Neptunium Neptunium is a chemical element with the Symbol (chemistry), symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after ...

(₉₃Np) is usually considered an

artificial element A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; ...

, although trace quantities are found in nature, so a

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, is ...

cannot be given. Like all trace or artificial elements, it has no stable isotopes. The first

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

to be synthesized and identified was ²³⁹Np in 1940, produced by bombarding with

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s to produce , which then underwent

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

to . Trace quantities are found in nature from

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

reactions by

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

atoms, a fact not discovered until 1951. Twenty-five neptunium

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s have been characterized, with the most stable being with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 2.14 million years, with a half-life of 154,000 years, and with a half-life of 396.1 days. All of the remaining

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

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 state A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ha ...

s, with the most stable being (t_1/2 22.5 hours). The isotopes of neptunium range from to , though the intermediate isotope has not yet been observed. The primary

decay mode Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

before the most stable isotope, , is

electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Thi ...

(with a good deal of

alpha emission Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

), and the primary mode after is

beta emission In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For exam ...

. The primary

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...

s before are

isotopes of uranium Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay ...

and

protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...

, and the primary products after are

isotopes of plutonium Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long ...

Uranium-237 Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The decay pro ...

and neptunium-239 are regarded as the leading hazardous radioisotopes in the first hour-to-week period following

nuclear fallout Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...

from a nuclear detonation, with ²³⁹Np dominating "the spectrum for several days."

List of isotopes

, - , , style="text-align:right" , 93 , style="text-align:right" , 126 , 219.03162(9) , 0.15(+0.72-0.07) ms , α , ²¹⁵Pa , (9/2−) , , - , , style="text-align:right" , 93 , style="text-align:right" , 127 , 220.03254(21)# , 25(+14-7) μs , α , ²¹⁶Pa , 1-# , , - , , style="text-align:right" , 93 , style="text-align:right" , 129 , , 380(+260-110) ns , α , ²¹⁸Pa , 1-# , , - , , style="text-align:right" , 93 , style="text-align:right" , 130 , 223.03285(21)# , 2.15(+100-52) μs , α , ²¹⁹Pa , 9/2− , , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 131 , rowspan=2, 224.03422(21)# , rowspan=2, 38(+26-11) μs , α (83%) , ^220m1Pa , rowspan=2, 1-# , rowspan=2, , - , α (17%) , ^220m2Pa , - , , style="text-align:right" , 93 , style="text-align:right" , 132 , 225.03391(8) , 6(5) ms , α , ²²¹Pa , 9/2−# , , - , , style="text-align:right" , 93 , style="text-align:right" , 133 , 226.03515(10)# , 35(10) ms , α , ²²²Pa , , , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 134 , rowspan=2, 227.03496(8) , rowspan=2, 510(60) ms , α (99.95%) , ²²³Pa , rowspan=2, 5/2−# , rowspan=2, , - , β⁺ (.05%) , ²²⁷U , - , rowspan=3, , rowspan=3 style="text-align:right" , 93 , rowspan=3 style="text-align:right" , 135 , rowspan=3, 228.03618(21)# , rowspan=3, 61.4(14) s , β⁺ (59%) , ²²⁸U , rowspan=3, , rowspan=3, , - , α (41%) , ²²⁴Pa , - , β⁺, SF (.012%) , (various) , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 229.03626(9) , rowspan=2, 4.0(2) min , α (51%) , ²²⁵Pa , rowspan=2, 5/2+# , rowspan=2, , - , β⁺ (49%) , ²²⁹U , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 137 , rowspan=2, 230.03783(6) , rowspan=2, 4.6(3) min , β⁺ (97%) , ²³⁰U , rowspan=2, , rowspan=2, , - , α (3%) , ²²⁶Pa , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 231.03825(5) , rowspan=2, 48.8(2) min , β⁺ (98%) , ²³¹U , rowspan=2, (5/2)(+#) , rowspan=2, , - , α (2%) , ²²⁷Pa , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 139 , rowspan=2, 232.04011(11)# , rowspan=2, 14.7(3) min , β⁺ (99.99%) , ²³²U , rowspan=2, (4+) , rowspan=2, , - , α (.003%) , ²²⁸Pa , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 140 , rowspan=2, 233.04074(5) , rowspan=2, 36.2(1) min , β⁺ (99.99%) , ²³³U , rowspan=2, (5/2+) , rowspan=2, , - , α (.001%) , ²²⁹Pa , - , , style="text-align:right" , 93 , style="text-align:right" , 141 , 234.042895(9) , 4.4(1) d , β⁺ , ²³⁴U , (0+) , , - , rowspan=2 style="text-indent:1em" , , rowspan=2 colspan="3" style="text-indent:2em" , , rowspan=2, ~9 min , IT , ²³⁴Np , rowspan=2, 5+ , rowspan=2, , - , EC , ²³⁴U , - , rowspan=2, , rowspan=2 style="text-align:right" , 93 , rowspan=2 style="text-align:right" , 142 , rowspan=2, 235.0440633(21) , rowspan=2, 396.1(12) d , EC , ''²³⁵U'' , rowspan=2, 5/2+ , rowspan=2, , - , α (.0026%) , ²³¹Pa , - , rowspan=3,

Fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...

nuclide , rowspan=3 style="text-align:right" , 93 , rowspan=3 style="text-align:right" , 143 , rowspan=3, 236.04657(5) , rowspan=3, 1.54(6)×10⁵ y , EC (87.3%) , ²³⁶U , rowspan=3, (6−) , rowspan=3, , - , β⁻ (12.5%) , ²³⁶Pu , - , α (.16%) , ²³²Pa , - , rowspan=2 style="text-indent:1em" , , rowspan=2 colspan="3" style="text-indent:2em" , 60(50) keV , rowspan=2, 22.5(4) h , EC (52%) , ²³⁶U , rowspan=2, 1 , rowspan=2, , - , β⁻ (48%) , ²³⁶Pu , - , rowspan=3, Most common nuclide , rowspan=3 style="text-align:right" , 93 , rowspan=3 style="text-align:right" , 144 , rowspan=3, 237.0481734(20) , rowspan=3, 2.144(7)×10⁶ y , α , ²³³Pa , rowspan=3, 5/2+ , rowspan=3, TraceProduced by

in uranium ore , - , SF (2×10⁻¹⁰%) , (various) , - , CD (4×10⁻¹²%) , ²⁰⁷Tl
³⁰Mg , - , , style="text-align:right" , 93 , style="text-align:right" , 145 , 238.0509464(20) , 2.117(2) d , β⁻ , ²³⁸Pu , 2+ , , - , style="text-indent:1em" , , colspan="3" style="text-indent:2em" , 2300(200)# keV , 112(39) ns , , , , , - , , style="text-align:right" , 93 , style="text-align:right" , 146 , 239.0529390(22) , 2.356(3) d , β⁻ , ²³⁹Pu , 5/2+ , Trace , - , , style="text-align:right" , 93 , style="text-align:right" , 147 , 240.056162(16) , 61.9(2) min , β⁻ , ²⁴⁰Pu , (5+) , TraceIntermediate decay product of ²⁴⁴Pu , - , rowspan=2 style="text-indent:1em" , , rowspan=2 colspan="3" style="text-indent:2em" , 20(15) keV , rowspan=2, 7.22(2) min , β⁻ (99.89%) , ²⁴⁰Pu , rowspan=2, 1(+) , rowspan=2, , - , IT (.11%) , ²⁴⁰Np , - , , style="text-align:right" , 93 , style="text-align:right" , 148 , 241.05825(8) , 13.9(2) min , β⁻ , ²⁴¹Pu , (5/2+) , , - , , style="text-align:right" , 93 , style="text-align:right" , 149 , 242.06164(21) , 2.2(2) min , β⁻ , ²⁴²Pu , (1+) , , - , style="text-indent:1em" , , colspan="3" style="text-indent:2em" , 0(50)# keV , 5.5(1) min , , , 6+# , , - , , style="text-align:right" , 93 , style="text-align:right" , 150 , 243.06428(3)# , 1.85(15) min , β⁻ , ²⁴³Pu , (5/2−) , , - , , style="text-align:right" , 93 , style="text-align:right" , 151 , 244.06785(32)# , 2.29(16) min , 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: *Alpha emission: the

decay energy The decay energy is the energy change of a nucleus having undergone a radioactive decay. Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy ...

is 5.2 MeV and the decay product is

protactinium-231 Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32, ...

. *Electron capture: the decay energy is 0.125 MeV and the decay product is

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...

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 Decay may refer to: Science and technology * Bit decay, in computing * Software decay, in computing * Distance decay, in geography * Decay time (fall time), in electronics Biology * Decomposition of organic matter * Tooth decay (dental caries ...

by the following methods: *Electron capture: the decay energy is 0.93 MeV and the decay product is

uranium-236 Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the rep ...

. This usually decays (with a half-life of 23 million years) to

thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay ...

. *Beta emission: the decay energy is 0.48 MeV and the decay product is

plutonium-236 Plutonium (94Pu) is an artificial element, except for trace quantities resulting from neutron capture by uranium, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. It was synthesized long b ...

. This usually decays (half-life 2.8 years) to

uranium-232 Uranium-232 () is an isotope of uranium. It has a half-life of around 69 years and is a side product in the thorium cycle. It has been cited as an obstacle to nuclear proliferation using 233U as the fissile material, because the intense gamm ...

, which usually decays (half-life 69 years) to

thorium-228 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, thorium-232, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer t ...

, which decays in a few years to

lead-208 Lead (82Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uraniu ...

. *Alpha emission: the decay energy is 5.007 MeV and the decay product is

protactinium-232 Protactinium (91Pa) has no stable isotopes. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32, ...

. 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 In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...

material with a

critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissi ...

of .Final Report, Evaluation of nuclear criticality safety data and limits for actinides in transport
, Republic of France, Institut de Radioprotection et de Sûreté Nucléaire, Département de Prévention et d'étude des Accidents. 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 as a nuclear fuel in weapons or reactors. Nevertheless, has been considered for use in

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

and as a

radioactive tracer A radioactive tracer, radiotracer, or radioactive label is a chemical compound in which one or more atoms have been replaced by a radionuclide so by virtue of its radioactive decay it can be used to explore the mechanism of chemical reactions by ...

, 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 In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Iso ...

. The most favorable reactions to accumulate were shown to be

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

and

deuteron Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one n ...

irradiation of

uranium-238 Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it ...

Neptunium-237

decays via the

neptunium series In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directly ...

, which terminates with

thallium-205 Thallium (81Tl) has 41 isotopes with atomic masses that range from 176 to 216. 203Tl and 205Tl are the only stable isotopes and 204Tl is the most stable radioisotope with a half-life of 3.78 years. 207Tl, with a half-life of 4.77 minutes, has the ...

, which is stable, unlike most other

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...

s, which decay to stable

isotopes of lead Lead (82Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium ...

. In 2002, was shown to be capable of sustaining a chain reaction with

fast neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

s, as in a

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

, with a critical mass of around 60 kg. However, it has a low probability of fission on bombardment with

thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...

s, which makes it unsuitable as a fuel for light water nuclear power plants (as opposed to

fast reactor A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed t ...

accelerator-driven system A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general c ...

s, for example). is the only neptunium isotope produced in significant quantity in the

nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in w ...

, both by successive

(which fissions most but not all of the time) and

, or (n,2n) reactions where a

occasionally knocks a neutron loose from

. Over the long term, also forms in

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...

as the decay product of

americium-241 Americium-241 (, Am-241) is an isotope of americium. Like all isotopes of americium, it is radioactive, with a half-life of . is the most common isotope of americium as well as the most prevalent isotope of americium in nuclear waste. It is com ...

. was projected to be one of the most mobile

nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state. The word ''nuclide'' was coined by Truman ...

s at the

Yucca Mountain nuclear waste repository The Yucca Mountain Nuclear Waste Repository, as designated by the Nuclear Waste Policy Act amendments of 1987, is a proposed deep geological repository storage facility within Yucca Mountain for spent nuclear fuel and other high-level radioac ...

Use in plutonium-238 production

When exposed to neutron bombardment can capture a neutron, undergo beta decay, and become , this product being useful as a thermal energy source in a

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...

for the production of electricity and heat in deep space probes (such as the

New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...

and Voyager probes) and, of recent note, the

Mars Science Laboratory Mars Science Laboratory (MSL) is a robotic spacecraft, robotic space probe mission to Mars launched by NASA on November 26, 2011, which successfully landed ''Curiosity (rover), Curiosity'', a Mars rover, in Gale (crater), Gale Crater on August ...

(Curiosity rover). 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.

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Navbox element isotopes Neptunium