Rubidium sounds exotic and Sci-Fi, but it is not rare - actually more abundant than copper on earth, yet it is hardly well-known in LENR. There are a number of reasons for that. Cost for one. It requires special handling as well.
Notably, rubidium is mildly radioactive in an unusual way - with a large proportion of unstable isotopes which have a very long half-life. 87Rb is a pure beta emitter with no gamma and the emitted electron is only 275 keV. Hot but not too hot, in the goldilocks tradition. The unstable isotope has an abundance of 28% of all rubidium - which is most remarkable, due to its long half-life. It will fog photographic film in a month, whereas U will do the same in an hour - yet the big unknown is this: can that slow decay rate be increased via LENR (possibly by UDH) so as to allow rubidium to operate as both a catalyst and as a fuel? Actually there is enough decay energy in Rubidium so that it could be the only fuel needed in LENR, if the decay rate could be drastically stimulated. Can it? There is one place where you have heard of this element most often. Rubidium-87 is the most popular atom for making Bose-Einstein-Condensates. As a BEC, the isotope is obviously a boson, though at first glance it seems like an odd choice to use a radioactive species. Plus 87Rb can be cooled with a common laser, like the kind used in DVD recorders - and. it is relatively high spin with a large nuclear magnetic moment. The dots are starting to connect. As a boson with a large magnetic moment, 87Rb is amenable to Efimov stability when combined with a photonic BEC and magnon BEC. This happens to a degree naturally with laser cooling and that is part of the reason why Rubidium-87 is the most popular isotope for making BECs, by far, in all of science. Obviously, helium seems like the likely choice for that, but it is much harder to condense. Among the alkali metals - Rb is also the best fit as a Mills' catalyst for the first level of redundancy at 27.2 eV. Yet potassium is more often used, since it is cheap and safe. Moreover, you will find few papers where Mills uses it so there could be more to the story. Why, if Rb is the best alkali catalyst due to its Rydberg level - is it not featured by BLP? The answer to that could lie in induced radioactivity. Mills is fully-invested in a non-nuclear explanation for thermal gain, so he tends to avoid anything nuclear. He avoids all talk of nuclear energy like the plague. Potassium is slightly radioactive, but it cannot be induced to emit at a higher level. If rubidium, due to having such a high percentage of unstable isotopes, is amenable, then it would spoil the "non-nuclear" sales pitch. Or maybe it is simply the high cost and low availability. Who knows. Anyway, in the context of LENR, where we would actually PREFER to have a mild beta decay candidate - rubidium could be the ideal alkali, since it is both a best-fit catalyst for Rydberg matter or fractional hydrogen - in place of, or in addition to potassium and lithium. but also if the unstable fraction can be induced into beta decay by the dense hydrogen which it forms - then the fuel value of rubidium is about 100,000 time greater than coal, pound for pound. The cost would drop substantially with higher demand. After all, Rb is more common than zinc and copper . and similar to nickel. Not a bad combination of features.
