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Thorium is thought of as radioactive and "dangerous" because its half life is 10 billion years. However, most rhenium is radioactive with a half life of 40 billion years even though rhenium does have a stable isotope which comprises a minority of natural rhenium atoms. Why don't we hear more about radioactive rhenium?

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  • $\begingroup$ I'm just guessing, but perhaps the abundance of thorium compared to rhenium has something to do with it? $\endgroup$ – JMac Sep 19 at 19:24
  • $\begingroup$ For the same reason we don't talk about radio-potassium? Potassium-40 (40K) is a radioactive isotope of potassium which has a long half-life of $1.251×10^9$ years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature. (Wiki) $\endgroup$ – Gert Sep 19 at 19:27
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    $\begingroup$ Wikipedia - “With an estimated average concentration of 1 part per billion (ppb), rhenium is one of the rarest elements in the Earth's crust.” $\endgroup$ – Farcher Sep 19 at 19:30
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    $\begingroup$ Danger may depend on what particles are emitted, what their energy is, and what happens to the daughter nuclei (if they too are radioactive). 187Rh has a pretty low energy beta decay to a stable daughter. 232Th decay kicks off a chain of 9 other decays before hitting 208Pb, and the longest half life of those is 228Ra at 5.7 years. $\endgroup$ – Jon Custer Sep 19 at 19:53
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    $\begingroup$ Iron-54, which comprises 5+% of naturally occurring iron, is suspected to have a half-life of 4.4x10^20 years. This means that over the course of the next 4,400,000,000,000,000,000,000 years, 2.5 percent of the iron in your car's engine may spontaneously decay to chromium. Experts are divided on whether this would constitute an illegal modification to the engine per racing rules, while team managers looking for an edge are attempting to forge engine blocks entirely out of iron-54 in the hopes that a self-chroming engine can be developed. Me? I'm not losing sleep over it, either way... $\endgroup$ – Bob Jarvis - Reinstate Monica Sep 20 at 3:28
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So $f=62.6$% of normal Rhenium is $^{187}$Re, decaying with a half-life of 40 billion years. That give a rate $r=\ln(2)/\text{40 Gyr}=7.927448\cdot 10^{-19}$ per second. That means that the decay rate per second is $f r N_A = 298,853$ Bq/mol.

$^{232}$Th has half life 14 Gyr and gives a rate per second as 1,364,005 Bq/mol. So it is about 4.5 times more active.

More importantly, it decays by $\alpha$ which tends to be nastier (if easier shielded) to biology than $\beta$. Also, it has a bunch of daughter isotopes that are gamma emitters and goes through radon, so they might get into the air. Rhenium just goes to osmium. Plus rhenium is rarely used, so people have not looked much at toxicity or radioactivity risks - they don't come up because rhenium rarely comes up.

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  • $\begingroup$ @JohnHennig - given that the US consumption of steel is in excess of 100 million metric tons per year, I'd put either 3 or 50 tons at 'rarely used', if not downright insignificant! $\endgroup$ – Jon Custer Sep 19 at 20:52
  • $\begingroup$ @JohnHennig - Sure, but I figured some context of the use of either, relative to common alloys, might be useful... Relative to millions of tons, the use of either Th or Rh is essentially nothing. Heck, the ~220 tons of beryllium, which can be quite dangerous itself, handily outweighs either of them. $\endgroup$ – Jon Custer Sep 19 at 21:13
  • $\begingroup$ Thorium used to be widely used as lamp screens back in the day, so it was also more present in the home than rhenium alloys. $\endgroup$ – Anders Sandberg Sep 19 at 23:53
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    $\begingroup$ I still have a bag of thorium lantern mantles in my camping kit. $\endgroup$ – hobbs Sep 20 at 3:27
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Thorium has the same decay energy as uranium with about 4 MeV. With 14 billion years its half live is even longer than uraniums half live of 4 billion years. That means the dose rate you get from thorium is even less than from uranium.

I see no reason why the radioactivity of thorium could be considered dangerous. Actually it should be safe to handle without much protection.

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