Nuclear Radiation

Some atomic nuclei are unstable. The nucleus gives out radiation as  it changes to become more stable. This is a random process called  radioactive decay. 

Activity is the rate at which a source of unstable nuclei decays.  Activity is measured in becquerel (Bq) 

Count-rate is the number of decays recorded each second by a detector  (eg Geiger-Muller tube). 

Nuclear radiation is always emitted from the nucleus. The nuclear radiation emitted may be alpha, beta, gamma or a neutron: 

alpha (α) emission

alpha particle (α): two neutrons and two protons emitted from the nucleus.
It is the same as a helium nucleus 

beta (β) emission

beta particle (β): a high speed electron ejected from the nucleus as  a neutron turns into a proton 

gamma (γ) emission

gamma ray (γ) : electromagnetic radiation emitted from the nucleus 

neutron (n) emission

neutron (n):a neutron emitted from the nucleus

When nuclear radiation passes through living cells it can ionise the atoms. This process of ionisation damages living cells causing mutations and cancer.

Nuclear Radiation Type Range (distance travelled) in airPenetration through materialsIonising Power
Alpha5 cmabsorbed by paper or skinstrong
Beta50 cm – 100 cmabsorbed by 2mm aluminiumweaker
Gammaunlimitedabsorbed by thick lead or 2m+ concretevery weak
Properties of nuclear radiation

Question: Explain the differences between the properties of alpha, beta and gamma radiations. (6 marks)

Changes in the Nucleus because of Nuclear Radiation

Nuclear equations are used to represent radioactive decay. 

The emission of the different types of nuclear radiation may cause a  change in the:

  • mass of the nucleus
  • charge of the nucleus

Alpha decay causes the mass to decrease (by 4) and charge of the nucleus to  decrease (by 2). 

Beta decay does not cause the mass of the nucleus to change but  does cause the charge of the nucleus to increase (by 1). 

The emission of a gamma ray does not cause the mass or the charge of  the nucleus to change. 

We use nuclear decay equations to show the changes in the nucleus of the unstable parent nucleus when nuclear decay happens;

Alpha decay equation. The alpha particle is represented by the ‘He’ symbol.
Beta decay equation. The beta particle is represented by the ‘e’ symbol.

Question: A nucleus of lanthanum-140 emits gamma radiation.
What happens to the mass number and the charge of the nucleus when gamma radiation is emitted? (2 marks)

Question: Uranium has 92 protons and a mass number of 238. It decays by alpha decay to make Thorium.
What is the atomic number and mass number of Thorium?

The Random Nature of Decay

Radioactive decay is random. We can not tell in advance which unstable nucleus will decay and when it will decay.

The half-life of a radioactive isotope is the time it takes for the number  of nuclei of the isotope in a sample to halve, or the time it takes for the  count rate (or activity) from a sample containing the isotope to fall to half  its initial level. 

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Half Life Graph

After one half life, the activity of a sample will have fallen by half.

After 2 half lives the remaining activity will have fallen by half – to one quarter of the original sample.

After ‘n’ half lives, there will be 1/2n of the original activity remaining

For example, after 3 half lives there will be 1/23 or 1/8 of the original activity remaining

Question: The half-life of carbon-14 is 5730 years.
Carbon-14 is used for carbon dating. Carbon dating can tell us how old some objects are.
A skeleton was carbon dated. The results showed that there was only 12.5% of the original amount of carbon-14 left in the skeleton.
Calculate the age of the skeleton. (3 marks)

Question: Radium-226 was discovered by Marie Curie in 1898.
Radium-226 has a half life of 1600 years.
The notebooks she used were contaminated with radium-226 and are still hazardous.
Explain why the notebooks are still hazardous. (2 marks)

Radioactive contamination is the unwanted presence of materials  containing radioactive atoms on other materials.

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The hazard from  contamination is due to the decay of the contaminating atoms. The type  of radiation emitted affects the level of hazard. 

Alpha radiation has the greatest hazard if it is inside your body as it is the most ionising. Outside the body is has least (but not insignificant) hazard because it can not pass through your skin.

Gamma radiation is able to pass through your body, damaging cells as it does. Unless you are behind a very thick shield it will always pose a hazard.

Irradiation is the process of exposing an object to nuclear radiation.

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The  irradiated object does not become radioactive. 

Suitable precautions must be taken to protect against any hazard that  the radioactive source used in the process of irradiation may present, for example using tongs to handle samples and limiting exposure time.

It is important for the findings of  studies into the effects of radiation on humans to be published and  shared with other scientists so that the findings can be checked by peer  review.

Question: People working with sources of nuclear radiation risk damaging their health.
State 3 precautions these people should take to reduce the risk to their health.