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

alpha radiation

  • an alpha particle is the same as a helium nucleus
  • alpha is the least penetrating
  • alpha is stopped by paper or skin
  • alpha has the shortest range in air
  • alpha will travel a few cm in air
  • alpha is most ionising
  • alpha has a charge of +2

beta radiation

  • a beta particle is an electron (emitted from the nucleus)
  • beta penetrates less than gamma and more than alpha
  • beta is stopped by a thin sheet of aluminium
  • beta has a shorter range than gamma
  • beta will travel up to 1m in air
  • beta is more ionising that gamma and less ionising than alpha
  • beta has a charge of -1

gamma radiation

  • gamma radiation is an electromagnetic wave
  • gamma is the most penetrating
  • gamma is reduced/stopped by several cm of lead or thick concrete
  • gamma has the largest range in air
  • gamma will travel very large distances in air
  • gamma is least ionising
  • gamma is uncharged

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.

mass number stays the same, charge stays the same

Atomic number of Thorium is 92 – 2 = 90

Mass number of thorium is 238 – 4 = 234

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

  • 12.5% is 3 half lives
  • 3 x 5730 = 17190
  • so the skeleton is 17190 years old
  • Less than 130 years have passed
  • so the activity has not dropped by much

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.

  • wear protective clothing
  • work behind lead/concrete/glass shielding
  • limit time of exposure
  • use remote handling