Fusion Reactions

The nuclear reactions that give the sun its heat can be duplicated on a small scale in a cyclotron or similar machine. With it the nuclei of light atoms are speeded up in an electric field to very high velocities. Their energies are measured in terms of a unit of one million electron volts (Mev). This is the energy acquired by an electron or a proton​—any particle with a single charge—​when it is drawn across an electric field of one million volts. A beam of such particles is directed against a target to induce a reaction between the nuclei in the beam and those in the target.

The accompanying table shows some of the fusion reactions that have been studied by nuclear physicists in their laboratories. In each case, one of the particles shown before the arrow is fixed in a target and the other strikes upon it at high velocity. To illustrate, in the first reaction shown, the nucleus of one hydrogen atom strikes another, fusing with it and throwing out a positive electron. There is a shrinkage of mass, which is converted into energy in accordance with Einstein’s famous equation E = mc². Consequently, the particles formed fly apart with more energy than the merging particles had. In this case the energy gained is 2,000,000 electron volts.

For comparison, when coal is burned the oxidation of a carbon atom yields only four electron volts. In nuclear reactions we are dealing with energies millions of times greater than in chemical reactions.

The first three reactions in the table are believed to be the principal ones taking place in the sun. Some of the other reactions may be easier to achieve in the laboratory. You will note that in reactions 3, 5 and 6, where helium-4 is produced, the energy gain is much greater. This results from the very tight bonds formed between two protons and two neutrons. Helium (He⁴) is a very stable element.

[Diagram on page 20]

(For fully formatted text, see publication)

H¹ PROTON

n¹ NEUTRON NUCLEAR BUILDING BLOCKS

H¹ HYDROGEN

H² DEUTERIUM HYDROGEN ISOTOPES

H³ TRITIUM

He³

He⁴ HELIUM ISOTOPES

ENERGY FROM FUSION REACTIONS

In the Sun:

(1)  H¹ + H¹ H² + e⁺ 2.0 Mev

(2)  H¹ + H² He³ 5.5 Mev

(3)  He³ + He³ He⁴ + H¹ + H¹ 12.9 Mev

Other Reactions:

(4)  H² + H² He³ + n¹ 3.2 Mev

H³ + H¹ 4.0 Mev

(5)  H² + H³ He⁴ + n¹ 17.6 Mev

(6)  H² + He³ He⁴ + H¹ 18.3 Mev