X~rays and gamma rays: These are electromagnetic waves of relatively high energy and therefore having relatively high penetrability through matter. They may be emitted from a nucleus, a gamma ray, or produced by electron rearrangements outside of the nucleus, Alpha particles: X-rays. 1 These are helium nuclei and are emitted from the nucleus of radioactive atoms. | These particles have a low penetrability and, when they are emitted from an atom inside the body, they may produce substantial local damage. Beta particles: These are low to high speed electrons given off by a radioactive material. Their penetrability in matter is between that of alpha particles and X-rars, Half-Life The rate of emission of a given type of particle from a sample of radioactive material decreases with time. when this decrease is rapid the of its initial value is called its half-life. half-lives of from a small fraction of a second to many thousands of years have been observed for different radionuclides, The half-life for plutonium 239 is about 24,000 years. Thus, after 24,000 years it will have emitted half of that remaining, and so on. It is as though we started out with $100 and after one half-life we have $50, after another half-life we are down to $25, another one leaves $12.50, and so on until nothing is left and the element has returned to its original stable form, pu24% is one of the longest-lived elements. and cesium 137 have half-lives of about 30 years, lives of a few months, days, minutes, or seconds. Strontium 90 Other isotopes have half- ahe The time for the rate of emission to reduce to one-half t to be long-lived, i am _*See' j__ i 4 wk isotope is said to be short-lived and when it is long the isotope is said