
Daughter isotope [n 5] |
Spin and parity [n 6] [n 7] |
Isotopic abundance | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Excitation energy [n 7] | |||||||||||||||||||
219 Np [5] [n 8] |
93 | 126 | 219.03162(9) | 0.15+0.72 −0.07 ms |
α | 215Pa | (9/2−) | ||||||||||||
220 Np [3] |
93 | 127 | 220.03254(21)# | 25+14 −7 μs |
α | 216Pa | 1−# | ||||||||||||
222 Np [6] |
93 | 129 | 380+260 −110 ns |
α | 218Pa | 1-# | |||||||||||||
223 Np [7] |
93 | 130 | 223.03285(21)# | 2.15+100 −52 μs |
α | 219Pa | 9/2− | ||||||||||||
224 Np [8] |
93 | 131 | 224.03422(21)# | 38+26 −11 μs |
α (83%) | 220m1Pa | 1−# | ||||||||||||
α (17%) | 220m2Pa | ||||||||||||||||||
225 Np |
93 | 132 | 225.03391(8) | 6(5) ms | α | 221Pa | 9/2−# | ||||||||||||
226 Np |
93 | 133 | 226.03515(10)# | 35(10) ms | α | 222Pa | |||||||||||||
227 Np |
93 | 134 | 227.03496(8) | 510(60) ms | α (99.95%) | 223Pa | 5/2−# | ||||||||||||
β+ (.05%) | 227U | ||||||||||||||||||
228 Np |
93 | 135 | 228.03618(21)# | 61.4(14) s | β+ (59%) | 228U | |||||||||||||
α (41%) | 224Pa | ||||||||||||||||||
β+, SF (.012%) | (various) | ||||||||||||||||||
229 Np |
93 | 136 | 229.03626(9) | 4.0(2) min | α (51%) | 225Pa | 5/2+# | ||||||||||||
β+ (49%) | 229U | ||||||||||||||||||
230 Np |
93 | 137 | 230.03783(6) | 4.6(3) min | β+ (97%) | 230U | |||||||||||||
α (3%) | 226Pa | ||||||||||||||||||
231 Np |
93 | 138 | 231.03825(5) | 48.8(2) min | β+ (98%) | 231U | (5/2)(+#) | ||||||||||||
α (2%) | 227Pa | ||||||||||||||||||
232 Np |
93 | 139 | 232.04011(11)# | 14.7(3) min | β+ (99.99%) | 232U | (4+) | ||||||||||||
α (.003%) | 228Pa | ||||||||||||||||||
233 Np |
93 | 140 | 233.04074(5) | 36.2(1) min | β+ (99.99%) | 233U | (5/2+) | ||||||||||||
α (.001%) | 229Pa | ||||||||||||||||||
234 Np |
93 | 141 | 234.042895(9) | 4.4(1) d | β+ | 234U | (0+) | ||||||||||||
234m Np |
~9 min [9] | IT | 234Np | 5+ | |||||||||||||||
EC | 234U | ||||||||||||||||||
235 Np |
93 | 142 | 235.0440633(21) | 396.1(12) d | EC | 235U | 5/2+ | ||||||||||||
α (.0026%) | 231Pa | ||||||||||||||||||
236 Np [n 9] |
93 | 143 | 236.04657(5) | 1.54(6)×105 y | EC (87.3%) | 236U | (6−) | ||||||||||||
β− (12.5%) | 236Pu | ||||||||||||||||||
α (.16%) | 232Pa | ||||||||||||||||||
236m Np |
60(50) keV | 22.5(4) h | EC (52%) | 236U | 1 | ||||||||||||||
β− (48%) | 236Pu | ||||||||||||||||||
237 Np [n 10] |
93 | 144 | 237.0481734(20) | 2.144(7)×106 y | α | 233Pa | 5/2+ | Trace [n 11] | |||||||||||
SF (2×10−10%) | (various) | ||||||||||||||||||
CD (4×10−12%) | 207Tl 30Mg | ||||||||||||||||||
238 Np |
93 | 145 | 238.0509464(20) | 2.117(2) d | β− | 238Pu | 2+ | ||||||||||||
238m Np |
2300(200)# keV | 112(39) ns | |||||||||||||||||
239 Np |
93 | 146 | 239.0529390(22) | 2.356(3) d | β− | 239Pu | 5/2+ | Trace [n 11] | |||||||||||
240 Np |
93 | 147 | 240.056162(16) | 61.9(2) min | β− | 240Pu | (5+) | Trace [n 12] | |||||||||||
240m Np |
20(15) keV | 7.22(2) min | β− (99.89%) | 240Pu | 1(+) | ||||||||||||||
IT (.11%) | 240Np | ||||||||||||||||||
241 Np |
93 | 148 | 241.05825(8) | 13.9(2) min | β− | 241Pu | (5/2+) | ||||||||||||
242 Np |
93 | 149 | 242.06164(21) | 2.2(2) min | β− | 242Pu | (1+) | ||||||||||||
242m Np |
0(50)# keV | 5.5(1) min | 6+# | ||||||||||||||||
243 Np |
93 | 150 | 243.06428(3)# | 1.85(15) min | β− | 243Pu | (5/2−) | ||||||||||||
244 Np |
93 | 151 | 244.06785(32)# | 2.29(16) min | β− | 244Pu | (7−) | ||||||||||||
This table header & footer: |
CD: | Cluster decay |
EC: | Electron capture |
IT: | Isomeric transition |
SF: | Spontaneous fission |
Actinides [10] by decay chain |
Half-life range ( a) |
Fission products of 235U by yield [11] | ||||||
---|---|---|---|---|---|---|---|---|
4n | 4n + 1 | 4n + 2 | 4n + 3 | 4.5–7% | 0.04–1.25% | <0.001% | ||
228Ra№ | 4–6 a | 155Euþ | ||||||
244Cmƒ | 241Puƒ | 250Cf | 227Ac№ | 10–29 a | 90Sr | 85Kr | 113mCdþ | |
232Uƒ | 238Puƒ | 243Cmƒ | 29–97 a | 137Cs | 151Smþ | 121mSn | ||
248Bk [12] | 249Cfƒ | 242mAmƒ | 141–351 a |
No fission products have a half-life | ||||
241Amƒ | 251Cfƒ [13] | 430–900 a | ||||||
226Ra№ | 247Bk | 1.3–1.6 ka | ||||||
240Pu | 229Th | 246Cmƒ | 243Amƒ | 4.7–7.4 ka | ||||
245Cmƒ | 250Cm | 8.3–8.5 ka | ||||||
239Puƒ | 24.1 ka | |||||||
230Th№ | 231Pa№ | 32–76 ka | ||||||
236Npƒ | 233Uƒ | 234U№ | 150–250 ka | 99Tc₡ | 126Sn | |||
248Cm | 242Pu | 327–375 ka | 79Se₡ | |||||
1.53 Ma | 93Zr | |||||||
237Npƒ | 2.1–6.5 Ma | 135Cs₡ | 107Pd | |||||
236U | 247Cmƒ | 15–24 Ma | 129I₡ | |||||
244Pu | 80 Ma |
... nor beyond 15.7 Ma [14] | ||||||
232Th№ | 238U№ | 235Uƒ№ | 0.7–14.1 Ga | |||||
|
Neptunium-235 has 142 neutrons and a half-life of 396.1 days. This isotope decays by:
This isotope of neptunium has a weight of 235.044 063 3 u.
Neptunium-236 has 143 neutrons and a half-life of 154,000 years. It can decay by the following methods:
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), [15] though precise experimental data is not available. [16]
236
Np
is produced in small quantities via the (n,2n) and (γ,n) capture reactions of 237
Np
,
[17] however, it is nearly impossible to separate in any significant quantities from its parent 237
Np
.
[18] 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.
[16] 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.
[19] 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.
[19]
237
Np
decays via the
neptunium series, which terminates with
thallium-205, which is stable, unlike most other
actinides, which decay to stable
isotopes of lead.
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.
[20] 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).
237
Np
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, 237
Np
also forms in
spent nuclear fuel as the decay product of
americium-241.
237
Np
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.
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. [21]
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 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. [22]
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." [23] [24]