Radioactivity and radioisotopes

1. Radioactivity and radioisotopes • The nature of radiation • Distinguishing Radiation • Background Radiation

2. Alpha particles Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha particles. These alpha particles are tightly bound units of two neutrons and two protons each (He4 nucleus) and have a positive charge. Emission of an alpha particle from the nucleus results in a decrease of two units of atomic number (Z) and four units of mass number (A). Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate. All alpha particles from a particular radionuclide transformation will have identical energies. Radioactive sample

3. Beta particles A radioactive nucleus that undergoes beta emission has a neutron in its nucleus converted into a proton and an electron, then it ejects the electron with high KE. The remaining nucleus has one more protons and one fewer neutron: the atomic number increases by one and the mass number stays the same. For example: Tritium, a radioactive isotope of hydrogen, undergoes beta decay. Its product is an isotope of helium Tritium  Helium High KE electron

4. Gamma - rays A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

5. Distinguishing radiations Every type of radiation has characteristic charge, mass and penetrative and ionizing power. The combination of these different properties can be used to detect and distinguish between a, band g radiation.

6. Penetrative power a b g Skin/paper stops ALPHA Thick lead stops GAMMA Thin Aluminium stops BETA

7. Deflection by B fields ALPHA: +ve charge + large mass  small deflection GAMMA no charge  no deflection B field into wall BETA: -ve charge + small mass  large deflection

8. Ionization by a-particles Alpha particles can ionize atoms by snatching electrons off the atoms when they pass by them. The alpha particles positive charge pulls the outer electrons away from the atoms as they approach them. An ion pair has been formed, where the electron is the negative member of the pair and the positive ion the positive member. Ion a-particle Ion pair Electron

9. What is background radiation? All around us there are radioactive substances that give low level radiation. All this radiation is called BACKGROUND RADIATION. These substances can be found: • in the earth's crust; • emanation of radioactive gas radon from the earth; • cosmic rays from outer space; • trace amounts of radioactivity in the body; • Radioactive waste.