Neptunium

Neptunium, Np, is the name of element 93. It forms in supernovae and neutron star mergers. Only two of its isotopes, ²³⁷Np and ²³⁶Np have half-lives longer than one year. Of these two, only ²³⁷Np can form via neutron capture (r process or s process), although gamma capture can produce a little ²³⁶Np. As an element present in today's earth without the aid of scientists, it is much less common than ²³⁹Pu.

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 - 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 ²³⁸Np are predicted or observed to decay primarily by beta emission. Half lives are predicted to increase, as A declines, from around 0.001 sec at the dripline to 7 sec at ²⁴⁹Np and peaking at 2.36 days in ²³⁹Np.

²³⁷Np decays by alpha emission, with a half-life of 2.144E06 yr. It is, by an order of magnitude, the longest-lived Np isotope.

²³⁶Np has a half-life of 1.54E05 yr. It decays in 3 ways: Electron capture is dominant with a 0.873 branch ratio, beta emission is second with a 0.125 branch ratio, and there is a weak alpha with a 0.002 branch ratio. This nuclide functions as a source for alpha-decay cascades at ²³⁶U and ²³⁶Pu.

²³⁵Np has a half-life of almost a year and decays by electron capture. Lighter Np isotopes decay rapidly. Near ²³⁵Np, positive beta decay (EC or positron emission) dominates, but alpha becomes important in the lighter isotopes. Half-lives decrease down to 0.00015 sec in ²¹⁹Np, the lightest observed.

None of ²³⁸U or ²³⁶U to ²³²U have reported beta-decay branches. Np isotopes lighter than ²³⁷Np can form by neutron-capture / beta-decay pathways.

& predict decay properties for isotopes as light as ²⁰⁷Np. Increased stability is predicted at ²¹⁹Np 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

Neptunium isotopes from the neutron dripline to ²³⁹Np plus ²³⁷Np can form in quantities comparable with the formable isotopes of uranium. Decay chains to ²³⁸Np, ²³⁶Np, and lighter isotopes are blocked by U. Long-lived ²³⁶Np does form, in small quantities in highly-evolved stars of all masses by the same mechanisms which produce it on earth.

b) On Earth

²³⁷Np 237 can form via the path: 1. neutron capture by a ²³⁵U nucleus produces an excited state ²³⁶U*. ²³⁶U* gamma decay occurs (0.2 branch ratio). ²³⁶U captures a neutron to form ²³⁷U, and finally ²³⁷U beta-decays to ²³⁷Np. ²³⁷Np can then capture a neutron to become ²³⁸Np. ²³⁸Np is short-lived, so the chain ends there. Terrestrial processes do not produce ²³⁶Np.

PERSISTENCE

As is true of the other elements, most Np isotopes are short lived. Only 3; ²³⁷Np, ²³⁶Np, and ²³⁵Np, have half-lives exceeding 4 days. Of these, ²³⁷Np will form in abundance and remain present for at least 2.2E08 years. This is sufficient time for it to be incorporated in planets within young stellar systems. Earth is too old to have any primordial ²³⁷Np, but does have a small quantity produced by slow neutron capture by ²³⁵U.

Small amounts of ²³⁶Np can form in supernovae and kilonovae (neutron star mergers) by a gamma-capture / neutron emission process. Since it cannot form in a cold (gamma-free) environment, it becomes extinct within as little as 16 million years after and event which led to its formation. ²³⁶Np can be expected to be present in very young stellar systems, but not in old ones.

ATOMIC PROPERTIES

Wikipedia's article "Neptunium" addresses the element's atomic properties and chemistry in some detail. Its natural environments include hot gas/dust and cold gas/dust.

REFERENCES

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".

(12-05-20)

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

(12-05-20)