<P> Elements heavier than iron, such as gold and lead, are created through elemental transmutations that can only take place in supernovae - as stars begin to fuse heavier elements, substantially less energy is released from each fusion reaction, and each fusion reaction that produces elements heavier than iron is endothermic in nature, and stars are incapable of carrying this out . </P> <P> One type of natural transmutation observable in the present occurs when certain radioactive elements present in nature spontaneously decay by a process that causes transmutation, such as alpha or beta decay . An example is the natural decay of potassium - 40 to argon - 40, which forms most of the argon in the air . Also on Earth, natural transmutations from the different mechanisms of natural nuclear reactions occur, due to cosmic ray bombardment of elements (for example, to form carbon - 14), and also occasionally from natural neutron bombardment (for example, see natural nuclear fission reactor). </P> <P> Artificial transmutation may occur in machinery that has enough energy to cause changes in the nuclear structure of the elements . Such machines include particle accelerators and tokamak reactors . Conventional fission power reactors also cause artificial transmutation, not from the power of the machine, but by exposing elements to neutrons produced by fission from an artificially produced nuclear chain reaction . For instance, when a uranium atom is bombarded with slow neutrons, fission takes place . This releases, on average, 3 neutrons and a large amount of energy . The released neutrons then cause fission of other uranium atoms, until all of the available uranium is exhausted . This is called a chain reaction . </P> <P> Artificial nuclear transmutation has been considered as a possible mechanism for reducing the volume and hazard of radioactive waste . </P>

Which reaction is an example of natural transmutation