Isotopes - atoms with the same number of protons but a different number of neutrons. This means they have the same atomic number, but different mass number. Most elements have isotopes, but only one or two are stable, the rest are radioactive. Examples of isotopes are carbon 12 and carbon 14.
Radiation - a random process. An unstable nucleus could decay at any point, and are completely unaffected by physical conditions.
Background radiation - present at all times. Comes from:
- Naturally unstable isotopes in air, food, rocks, and building material.
- Space (cosmic rays mostly from the sun)
- Man made sources such as nuclear weapons, accidents, and waste.
Exposure - radiographers, commercial pilots, and miners all have increased risk of exposure to radiation.
Alpha, Beta, and Gamma Radiation
- 2 neutrons, 2 protons
- Electric charge of +2 and a relative atomic mass of 4
- Penetration power: stopped by paper or a few cm or air, strongly ionising, weakly deflected
- High energy electrons
- Electric charge of -1
- Penetration power: stopped by a few mm of metal, weak - moderately ionising. strongly deflected
- High energy electromagnetic radiation
- Electric charge and relative atomic mass of 0
- Penetration power: reduced by several cm of lead or ms of concrete, very weakly ionising, no deflection
When nuclei decay by alpha or beta emission, they change from one element to another. This can by written as nuclear equations, and they must balance on each side.
- The beta particle is the electron that is produced
- Like in alpha decay, the mass and atomic number of the new products add up to the mass and atomic number of the original element.
- However an electron is produced instead of He.
- Electrons have a mass number of 0 and an atomic numer of -1.
- Therefore the mass number of the new product (N) will remain the same and the atomic number will increase by one. 14 = 14, 6 + 1 = 7
Alpha decay is quite similar to beta decay in the sense that the atomic and mass number add together, but alpha decay produces different things
- The helium is the alpha particle as they both have the same amount of protons
- The mass number of the new product (Po) and He add up to the mass number of the mass number of original element (Rn) 215 + 4 = 219
- The atomic number of the new product (Po) and He must add up to the atomic number of the origianl element (Rn) 84 + 2 = 86
Half life - the average time is takes for the number of nuclei in a radioactive isotope sample to halve.
- Each time decay happens, a radioactive nucleus disappears as alpha, beta, or gamma is given out. As this happens more, it will emit less radiation as the activity decreases.The activity never reaches zero
"The activity of a radioisotope is 640 cpm. Two hours later, it is 80 cpm. Find the half life."
To do this, divide 640 by 2 until you reach 80 (640, 320, 160, 80)
You can see it took 3 division to get to 80. This means two hours (120 mins) represents 3 half lives. Therefore the half life of the radioisotope is 120/3 = 40 minutes.