Nucleosynthesis

Discussion

big bang nucleosynthesis

By the first millisecond, the universe had cooled to a few trillion kelvins (10¹² K) and quarks finally had the opportunity to bind together into free protons and neutrons. Free neutrons are unstable with a half-life of about ten minutes (614.8 s) and formed in much smaller numbers. The abundance ratio was about seven protons for every neutron. Before one neutron half-life passed nearly every neutron had paired up with a proton, and nearly every one of these pairs had paired up to form helium. By this time the universe had cooled to a few billion kelvins (10⁹ K) and the rate of nucleosynthesis had slowed down significantly. By the time the universe was three minutes old the process had basically stopped and the relative abundances of the elements was fixed at ratios that didn't change for a very long time: 75% hydrogen, 25% helium, with trace amounts of deuterium (hydrogen-2), helium-3, and lithium-7. Big Bang nucleosynthesis produced no elements heavier than lithium. To do that you need stars, which means waiting around for at least 200 million years.

we are all made of stars

More than ninety per cent of the universe is composed of hydrogen and helium. Both elements have been around since shortly after the beginning of the universe. Yet, hydrogen and helium together won't make anything as complex and as interesting as the Earth, or a bacterium, or a refrigerator, or you and I. To do that we need carbon and oxygen and nitrogen and silicon and chlorine and every other naturally occurring element. Almost all the hydrogen and helium present in the universe today (and some of the lithium) were created in the first three minutes after the big bang. All of the other naturally occurring elements were created in stars.

Stars like the Sun

Details were discussed in the section on Fusion. The basic parts of the reaction are…

Which overall yields…

4¹₁H → ⁴₂He + 2[⁰₊₁e + ⁰₀γ + ⁰₀ν] (26.7 MeV)

Stars heavier than the Sun use ¹²₆C as a catalyst.

You need really massive stars for this — say 20 to 120 times the mass of the Sun.

Really, really heavy stars do something different.

The Mass-5 and Mass-8 Bottlenecks. There are no stable isotopes (of any element) having atomic masses 5 or 8. But there is always a small amount of ⁸₄Be at any moment that is available to fuse with a third helium to produce ¹²₆C. This improbable sequence is called the triple-alpha process because the net effect is to combine 3 alpha particles to form a ¹²₆C nucleus. The triple-alpha process is not relevant in main sequence (normal) stars like the Sun because their core temperatures are too low. However, in the red giant phase, after many stars have accumulated vast amounts of helium in their core, the central temperature can rise high enough (10⁸ K) to initiate the triple-alpha process.

Overall

3[⁴₂He] → ¹²₆C + ⁰₀γ (7.4 MeV)

In order of increasing alpha number, the following forms of fusion take place…

How to Cook Everything

Mix it all up and get everything from hydrogen to uranium (and maybe even up to californium).

• slow neutron capture within stars (s-process) will not synthesize elements heavier than iron
• rapid neutron capture during supernova explosions (r-process) heavy hydrogen through uranium

how like a god

Ernest Rutherford (1871–1937) New Zealand–Canada–England was the first to transform one element into another.

¹⁴₇N + ⁴₂He → ¹⁷₈O → ¹₁H

technetium

• Discovered by Perrier & Segre in a sample of Molybdenum that had been irradiated by deuterons at the UC Berkeley cyclotron by E.O. Lawrence and then sent to Italy.

  9642Mo + 21H →	9843Tc
  9843Tc →	9844Ru + 0−1e + 00ν

• Contamination hazard

  9842Mo + 10n →	9942Mo
  9942Mo →	9943Tc + 0−1e + 00ν
  9943Tc →	9944Ru + 0−1e + 00ν + 00γ

• Radioactive tracer

  ? →	9543Tc*
  9543Tc* + 0−1e →	9644Ru + 00ν + 00γ

• From New Scientist, "Famed scientist P.W. Merrill fifty years ago observed the signature of live technetium - an element that has no stable isotopes - in the starlight from certain types of stars, thereby proving the then-controversial theory that stars make atoms via a process called nucleosynthesis."

promethium

• First synthesized at Ohio State in 1941

  14660Nd + 10n →

  14761Pm + 0−1e + 00ν

• Identified chemically by Larry Glendenin, Jacob Marinsky, and Charles D. Corell in 1944

transuranic, cisuranic, superheavy