Americium

Americium
The location of Americium
Symbol	Am
Number	95
Group, period	N/A, 7
Electrons per shell	2, 8, 18, 32, 25, 8, 2
Discoverer(s)	Glenn T. Seaborg, Leon Morgan, Ralph James, Albert Ghiorso
Date discovered	1944
Location discovered	Metallurgical Laboratory, University of Chicago
Atomic weight	(243)
Category	actinide

A smoke dectector, an application of americium.

Americium, Am, is the name of element 95. It forms in surpernovae and neutron star mergers. One of its isotopes, ²⁴³Am, has a half-life near 7400 years, which means it will persist long enough to be incorporated into collapsing cloud cores that are turning into solar systems, but not long enough to remain in mature ones. Am is also a terrestrial element - it is present in today's earth without the aid of scientists.

Naturally-occurring terrestrial Am has not been detected.

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 Am isotopes. Other references used are cited.

Pridicted and observed properties

Isotopes from the neutron dripline down to ²⁴⁴Am 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 20 sec at ²⁵⁵Am, reaching 2 hrs by ²⁴⁵Am, and peaking at 10 hrs in ²⁴⁴Am.

²⁴³Am and ²⁴¹Am decay almost entirely by alpha emission; neither has a beta-emission decay branch. Their half-lives are near 7400 yr & 432 yr respectively.

²⁴²Am, in its ground state, has both a negative and a positive beta decay branch. Its half-life is close to 16 hrs. However, it has an isomer, ^242m1Am, whose half-life is 141 yr and which decays almost exclusively (0.995 branch ratio) by emitting a gamma ray and dropping to ground-state ²⁴²Am. In effect, ²⁴²Am decays by a mix of beta emission and electron capture, and has a 141 yr half-life.

Positive beta decay sets in at ²⁴⁰Am Half-lives drop, as A declines, from 50 hrs at ²⁴⁰Am to 1.8 sec at ²²⁹Am.

A very light isotope, ²²³Am, has been reported. It decays by alpha emission with a millisecond-scale half-life. This is far longer than the half-life predicted by Ref. 1.

Ref. 1 predicts decay properties for isotopes as light as ²¹⁸Am. Increased stability is predicted at ²²¹Am and below, due to the neutron shell closure at N = 126. Ref. 3 extends prediction to ²¹⁴Am. All these decay by either fission or alpha emission and have half lives under 1 sec.

Occurence

Formation

a) Outside Earth

Am isotopes from the neutron dripline down to ²⁴⁵Am, as well as ²⁴³Am and ²⁴¹Am can form via rapid neutron capture and fission infall, followed by beta decay.

Neither ²⁴²Am nor ^242m1Am can form by rapid neutron capture followed by chains of beta decays.

Formation of ²⁴⁴Am, ²⁴⁰Am, and lighter isotopes is 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 Am isotopes which can form.

b) On Earth

Spontaneous fission of ²³⁸U produces free neutrons. Those neutrons sustain a slow process of neutron capture followed by beta decay back to a relatively stable nucleus prior to the next neutron capture. Beta decay branches at ²⁴³Pu and ²⁴¹Pu allow ²⁴¹Am and heavier isotopes to form, up to a limit at ²⁴⁴Am. The isotopes which can form include ²⁴²Am and ^242m1Am (See chart in Wikipedia article Curium.) The amount of Am produced this way is small. Synthetic production produces the same isotopes as natural, and in greater quantity.

Naturally-occurring terrestrial americium 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 Am widely. Any effort to detect "natural" Am must eliminate possible contamination by synthetic americium.

Persistence

The amount of Am 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 the long-lived isotopes from²⁴³ Am and ²⁴¹Am.

Following a supernova or neutron star merger, ²⁴¹Am will persist at potentially observable concentration for a few thousand years, and will, in principle, be present for up to 40000 yrs. ²⁴³Am will remain in potentially detectable amounts throughout the lifetime of a supernova or kilonova remnant. The isotope will, in principle, be present for up to 700000 yr.

Although ^242m1Am is long-lived, it does not form as a consequence of supernova explosion or neutron star merger.

Other isotopes which can form have half-lives of up to 50 hr, which means they are gone in less than a year.

Slow neutron capture in rocky bodies active enough to concentrate uranium can produce all the important Am isotopes. Since production is ongoing, these will persist as long as U remains. Naturally-occurring terrestrial Am has not been detected.

Atomic properties

Wikipedia's article "Americium" addresses the element's atomic properties and chemistry in some detail. The element persists long enough to reach cool environments. By that time, pressure has become low. Some Am will be incorporated into dust grains, but will become extinct long before any collapse into a new star can occur.

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 Americium", 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

(11-26-20)