Radiation

Radiation Glossary Slide 3: Radioactive Sources Slide 4: Types of Radiation Slide 5: Activity Slide 6: Ionisation Slide 7: Shielding Slide 8: Half-life Slide 9-10: Absorbed Dose Slide 11-12: Equivalent Dose Slide 13-14: Nuclear Fission Slide 15: Nuclear Fusion

Radioactive Sources A radioactive source gives out radiation. The radiation given out (emitted) comes from the nucleus of atoms inside the radioactive source. The three common types of radiation that appear in National 5 Physics are alpha, beta and gamma.

Types of Radiation An alpha particle is made up of two protons and two neutrons. It is a helium nucleus. A beta particle is a fast-moving electron. A gamma ray is a high energy electromagnetic wave. + -

Activity The activity of a radioactive source is a measure of how much radiation it gives off per second. Activity = Number of radiations emitted per second There were 8 radiations/decays emitted in the 10 seconds. A = N/t Consider this radioactive uranium nucleus. If we count how many decays it gives out in 10 seconds we can determine the activity of the source. When you next click your mouse the timer will begin. Be ready to count the radiations emitted. Activity = Number of radiations/time A = 8/10 - + + A = 0.8 Bq A = N/t s + - 0.8 decays per second. Bq (Becquerels) decays - In reality, because a radioactive source is made up of so many atoms, there are millions of decays per second, so it is more common to see MBq which is mega Becquerels (a million Becquerels). Time (s) Uranium Nucleus 10 2 1 6 3 4 8 9 5 7

Ionisation Beryllium Atom Ionisation occurs when an atom gains or loses an electron. All types of radiation can cause ionisation which is why they can be dangerous. Alpha particles are the most ionising of the radiations. Beryllium Atom When the alpha particle passed through the Beryllium atom it caused ionisation (the atom lost an electron). +

Shielding We can protect ourselves from radiation by shielding ourselves. Thin sheet of paper A few mm thick aluminium A few cm thick lead NOT TO SCALE! + The alpha particle gets absorbed easily merely by paper never mind aluminium or lead. The beta paticles passes through paper but gets absorbed by a few millimetres of aluminium. The gamma ray passess through paper and aluminium easily but gets absorbed by a few cm thick of lead. -

Half-Life Half-life is a measure of the time it takes for the activity of a radioactive source to decrease to half its original value. Over time a radioactive source becomes safer because its activity decreases over time. Example If a radioactive source had an activity of 500 Bq (500 radiations emitted per second) then you checked it after 10 minutes and the activity was 250 Bq then the half-life of the source must be 10 minutes. If you checked it again in another 10 minutes the activity would be 125 Bq. In another 10 minutes, 67.5 Bq. It is becoming safer over time!

Absorbed Dose Absorbed dose is a measure of how much energy from radiation is absorbed per kilogram of mass. Consider Dwayne “The Rock” Johnstone being exposed to a radioactive source. If we know the energy of the radiation he absorbs and his mass then we can determine his absorbed dose. Say each radiation has an energy of 5J and The Rock has a mass of 100kg. As he absorbed 4 radiations then the energy he absorbed is 20J in total. Absorbed Dose = The energy absorbed per kilogram of mass Absorbed Dose = Energy/mass D = E/m D=E/m Radioactive Source kg + - Gy (Grays) J (Joules) D = 20/100 - D = 0.2 Gy 0.2 joules of energy per kilogram of mass In reality, each radiation has a very small energy, usually in mJ (milli-joules, divide by 1000) or μJ (micro-joules, divide by 1000,000). Be careful of this.

Absorbed Dose Now consider a thinner man. He is exposed to the same radioactive source as The Rock but his mass is smaller. Say each radiation still only has 5J of energy but this man’s mass is 50kg. The total energy absorbed is still 20J. D = E/m D = 20/50 D = 0.4 Gy Radioactive Source + - 0.4 joules of energy per kilogram of mass. - Significantly more than The Rock’s absorbed dose because his mass is less and the energy is the same.

Equivalent Dose The absorbed dose does not take into consideration the type of radiation which is absorbed and the extra harm this can cause. The equivalent dose does do this. It factors in the absorbed dose and the type of radiation. Equivalent Dose = Absorbed Dose x Radiation Weighting Factor H = Dwr (no units) The Radiation Weighting Factor (wr) can be found on the back of your data sheet. It is specific to the type of radiation absorbed. Gy Sv (Sieverts)

Equivalent Dose In National 5 Physics you won’t be given a source which gives out different types of radiation; it will just be a source with one type so that the equivalent dose is easy to calculate. Alpha particles have a radiation weighting factor of 20 (check data sheet). So The Rock’s equivalent dose can now be calculated. Consider The Rock one final time. He is exposed to a radioactive, alpha particle source. Each radiation still has 5J of energy and he still has a mass of 100kg. Radioactive Source + + + + As he still only absorbs four radiations then his absorbed dose is still 0.2 Gy. H = Dwr H = 0.2 x 20 H = 4 Sv

Nuclear Fission Nuclear fission occurs when a large nucleus breaks into two smaller nuclei, releasing a few individual neutrons and energy as well.

Nuclear Fission When a neutron is absorbed by a large nucleus it can cause it to become unstable. When this is the case, nuclear fission can occur. After fission, smaller nuclei are formed and neutrons and energy are released. The neutrons released then go on to cause more fission reactions with surrounding nuclei. This causes a chain reaction as more neutrons are released with every fission reaction.

Nuclear Fusion ENERGY Nuclear fusion occurs when two small nuclei fuse (merge) together to create a larger nucleus, releasing individual neutrons and energy at the same time.