Radioactivity

Definition - nuclei of unstable isotopes breaking down at random

Each nucleus decays qute spontaneously at it's own good time. Completely unaffected by physical and chemical reactions

In process , nucleus often changes into new elements

All around us

Comes from

Substances here on Earth - some radioactivity comes from building,materials , rocks.

Radiation from space e.g cosmic rays

living things - ther's a bit of radioactivity in all of us

human activity - nucleur explosions , nuclear waste.

Nucler radiation occurs : sub -atomic particles start bashing into atoms and knocking electrons off. Causing the formation of Ions

Further the radiation can penetrate through a wall before bashing into an atom and being stopped , less damage it wil be and less ionising

α

Helium Nuclei : 2 protons and 2 neutrons

big , heavy , slow moving 

Don't pentrate far into walls but ar stopped quickly

Large size , strongly ionising as they bash into lots of atoms

Electricaly positive charge . They are deflected when bumping into electric and magnetic fields

Emiting alpha particle decrease mass number by 4 and atomic number by 2

Electron emitted from nucleus

Beta particle emitted , proton number increases so atomic number increases but mass number stays the same

Move quite fast as they are quite small

Moderatly penetrating , Moderatly ionising

Negative charge - deflected by electromagnetic fields. 

β

they have no mass , just energy

Penetrate farthest in material wothout being stopped

Weakly ionising as they pass through ions instead of colliding with them

no charge , not deflected by electric fields

gamma ray emmision no effect on mass or atomic number. If there is excess energy , it loses it's energy by emitting a gamma ray

γ

Alpha - blocked by papaer , skin , few cm of air

Beta - blocked by thin metals . 

Gamma -  thick lead or very thick concrete. 

radiactive sources should be kept in a lead lined box

picked up using long handled tongs

keep a safe distance from them

Definition - time taken for half of the radiactive atoms now present to decay

Unstable nuclei eventually dissappears , the activity will decrease. So older the sample becomes , less radiation it will emit.

How quickly activity drops varies. Some isotopes take few hours all unstable nuclei to dissappear but others millions of years

Short half life - nuclei decay quickly

long half life - nuclei decay slowly as most nuclei don't decay for a long time . 

count rate mesaure half life 

Count rate number of alpha and beta emmisions per second

Count rate measured in  Becquerels , Bq

1000 Bq -  after 15 years fallen to 125Bq - work out half life

1000/2 = 500/2 = 250/2 = 125

three half lives

15/3 = 5

1 half life =  five years. 

- half life found by time interval at bottom axis corresponding to halving activity of vertical axis. E.g coordiante 2,100 and 6,50 and 10,25. 10-6 = 4 6-2 =4 Half life 4 years

Make sure to subtract backround radiation from results collected . 

To find how much activity depending on how much half-lifes particle have experienced, you have number of half-lifes as indices to two 

e.g 3/4 half life

2^3/4 

900 half life

2^90

Definition - splitting of an atom which releases energy. It can be random , but in a nuclear reactor it's made to happen

In nuclear power stations - nuclear fissions occur in uranium-235

Slow moving neutrons absorbed by uranium-235 , the nucleus can split. 

Each time it happens , small neutrons are split and they move towards other uranium nuclei. This causes them to split and so on , so fourth casung a chain reaction

Uranium 235 splits into two - form two daughter nuclei , both lighter elements than uranium. These new nuclei are usually radioactive

Problem with Nuclear power - radioactive waste

Kinetic Energy released from splitting of nucleus.

The KE is transferred to the thermal energy stores to produce steam to drive turbine. 

nuclear fission occuring in nuclear reactors release a lot of energy

Release of neutrons need to be slowed down inorder for right amount of neutrons to be absorbed for uranium nuclei to sustain chemical reaction

Moderator usually graphite or water slows down neutron movement. 

Control rods often made from Boron which absorb excess neutrons to limit rate of fission

High energy neutrons and gamma rays released in fission are highly penetrating ionising radiation. Therefore shielding used as a thich concrete structure , which contains lead and other metals

CO2 pumped around reactor to transfer thermal energy to water in heat exchanger . Produces steam caused turbine to turn , geneator turns producing electriciy. 

Definition - two light nuclei collide at high speed and fuse to create larger heavier nucleus.

Hydrogen nuclei fuse together to produce helium nucleus

Heavier nucleus doesn't have much more mass than two lighter nuclei as some mass of lighter nuclei converted to energy and released as radiation

fusion release lots of energy. All energy from stars comes from fusion. 

Fusion only happens really high pressure and temperature  (10 million degrees)  as positively charged nuclei need to get very close to fuse. Need to be moving very fast overcome electric repulsion

Scientists haven't found a way for of using fusion to generate energy for human activity. Fusion reactors expensive to build as fusion requires high temp and pressure

Radioactive souce , material , geiger muller detector

First measure count rate with material 

Second measure count rate without material  with soucre

radiation with material - radiation without material

1. If this is same , all radiation is passing thorugh material.

2. ordinary radiation lower backround radiation - some radiation is being absorbed

3 If radiation drops to zero , all radiation is absorbed.