Merging with other stable nuclei is a fusion trick which would require overcoming significant coulomb barriers, and those energies and containment are generally only available in the core of stars nearing end of life.
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So then it must be the alpha particle itself that causes the discoloration, not the path it takes.
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Helium is colorless and noble, so I would think not directly. An alpha particle would swiftly win the contest for electrons. I cannot find where Gentry specifies anything more specific than damage, not to say he hasn’t.
Well, I mean the location of the alpha particle, which would become colorless helium. However, perhaps taking electrons from the neighbors is what disrupts the crystal? Or the presence of helium itself disrupts the crystal?
I’m not an expert in geochemistry (I took one course in college) but my reading of this is that the discoloration is due to a disruption of the crystal lattice within the mineral. The color of rocks and minerals is very sensitive to disruptions or impurities.
I don’t think it’s causing no damage and then only causing damage when it hits a certain radius. I think it’s creating overlapping disks of quantized (stepped) radius rather than different diameter rings.
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Many of the thin section micrographs used to show halo colors are from crossed-polarized light, and the colors we see are do from optical effects, not intrinsic colors of the minerals.
Most minerals exhibit two (or three) indices of refraction, a property known as birefingence. The different indices are aligned with the crystal structural axes.
A petrographical microscope as two polarizing filters, one below the stage (the source) and a second polarizer (the analyzer) above the stage. With the polarizers crossed (out of phase), we will see a black view, as no light passes the upper polarizer. Likewise, if we insert a blank glass slide, the view is still black.
The picture changes when we introduce a birefringent mineral on the stage. The source polarized light moving through the sample is split into two beams, and their path differences rotate the light away from the polarized orientation of the source beam, allowing light to pass through the upper polarizer.
The good part of this is that the wavelength of the light passed by the birefringent mineral is given as:
r = t(n2-n1) where r is the wavelength, t is the thickness of the sanple, and n1 and n2 are different refractive indices. If we use nanometers for r and t we can calculate the “color” (wavelength) of the resultant light.
This relationship was use in the 1880’s by a French geologist Auguste Michel-Levy to create his interference color chart, that displayed the colors for various minerals based on their birefringence and thickness.
Since most thinsections are ground down to 30 microns, many minerals can be identified by inspection with a bit of training and practice. The common mineral quartz shows a light straw-yellow color when your section is the right thickness.
You can see the same phenomina on soap bubbles and oil slicks,
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So the color is changed by disruptions to the crystal lattice, visible as opacity in polarized light, and caused by the presence of helium?
Ah, I’m fairly familiar with this, just not in the mineral context. I worked with polarization-resolved spectroscopy of “nanomachines”. We needed to make sure our substrates were not birefringent. We did have several polarization rotators and a photoelastic modulator that used birefringence.
I was thinking about this being bulk rather than cross-polarized thin sections. Thanks for the clarification.
I’m still not exactly sure the physical mechanism that’s happening when the alpha particle interacts with the crystal lattice in the mineral. I would guess it’s just simply physically running into the atoms and breaking up the lattice as it moves through the mineral. Thoughts?
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Good question Joshua
Alpha impacts in the crystal lattice causes changes in local chemistry, the geometry of the lattice in the neighborhoood of the impact, and hence the refractive indices and the the path difference between the “slow” and “fast” wave = bright colors.
I’ve looked around but have not found a definitve reference on the final fate of the original He2+ state of the alpha in the mineral. It does not appear that the helium (as an ion) is a reactive species.
Perhaps the alphas are “runnaway drivers” that ding the lattice, then slip away?
Alpha particles are high energy charged particles and so they ionize the atoms as they collide. I think it would be analogous to the ballistic gel example @AllenWitmerMiller. The alpha particle would ionize (and damage) atoms as it careens through the lattice until it loses its kinetic energy and turns into Helium.
I do believe that helium is produced.
http://www.ajsonline.org/content/313/3/145.short
Helium in zircon’s is another YEC argument, which neglects the production of helium by alpha particles.
I am not sure “impact” is the right descriptor. It seems that the particle is slowed by the electrons in the rock, without really impacting anything. Rather it is the final resting place of the alpha particle were the disruption occurs.
A contrast to this would be fission track dating, where (it seems) the total path of the particle is disrupted.
Am I understanding this correctly?
But to turn alphas into Helium we must “steal” 2 electrons (from where?) for each alpha , leaving electron “holes” in the lattice.
But that would create damage through the whole path…
I’m trying to reconcile the fact that halos only are recording the end of the path, while fission tracks record the whole path, and seem to go much farther. I wonder if it is the difference between Alpha vs. gamma radiation?
Have you ever seen the scatter plots of the landing points of tennis serves or baseball hits?
What we see here with alpha particle paths into solid matter: these are mean-path statistics. There will be some distance from the source where there is a maxima of “damage” , but also some “damage” will also occur before and beyond the peak.
Yeah, I think I’m getting a better idea of what could be happening. So the alpha particles are pretty small compared to the atoms they are moving through, but they can cause some damage (leaving trails), however when they do lose their kinetic energy and stop, they are still positively charged ions and so will be highly reactive, stealing electrons from nearby atoms. This would be a chemical disruption to the lattice, that could be what causes the birefringence to change at the end of the path? I think should have a Boltzmann-like distribution around a ring with radius governed by the average kinetic energy of the alpha particles ejected.
This is an interesting conversation 
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True in passing, but electrons are flowing through rock all the time; back to power stations, away from the now unbalanced alpha emitting atom electron shells, under thunderclouds. Holes are promptly filled, and it is not necessary to disrupt bonds to do so.
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As the quote stated, most of the energy is expended when the particle is finally stopped. Perhaps the speeding nucleus bumps into a few other atoms before hitting something squarely and dumping most of its velocity in that final collision. That energy is enough to break molecular bonds which changes the chemical structure of the molecule it strikes. I doubt there is a nuclear fusion reaction, so I would suspect that the discoloration is due to changes in covalent bonds.
After reading further into the thread, what @Jordan said.
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Why should alpha particles with more energy travel farther before colliding with an atom? That can’t be it. Are there no physicists reading this stuff?
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That doesn’t make sense though. The place where it hits something squarely should not be affected by energy level. @physicists?
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From the Talk.Origins web site:
The site mentions a number of problems with the explanation too.
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