different radioisotopes give off different kinds of radiation—alpha, beta, gamma, X ray, or neutron emissions—all of which behave differently. Alpha emitters, such as plutonium and radon, are intensely ionizing but don’t penetrate very far and generally can’t get through the dead layers of cells covering skin. But when they are inhaled from the air or ingested from radiation-contaminated food or water, they emit high-energy particles that can do serious damage to the cells of sensitive internal soft tissues and organs. The lighter, faster-moving beta particles can penetrate far more deeply than alpha particles, though sheets of metal and heavy clothing can block them. Beta particles are also very dangerous when inhaled or ingested. Strontium-90 and tritium, a radioactive form of hydrogen, are both beta emitters. Gamma radiation is a form of electromagnetic energy like X rays, and it passes through clothing and skin straight into the body. A one-inch shield of either lead or iron, or eight inches of concrete are needed to stop gamma rays, examples of which include cobalt-60 and cesium-137—one of the radionuclides of most concern in the Fukushima fallout