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Radioactivity


Notes


Background radiation

Radioactive elements are naturally found in the environment and are continually emitting radiation. This naturally occurring radiation is called background radiation, which we are all exposed to throughout our lives.

Background radiation comes from a number of sources. (Note that these are averaged across the population and may differ for different groups, for example depending on any medical treatment you may have, or whether you make many aeroplane flights.) 

One of the major sources of background radiation is radon gas. This is produced by minute amounts of uranium, which occurs naturally in rocks, and is present in all parts of the country. It disperses outdoors so is only a problem if trapped inside a building. Exposure to high levels of radon can lead to an increased risk of lung cancer. 



Types of Nuclear Radiation

There are three types of radiation emitted by radioactive materials. They are all emitted from unstable nuclei



Most nuclei never change; they are stable. Radioactive materials contain unstable nuclei. These can break up and emit radiation. When this happens, we say the nucleus has decayed. The result for alpha and beta decay is the nucleus of a different element. For gamma decay, it is the same element but it has less energy. 

Alpha decay



Mass number decreases by 4 (2 protons + 2 neutrons lost). Atomic number decreases by 2 (2 protons lost). 

Beta decay



Gamma emission

Often after either alpha or beta decay the nucleons have an excess of energy. By rearranging the layout of their protons and neutrons, they reach a lower energy state and the excess energy is emitted in the form of a gamma ray. 


Half-life

  • Most types of nuclei never change; they are stable. However, radioactive materials contain unstable nuclei. The nucleus of an unstable atom can break up (decay) and when this happens, it emits radiation.
  • A nucleus of a different element is left behind.

  • As time goes by radioactive materials contain fewer and fewer unstable atoms and so become less and less radioactive and emit less and less radiation.
  • There is no way of predicting when an individual nucleus will decay; it is a completely random process. A nucleus may decay in the next second or not for a million years. This means it is impossible to tell how long it will take for all the nuclei to decay.
  • Like throwing a die, you cannot predict when a six will be thrown. However, given a very large number of dice you can estimate that a certain proportion, 1/6th, will land as a six.
  • We define activity as the number of nuclei that decay per second (N.B. 1 decay per second = 1 Bq). The time it takes for the activity of a radioactive material to halve (because half of the unstable nuclei that were originally there have decayed) is called the half-life

  • We see the activity falling as there are fewer nuclei available to decay. However, note that the time taken to halve is independent of the number of nuclei, in this case 2 seconds. Half-lives are unique to each individual isotope and range from billions of years to fractions of a second.
  • The half-life of a radioactive isotope is formally defined as: 
    • ‘The time it takes for half the nuclei of the isotope in a sample to decay, or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.’ 

N = Amount of radioisotope particles after nth half life.
N0 = Initial amount of radioisotope particles.
n = number of half life


Graphically 

A graph of activity vs. time can be plotted from experimental measurements. We must remember to subtract the background count from the actual count to find the count due to the source alone. We call this the corrected count rate



Nuclear radiation never completely dies away, but eventually drops to a negligible level, close to the background. At this point, a source is considered safe. Consideration of half-life therefore, has importance when considering which isotopes to use for various applications and the disposal of radioactive waste – see section on applications of radioactivity. 

Nuclear Energy - Einstein Formula

m = mass change (kg)
c = speed of light (m s-1 )
E = energy changed (J)


Rules for nuclear equations

The total mass number must be the same on both sides of the equation. The total atomic number on both sides of the equation must be the same. The total charge must be the same on both sides of the equation. 

N/Z Curve

Nuclei have positive charge due to the protons in them. All the protons repel, so why does the nucleus not explode?

There is another force acting called the strong nuclear force. This acts between all nucleons, both protons and neutrons. 



Fundamental Particles

  • A fundamental particle is one that cannot be split into anything simpler.
  • The word atom means ‘indivisible’ because scientists once thought atoms were fundamental particles.
  • Similar experiments to Rutherford’s alpha scattering using electrons fired at protons and neutrons reveals that they are made up of smaller particles – quarks. 


Beta decay

In beta decay, one of the up quarks changes to a down quark or vice versa


Protons and neutrons are made of just two types of quark, the up and the down. Other particles have to be created in special machines called particle accelerators. 


Is Radiation Dangerous?

  • All nuclear radiation is ionizing. It can knock electrons out of atoms, or break molecules into bits. If these molecules are part of a living cell, this may kill the cell. 
  • Radiation dose is measured in Sieverts. This unit measures the amount of energy deposited in the tissue by the radiation, and takes account of the type of radiation, because some particles are more effective at damaging cells than others. It is a measure of the possible harm done to your body. 


Nuclear Fission

  • Nuclear fission is the splitting of an atomic nucleus.
  • A large parent nucleus, such as 235-uranium or 239-plutonium, splits into two smaller daughter nuclei, of approximately equal size. 
  • This process also releases energy (heat) which can be used to generate electricity. Normally, this will happen spontaneously but can be speeded up by inducing fission. 
Chain reaction

  • If uranium were burned chemically to uranium oxide, it would release about 4500 J/g. The equivalent energy release from nuclear fission is 8.2 × 1010 J/g. 
  • The daughter products themselves are radioactive because they still tend to be neutron rich (i.e. lying above the N/Z curve), and decay, releasing more thermal energy and nuclear radiation. They have a wide range of half-lives. These factors need to be taken into account when considering their disposal.

Nuclear Fusion

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. It is the process by which energy is released in stars.

 


Bonus: History of Radioactivity


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