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Primordial U composition

  1. Nov 13, 2013 #1
    What was the primordial composition of U isotopes in solar system?

    Solar system contained isotopes less than a million year halflife, because meteors have clear traces of having crystallized with abundant aluminium 26.

    So, what was the actinide composition?

    We know the exact amount of uranium 238 because her longest lived mother is Pu-242, half-life 373 000 years. All uranium 238 was in Solar System primordially - she has only decayed since, at a predictable rate and known final amount.

    Not so with uranium 235. For though the halflife of U-235 and remaining amount is known, U-235 unlike U-238 has a long lived mother - Cm-247 with half-life of 15,6 million years.

    Since Cm-247 is now completely extinct, is it known what fraction of U-235 was primordial and what fraction is daughter of Cm-247?

    Now, concerning uranium...
    Uranium 236 is also an isotope of uranium. She is herself long lived - 23,5 million years half-life - and daughter of even longer lived - 80 million years - plutonium 244.
    Considering uranium 236 and plutonium 244 are both long lived, did primordial solar system contain excess uranium 236 (primordial amounts) or was the solar system short of uranium 236 till she formed by decay of plutonium 244?

    Finally, neptunium 237 has a half-life 2,2 million years. Her daughters include uranium 233.

    Therefore, the primordial U composition would have been:
    U-238 (all there, known amount)
    U-234 (already in equilibrium as daughter of U-238, so known amount)
    U-235 (how much had formed?)
    U-236 (how much was there?)
    U-233 (same question)

    Can anyone provide the numbers for that isotopic composition?
     
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  3. Nov 15, 2013 #2

    Chronos

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    These isotopes are mainly by products of U238 and thorium decay. Their natural abundance is related to their half life and abundance of progenitors. All elements heavier than iron/nickel are the result of supernovae. It is reasonable to assume they originated in relatively nearby supernovae before the solar system evolved.
     
  4. Nov 15, 2013 #3
    No. Only uranium 234 is. The others - uranium 235, uranium 233, uranium 236 and also traces of uranium 240 - most emphatically cannot be formed by decay of uranium 238, nor thorium.
    Rather they are products of neptunium, plutonium and curium decay.
    Half lives of progenitors are known. I quoted all of them. But my question is, what were the primordial abundances of progenitors?
     
  5. Nov 15, 2013 #4

    Chronos

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    Apologies for the decay chain confusion. Only two uranium isotopes are primordial [birthed by supernova] - U238 and U235. Plutonium 244 is the primordial nuclide that 'mothers' a decay chain producing U240, U236, U233, and U232. U233 is not produced by any known natural decay process. It is known from meteorite studies that no less that 10 progenitor stars contributed to primordial nuclides in the solar system. If you were to assume [erroneously] a single progenitor supernova was responsible for all the uranium in the solar system, it must have occurred about 6.5 billion years ago. See http://world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/The-Cosmic-Origins-of-Uranium/ for discussion.
     
    Last edited: Nov 15, 2013
  6. Nov 16, 2013 #5
    Look at the contradiction between your two consecutive sentences!
    U-232 is not produced as a daughter of Pu-244, because U-236 undergoes alpha decay to Th-232, which does not undergo beta decay, but alpha decay (to Ra-228).
    U-233 is, as I posted above, produced as a daughter of Np-237 so long as Np-237 (half-life 2,2 million years) remains in nature.
     
  7. Nov 16, 2013 #6

    Chronos

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    Agreed, inclusion of U233 in the Pu244 decay chain was unintended, as evidenced by the sentence that followed. Take a look at http://periodictable.com/Isotopes/094.244/index.full.html. Given that Thorium 232 is considered primordial, it is not necessary to trace its ancestry back to Pu244 to account for natural production of U232. It is, however, necessary to account for natural production of U236 and U240 in a primordial nuclide decay chain. I may have missed something, but, it does not appear there is any primordial nuclide that naturally produces U233 in its decay chain. It should be noted that Np237 is not considered a primordial nuclide. For a listing of primordial nuclides see http://en.wikipedia.org/wiki/Primordial_nuclide.
     
    Last edited: Nov 16, 2013
  8. Nov 17, 2013 #7
    But that´s a fine line. Very fine considering that it is double beta - in fact, there is no actual decay fraction given, so the claim that thorium 232 might undergo double beta is unproven.
    But that list also omits Al-26, I-129 etc., which are proven to have existed in young solar system. So the omission of Np-237 merely shows that it has decayed since, not that it was not present in nature.
    What then were the original ratios in young solar system of:
    Pu-244/Th-232?
    Cm-247/U-235?
    Np-237/Bi-209?
     
  9. Nov 17, 2013 #8

    Chronos

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    As already explained, the abundance of primordial nuclides in the solar system is not calculable. They can only be measured. This is problematic for nuclides with short half lives [i.e. less than 80 million years] which are considered non-primordial - as is the case for Al26, I129 and Np237. Any such nuclides that currently exist in the solar system are necessarily the result of the decay chain of longer lived primordial nuclides. [see http://en.wikipedia.org/wiki/List_of_nuclides] [Broken]. Pu244 is at the edge of detectability on this basis. It appears you are less interested in the original question than arguing the details.
     
    Last edited by a moderator: May 6, 2017
  10. Nov 17, 2013 #9
    Precisely. I explained it.
    Precisely.
    Or formed recently in different manners, like radiogenic. So measuring their present abundance measures their present production mechanisms, not their primordial amounts.
    No. I am interested exactly in the original question. I need to argue the details because they are offered instead of the original question.
    It is possible to measure primordial abundances of extinct isotopes by measuring their daughters in meteors.
    What have been the measured primordial abundances of the long-lived extinct actinids - Pu-244, U-236, Cm-247 and Np-237?
     
  11. Nov 17, 2013 #10

    mfb

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    Tricky. You can measure the current abundances, but it is hard to impossible to determine which (relatively) short-living isotope started the chain.
     
  12. Nov 17, 2013 #11
    But if they are different element then they should have different chemical behaviour.
    Extant actinides:
    Th is always 4+. Impossible to oxidize or reduce. Crystal radius 108 nm. It is a highly incompatible element - a strong field cation.
    U has oxidation state options. But U3+ is a very strong reducer. UO2(2+) is common on modern Earth - with free dioxygen available. In ordinary space, U would also be U4+ - crystal radius 103 nm.

    By contrast, look at the extinct actinides:
    Np is on modern Earth easily oxidized to Np4+ or NpO2+, and with some difficulty to NpO2(2+). But under ordinary reducing conditions, where U is U4+ rather than UO2(2+), Np is Np3+ rather than Np4+. And the crystal radius of Np3+ is 115 nm - compare Ce3+, also 113 nm.
    Pu is oxidized to Pu4+, PuO2+ or PuO2(2+), with some difficulty - and easily reduced to Pu3+, crystal radius 114 pm, compare Pr3+, 113 pm.
    Cm is very hard to oxidize, and is Cm3+ under any common conditions - crystal radius 111 nm, compare Nd3+, 112 nm, and Sm3+, 110 nm.

    It seems to me that there is a gross difference in chemical behaviour - long-lived U and Th (together with their extinct isotopes like U-236) should have entered minerals as strong field incompatible cations, whereas the extinct transuranium elements should have partitioned with ceria earths.
     
  13. Nov 19, 2013 #12

    Chronos

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    Just to ensure we are talking about the same thing, here is the OP:
    Let's start with the initial question - "What was the primordial composition of U isotopes in solar system?" There are 288 nuclides that were produced in stars AND have half lives long enough to remain in detectable amounts [i.e., are PRIMORDIAL]. A list of these nuclides is here: http://en.wikipedia.org/wiki/List_of_nuclides. You will note that U238 and U235 are the only isotopes of uranium listed as primordial. All other uranium isotopes are either of cosmogenic or radiogenic origin, and NOT PRIMORDIAL.

    Moving on to the next statement - " Solar system contained isotopes less than a million year halflife, because meteors have clear traces of having crystallized with abundant aluminium 26. So what was the actinide compositiont?" Al26 is believed to be of cosmogenic origin. This article might be of interest: http://iopscience.iop.org/0004-637X/489/1/346/fulltext/

    Moving on to the next statement - "We know the exact amount of uranium 238 because her longest lived mother is Pu-242, half-life 373 000 years. All uranium 238 was in Solar System primordially - she has only decayed since, at a predictable rate and known final amount." U238 is primordial whereas none of its mother isotopes are considered primordial.

    And next - "Not so with uranium 235. For though the halflife of U-235 and remaining amount is known, U-235 unlike U-238 has a long lived mother - Cm-247 with half-life of 15,6 million years.
    Since Cm-247 is now completely extinct, is it known what fraction of U-235 was primordial and what fraction is daughter of Cm-247?" It is likely impossible to deduce how much U235 was a daughter of Cm247 and how much was produced directly by supernova. See http://world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/The-Cosmic-Origins-of-Uranium/ for further discussion.

    And lastly- "Now, concerning uranium...
    Uranium 236 is also an isotope of uranium. She is herself long lived - 23,5 million years half-life - and daughter of even longer lived - 80 million years - plutonium 244.
    Considering uranium 236 and plutonium 244 are both long lived, did primordial solar system contain excess uranium 236 (primordial amounts) or was the solar system short of uranium 236 till she formed by decay of plutonium 244? Finally, neptunium 237 has a half-life 2,2 million years. Her daughters include uranium 233. " The amount of Pu244 currently present in the solar system is at our limits of detectability. The amount of U236 and Np237 currently present is beyond detectable limits. It is thus very difficult/impossible to assess the abundance of non primordial nuclides in the pre solar nebula. Chances are their abundance was negligible in the pre solar nebula. This is true of any non primordial nuclide.
     
  14. Nov 20, 2013 #13
    Yes, but that depends on the definition of "primordial" adopted by Wikipedia. I did not ask what the "primordial" U isotopes are now, I asked what they were, then. So what term would you prefer for isotopes formed in star, neither cosmogenic nor radiogenic, that existed in young solar system but have since decayed?
    Al-26 is easily formed cosmogenically and small amounts are formed now. Yet young solar system contained much bigger amounts than was around later.
    Discussion that makes no mention whatever of Cm-247
    Manifestly untrue for Al-26, therefore why not for U-236?
     
  15. Nov 20, 2013 #14

    Chronos

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    The definition of a primordial nuclide is "In geochemistry and geonuclear physics, primordial nuclides, also known as primordial isotopes, are nuclides found on the Earth that have existed in their current form since before Earth was formed. Primordial nuclides are residues from the Big Bang, from cosmogenic sources, and from ancient supernova explosions which occurred before the formation of the solar system. They are the stable nuclides plus the long-lived fraction of radionuclides surviving in the primordial solar nebula through planet accretion until the present. Only 288 such nuclides are known." [re: http://en.wikipedia.org/wiki/Primordial_nuclide] [Broken].

    Extinct nuclides is evidently the term we are struggling with to reach concensus - "An ‘extinct radionuclide’ is understood to be one that was formed by a process of stellar nucleosynthesis prior to the coalescence of the solar-system, and which has subsequently decayed away to zero." [re: http://www.onafarawayday.com/Radiogenic/Ch15/Ch15-1.htm] [Broken] Estimated abundances of some extinct nuclides in the pre solar nebula is given by table 1 of this paper - http://arxiv.org/abs/1105.5172. Contributions to daughter nuclides by sources other than extinct nuclides, such as Al26 and I129, complicate quantification of extinct nuclide abundances. See http://arxiv.org/abs/astro-ph/9605128.

    I was unable to find any information on the estimated abundance of U236 in the pre solar nebula.
     
    Last edited by a moderator: May 6, 2017
  16. Nov 21, 2013 #15
    Thanks! Finally roughly what I wanted.
     
  17. Nov 21, 2013 #16

    Chronos

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    I'm pleased we arrived on the same page. I accept most of the credit for obfuscating matters.
     
  18. Nov 21, 2013 #17
    So:
    Cm-247 has been found - to one star confidence. The amount was small, however. By my calculations, it is making the difference between original U-235 concentration of either 24,2197 % or 24,2156 % - so just 0,018 % of U-235 originates from decay of primordial Cm-247. This comes from detected variations of U-235/U-238 ratios in meteors.
    By my computations, neglecting the primordial U-236 and Np-237 (not mentioned in the article quoted), the original U composition 4566 million years ago should have been, within tenths of %:
    75,6 % U-238
    24,2 % U-235
    0,2 % U-236
    Is it correct, or do you want to see the details of my computations?
     
  19. Nov 21, 2013 #18

    Chronos

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    The present ratio of U238 to U235 is about 99.3% vs 0.7%. Did you derive their primordial abundances by extrapolating back using standard decay rates? As far as I know this will only yield approximate abundances at any given point in history. Meteorite studies suggest variations in the abundance of pristine and 'mother' nuclides over time - probably a consequence of injection of these isotopes in variable quantities from multiple supernovae sources. I have no clue how to calculate primordial U236 abundances. I am curious about that computation.
     
  20. Nov 21, 2013 #19
    Yes.
    Calculate or measure?
    As I stated, I found no information about primordial U-236, and said I neglected it.
    So I assumed that primordial U-236 was in equilibrium with her mother Pu-244.

    Now if a daughter is longer lived than mother then she can never come into equilibrium with mother. The ratio of daughter to mother will diverge to infinity as mother decays exponentially and the daughter, not so fast.

    Equilibrium is impossible no matter how slightly longer lived the daughter is. Therefore the ratio of daughter to mother will diverge to infinity even when their lifetimes are exactly equal.
    My guess was that the equilibrium ratio of U-236 to Pu-244, given their half-lives 23,48 and 80 million years respectively should have been 23,48/(80-23,48). Can anyone tell if the guess is correct?
    Now given that the primordial ratio of Pu-244 to U-238 was measured as 0,68%+-0,10 % (in the source), the primordial ratio of U-236 to U-238 comes out as 0,28 %. And given about 75 % total U-238 (most of the rest primordial U-235) is how I got 0,21 % U-236.
     
  21. Nov 21, 2013 #20

    Chronos

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    Such calculations are difficult because of contributions from nearby supernovae and galactic sources, which are necessarily of different ages. The estimated abundances U235, U238 and Th232 resulting from nucleosynthesis are summarize here, http://adsabs.harvard.edu/full/1964ApJ...139..637C. The initial ratio of Pu244 to U238 is estimated to be about 7x10E-3 per http://www.uapress.arizona.edu/onlinebks/PPIV/chap35.pdf. The relative contribution by the ISM vs supernovae is, however, difficult to quantify. My best guess as to abundances in the presolar nebula, based solely on nearby supernovae contributions, would be about 61% U235, 39% U238 and .003% Pu244.
     
    Last edited: Nov 21, 2013
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