Plutonium

Plutonium, Pu, is the name of element 94. It forms in supernovae and neutron star mergers. One of its isotopes, ²⁴⁴Pu, has a half-life near 80 million years, which means it will be present in young solar systems. There is hard evidence that ²⁴⁴Pu once existed on earth in significant amounts.

Pu is also a terrestrial element - it is present in today's earth without the aid of scientists. Its most common isotope, ²³⁹Pu, is abundant enough in uranium ores to be readily detectable via alpha spectroscopy. Samples of one mineral, unofficially known as muromontite, have Pu concentrations exceeding 0.01 (per unit) of U concentrations. the mineral incorporates both Be and U, which means alpha decay of U contributes to the neutron flux which generates Pu. This accounts for its high concentration.

Wikipedia has an article which provides a lot of information about the element.  This article will focus on things Wikipedia does not stress: formation and the element's natural presence on earth..

NUCLEAR PROPERTIES

INFORMATION SOURCES

Japan Atomic Energy Agency (JAEA) maintains an on-line chart of nuclides which includes decay properties of many predicted nuclides⁽¹⁾ - unlike charts published by Korea Atomic Energy Research Institute (KAERI) or the (U.S.) National Nuclear Data Center (NNDC). This chart gives separate numerical values for partial half-lives against fission, beta emission (both b^- and b⁺), and alpha emission. This reference provides the most focused look at the most significant predicted Pu isotopes. Other references used are cited.

PREDICTED AND OBSERVED PROPERTIES

Isotopes from the neutron dripline down to ²⁴⁵Pu are predicted or observed to decay primarily by beta emission. Half lives are predicted to increase, as A declines, from around 0.001 second at the nucleus to 30 sec at ²⁵¹Pu. then peaking at 10 days in ²⁴⁶Pu.

²⁴⁴Pu decays almost entirely by alpha emission, but does have a fission branch with branch ratio of 0.0012 per unit. It has no beta-decay branch. Its half-life is 8.0e07 yr.

²⁴³Pu and ²⁴¹Pu decay by beta emission. ²⁴³Pu has a half-life around 5 hrs, but ²⁴¹Pu is long-lived for a beta-decaying nuclide, with a half-life of 14.3 yr.

²⁴²Pu, isotopes in the range ²⁴⁰Pu to ²³⁸Pu, and ²³⁶Pu decay by alpha emission almost exclusively. None has a beta-decay branch. Half-lives range from 375000 yr in ²⁴²Pu down to 3 yr in ²³⁶Pu.

Positive beta decay sets in at ²³⁷Pu, and is part of decay properties for all isotopes ²³⁵Pu and lighter. Half-lives drop, as A declines, from 45 days at ²³⁷Pu to 25 min at ²³⁵Pu to 1.1 sec at ²²⁹Pu⁽⁴⁾.

Ref. 1 & Ref. 3 predict decay properties for isotopes as light as ²¹³Pu. Increased stability is predicted at ²²⁰Pu and below, due to the neutron shell closure at N = 126. All these decay by either fission or alpha emission and have half lives under a few seconds.

OCCURRENCE

FORMATION

a) Outside Earth

Pu isotopes from the neutron dripline down to ²³⁹Pu can form via beta decay chains from initial nuclides originating in a disintegrating neutron star or, built up from lighter nuclides via rapid neutron capture. No lighter isotopes can form in this way, decay chains being blocked by U and by ²³⁷Np⁽⁴⁾.

b) On Earth

Another method exists by which Pu can form. Spontaneous fission of ²³⁸U produces free neutrons, although at a low rate. Some of these are captured by other U nuclei. This slow capture process can build all the isotopes of Pu which can form in supernovae and neutron star mergers (kilonovae). By far the most abundant isotope produced this way is ²³⁹Pu, which forms when a ²³⁸U nucleus captures a single neutron.

This simplicity allows an interesting piece of information to be estimated. Estimates of elemental composition of the crust is readily available by mass⁽⁵⁾, as are data for molar mass⁽⁶⁾. Concentration as mass/mass can thus be converted to moles/mass. Slow neutron cross sections by element (rather than isotope) are also available⁽⁷⁾, which allows total neutron capture cross section in a unit mass of earth's crust. From that the probability that a neutron emitted by a decaying ²³⁸U nucleus will be captured by a second ²³⁸U nucleus can be estimated to be 5.4E-06. The branch ratio for fission in ²³⁸U is 5.5E-07⁽⁸⁾ and its neutrons/fission ratio is 2.07⁽⁹⁾, so an effective "branch ratio" of 1.14E-06 connecting ²³⁸U activity (decays/time) to ²³⁹Pu formation rate can be used. Multiplying the fraction of emitted neutrons which are captured by ²³⁸U by this "branch ratio" gives 6.2E-12 as the ratio at which ²³⁹Pu forms. Equilibrium concentration ratio [²³⁹Pu]/[²³⁸U] can be computed from that by multiplying by t_1/2(²³⁹Pu)/t_1/2(²³⁸U) = 5.4E-06 or 3.3E-17 mol(Pu)/kg(overall) or 7.9E-18 kg(²³⁹Pu)/kg(crust). Given that bare sphere critical mass of ²³⁹Pu is 10 kg^(10),(11), ²³⁹Pu concentration can be expressed as 7.9E-19 bombs/kg(crust). Since crustal density is 2700 kg/m³, this is equivalent to 2.1E-15 bombs/m³ or 2.1E-06 bombs/km³. Anyone who wishes to make a ²³⁹Pu bomb need only mine the entire state of West Virginia to a depth of 7.5 meters to get enough for one. Thus does nature hide a dangerous toy in plain sight.

Outside the laboratory ²³⁸Pu forms in two ways. In an environment where gamma radiation is intense, the reaction gamma + ²³⁹Pu --> ²³⁸Pu + n [ or gamma(²³⁹Pu,²³⁸Pu)n ]. In an s process (slow neutron capture) environment, when a ²³⁵U nucleus captures a neutron, it forms an excited state of ²³⁶U. In roughly 80% of cases, this excited nucleus fissions. In about 20% of cases, it emits a gamma to become ²³⁶U in its ground state. This nucleus can then capture a second neutron to become ²³⁷U, which decays to ²³⁷Np. ²³⁷Np can then capture a neutron to become ²³⁸Np, which decays to ²³⁸Pu. Little of it forms, but it does exist outside the laboratory.

PERSISTENCE

As is true of the other elements, most Pu isotopes are short lived. Only 7; ²⁴⁴Pu, ²⁴²Pu through ²³⁸Pu, and ²³⁶Pu persist for more than 10 yr. Three of these (²⁴¹Pu, ²³⁸Pu, and ²³⁶Pu) will enter the expanding, cooling diffuse remnant produced by a supernova or neutron star merger (kilonova), but will not survive to become incorporated into a molecular cloud core (the predecessor of a star). Two isotopes more (²⁴⁰Pu, and ²³⁹Pu) can survive to the planetesimal stage of stellar system formation, Only two survive long enough to become part of fully-formed planets: ²⁴²Pu will survive about 38 million years after an event which forms it, while ²⁴⁴Pu, will still be present at 0.001 of its original concentration 800 Myr after it forms. Earth is a little less than 60 ²⁴⁴Pu half-lives old. It is still present at concentration above one nucleus per mole of initial nuclei. It is sometimes classed as a "primordial nuclide". Efforts to observe it have not been successful. Evidence that ²⁴⁴Pu existed in the early solar system is strong⁽¹²⁾.

The amount of Pu produced during a supernova, neutron star merger, or comparable event will be comparable with the amount of uranium produced. Even at three orders of magnitude down, there may be enough ²⁴⁴Pu present to contribute to heating in young solar systems.

ATOMIC PROPERTIES

Wikipedia's article "Plutonium" addresses the element's atomic properties and chemistry in some detail. The element is spectacular for its oddities, and disturbing for its pyrophoricity.

REFERENCES

1. "Chart of the Nuclides, 2014", Japan Atomic Energy Agency; website available using "chart of nuclides" and "JAEA" as internet search terms.

2. "Nuclear Properties for Astrophysical Applications"; P. Moller & J. R. Nix; Los Alamos National Laboratory website; search by "LANL, T2", then "Nuclear Properties for Astrophysical Applications".

3. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011.

4. "Isotopes of Plutonium", Wikipedia article.

5. "Abundances of the elements (data page)" in Wikipedia summarizes the data and cites references.

6. most chemistry books

7. "Neutron scattering lengths and cross sections"; NIST Center for Neutron Research; www.ncnr.nist.gov/resources

8. "Isotopes of Uranium" in Wikipedia provides original sources.

9. "Fundamentals of Nuclear Science and Engineering", p 141 (Table 6.2); Shultis, J Kenneth, Richard E. Faw; CRC Press; ISBN 978-1-4200-5135-3; 2008.

10. "Critical Mass", Section 4; Wikipedia.

11. "Expensive Shoebox" in "What If"; xkcd.

12. "Plutonium 244 in the Early Solar System and the Pre-Fermi Natural Reactor; P.K. Kuroda; Geochemical Journal, Vol. 26, pp. 1 to 20; 1992.

9-Period Periodic Table of Elements
1	1 H																																																																	2 He
2	3 Li	4 Be																																																																	5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg																																																																	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca																																																																	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Ge	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr																																																																	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba																																																					57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra																																																					89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Cn	113 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og
8	119 Uue	120 Ubn																															121 Ubu	122 Ubb	123 Ubt	124 Ubq	125 Ubp	126 Ubh	127 Ubs	128 Ubo	129 Ube	130 Utn	131 Utu	132 Utb	133 Utt	134 Utq	135 Utp	136 Uth	137 Uts	138 Uto	139 Ute	140 Uqn	141 Uqu	142 Uqb	143 Uqt	144 Uqq	145 Uqp	146 Uqh	147 Uqs	148 Uqo	149 Uqe	150 Upn	151 Upu	152 Upb	153 Upt	154 Upq	155 Upp	156 Uph	157 Ups	158 Upo	159 Upe	160 Uhn	161 Uhu	162 Uhb	163 Uht	164 Uhq	165 Uhp	166 Uhh	167 Uhs	168 Uho	169 Uhe	170 Usn	171 Usu	172 Usb
9	173 Ust	174 Usq
Alkali metal Alkaline earth metal Lanthanide Actinide Superactinide Transition metal Post-transition metal Metalloid Other nonmetal Halogen Noble gas predicted predicted predicted predicted predicted predicted predicted predicted predicted

(11-27-20)