Simple Nature - Light and Matter

energy as would be produced in a nuclear power plant by one kg offuel.Although nuclear reactions that use up energy (endothermicreactions) can be initiated in accelerators, where one nucleus isrammed into another at high speed, they do not occur in nature, noteven in the sun. The amount of kinetic energy required is simplynot available.To find the amount of energy consumed or released in a nuclearreaction, you need to know how much nuclear interaction energy,U nuc , was stored or released. Experimentalists have determined theamount of nuclear energy stored in the nucleus of every stable element,as well as many unstable elements. This is the amount ofmechanical work that would be required to pull the nucleus apartinto its individual neutrons and protons, and is known as the nuclearbinding energy.A reaction occurring in the sun example 8The sun produces its energy through a series of nuclear fusionreactions. One of the reactions is1 H + 2 H → 3 He + γThe excess energy is almost all carried off by the gamma ray (notby the kinetic energy of the helium-3 atom). The binding energiesin units of pJ (picojoules) are:1 H 0 J2 H 0.35593 pJ3 He 1.23489 pJThe total initial nuclear energy is 0 pJ+0.35593 pJ, and the finalnuclear energy is 1.23489 pJ, so by conservation of energy, thegamma ray must carry off 0.87896 pJ of energy. The gamma rayis then absorbed by the sun and converted to heat.self-check EWhy is the binding energy of 1 H exactly equal to zero? ⊲ Answer, p.925Figure a is a compact way of showing the vast variety of thenuclei. Each box represents a particular number of neutrons andprotons. The black boxes are nuclei that are stable, i.e., that wouldrequire an input of energy in order to change into another. Thegray boxes show all the unstable nuclei that have been studied experimentally.Some of these last for billions of years on the averagebefore decaying and are found in nature, but most have muchshorter average lifetimes, and can only be created and studied inthe laboratory.The curve along which the stable nuclei lie is called the line ofstability. Nuclei along this line have the most stable proportionSection 8.4 Nuclear Energy and Binding Energies 497