physics unit 5

Radioactivity

Discovery of the Nucleus

Ionisation

• Alpha radiation is the MOST ionising. Has a range in air of no more than a up to 100mm.
• Beta radiation has much weaker ionising effect. Range in air is up to a METRE. A beta particle therefore produces fewer ions per mm alon gits path than an a particle does.
• Gamma radiation has the WEAKEST ionising effect. Photons carry no charge so they have less effect than alpha or beta particles do. Range follows the inverse square law.

In a cloud chamber ALPHA and BETA leave trails of droplets due to ionising particles produce ions that trigger droplets to be formed within the supersaturated vapour.

Properties of a, B and y radiation

Nature

Alpha- Positively charged nucleus with 2 protons and 2 neutrons similar to helium. 

Beta- from naturally occurring radioactive substances containing electrons. 

Gamma- Photons with wavelength of 10^-11m

Inverse Square Law for Gamma radiation

Intensity of radiation is the radiation energy per second passing normall through unit area.

• For a source that emits n gamma photons per second with energy Hf. The radiation energy per second = nhf 
• At distance r away from the source all the photons from the source pass through area 4PIr^2 (Surface area of sphere)

SO... the intensity of the radiation at this distance = nhf/ 4PIr^2 

therefore I= k/r^2 where constant K= nhf/4PI

THUS i VARIES WITH THE INVERSE SQUARE OF DISTANCE r 

The equations for radioactive change

a- emission

The equation above shows when an alpha particle is emitted for form a new nuclied

Beta minus emission

Neutron in a neutron rich nucleus changes to a proton (down quark to an up) and emmits an electron/beta minus particle and an ANTI neutrino.

Beta plus emission

A proton in a proton rich neucleus changes into a neutron emitting a positron and neutrino.

Electron E

Proton rich nuclei capture an inner shell electron and turn into a neutronemitting an electron neutrino at the same time. The inner shell vacancy is then filled by an electron orbitting in an outer shell, this change in energy level causes photons to be emitted.

Y emission

No change occurs with protons and neutrons when a y photon is emitted. This occurs if a nucleus has too much excess energy after it has emitted an a or b MINUS particle.

The dangers of radioactivity

Hazards of ionising radiation

Can be dangerous as it damages living cells;

• It can destroy cell membranes which cause cells to die.
• It can damage vital molecules such as DNA by creating 'free radical' ions which react with vital molecules. Normal cell division is affected and nuclei become damaged. Can cause cells to divide and grow uncontrollably, causing possible cancerous tumours. Damaged DNA in a sex cell can cause infertility or mutation of a foetus.

Background Radiation

• medical
• air eg. radon gas
• ground and buildings
• food and drink
• cosmic rays
• nuclear weapons
• air travel
• nuclear power

Safe use of radiation

Storage

• …