Transmutation and Radioactivity in Astronomy
The greatest source of energy on Earth is due to collisions and changes of atomic nuclei, or transmutation. It may first be guessed that heavy elements such as uranium and plutonium are the chief sources of this energy, because they are extensively used in nuclear power. In actuality, the potential of solar power far exceeds that of nuclear, even though the transmutations that produce solar radiation occur between some of the smallest atoms possible.
The core of the Sun is a center of nucleic contact; nearly three-fourths of its mass is comprised of the lightest element, hydrogen. Collisions between these trillions and trillions of particles occurs about 9.2 × 1037 times every second. Some instances of fission and fusion produce gamma rays by the annihilation of matter and antimatter, which account for the energy radiated from the star.
The Proton-Proton Chain Reaction
The common hydrogen atom consists of one proton and one electron. During the chain reaction between hydrogen atoms, however, many isotopes and subatomic particles are involved, resulting in several steps of fission and fusion, detailed in the image below. Recall from Isotopes in Astronomy that isotopes are designated in the MNE format, where N is the atomic number, number of protons; M is the atomic mass, (protons + neutrons); and E is the element symbol.
The first step consists of pairs of hydrogen atoms colliding with other. In each collision, one proton loses a positron (the particle that gives the proton its positive charge) and turns into a neutron. The resulting atom has an nucleus with one proton and one neutron; it is the hydrogen isotope Hydrogen-2, deuterium.
11H → 11H + 21H + e+ + ve
Another Hydrogen-1 (Protium) atom then collides with the deuterium, producing the Helium-3 isotope and releasing gamma radiation.
11H + 21H → 32He + γ
Finally, two 32He atoms from identical reactions then collide to release two protons (protium) and form a Helium-4 isotope.
32He + 32He → 11H + 11H + 42He
The proton-proton chain reaction is the main producer of energy in the Sun, but variations and other transmutation chains exist as well.
EXAMPLE 1: An alternate branch of the Proton-Proton Chain involves 32He colliding with an existing 42He, synthesizing a new atom. If the only other byproduct of this reaction is gamma ray energy, give the isotope symbol of the created atom.
ANSWER: Since the fusion of 32He and 42He release no subatomic particles, the resulting atom will retain all the protons and neutrons present in its two components. Helium-3 has two protons and one neutron, and Helium-4 has two protons and two neutrons. These subatomic particles will combine to form the nucleus of an atom (2+2)+(2+1)(2+2)A = 74A. The element with atomic number 4 is Beryllium, therefore, the fused isotope is 74Be.
EXAMPLE 2: In stars many time the mass of the Sun, more complex nucleic interactions take place. One such chain is termed the CNO cycle, which consists of six steps in a loop.
a) In one step, 136C is bombarded by an unknown atom, and the transmutation produces gamma ray energy and 147N. What is the unknown atom?
b) In another step, 157N and a protium atom collide. One of products is 126C. What is the other?
a) We can see from the symbolic notation that 147N has an extra proton compared to 136C, and that the ntirogen isotope's atomic weight is only one amu more than the carbon isotope's. Therefore, there is no change in the number of neutrons in the nucleus. The unknown atom is 11H, a protium atom.
b) 157N and 11H collectively have 8 protons and 8 neutrons. The product 126C has 6 protons and 6 neutrons; Thus, the remaining subatomic particles combine to form an atom A with configuration (8-6) + (8-6)8-6A = 42A. From the bottom number we can identify the number of protons in the atom and the identity of the element; the element with atomic number 2 is He. The other product is 42He.