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Thread: Question about the CNO fusion cycle

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    Question about the CNO fusion cycle

    At the end of the cycle when N-15 absorbs a proton. To become O-16, why does it emit an alpha particle?

    Is the O-16 excited?

    https://en.wikipedia.org/wiki/CNO_cycle

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    Member Peez's Avatar
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    Quote Originally Posted by repoman View Post
    At the end of the cycle when N-15 absorbs a proton. To become O-16, why does it emit an alpha particle?

    Is the O-16 excited?

    https://en.wikipedia.org/wiki/CNO_cycle
    Wouldn't you be on your birthday?

    As I understand it, O16 emits an alpha particle because it absorbs a proton.

    Peez

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    Elder Contributor barbos's Avatar
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    Quote Originally Posted by repoman View Post
    At the end of the cycle when N-15 absorbs a proton. To become O-16, why does it emit an alpha particle?

    Is the O-16 excited?

    https://en.wikipedia.org/wiki/CNO_cycle
    Yes. And in general if some nucleus is a result of a fusion it would be in excited state which then relaxed by emitting photon or if there is enough energy that could go through nuclear force decay which is statistically preferrable
    in this particular case we have (15.0001088989+1.00782503224)-(15.99491461960) = 0.0130193115400026u
    this means O16 is about 13 MeV excited which is enough for nuclear force decay into C12 and He4.

    Also, I am not sure if we can call it O16 in excited state because it does not last any meaningful time as a nucleus.
    Last edited by barbos; 09-23-2019 at 04:16 PM.

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    No, N-15 absorbs a proton becoming some form of O-16 (normally stable) which has to shed some momentum or energy(?) Because of how it formed and then emits an alpha particle.

    I guess I have a lot more to learn about this stuff even on a non technical layman level.

    I

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    Elder Contributor barbos's Avatar
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    Quote Originally Posted by repoman View Post
    No, N-15 absorbs a proton becoming some form of O-16 (normally stable) which has to shed some momentum or energy(?) Because of how it formed and then emits an alpha particle.
    Why No?

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    Fair dinkum thinkum bilby's Avatar
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    Quote Originally Posted by repoman View Post
    No, N-15 absorbs a proton becoming some form of O-16 (normally stable) which has to shed some momentum or energy(?) Because of how it formed and then emits an alpha particle.

    I guess I have a lot more to learn about this stuff even on a non technical layman level.

    I
    Given the speed with which all this occurs, it's just as reasonable to describe these events as 15N is struck by a high energy proton, and splits into an alpha particle and a 12C nucleus.

    You can include 16O as an intermediate step if you like. But as barbos says, it has so much energy above it's ground state that it doesn't last long before it spits out an alpha to shed that excess.

    The whole process occurs at high temperature (a dozen or more megakelvins) so all the particles have a lot of momentum, and the entire cycle is very strongly exothermic. It's hardly surprising that most of the nuclei involved are in an excited state.

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    Elder Contributor barbos's Avatar
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    Quote Originally Posted by bilby View Post
    Quote Originally Posted by repoman View Post
    No, N-15 absorbs a proton becoming some form of O-16 (normally stable) which has to shed some momentum or energy(?) Because of how it formed and then emits an alpha particle.

    I guess I have a lot more to learn about this stuff even on a non technical layman level.

    I
    Given the speed with which all this occurs, it's just as reasonable to describe these events as 15N is struck by a high energy proton, and splits into an alpha particle and a 12C nucleus.

    You can include 16O as an intermediate step if you like. But as barbos says, it has so much energy above it's ground state that it doesn't last long before it spits out an alpha to shed that excess.

    The whole process occurs at high temperature (a dozen or more megakelvins) so all the particles have a lot of momentum, and the entire cycle is very strongly exothermic. It's hardly surprising that most of the nuclei involved are in an excited state.
    Actually this is all rather incorrect. On the scale of nuclear forces ~20 megakelvns temperatures are effectively zero because characteristic nuclear energy is around 2-5 MeV which corresponds to 200-500 billion kelvins. So proton does not really strike N15, it's more like get there with little energy and then triggers release of a lot of energy in the form of alpha-decay.
    Also I said I was not sure whether or not it can be called excited state, I was probably wrong.

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    Fair dinkum thinkum bilby's Avatar
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    Quote Originally Posted by barbos View Post
    Quote Originally Posted by bilby View Post
    Quote Originally Posted by repoman View Post
    No, N-15 absorbs a proton becoming some form of O-16 (normally stable) which has to shed some momentum or energy(?) Because of how it formed and then emits an alpha particle.

    I guess I have a lot more to learn about this stuff even on a non technical layman level.

    I
    Given the speed with which all this occurs, it's just as reasonable to describe these events as 15N is struck by a high energy proton, and splits into an alpha particle and a 12C nucleus.

    You can include 16O as an intermediate step if you like. But as barbos says, it has so much energy above it's ground state that it doesn't last long before it spits out an alpha to shed that excess.

    The whole process occurs at high temperature (a dozen or more megakelvins) so all the particles have a lot of momentum, and the entire cycle is very strongly exothermic. It's hardly surprising that most of the nuclei involved are in an excited state.
    Actually this is all rather incorrect. On the scale of nuclear forces ~20 megakelvns temperatures are effectively zero because characteristic nuclear energy is around 2-5 MeV which corresponds to 200-500 billion kelvins. So proton does not really strike N15, it's more like get there with little energy and then triggers release of a lot of energy in the form of alpha-decay.
    Also I said I was not sure whether or not it can be called excited state, I was probably wrong.
    Thanks for the correction. It's a good thing I don't work in stellar construction.

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    So how do you hold the geometry of "novel" atoms together so they don't fly apart after making "transparent aluminum", for example? Magnets?

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