Bismuth No Longer the Heaviest Stable Element

forii writes "Bismuth-209 was commonly thought to be the heaviest stable element. But now Physicists have discovered that Bi-209 actually is unstable and decays with a halflife of 2*10^19 years. This means that the average 8oz (237ml) bottle of Pepto-Bismol contains one decay event every 36 hours or so."

Comments

Proton decay

[reverseengineer]

"Other kinds of decays such as protons from proton-rich nuclei could be studied by the same method but this will have to be proved!"

This could prove to be the most important use of this technique, as most proposed Grand Unified Theories have interactions that can turn quarks into leptons, so that a proton would be expected to eventually decay into a positron and a meson. Unfortunately, this process has never been observed (well, only somewhat unfortunately, as high proton stability is definitely a Good Thing in most ways), and experiment and theory have thus set a lower bound on the lifetime of a proton of roughly 10^33 years, about 23 orders of magnitude greater than the estimated current age of the universe.

As you can see, compared to the suggested lifetime of a proton, even Bi-209 seems unstable. The expected extreme rarity of a proton decay event, however, is somewhat balanced by the overwhelming abundance of protons in the universe.The "lifetime" for an individual proton is more like a life expectancy, an average figure- given a suitably large collection of protons, odds are good that at least one would decay in a reasonable timeframe. If you carefully watch 10^33 protons for a year, for example, and reality agrees with theory (big if), then it is likely (certainly not guaranteed though) you will see at least one decay event. Now, 10^33 may sound like a tremendous amount, but remember that each proton has a mass of only 1.67*10^-27 kilograms, so that 10^33 protons would have a mass of about 1,600 metric tons- a lot, but not outrageous.

The real problem lies in that "carefully watching" part. So many other forms of radiation are much more prevalent, and so might mask the signature of proton decay. Cosmic rays, naturally occuring radioisotopes in places you'd never think to look, solar neutrinos, that sort of thing. Ah, why yes, this is one of those experiments they do in a salt mine and uses a gigantic tank of ultrapure water (your proton source). However, as of yet, no one has found concrete evidence for proton decay from one of these experiments. Go here for a excellent site about a proton decay detector that ran in the 80s, and here for one currently in use.

Perhaps this process will detect this very rare event, lending profound support to one of the many supersymmetric models out there. Unfortunately, if it does not detect proton decay, it will be much more difficult to say just what the result means, it being difficult to prove a negative and all.