Curium

Curium, Cm, is the name of element 96. Its isotope ²⁴⁷Cm has the highest Z and A of any nuclide which has a geologically long half-life, 1.56E07 yr. There is evidence that ²⁴⁷Cm was present early in the formation of our solar system. Cm is also a terrestrial element - it is present in today's earth without the aid of scientists.

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 Cm isotopes. Other references used are cited.

PREDICTED AND OBSERVED PROPERTIES

Isotopes from the neutron dripline down to ²⁵³Cm are predicted to decay primarily by beta emission. Half lives are predicted to increase, as A declines, from around 0.001 sec at the dripline to 2 min at ²⁵⁵Cm and staying below 1 hr in ²⁵⁴Cm and ²⁵³Cm.

²⁵²Cm is predicted to decay almost entirely by fission, and without a beta-decay branch. Comparison with observed ²⁵⁰Cm indicates that there may be a weak beta decay branch from ²⁵²Cm. (See "Californium", this Wiki.) (Note: see Ref, 5.)

²⁵¹Cm and ²⁴⁹Cm decay by beta emission. Their half-lives are short, compared to adjacent Cm isotopes.

²⁵⁰Cm has been observed to decay predominantly by fission, but with significant alpha decay and beta decay branches. At 8300 yr, it has a very long half-life. It appears to be the longest-lived nuclide which decays mainly by fission.

Between ²⁴⁸Cm and ²⁴⁴Cm, all isotopes decay predominantly by alpha emission. ²⁴⁸Cm has a branch ratio of 0.083 for fission, but fission branch ratio of the others is under 0.002. None of these isotopes have a beta-decay branch. Isotopes in this band are notable for their long half-lives, Only ²⁴⁴Cm has a half-life under 5000 yr, and ²⁴⁷Cm has a half-life of 1.56E07 yr.

Positive beta decay sets in at ²⁴³Cm. As A declines, half-lives decline to 27 sec for the lightest observed isotope, ²³³Cm.

Ref 1 predicts isotopes as light as ²¹⁹Cm. It also predicts stabilization at and below ²²²Cm, a consequence of neutron shell closure at N = 126. Ref 3 extends that prediction to ²¹⁷Cm. Half-lives of these very light isotopes are all short.

OCCURRENCE

FORMATION

a) Outside Earth

Cm isotopes from the neutron dripline down to ²⁴⁵Cm can form via rapid neutron capture and fission infall, followed by beta decay. This includes all the long-lived isotopes in the band ²⁴⁸Cm to ²⁴⁵Cm. This includes the very long-lived ²⁴⁷Cm and ²⁴⁸Cm.

²⁴⁴Cm and lighter isotopes are blocked from forming via rapid neutron capture & beta decay⁽⁴⁾.

While Cm is produced in quantity in neutron star mergers and supernovae, all but two of its isotopes will vanish within the geologically short time of a million years. ²⁴⁷Cm and ²⁴⁸Cm will last long enough to be incorporated into planets. Evidence that ²⁴⁷Cm once existed in quantity in the solar system is available from meteorites. Curium has also been detected in the atmosphere of one star, HD101065 (Przybylski's star) and the observation of Pm spectral lines in GY Andromeda indicates that Cm is also present (although not reported). Neither star is producing the element in its interior because transporting material to the surface would take longer than the element lasts. Kilonova (neutron star merger) remnants have low mass, and will quickly become invisible. It appears likely that these two stars have hit a kilonova remnant.

b) On Earth

Spontaneous fission of ²³⁸U produces free neutrons. This small flux of neutrons will produce Cm via neutron capture, first to Pu, then to Am, and finally to Cm. (Wikipedia's article "Curium" includes a reaction network for synthesis of ²⁴⁴Cm and ²⁴⁵Cm.) Although not shown on that network, neutron capture by ²⁴¹Am can produce (branch ration around 0.1) ^242m1Am, which has a beta-decay branch to ²⁴²Cm (where another neutron capture leads to ²⁴³Cm). Slow neutron capture can generate light isotopes of Cm which cannot form via beta-decay chains from the dripline. Concentration of Cm will be many orders of magnitude smaller than uranium concentration; so only geologically-active rocky bodies like the earth, which can concentrate uranium, will contain significant quantities of Cm.

Naturally-occurring terrestrial curium has not been detected. Unlike Bk and higher actinides, this is probably not due to the small concentration of the element, but to interference. Nuclear tests and leaks have distributed synthetic curium widely. Any effort to detect "natural" Cm must eliminate possible contamination by synthetic curium.

PERSISTENCE

The amount of Cm produced during a supernova, neutron star merger, or comparable event will only be a few orders of magnitude less than the amount of uranium produced. This material includes all the long-lived isotopes from ²⁵⁰Cm to ²⁴⁵Cm.

²⁴⁷Cm will retain 1% of its initial activity for around 100 million years. That's long enough for ²⁴⁷Cm to remain throughout the process of planet formation. There is evidence that it was present early in the formation of our own⁽⁶⁾.

A second isotope, ²⁴⁸Cm, is still present at the atoms per initial mole level for nearly 30 Myr. It, too, is expected to be present during the initial phases of planet formation. ²⁴⁶Cm will last close to 0.4 Myr, while ²⁵⁰Cm 250 and ²⁴⁵Cm 245 will persist for close to 0.7 Myr. They are unlikely to survive long enough to be incorporated in planetesimals. Other isotopes which can form have half-lives of up to an hour, which means they are gone in 4 days..

Slow neutron capture in rocky bodies active enough to concentrate uranium can produce Cm 242 and heavier isotopes up to Cm 249. This production continues as long as U remains.

ATOMIC PROPERTIES

Wikipedia's article "Curium" addresses the element's atomic properties and chemistry in some detail. Some Cm is present at all stages in the return of material from a supernova or neutron star merger to the interstellar medium - and its later collapse into stars and planets. It will be cool enough during most of that process for Cm to be able to interact chemically. Near the end, some Cm may be incorporated into refractory grains (presolar grains) which form in close proximity to young stars. Cm has an extensive, if not terrestrial, natural chemistry.

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 Curium", Wikipedia article.

5. A table of nuclides maintained on-line by KAERI (Korea's atomic energy agency.) reports short life and decay by beta emission for Cm 252. That appears to be a prediction, not observed data. It supports the idea of a beta-decay branch for Cm 252.

6. "Origin of Uranium Isotope Variations in Early Solar Nebula Condensates"; François L. H. Tissot, Nicolas Dauphas, and Lawrence Grossman; Science Advances, Vol 2, No. 3; 04-Mar 2016; DOI: 10.1126 / sciadv.1501400

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

(11-25-20)