C HAPTER 10: R ADIOACTIVITY 10.1 Radioactivity 10.2 Rates of Nuclear Decay

D ISCOVERY OF R ADIOACTIVITY Radioactivity was discovered by the French physicist Henri Becquerel. He had left some uranium salts on a photographic plate overnight and later developed the photograph. He discovered that the uranium contained some type of energy that left an image on the photograph. That energy was radiation. Radioactivity is the process where an unstable atomic nucleus emits particles and energy.

T HE TYPES OF N UCLEAR R ADIATION Alpha particles  An alpha particle is composed of 2 protons and 2 neutrons. Recall that protons and neutrons have an atomic mass of one, so the total mass number of an alpha particle is 4. Since there are no electrons present, the charge of the particle is a 2+ (2 protons with a 1+ charge) Alpha particles are the least penetrating type of radiation. These particles can not even pass through a single sheet of paper. These particles are only dangerous to us if inhaled or ingested.

T HE TYPES OF N UCLEAR R ADIATION Beta particle  A beta particle is equal to an electron (so the mass is disregarded as electrons are ignored). Beta particles have a charge of 1- (same as an electron). What actually happens is that one neutron in an atom breaks down to form a proton and an electron. The proton is kept in the nucleus and the electron is emitted from the atom. A beta particle can pass through a sheet of paper, but is stopped by a thin sheet of aluminum. Beta particles are more dangerous to the body than alpha particles.

T HE TYPES OF N UCLEAR R ADIATION Gamma ray  Unlike alpha and beta, gamma rays have no mass and no charge. Gamma rays move at the speed of light. They can penetrate paper and aluminum but will be stopped by a thick piece of lead or several meters of concrete. Gamma rays can penetrate deep into the body through the skin and tissues to cause damage to all organs of the body,

T HE R ATE OF N UCLEAR D ECAY Half-life is the rate at which a radioactive substance decays. Half-life describes the time it takes for one half of the radioactive substance to give off its radiation (decay). Nuclear decay produces a constant rate of decay. Not all of the atoms give radiation at the same time and the variety of radioactive elements decay at different rates. See the chart on page 299 to see the different half-life period for some select radioactive elements.

C ALCULATING H ALF -L IFE Calculating half-life can be a little tricky at first but can be easily mastered. The main idea to understand about half-life is that a radioactive substance never fully loses all of its radioactivity. As a fraction, you can calculate the amount of a substance that is still radioactive after a certain number of half-lives. Let’s say a radioactive substance undergoes 3 half-lives of decay. What fraction of the substance is still radioactive? Multiply ½ times itself for each half-life ---- ½ x ½ x ½ = 1/8  so 1/8 th of the substance is still radioactive after 3 half-lives.

C ALCULATING H ALF -L IFE BY MASS Iodine-131 has a half-life of about 8 days If we started out with a 200 gram sample of Iodine-131 and held it for about 32 days, the substance would undergo 4 half-lives (32/8 = 4) So how many grams of the iodine would still be radioactive after 4 half-lives? 200 g  100 g  50 g  25 g  12.5 g Each arrow above symbolizes one half-life