The Universe as a whole is the best labratory for studying nuclear physics. Where else can you find the 10⁹ K tempatures and a 15 billion year old experiment?

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Alpha-Carbon Reaction

After three helium nuclei have combined to form ¹²C in a red giant, another alpha particle is captured by the ¹²C to form ¹⁶O and gamma rays. These particles, ¹²C and ¹⁶O, are the ashes from a dying star. They are also the basis of life on Earth.

Big Bang Nucleosynthesis (see also Universe Expansion)
At the big bang, the universe had a temperature of 100,000,000,000 K. It was so hot that only protons and neutrons were formed. However, as the universe cooled, the amount of deuterium and helium nuclei increased. Just below 1 billion degrees there is a significant increase in deuterium and helium, and a decrease in the abundance of protons and neutrons. This was the deuterium "bottleneck". This used up the all the free neutrons and some protons, and caused the neutron line to drop off, and the proton line to dip (relatively few protons are used up). The deuterium abundance only increased to a point because it is an intermediate to the formation of helium. So as it was created, it is quickly consumed to complete the process of helium nucleosynthesis. Once all the neutrons were been used up, its presence dropped off.
For more information, see Formation of the Elements/ Nucleosynthesis in the early universe.

1. Pair Production
2. Positron Annihilation
3. Proton/Neutron Conversion
4. Proton/Neutron Collision
5. Helium Nucleosynthesis
6. Heavier Elements Nucleosynthesis
7. Electron Capture

Carbon-Nitrogen-Oxygen Cycle
In stars more massive than the sun (>1.1 Solar masses), this cycle is the primary process which converts hydrogen into helium. ¹²C serves as a catalyst, an ingredient which is necessary for the reaction but is not consumed.
Listing of the steps

Electron Capture
The nuclei formed early in the birth of the universe still had not enough electrons to form a neutral atom. But, the free electrons had plenty of energy. After 700,000 years of cooling, the nuclei started to capture electrons. These electron clouds are important. They stopped radiation from passing through the atoms.

• Helium Burning (triple alpha process)
  When temperature in the core of a star reaches 100 million degrees, three colliding helium nuclei fuse to form a carbon nucleus. This process occurs when the star is a red giant. The alpha-carbon reaction is the next step.

  Helium Nucelosynthesis
  When proton/neutron collisions produced deuterium, this sets up the way for the synthesis of the helium-4 nucleus. ⁴He was the largest nuclei produced in this stage of the universe. The energy density was too low to allow heavier nuclei to stick. At the start of nucelosynthesis, the relative abundance of protons and neutrons was 87% and 13% respectively. All of the neutrons were incorporated by the ⁴He. At the end of this wave, the universe consisted of roughly 25% helium and 75% hydrogen.

  • Pathway #1
  • Pathway #2

  Hydrogen Burning
  Hydrogen burning is the fusion of four hydrogen nuclei (protons) into a single helium nucleus (two protons and neutrons.) The process is a series of reactions. The type of reactions depend on the mass of a star and its core temperature and density. In our Sun, the process is a proton-proton chain. In more massive stars, the C-N-O cycle (Carbon-Nitrogen-Oxygen) serves to fuse hydrogen into helium.

  Nucleosynthesis of the largest Big Bang Elements
  The largest elements formed in the Big Bang are lithium and beryllium. The rest of the periodic table was formed in dying stars.

  • Lithium 6
  • Lithium 7
  • Beryllium 7

  Nucleosynthesis of Heavy Isotopes
  The processes described of stars only produce nuclei with mass up to 70. Gold and uranium came out of three additiional processes:

• s-process
• r-process
• rp-process

  Pair Production
  A collision process for gamma rays with energies greater than 1022-keV (two electron masses) where an electron /positron pair is produced. A heavy nucleus must be present for pair production. For high-energy gamma rays the pair production process is proportional to Z² and ln(gamma).

  Positron Annihilation

  Positron decay in matter by annihilation with an electron. Usually and "atom" of positronium (e+e-) forms which annihilates to produce two 511-keV photons. Occasionally, the positron will annihilate in flight to produce on or more photons sharing the total rest mass and kinetic energy of the positron and electron.

  Proton/Neutron collision
  After about 14 seconds after "the beginning" of the universe, the temperature dropped to 3,000,000,000 K. When neutrons and protons collided, it was to hot for them to stick together. Since deuterium (a proton and a neutron) could not be formed, there were no large molecules. As the universe cooled down, (see Universe Expansion) the nucleons (protons and neutrons) were able to stick together. Then, deuterium was able to form into helium.

• Hot Universe
• Cool Universe

  Proton/Neutron conversion
  Because of the high energy density at the start of the universe, the collisions which produced neutrons and protons balanced each other. As the universe cooled, the balance was skewed towards the protons. Neutrons have slightly more mass than protons. Because of the extra mass, they need more energy to form. When the energy available decreased, the reaction with more entropy was favored.

• Reaction #1
• Reaction #2

  Proton-Proton Chain
  In the Sun and other less massive stars, this chain is the primary source of heat and radiation. The proton-proton chain converts hydrogen into helium releasing energy in the form of particles and gamma-rays. Hydrogen is converted into helium in a chain of reactions. The first reaction takes an average of 1 billion years to occur while the others are much shorter. One step is only 1 second long. In the Sun, there are so many hydrogen nuclei that the 1 billion year waiting period does not stop it from producing tremendous radiation.
  List of the steps

  Proton Separation Energy
  The energy required to remove a proton from a nucleus.

  R-Process
  The r-process is a rapid sequence of neutron absorptions. It starts with a seed nuclide Z,A from the iron region, and one neutron is absorbed to give A+1. However, in the very rapid r-process, it is assumed that A+1 may not have time to decay before it absorbs another neutron and goes to A+2. This sequence continues moving toward the so-called "neutron drip line" until the probability for absorbing a new neutron is overwhelmed by the probability that a neutron will be knocked off by photodisintegration. This balance point defines the equilibrium A value for this Z. Some of the nuclei will decay to Z+1 by - emission during this equilibrium and provide a seed for a new series of neutron captures. The path of nucleosynthesis therefore moves up along a line somewhere between the valley of stability and the neutron drip line (the offset depending on conditions such as temperature, neutron flux, and photon flux) until finally fission kills off the chain up in the actinide region. This process clearly requires a huge neutron flux, and it is assumed to take place during the explosion of a supernova. That lasts only a few seconds; when the neutron flux shuts down, all the unstable nuclei produced along the r-process line decay by - emission to a stable final point. An analysis of this process indicates that the relative abundance of each nuclide should be proportional to the decay lifetime of its progenitor on the r-process line.
  -Educational Tour of Nuclear Data

  RP-Process
  The rp-process is very similar to the r-process, except it goes by successive proton absorption and + decay; thus, it tracks somewhere between the valley of stability and the "proton drip line."
  -Educational Tour of Nuclear Data

  S-Process
  In the s-process, one starts with existing iron-group nuclei. Therefore, it would only be expected to take place in second-generation stars that collapsed out of the residue of a previous supernova explosion. A flux of neutrons is required, and it is most likely that these neutrons come from various (,n) reactions in the helium-burning region of a red giant star. The seed isotope Z,A from the iron region absorbs a neutron, changing from A to A+1. If the new isotope is stable, it can absorb another neutron, going to A+2. If it is unstable, it is assumed that the neutron capture rate is low enough that the nuclide has plenty of time to decay to Z+1 by - emission before the next capture. The same neutron-absorption process is then repeated for Z+1. Thus, the nuclides produced lie in the "valley of beta stability" of the chart of the nuclides.
  -Educational Tour of Nuclear Data

  Triple Alpha Process (see helium burning)

  Universe Expansion
  In "the beginning", the universe was densely packed with radiation in the forms of photons and neutrinos. This energy collided with itself to create a dynamic wall. After each collision, the universe would get bigger. As the universe grew, the total energy stayed the same. Thus, the average energy or the temperature dropped to allow several important reactions proceed: Proton/Neutron collisions, Helium Nucleosynthesis.

  *The background image is a chart of the decays. Each box represents an nuclide. The neutrons are on the x-axis and the neutrons are the y-axis. Each color represents different decays. The white band in the middle is for stable isotopes. Notice how the isotopes farther from the stable band are more unstable.

  Glossary of Nuclear Science Terms

  Back to Exploring the Isotopes
  

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