Californium

Californium, Cf, is the name of element 98. 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..

Cf forms in supernovae and neutron star mergers via rapid neutron capture followed by chains of beta decays. ²⁴⁹Cf and heavier isotopes, except maybe for ²⁵²Cf, can form in this way. Significantly, the isotopes ²⁵¹Cf and ²⁴⁹Cf, with half-lives of 900 and 351 yr respectively, can form. Cf can be expected to be present in supernova or kilonova (neutron star merger) remnants. The element may be observable in young remnants. It will become cool enough to have a natural chemistry.

On earth, neutrons from ²³⁸U spontaneous fission drive a slow process of neutron capture followed by beta decay followed by another capture. This process builds from ²³⁸U itself, and produces the isotopes ²⁵²Cf through ²⁴⁹Cf. Their concentration in the earth is too low to be detectable.

Synthetic Cf is largely ²⁵²Cf. Its half life of 2.6 yr is long enough to make it practical for use without being difficult to dispose of, while its 3% fission branch ratio makes it a good neutron source.

NUCLEAR PROPERTIES

If transmuting from nuclide (Z₁,A₁) into (Z₂,A₂) releases energy, there is some chance that the transmutation will occur. In some cases, the transmutation is likely, in others, it is rare. With large nuclides, there are usually four kinds of transmutation available. If they are neutron-rich, neutrons are heavier than protons. Turning a neutron into a proton releases energy, so these nuclides will beta-decay, releasing energy in the form of an electron and an antineutrino. If they are proton rich, they have three main choices, The first is positive beta decay Turning a proton into a neutron reduces electrical potential energy enough to pay the mass-energy debt the transformation requires. There are two ways to do this, positron emission and electron capture. In both cases, beta (b-) and positive beta (b+) decay half-lives increase as energy yield declines. Heavy isotopes of any Z try to decay by (b-) emission. As A goes down, half-lives increase until the reaction becomes impossible. Then, after a gap of at least one isotope, (b+) decay sets in and becomes increasingly short-lived in the lightest isotopes.

²⁵⁵Cf and ²⁵³Cf decay by beta emission, indicating that the valley of beta stability is nearing its maximum A. ²⁵⁶Cf fissions, which makes use of predicted data necessary for heavy isotopes.

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 Cf isotopes. Other references used are cited.

PREDICTED AND OBSERVED PROPERTIES

Isotopes from the neutron dripline down to ²⁵⁹Cf 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 ²⁶²Cf and peaking around 15 min at ²⁵⁹Cf.

²⁵⁸Cf is predicted to decay primarily by fission with a half-life near 6 days. It is not predicted to have any beta-decay branch.

²⁵⁷Cf is predicted to decay primarily by beta emission, with a half-life around 2.5 days. It is predicted to have weak fission and alpha-emission branches..

Among observed isotopes, ²⁵⁵Cf and ²⁵³Cf decay mainly by beta emission. ²⁵³Cf is longer-lived at around 40 day half-life.

²⁵⁶Cf, ²⁵⁴Cf, ²⁵²Cf, and all isotopes lighter than ²⁵²Cf are not observed to decay by beta emission.

The longest-lived Cf isotopes are found in the band ²⁵²Cf to ²⁴⁸Cf, all of which decay principally by alpha emission. Most have an observed fission decay branch, and none have a positive beta decay branch. Peak half-lives occur at ²⁵¹Cf (900 yr) and ²⁴⁹Cf (351 yr).

A positive beta decay branch has been observed at ²⁴⁷Cf. This and lighter isotopes decay primarily by alpha and positive beta decay. Fission plays a minor role, except in the lightest isotopes.

Isotopes as light as ²³⁷Cf have been observed. Additional isotopes down to ²²⁵Cf are predicted in Ref. 1.

Ref. 3 predicts that an a set of isotopes, ²²⁷Cf to ²²¹Cf, will have half-lives exceeding 10^-09 sec. All are short lived, with half-lives under 0.001 sec. These isotopes are stabilized by the N = 126 neutron shell closure.

OCCURRENCE

FORMATION

a) Outside Earth

It appears likely that Cf isotopes from the neutron dripline down to ²⁵³Cf can form. In this band, even-N isotopes have no beta decay branch. They decay primarily by fission, and their half-lives rise as A declines, reaching a peak of 60.5 days at ²⁵⁴Cf,

Ref. 1 does not predict a beta-decay branch for ²⁵²Cm, which implies ²⁵²Cf cannot form. ²⁵⁰Cm, on the other hand has an observed beta-decay branch ratio BR(b) = 0.08, From ²⁵⁰Cm data, it is possible to construct partial half-lives for its various decay modes via t12(m) = t12/BR(m), where t12 is half-life, t12(m) is partial half-life against decay by mode m, and BR(m) is branch ratio for mode m. When ²⁵²Cm and ²⁵⁰Cm are compared in this way their alpha decay half-lives are to be expected, and the sudden drop in fission half-life not surprising. Beta decay behavior of ²⁵²Cm is uncertain. It would normally have a shorter half-life against beta decay than ²⁵⁰Cm, which implies some formation of ²⁵²Cf Perhaps the best that can be said is that very small amounts of ²⁵²Cf can form.

The isotopes ²⁵¹Cf to ²⁴⁹Cf can form. With half-lives of 900, 13.1, and 351 yr respectively, these are by far the longest-lived Cf isotopes.

²⁴⁸Cf and lighter isotopes are blocked from forming via rapid neutron capture & beta decay⁽⁴⁾.

Both high-A nuclides ejected during a neutron star merger and rapid neutron capture contribute to the production of those Cf isotopes which can form. Slow neutron capture can also produce Cf isotopes in the ²⁵²Cf to ²⁴⁹Cf band.

b) On Earth

While Cf is produced in some quantity in neutron star mergers and supernovae, those nuclides will decay quickly. After a short time, the only Cf remaining in a body will be what is produced continuously by slow neutron capture. Spontaneous fission of ²³⁸U produces free neutrons. This small flux of neutrons will produce isotopes in the ²⁵⁰Cf to ²⁴⁹Cf band. Concentration of Cf 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 potentially-detectable quantities of Cf.

PERSISTENCE

The amount of Cf 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. Most of it decays until it either vanishes or comes into equilibrium with the continuous, slow production resulting from ²³⁸U fission.

With its half-life of 900 yr, ²⁵¹Cf may persist, in principle, for around 1.6E05 yrs, although a more realistic figure would be 70000 yrs. This is sufficient time for a supernova or kilonova (neutron star merger) remnant to become part of a protostar.

Cf persists long enough to participate in dust formation and other chemical interactions in interstellar space.

Slow neutron capture in rocky bodies active enough to concentrate uranium does not produce isotopes below ²⁴⁹Cf, so does not introduce any additional Cf isotopes into the picture. Formation of those isotopes ²⁵²Cf to ²⁴⁹Cf continues while U remains.

ATOMIC PROPERTIES

Wikipedia's article "Californium" addresses the element's atomic properties and chemistry in some detail. Chemistry of Cf is not limited to the laboratory.

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 Californium", Wikipedia article.

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

(11-22-20)