b'P ROVING THE P RINCIPLEchemically with other elements, the machinesatom smashersto find out arranges itself into pairs of elementsprotons and neutrons in their nuclei can what would happen when they fired with roughly half the mass of uranium,behave very differently upon being pro- streams of neutrons into various ele- such as (but not always) barium andvoked in certain ways.ments at high speeds. krypton. Energy is released in the formof twosometimes threeneutrons,Scientists soon found that they could Upon bombarding uranium with neu- heat, and other leftover particles. Manystrip the electrons or neutrons from cer- trons moving at a certain speed, they of the fission products are themselvestain atoms and bombard other elements found that some of the uranium atoms radioactive. Of all the uranium iso-with them. In the 1930s they built great break apart (fission). The debris topes, it appeared that only uranium-235 fissioned.The ancient quest for some sorcery thatPlaying Marbles with Atomic Pa r t i c l es would transmute one kind of matterinto another was over. Traditionally theA fter physicists had sorted out the Van de Graaff generators, cyclotrons, quest had been to make gold. Perhapskinds of particles that made up an synchrotrons, and betatrons are types ironically, however, it turned out thatatom, they learned that they could of particle accelerators. the nuclear sorcerers could turn goldseparate electrons or neutrons or pro- into mercury, but couldnt turn mercurytons from certain atoms. Then they Nuclear reactors split atoms apart, (or lead) into gold.invented ways to shoot the particles at producing streams of neutrons (andsamples of other elements. large amounts of heat). Targetsare placed inside theThe faster a beam could travel, the reactor close to themore energy it had when it struck the flow of neutrons. target, and the more interesting theresults. Most of what we have learned The neutronabout energy and matter in the twenti- was discov-eth century, we have learned by bom- ered in thebarding samples of matter with 1930s, theatomic particles. last of thetrio of elec-Particle accelerators, also called trons, protons,atom smashers, shoot streams of and neutrons toelectrons, protons, deuterons, alpha be found out.particles, or heavy ions at their tar- Because it had no elec-gets. The machines that create these tric charge, the neutron couldfast-moving streams rely on the appli- penetrate the nucleus of atoms, some- The central portion of the atom depicts the nucleus,cation of an electric field, which thing other particle accelerators had consisting of protons and neutrons, which give theeither attracts or repels particles of not been able to do. The big surprise atom most of its weight. Electrons orbit the nucleuslike charge. in the 1930s was to learn that shoot- and are considered weightless.ing a neutron at uranium-235 atomscaused them to split apart.2 0'