Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Eternal Student on 07/10/2023 22:20:57
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Hi.
So assume you have an atom that decays by alpha emission. Example:
241 Am 95 -----> 237 Np 93 + 4 He 2
Americium decays to Neptunium. It's tricky to get mass and proton numbers both lined up properly on the LHS so I hope you'll forgive me putting them left and right.
The Americium started off as a neutral atom, so it had 95 electrons. On completion the Neptunium only wants 93 electrons. The decay is an essentially nuclear process, the electrons were never really involved or changed. So what happens to the surplus electrons? Do they just drift off somewhere?
Best Wishes.
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The negative charge on the electrons are balanced by the positive charge on the alpha particle. In principle, they could combine to form a helium atom. Alternatively, they might interact with other atoms in the environment to form ions.
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Hi.
Sounds reasonable @Kryptid, thank you. However, we have the general impression that alpha particles do tend to be positively charged. So when they are emitted, the surplus electrons do not seem to hang around or keep up with the alpha particle and form a neutral Helium atom (in the general case).
I would guess that the second option you mentioned is more likely. I'm guessing that for a short while you have something like a Neptunium ion with -2 charge in existance. Shortly afterwards the electrons become involved in something else. I've never seen any textbooks mention that anything like an energetic electron (so much like a Beta emission) always tends to accompany an alpha emission, so I'm guessing that even if some energy is liberated from the nuclear reaction to free the electrons from the Neptunium atom, there isn't a lot of spare energy - i.e. those electrons have a very low kinetic energy and don't behave much like Beta particles.
Note that I don't really know, that's why I'm asking. In practice, an alpha particle is strongly ionising - it knocks electrons into and out of most atoms (e.g. atoms that are in air) it finds along its path - so when the decay happens in air it shouldn't be too difficult for the surplus electrons to end up being picked up by something. However, nuclear decay will happen everywhere and there is some happening in space (a vaccum more or less) - where then would the electrons go? Do charged Daughter atoms from the decay persist for quite a while? I don't know.
Best Wishes.
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The remaining atom is indeed ionised, and an insulated lump of any alpha or beta emitter gradually acquires a charge of opposite sign to the emitted particle.
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If one charges up any arbitrary item with either + or - and leave it undisturbed one will find the charge ebbs away as virtually all so called insulators do conduct to some very minor degree. A totally isolated alpha emitter, in vacuo, would, I expect, become highly charged over time and at some point might inhibit or slow the alpha emission(wild speculation).
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Hi.
Thanks @paul cotter . Seems reasonable.
This bit I know a little about and can can comment on:
... become highly charged over time and at some point might inhibit or slow the alpha emission(wild speculation).
The simple view of nuclear reactions is that they are almost completely independant of all conditions external to the nucelus. For example, unlike chemical reactions, pressure and temperature does not influence nuclear reactions. Similarly the electronic configuration or ionisation state of the atom should have no effect.
The reasoning is that nuclear reactions are going on in the nucleus where the strong force dominates and nothing else is significant by comparison. For example, the Coulomb force due to charge on the atom should be insignificant. So nuclear decay was traditionally considered as a random process, you can't predict exactly when it will happen and you cannot control it or influence it by altering the environment. This was an important assumption for things like Carbon dating and more generally for estimating the age of anything (say something out in space) based on the proportion of a substance that had decayed.
However, it is now known that there is at least one type of nuclear change which is influenced by the charge of the atom. This is a type of decay process called "electron capture". See https://en.wikipedia.org/wiki/Electron_capture for more info if required. This is where an inner electron of the atom is drawn into the nucleus and does result in a genuine nuclear change (a proton --> neutron). It has been noticed that you can influence the half-life of substances like Lithium undergoing electron capture JUST by having the Lithium in the form a salt, like Lithium Oxide, where the Lithium would actually be an ion (i.e. generally deficient in electrons). The influence is small (about 2%) on the half-life but it has been rigourously and accurately tested and confirmed to exist. This has slightly changed the perception that nuclear decay can't be influenced by external enviromental conditions. However, keeping it very simple, most nuclear changes should be unaffected by the electronic configuration of the parent atom.
LATE EDITING: The spirit of what was said remains unchanged. However it seems that it was Berylium-7 decaying into Lithium-7. Specifically, Lithium was the decay product not the reactant. Thus, the reaction rate is influenced by using a Berylium salt instead of un-ionised Berylium etc.
Best Wishes.
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Thes energy involved in expelling an alpha is enormous compared with the ionisation energy of any atom you can think of, but this does raise an interesting question in the very long term. Suppose we have a planet composed entirely of alpha emitter: at what point does the internal coulomb repulsion overcome the binding energy of the delocalised electrons and blow the chunk of metal apart?
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Let's assume that the alpha emitting planet is bigger than a pea.
And let's assume that the range of alpha particles in solid matter is 100 micrometers.
What fraction of the alpha particles can actually escape?
After a while the planet would be hot.
And either thermionic emission of electrons or emission of helium ions (making their way out via diffusion) would ensure that the planet was electrically neutral and thus safe from that fate- though it might manage to boil itself.
The process where "internal coulomb repulsion overcome the binding energy of the delocalised electrons and blow the chunk of metal apart?"
Is (more or less) used in chemistry.
https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Biophysics_241_-_Membrane_Biology/06%3A_Experimental_Characterization_-_Mass_Spectrometry_and_Atomic_Force_Microscopy/6.03%3A_Electrospray_Ionization_(ESI)_Mass_Spectrometry
It's called a coulomb explosion.
(Though, it's a very quiet "bang".)
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What fraction of the alpha particles can actually escape?
Not a lot, but we are in no hurry - this is cosmology! However to speed things up you might consider a tiny grain of an energetic beta emitter like Yttrium-90.
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consider a tiny grain of an energetic beta emitter like Yttrium-90.
I more or less did.
it might manage to boil itself.
Once the thermal energy is enough to " overcome the binding energy of the delocalised electrons " you no longer have a lump.
And I think that's about 6 or 7 orders of magnitude less than the nuclear energy available.There's also the question of "what is the boiling point of something in an absolute vacuum?" (It needs to be that good, in order to act as an electrical insulator.
It's interesting that your idea about a planet only works for planets that are smaller than a grain of salt.
Unless, of course they are flat-earth shaped.
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At some point during the evolution of the cosmos, most planets were indeed smaller than a grain of salt. But the density of beta emitters was very small indeed.
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At some point during the evolution of the cosmos, most planets were indeed smaller than a grain of salt. But the density of beta emitters was very small indeed.
Planet means "wanderer" and at that stage, they didn't.
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... become highly charged over time and at some point might inhibit or slow the alpha emission(wild speculation).
The simple view of nuclear reactions is that they are almost completely independant of all conditions external to the nucelus.
Agree that the decay will (usually) be unaffected by any acquired charge. If the radioactive material emits enough alpha particles, it will become sufficiently negatively charged to rip the radioactive material apart (unlikely), or it will simply eject most of the surplus electrons (likely), aided by the negative charge repelling them.
That leaves a bunch of nuclei and electrons loose in space, too far apart to find each other. So what? It just takes longer for these things to become parts of whole atoms again, and not very likely the same atoms they once were.
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We sort of did an experiment of this sort in school. It didn't involve radioactivity, but it did involve stripping away electrons from molecules and sorting them into left and right buckets. It was just water, and after a while the dripping water just plain refused to fall into the bucket below, preferring to circle like all the airplanes in the 2nd Die Hard movie.
The lab table got quite wet in the process since gravity was no longer doing its job. Similarly, one can drop protons into a black hole only for so long before it will refuse any more.
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Hi.
If the radioactive material emits enough alpha particles, it will become sufficiently negatively charged to rip the radioactive material apart (unlikely), or it will simply eject most of the surplus electrons (likely), aided by the negative charge repelling them.
Yes I think we're reaching some general agreement. The stuff could be ripped apart in that atoms fly away from other atoms and even the electrons could be stripped and ejected from the rest of an atom - but each individual nucleus pretty much stays together, they are not likely to be ripped apart just by Coloumb forces (for the reasons previously mentioned - the strong nuclear force is all that really matters inside a nucleus).
So, just to be clear, I would think that the nucleii of Americium (in my example) will persist and continue decaying in much the same way as they ever did irrespective of what-ever soup of exploded, diffuse material with a peppering of clouds or fast moving rays of electrons they ultimately end up being in.
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On a minor note though, what may influence the decay rate much more noticeably is the presence of a large number of alpha particles already outside the nucleus. I've read a bit more since starting this thread and had some thinking time. The current best models for alpha decay are based on assuming that a particle like an alpha particle can form inside the nucleus. Taking a simple model, we assume all the nucleons are in something resembling motion and constant re-arrangement, so that sometimes there is something resembling an alpha particle that is just a little distinct from the bundle of other nucleons. Both the bundle of remaining stuff and the alpha particle are effectively inside a region you would still describe as the nucleus, in that they are still in a region of a deep potential well. In that deep well region, the strong force still dominates. However the alpha particle, like any quantum mechanical particle, can tunnel through the potential barrier that normally (or classically) confines the nucleons to this deep potential well.
Anyway, the general idea is that there really is no reason why an alpha particle travelling the other way can't tunnel in to the nucleus (instead of the tunneling out as in alpha decay). So flooding a region with alpha particles could bring the net rate of decay ( Americum-241 --> Neptunium-237 ) to 0.
NOTE: I've not seen any experiment or practice where this was done. It is just speculation that seems to fit the (mainstream) theory of alpha decay as a form of tunneling.
Best Wishes.
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We sort of did an experiment of this sort in school. It didn't involve radioactivity, but it did involve stripping away electrons from molecules and sorting them into left and right buckets. It was just water, and after a while the dripping water just plain refused to fall into the bucket below, preferring to circle like all the airplanes in the 2nd Die Hard movie.
The lab table got quite wet in the process since gravity was no longer doing its job. Similarly, one can drop protons into a black hole only for so long before it will refuse any more.
Was it one of these?
https://en.wikipedia.org/wiki/Kelvin_water_dropper
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As far as I can tell, the thermodynamics that applies to chemical kinetics and equilibria would also apply mutatis mutandis to nuclear physics.
But the energies are something like a million times higher so the temperatures needed to affect the outcome would be about a million times higher.
The test tubes would melt.