Click on a number in the grid to see clues.

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Across: 1. When one nuclear reaction causes another, which causes another, etc. It allows nuclear reactions to sustain themselves. 2. When an atom gives off energy or nuclear particles (like alpha particles). 4. Together the protons and neutrons are called this: 5. When a radioactive atom gives off a helium atom (2 protons and 2 neutrons). Lowest in energy. Can be stopped by paper or skin. 7. Heaviest natural element; radioactive; used in nuclear fission reactors. 9. High energy radiation. Can cause great harm to biological organisms. Hard to stop (takes lead or many feet of concrete). 10. The splitting of an atom into smaller atoms. 11. An atom of the same element with a different number of neutrons.

Down: 1. A method of using the half-life of a radioactive isotope to determine the age of ancient objects up to 50,000 years old. 3. Radioactive decay of a neutron splitting into a proton and an electron. Medium energy: wood or clothing can stop them. 6. Bringing together two nuclei to make a bigger atom. 8. The time necessary for half of a radioactive material to decay.