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, electron rearrangements outside of the nucleus, Alpha particles: or produced hy X-rays. 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-rays. 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 isotope is said to be short-lived and when it is long the isotope is said to be long-lived. The time for the rate of emission to reduce to one-half 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. thus, ‘The half-life for plutonium 239 is about 24,000 years. 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 wo 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. and bvu23% is one of the longest-lived elements. cesium 137 have half-lives of about 30 years. lives of a few months, days, minutes, gg 268 , or seconds. 4 Strontium 90 Other isotopes have half-