Alpha radiation can be stopped by a piece of paper, but what happens to a helium nucleus after it loses all its energy in matter? Does it became part of the material or does it change nuclear properties?
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4 Answers
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An alpha particle in matter will interact with electrons and nuclei by scattering. Those scattering interactions will tend to redistribute the alpha particle's kinetic energy until it is in thermal equilibrium with its surroundings.
An alpha particle at room temperature is just the same as a twice-ionized helium nucleus. Eventually it will steal two electrons from other atoms in its environment, which turns it into a neutral helium atom. Helium is hard to trap, so it will generally escape from any solid material and become part of Earth's atmosphere.
At Earth's surface, helium makes up about five parts per million of Earth's atmosphere. Because of helium has such low mass, it has a taller scale height than other gas species and is a major component of Earth's exosphere, where some of the more energetic helium atoms find themselves with enough energy to escape forever into space. A commenter links to a 2012 reference which claims that about $10^{-6}$ of Earth's atmospheric helium is replaced every year by radioactively-produced helium escaping from Earth's interior; a back-of-envelope estimate gives $10^{6\text{-ish}}$ years for helium to diffuse from the surface to the top of the atmosphere, as well. So your stopped alpha particle will hang around for a long while.
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3$\begingroup$ Moreover, this is also where most (if not all?) helium available on Earth comes from. The stuff that fills your party balloons is stale, stopped radiation. Your balloons are full of radiation! It's just been made safe now by losing all its energy. :D $\endgroup$ May 2, 2020 at 19:14
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3$\begingroup$ @The_Sympathizer Right: given the rates in the answer, the helium in Earth's atmosphere has been completely replaced several thousand times over Earth's history. $\endgroup$– rob ♦May 2, 2020 at 19:27
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2$\begingroup$ @The_Sympathizer Most, not all. From en.wikipedia.org/wiki/Helium-3#Terrestrial_abundance which cites L.J. Wittenberg (July 1994). "Non-Lunar ³He Resources", around 7% of the He in the mantle is primordial, considered to have become entrapped within the Earth during planetary formation, not produced in the mantle by alpha decay. I'm a little surprised that that percentage is so high. $\endgroup$– PM 2RingMay 11, 2020 at 4:17
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$\begingroup$ @PM2Ring Interesting find. I was surprised to estimate $10^6$ years for helium to diffuse from the bottom to the top of the atmosphere; if I ballpark $10^9$ years to diffuse from the bottom to the top of the mantle, then having a substantial fraction of primordial helium still trapped within Earth is not crazy. (But your 7% depends on production rates, too.) Helium-3 in Earth's atmosphere must be a mixture of primordial helium from the mantle and the detritus of tritium produced when cosmic ray spallation neutrons (rarely) make tritium in the ocean. $\endgroup$– rob ♦May 11, 2020 at 17:56
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$\begingroup$ Some primordial He-3 + He-4 is also delivered via meteoric dust. I did see an estimate for that, but I can't remember where (oops). Some of that dust makes its way to the ocean floor, and may be subducted into the mantle. But of course that helium would have a very similar He-3 : He-4 ratio to the original primordial helium in the mantle. $\endgroup$– PM 2RingMay 11, 2020 at 22:54
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When a radioactive atom emits an alpha particle it loses two protons' worth of electric charge. It therefore becomes a negatively-charged ion and soon loses a couple of orbiting electrons too.
The charges soon redistribute themselves; the electrons find a couple of positive ions, the alpha particle picks up a couple of electrons to become a helium atom.
Helium is unreactive and will diffuse out of any solid that slowed it down. Once in the air, it hangs around for many years, but eventually it has an unfortunate habit of drifting off into space.
answered Apr 30, 2020 at 19:24
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1$\begingroup$ Re "soon loses a couple of orbiting electrons": What loses? The original (radioactive) atom? $\endgroup$ May 1, 2020 at 18:40
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1$\begingroup$ Re "has an unfortunate habit of drifting off into space": Yes, but how long does it take on average? 3 billion years? $\endgroup$ May 1, 2020 at 18:43
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1$\begingroup$ @PeterMortensen, Re. 1, yes that is the grammar of my sentence. Re. 2. I do not know the half-life of helium loss from the atmosphere, I'd hazard a few hundreds of years. I do know that the loss is significant to the Earth's helium reserves and impacts the long-term viability of say the helium-filled airship as a cost-effective technology. $\endgroup$ May 2, 2020 at 9:20
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Helium irradiation is a key problem in fusion devices, as the plasma can implant helium ash into reactor components. These tend to form helium bubbles, which in turn can act as binding sites for hydrogen. It helps to embrittle materials like W, where this can cause flaking or cracking of the W surface.
The helium is pretty stable in there (W), and the bubbles are a few nm in size. This only forms under heavy irradiation conditions. See e.g. Figure 5 of this 2016 paper: the He is not mobile until ~1500K.
I do not have any affiliations with the above work, nor work in the field.
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$\begingroup$ I suppose this info is relevant, although I assume the OP is asking about alpha particles from radioactive decay, not the high KE alphas that fusion can produce. $\endgroup$– PM 2RingMay 1, 2020 at 22:52
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3$\begingroup$ I was mostly concerned that people seem to think it automatically diffuses out of materials. This is still a problem in spent fuel storage and reactor pressure vessels for example, but not as marked.The irradiation energy mainly changes the depth of implantation, rather than the diffusion. $\endgroup$– HeBubbleMay 1, 2020 at 22:55
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1$\begingroup$ Fair enough. And even fresh plutonium damages its own structure pretty quickly. I know that alloying Pu with gallium makes it a little more workable, but I have no idea what impact that has on the He diffusion. $\endgroup$– PM 2RingMay 1, 2020 at 23:22
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$\begingroup$ Bubble lifetimes in tungsten are measured in picoseconds (Fig 3), which may be long to a nuclear fusion physicist but hardly so when compared to diffusion. I am more familiar with annealing newly-worked mu-metal to diffuse the hydrogen out, which can take weeks or even months. $\endgroup$ May 11, 2020 at 9:06
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$\begingroup$ @Guy : No, they are observable by TEM and other techniques (FIB/SIMS), where prep t observation times are measured in days to years. You might be looking at the simulation lifetimes? Heres some picture : researchgate.net/figure/… $\endgroup$– HeBubbleMay 11, 2020 at 23:17
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As far as I know, an alpha particle which penetrates into a material will just kind of hang out there, until it can get two electrons to become a stable helium atom. Along the way, it may have affected neighboring atoms/molecules via ionization.
The reason alpha radiation can't penetrate too far is that the alpha particle is (relatively) fairly massive and so interacts strongly with the material by 'bumping' into other atoms and interacting with them electromagnetically. In order for it to become part of the material or affect it chemically would require extra energy so I wouldn't expect that to happen.
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1$\begingroup$ Electromagnetically sounds right, but isn't electrostatic enough to explain it (not a rhetorical question)? $\endgroup$ May 1, 2020 at 18:35
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$\begingroup$ @PeterMortensen Electrostatic is not enough; you also need the strong force. (But the energy dependence of the strong force is counterintuitive in a way that doesn't fit neatly into a comment.) $\endgroup$– rob ♦May 2, 2020 at 3:53