P6 - Radioactive Materials

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Radioactivity

nucleus -> protons and neutrons -> most of mass

electrons -> in shells

atom of element -> same no. protons

isotopes -> same no. protons but diff. no. neutrons

Each element -> only 2 stable isotopes

rest radioactive & unstable -> decays -> emits radiation -> e.g carbon-14

unstable atoms -> decay randomly

ionising radiation -> alpha, beta, gamma

Radiation

Alpha

Big -> heavy -> slow

don't penetrate far

helium nucleus -> 2 protons & 2 neutrons

released by very heavy nuclei -> e.g uranium

mass of 4 -> charge of +2

blocked -> paper

Beta

quite fast -> quite small

penetrates moderately

released by nuclei w/ too many neutrons

during decay -> nucleus turns into proton -> element changes

identical to electron -> no mass -> charge -1

blocked -> aluminium

Gamma

After emitting an alpha/beta particle -> nucleus may need to get rid of extra energy

Does it by emitting gamma ray -> electromagnetic wave

No mass

Penetrate a long way

Element doesn't change -> electromagnetic radiation

Blocked -> thick lead

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Half-Life

Time taken for half of the radioactive nuclei now present to decay

Unstable nucleus decays -> 1 more radioactive nucleus isn't there to decay

More unstable nucleus decay -> whole radioactivity decreases

How quickly decreases -> varies

Short half-life -> activity falls quickly -> lots of nuclei decay in short time

Long half-life -> activity falls slowly -> most of nuclei doesn't decay for long time

Look in physics book for how to calculate half life

The atom and nuclear fission

Rutherford's Scattering

1909 -> Rutherford, Geiger & Marsden

Fired positively charged alpha particles at thin gold foil

Most alpha particles -> straight through gold foil

A couple -> reflected off gold foil

Found:

Atoms had nucleus -> most went through but some reflected -> something in the centre

Lots of empty space -> most alpha particles went straight through gold foil

Nucleus is positively charged -> otherwise positively charged alpha particles wouldn't be repelled by the nucleus and wouldn't scatter

Nucleus -> positively charged -> repel each other

Nucleus held together -> attractive force -> greater than repulsive electrostatic force

Called: strong force

Strong force -> short range

Larger separation -> strong for is so weak -> disappears

Nuclear Fusion

2 nuclei -> combine (fuse) -> create larger nucleus -> releasing energy

e.g hydrogen nuclei fuse -> helium nuclei

Nuclei only fuse -> overcome repulsive electrostatic force -> & get close enough for strong force to hold together

Lots of energy & high temps. needed

Nuclear Fusion -> converts mass into energy

Einstein -> mass is a form of energy -> so can convert into other forms energy

Energy = mass x (speed)^2 -> E = mc^2

Nuclei -> nuclear fusion/fission -> lose mass -> energy released

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Nuclear fission and Nuclear power