physics - topic 6: radioactiity

What is the average size of an atom?

1 x 10^-10 metres

What defines an atom’s element?

The number of protons.

What is an element’s mass number?

The total number of protons and neutrons.

What is an element’s atomic number?

The total number of protons

What is an isotope?

Atoms of the same element with a different number of neutrons.

What does it mean if a nucleus is ‘unstable’?

It has an imbalance of forces within the nucleus.

Define ‘ionising’.

The ability to knock electrons off of atoms.

What are the four types of radioactive decay?

Alpha, beta, gamma, and neutron emission/decay.

What is an alpha particle? (α)

• A helium nucleus

• Contains two neutrons and two protons

• Relative charge of +2

• Relative mass of 4

What is a beta particle? (β)

• Contains a fast-moving electron

• Relative charge of -1

• Relative mass of 1/2000

What is a gamma ray? (γ)

• An EM wave

• Relative charge of 0

• Relative mass of 0

How penetrating and ionising is an alpha particle?

Most ionising, least penetrating.

How penetrating and ionising is a beta particle?

Fairly ionising, fairly penetrating.

How penetrating and ionising is gamma?

Least ionising, most penetrating.

What is a nucleon?

A particle found in the nucleus.

Define activity.

The rate at which the unstable nuclei decays.

What do we use to detect radiation?

A GM (geiger-muller) tube.

What is count rate?

The number of decays per second.

Explain alpha decay.

• An alpha particle is emitted from an unstable nucleus.

• Produces a new element because the mass & atomic number changes.

Explain beta decay.

• A neutron changes into a proton and an electron.

• The electron is emitted from the unstable nucleus.

• Produces a new element because the atomic number changes.

Explain gamma decay.

• A gamma ray is emitted from an unstable nucleus.

• This reduces the energy of the nucleus, but doesn’t change the element.

Explain neutron decay.

• A neutron is emitted from an unstable nucleus.

• The element remains the same, however it is now an isotope.

• Note: this is not considered ionising because a neutron has no charge.

What stops alpha particles from penetrating?

Paper or skin.

What stops beta particles from penetrating?

Thin aluminium

What stops gamma rays from penetrating?

Thick lead

Define irradiation.

Exposure to ionising radiation.

Define contamination.

A radioactive substance has been released into the environment.

Define radioactive.

An atom with an unstable nucleus.

Radioactive decay is ___ and ___.

Random and spontaneous.

Define half-life.

The time taken for half of the radioactive nuclei to decay.

OR

The time taken for the activity to halve.

What is the half-life of carbon?

5700 years.

Why is having a short half-life dangerous?

It decays very quickly, so a lot of radiation is emitted.

Why is having a long half-life dangerous?

It decays slowly, however it’ll remain radioactive for a very long time.

Why is ionising radiation dangerous?

• Can damage living cells by causing mutations in DNA.

• Cells can divide uncontrollably.

• This causes cancer.

What are the three main uses of radiation?

Medical tracers, radiotherapy, and smoke alarms.

Describe medical tracing.

• A radioactive substance that gives out beta or gamma radiation is ingested/injected (a tracer).

• Alpha isn’t used because it is the most ionising.

• The path of the tracer is able to be observed by detecting it’s path via a GM tube outside the body.

• This produces an internal image of the body.

Describe radiotherapy.

• This is the treatment of cancer via radiation, which can kill living cancerous cells.

• Beams of gamma rays are directed at a cancerous tumour from all angles.

• This minimises the exposure of normal cells so the damage is limited.

• Treatment is given in doses to give healthy cells time to repair.

Describe how smoke alarms work.

• Contain an alpha source with a long half-life.

• Alpha particles ionise the air and create a current.

• If smoke particles enter the detector, they absorb some of the radiation and less current flows.

• This causes the alarm to go off.

Define nuclear fission.

The splitting of a large unstable nucleus into 2 smaller daughter nuclei and 2-3 neutrons.

Isotopes of what elements undergo fission?

Uranium and plutonium.

Describe the process of nuclear fission?

• A neutron is fired at a (uranium) nucleus.

• The nucleus absorbs the neutron and becomes unstable, causing it to split.

• Every time it splits, it spits out 2/3 neutrons, which could be absorbed by other nuclei.

• This could cause a chain reaction.

How is fission used in power stations?

• Power stations use uranium as fuel, since the energy released from nuclear fission heats water to make steam, which drives a turbine.

• This is attached to a generator, which generates electricity.

What is nuclear fusion?

The joining of two small nuclei to form a larger stable nucleus.

What does nuclear fusion require?

High temperatures and high pressures.

Stars release energy from ___.

(Nuclear) Fusion.

What is excitation?

When an electron moves up an energy level.

Describe the process of excitation.

• An electron absorbs a photon of light with the exact amount of energy needed to move up. f the photon has too little energy it will not be absorbed.

• It transitions to a higher energy level.

• However excited electrons are unstable on higher energy levels, so they emit a photon of light and de-excite spontaneously.

How do we get an absorption spectra?

• White light passes through a cold gas

• Photons with frequencies that match the difference in energy levels are absorbed.

• Those absorbed photons are missing from the spectrum after it passes through the prism.

• This makes black lines appear.

• This is an absorption spectrum.

How do we get an emission spectra?

• White light passes through a hot gas

• Photons with frequencies that match the difference in energy levels are emitted.

• Those emitted photons are present in the spectrum after it passes through the prism.

• This makes coloured lines appear on a black background.

• This is an emission spectrum.

What would an electron require to leave an atom completely?

A very high energy photon of light.