Valentine Pontifex

Here is another article on Po Halos.

<a href="http://www.ebonmusings.org/evolution/gentry.htm" target="_blank">http://www.ebonmusings.org/evolution/gentry.htm</a>

Patrick and John, since you two know geology, what do you think about it?

ps418

Looks great to me. From my biased perspective, though, there is no need to even get into nuclear physics, since Gentry's interpretation cannot even be reconciled with the easy-to-understand field evidence. Maybe if I was a physics buff I'd see things differently though.

The article makes some good points regarding Gentry's assumption of decay rate change:

Gentry makes the erroneous statement that "the probability for alpha decay is dependent on the energy with which the particle is emitted from the nucleus," and then proceeds to argue that radioactive decay rates have varied over time. He claims that polonium haloes are evidence that decay rates of isotopes have not been invariant over Earth history. In other words, to reconcile the young age of the Earth represented in his hypothesis with reported isotopic age dates for rocks around the world, Gentry is forced to conclude that decay rates for polonium have remained constant while those of all other radioactive isotopes were many orders of magnitude greater 6,000 to 10,000 years ago. This of course gives rise to several major inconsistencies:

all of the different decay schemes for all radioactive isotopes must have been accelerated by just the exact (but different) amounts to give the consistent age dates we find for rocks today. For example, the decay rate for potassium-40 (half life = 1.25 b.y.) would have to be accelerated by nearly four times the rate for uranium-238 (half life = 4.5 b.y.). Given the number of radioactive isotopes affected, the chance of this coincidence taking place is essentially zero.

speeding up radioactive decay rates to be consistent with a 6,000 - 10,000 year age for the Earth requires that the energies of decay 10,000 years ago would have been extreme, keeping the Earth in a molten state to the present day.
if one is going to propose that radioactive decay rates varied, and varied differently over time, there is no reason why the decay rates of one particular element - polonium - should not also have varied. Under a variable decay rate model, it can even be proposed that polonium decay rates were much longer than today. In fact, once the idea of variable decay rates is introduced, it becomes impossible to assign pleochroic haloes to any specific isotope or isotopic series, and Gentry's hypothesis falls completely apart.

The decay rate and the energy of emitted particles are both related to the imbalance of neutrons and protons in an atomic nucleus, and are controlled by the weak nuclear force and the binding energy for the particular nuclide. Anything more than a fractional change in the decay rate over time would require variation of the fundamental forces of nature and the relationship of matter and energy. There is no evidence that anything of the sort has ever occurred.


Giving Gentry the benefit of the doubt that the concentric ring haloes are actually due to radiation damage, an immediate problem arises with the short half-life of the polonium isotopes themselves. In order to leave a visible radiation damage halo, the affected mica (or fluorite) grains would have to crystallize before the polonium decayed away - about 10 half lives. For polonium isotopes, this correlates to between a fraction of a second (Po-212, -214, -215) and 3.79 years (Po-210). Magmas which cool rapidly form glasses (e.g., obsidian), or show quench textures (as in komatiite basalts). Laboratory tests and field studies of heat flow show that granites take from thousands to 100s of thousands of years to crystallize ­ with hydrated minerals such as micas forming in the later stages of crystallization (Hyndman, 1972). The rocks Gentry calls granites show neither of these features. Rather, the samples show medium-grained to large-grained texture. Field studies show that Gentry's samples are from much younger rock units than the oldest portions of the sample area. "Primordial" polonium would have been long gone by the time of crystallization of these rocks and could not possibly be the source of any radiation damage haloes.

John Solum

Good catch LordValentine, but I don't know that it's "the definitive response".

I was happy to see the links to John Brawley, Richard Wakefield, and Laurence Collins' web sites, I think they're good resources too. Those websites also have pictures of the haloes, which I think is valuable.

I'm not comfortable with this. Observations of radiation-damage-haloes show that concentric rings do exist. Mr. Baillieul doesn't deny this, but I don't think his explanations explain them.
I don't think Mr. Ballieul is correct in claiming that alpha decay can't result in discontinuities (i.e., the concentric rings). As he points out higher energy particles can penetrate further into the crytal from the source than lower energy particles, and he is correct that the particles will interact with the crystal along the entire path length (i.e., some particles will lose their energy before they reach the end of their maximum possible path length). However, I don't think this invalidates the idea that alpha decays of different energies are responsible for the concentric rings. Lets say a low energy alpha particle can penetrate a maximum of .1mm into the sample (I'm just making these numbers up) and that a high energy alpha particle can penetrate a maximum of .3mm. The damage to the crystal structure caused by the low energy decay will occur in a sphere with a radius of .1mm (the damage will be distributed throughout that sphere since some alpha particles will lose their energy before they travel the maximum distance of .1mm). The damage associated with the high energy decay will result in a sphere with a radius of .3 mm (again, the damage will be distributed throughout the entire sphere). The final result will be a spherical zone of damage with a radius .3 mm, but imprinted on that sphere will be a smaller concentric sphere with a radius of .1mm. Mr. Ballieul is correct that the damage won't occur soley at a distance of .1 and .3mm, but alpha decays with different energies will result in damage spheres of different radii, which will result in concentric rings.

This isn't meant as a defense of Gentry's work. Even if the cocentric rings are the result of alpha decays of different energies, as others have pointed out (go to the article by John Brawley for example), Gentry hasn't demonstrated that the haloes are Polonium haloes and not Radon-222 haloes.

Basically my comments are a long way of saying that I don't agree with Mr. Ballieul's first conclusion. Other than that, I think it's a fine paper.