Lithium, Big Bangs and Stellar Nucleosynthesis

When it comes to nuclear weapons a very important fusion reaction is

H3 + H2 > He4 + n + 17.6 Mev

It is a handy reaction because it occurs at relatively low temperatures, that is relative to other fusion reactions.

Tritium is produced in nuclear reactors by bombarding lithium-6 with neutrons

Li6 + n > He4 + H3 + 4.8 Mev

Tritium for US nuclear weapons is currently produced in civil nuclear reactors, and presumably the same applies to Russia and China (H3 has a half life of 12.32 years).

Lithium is the lightest solid element in the universe, and this fact makes it important for thermonuclear weapons. In a thermonuclear device it is convenient to have a solid material of Li6 and H2 whereby the neutrons produced from the explosion of the fission primary produce tritium then and there, so to speak, and the process of fusion itself produces more neutrons which also leads to the further production of tritium.

Hydrogen, helium and lithium were first produced not long after the big bang, a process known as big bang or primordial nucleosynthesis. 75% of the H in the universe was produced by primordial nucleosynthesis, 25% He and 25% Li.

Primordial nucleosynthesis was developed in a famous paper known as the Alpher-Bethe-Gamow paper even though Hans Bethe, who was the head of the theoretical division at Los Alamos during the Manhattan Project, was not involved in the development of the theory.

George Gamow, who was somewhat the joker, wanted Bethe’s name affixed to the paper so that it would go down in the annals of physics as the Alpha-Beta-Gamma paper, appropriate enough given the subject matter. The interesting thing about primordial nucleosynthesis and ABG is that it was an early portent of an important concept, namely how the study of the very small, in this case nuclear physics, can illuminate features of the universe as whole, the preserve of cosmology.

Another crucial paper was, of course, the B2FH paper.

It was not known for certain how much of the remaining lithium in the universe was produced, although it was understood that much of it had to have been created in some process of stellar nucleosynthesis.

A paper just published in the Monthly Notices of the Royal Astronomical Society shows that lithiium, to be precise Li7, is produced via stellar nucleosynthesis in novae,

The solution to the enigma of the origin of lithium lies, according to this study, in the novae, explosive phenomena occurring in binary star systems in which one of the stars is a white dwarf. The white dwarf can nab material from its twin star and form a superficial layer of hydrogen which, when it reaches a certain density, will trigger an explosion — a nova — which can increase the brightness of a star up to one hundred thousand times. After a few weeks the system stabilises and the process starts again

The process can continue so long as the white dwarf does not exceed the Chandrasekhar Limit. To continue our little theme, I can’t help but point out that the Chandrasekhar Limit is analogous to the Oppenheimer-Volkoff Limit. It is interesting that this is probably Oppenheimer’s most important scientific contribution, and it is more astrophysics than particle physics.

Back to our Nova and Li

The researchers studied nova Sagittarii 1015 N.2 (also known as V5668 Sgr), which was detected on 25 March 2015, and remained visible for more than eighty days. The observation, made with the UVES instrument of ESO’s Very Large Telescope over the course of 24 days, made it possible for the first time to follow the evolution of the beryllium-7 signal inside a nova and even to calculate the amount of it present. “Beryllium-7 is an unstable element which decays into lithium in 53.2 days, so its presence is an unequivocal sign of the existence of lithium”, says Christina Thöne, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC).

The cool thing about Sagittarii 105 N.2 is that it was discovered by a citizen scientist, which goes to show that there is still hope yet for amateur natural philosophers in the era of the Anthropocene, which hitherto has been dominated by big science.

Beryllium, of course, was also used as a source of neutrons and a neutron reflector in nuclear weapons, especially in the earliest fission explosives.

As a nuclear analyst, I can’t help but notice nifty little papers/results such as this.

While we are here, why not throw in a well known game of chess between Einstein and Oppenheimer. Einstein owns Oppenheimer in this game, and he opened with the Spanish Defence and took care to develop his pieces even though it cost him early in the game.

Einstein played like a true European intellectual of the classical pre-World War Two era.