Half life

L.O: explain the decay of radioactive atoms

What you need to know...

Radioactive material• What makes an element radioactive?

• Unstable nuclei

• What is emitted from an unstable nucleus?

• Radiation (and sometimes particles)

• What is left behind?

• A different element

• Why?

• Nucleus has changed – in terms of atomic number

Questions

• What three conclusions did Rutherford make about the structure of the atom from his experiments?

1. The atom is made up mainly of empty space

2. The mass of the atom is concentrated in the centre of the atom

3. The central mass has a positive charge

Radioactive decay

• When a radioactive element emits radiation the nucleus changes and gives off energy

• The nuclei decays (breaks down to a smaller nuclei)

• It has lost radiation – has less radiation

• This is measured by something called “half life”

Half life

The time it takes the number of radioactive nuclei in a sample to decrease by 50%.

Definition of half life:

A short half life means the nuclei decay quickly, and so the sample becomes less radioactive in a shorter space of time

Activity of a radioactive atom

The number of atoms that decay per second

The unstable parent atom loses a particle and becomes a different atom

Because the number of parent atoms goes down, the activity of the sample decreases

Half life of a radioactive sample

• Decay is random – you cannot predict when an atom will decay

• Only know that after a certain period of time, half of the nuclei present would have decayed

• They are no longer the radioactive nuclei they were

Graphical representation of half life

Decay rate (counts/min

)

Time (min)

80

60

40

20

2 4 6 8

What is the half life of the radioisotope represented by the following graph?

The time it takes the count rate to decrease from 80 per min to 40 per min is what? 2 mins

Double check, the time it takes the count rate to decrease from 40 per min to 20 per min is?

2 mins

The half life of the radioisotope is 2 mins.

If the half life of a sample is always the same, why dies the graph have a curved shape?

The number of atoms left to decay decreases each time

Calculating half life

• A radioactive sample is measured at 1280 bequerel (Bq)

• Two hours later the sample has decayed to 80 Bq

• Find its half life

Half life questions

1. What is the definition of half life?

2. If 1/64th of an original radioisotope is left after 1 hour, what is the half life of the sample?

3. A radioisotope has a half life of 12 minutes. What fraction of the radioisotope will be left after 2 hours?

4. The background radiation in a laboratory is 12 counts per minute. The count rate from a radioisotope is measured and it has a reading of 120 counts per minute. If the half life of the radioisotope is 10 minutes, what will be the radioisotope level 20 minutes later?

Ionising radiation

If the exposure is high, it can kill the cell.

If the exposure is lower it can cause cancer.

The higher the exposure, the higher the risk of cancer.

Alpha is the most ionising radiation, gamma is the least.

What happens if radiation is incident upon a living cell?

Ionising radiation can be used to kill cancer cells.

Radiation can ionise cells which causes cellular damage.

Radiation effects• Ionising – breaks molecules in cells into smaller ions that can take part in

other chemical reactions• Can form cancer• All types of radiation are ionising• Alpha most ionising – but least dangerous – why?• Penetrative effect also important• Irradiation – exposure to radiation – e.g. x-ray – can walk away from

source and are no longer exposed• Contamination – have radioactive source on you – e.g. consume/breathe

in, enters skin – carry radiation with you as it is in/on your person• Which is more dangerous?• Radiation dose – measured in Sieverts• Exposed constantly by background radiation (Radon gas – alpha – homes)• Hospital/nuclear power workers more at risk

Sterilisation

Gamma rays are used to kill bacteria, mould and insects in food. This can be done even after the food has been packaged. It can affect the taste, but supermarkets like it because it lengthens the shelf life.

Gamma rays are also used to kill bacteria on hospital equipment. It is particularly useful with plastic equipment that would be damaged by heat sterilisation.

Gamma rays are used to kill bacteria, mould and insects in food. This can be done even after the food has been packaged. It can affect the taste, but supermarkets like it because it lengthens the shelf life.

Gamma rays are also used to kill bacteria on hospital equipment. It is particularly useful with plastic equipment that would be damaged by heat sterilisation.

Gamma Sourceunsterilised sterilised

Radiotherapy

A carefully controlled beam of gamma rays can be used to kill cancer cells. It must be directed carefully to minimise the damage to normal cells.

However, some damage is unavoidable and this can make the patient ill.

It is therefore a balancing act - getting the dose high enough to kill the cancerous cells, but as low as possible to minimise the harm to the patient.

A carefully controlled beam of gamma rays can be used to kill cancer cells. It must be directed carefully to minimise the damage to normal cells.

However, some damage is unavoidable and this can make the patient ill.

It is therefore a balancing act - getting the dose high enough to kill the cancerous cells, but as low as possible to minimise the harm to the patient.

Sterilisation

Gamma radiation can also be used to sterilize food.

The radiation will kill any living microbes that are on the food/packaging without damaging the food.

Will the radiation have a long or short half life?

Gamma radiation can also be used to sterilize food.

The radiation will kill any living microbes that are on the food/packaging without damaging the food.

Will the radiation have a long or short half life?

Medical tracer

Radiation can also be used as a medical tracer.

The patient will be injected with a radioactive source and then scanned to detect the radioactivity.

This can show blockages (tumour’s) and other illnesses.

What type of radiation would be most suitable for a tracer?

Will the radiation have a long or short half life?

Radiation can also be used as a medical tracer.

The patient will be injected with a radioactive source and then scanned to detect the radioactivity.

This can show blockages (tumour’s) and other illnesses.

What type of radiation would be most suitable for a tracer?

Will the radiation have a long or short half life?