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Learning objectives
describe the composition of an atom in terms of a positively charged nucleus (with protons and neutrons) and negatively charged electrons use the terms proton (atomic) number Z, nucleon (mass) number A and isotope
use and interpret the term nuclide and use the nuclide notation
show an understanding that nuclear decay is a random and spontaneous process whereby an unstable nucleus loses energy by emitting radiation
show an understanding of the nature of alpha (α), beta (β), and gamma (γ) radiation (including ionising effect and penetrating power) [β-particles are assumed to be β– particles only]
use equations involving nuclide notation to represent changes in the composition of the nucleus when radioactive emissions occur
show an understanding of background radiation
use the term half-life in simple calculations, which might involve information in tables or decay curves
discuss the applications (e.g. medical and industrial uses) and hazards of radioactivity based on:
(i) half-life of radioactive materials,
(ii) penetrating abilities and ionising effects of radioactive emissions
state the meaning of nuclear fusion and nuclear fission and relate these nuclear processes with the release of energy from nuclear fuels (recall of the energy-mass equivalence and details of technologies in nuclear power plants are not required)
The atom and the nucleus
The atom and the nucleus
The atom consists of electrons that orbit around a tiny core called the nucleus.
The electrons are negatively charged and are responsible for chemical reactions, chemical bonding, characteristic X-ray emissions, absorption/emission spectra etc.
The nucleus is positively charged and has a mass that is much larger than that of an electron. The atomic nucleus does not participate in chemical reactions.
The charge unit 𝑒 = 1.6022 × 10^−19 C. This constitutes the charge magnitude of both electrons and protons.
Nuclide Notation
Nuclide Notation
A nuclide is a species of atom characterized by the specific constitution of its nucleus (i.e., its number of protons and its number of neutrons).
The proton number, Z, is the number of protons in the nucleus. is also called the charge number or the atomic number.
The nucleon number, A , is the total number of nucleons. A nucleon is either a neutron or proton in the nucleus. is also known as the mass number.
Therefore, the number of neutrons in the nucleus is
N = A - Z
Isotopes
Isotopes
Isotopes are atoms that have the same number of protons but different numbers of neutrons.
Isotopes have nearly identical chemical properties (same proton number), but different physical properties (different nucleon number).
Radioactive isotopes, also known as radioisotopes, are unstable forms of elements that emit radiation due to an imbalance of protons and neutrons in their nucleus.
Nuclear reactions
Nuclear reactions
The nuclei of atoms can change, or be made to change, in various ways. Changes to the nuclei of atoms are called nuclear reactions.
Examples of nuclear reactions:
o Nuclear fission occurs when a massive nucleus splits into two less massive nuclei of approximately the same nucleon number and releases a huge amount of energy. The daughter nuclei may be stable or may decay further.
o Nuclear fusion occurs when two less massive nuclei combine (fuse) to form a single more massive nucleus and releases a huge amount of energy. It can occur when the nuclei have very high energy (e.g., at very high temperatures). The reason for this is that, as the nuclei approach each other, kinetic energy is converted into electrical potential energy. Fusion cannot occur if the nuclei lose all their kinetic energy before they bind.
o Radioactive decay is a process where an unstable nucleus loses energy and changes into a more stable nucleus by emitting radiation spontaneously and randomly.
Nuclear fission
Nuclear fission
Nuclear fission is the process in which a heavy nucleus of an atom is split into two or more smaller nuclei of comparable sizes after being bombarded by fast moving neutrons.
Nuclear fission is the process in which a heavy nucleus of an atom is split into two or more smaller nuclei of comparable sizes after being bombarded by fast moving neutrons.
Nuclear fusion
Nuclear fusion
Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus at very temperature.
Radioactive decay
Radioactive decay
• Some nuclei are unstable. One of the possible routes via which unstable nuclei can be transmuted into more stable nuclei is via radioactive decay.
• Radioactive decay is a spontaneous and random process where an unstable nucleus changes into a more stable nucleus, emitting radiation as it does so.
o When we say that radioactive decay is a spontaneous process, it means that radioactive decay is not triggered by external factors or influences and that the rate of decay is not affected by environmental conditions.
o When we say that radioactive decay is a random process, it means that it is not possible to predict which nucleus will decay next or when a particular nucleus will decay, while the probability of decay per unit time is constant and the same for all nuclei in the sample.
• Hence, the decay of a radioactive atom is not affected by any chemical reaction. It can neither be sped up nor slowed down and is independent of physical conditions such as temperature, pressure, and the decay of other atoms.
Types of Radioactive Decay
Types of Radioactive Decay
When an atom decays be emitting an alpha particle, its mass number decreases by 4 and its atomic number decreases by 2.
When an atom decays by emitting a beta particle, its mass number remains the same but its atomic number increases by 1. During beta decay, a neutron in the nucleus splits into a proton, which remains in the nucleus, and an electron which is emitted at high velocity as a beta particle.
When an unstable atom decays, the atom is sometimes left with excess energy. This is given out in the form of gamma radiation. The emission of gamma rays has no effect on either the mass or atomic number of the atom. (the excited nucleus is denoted by an asterisk * )
Laws of Radioactive Decay
Laws of Radioactive Decay
Activity and Count Rate
The rate of decay or activity A of a radioactive sample is the number of disintegrations per unit time. The S.I. unit of activity is becquerel (Bq) or inverse second (s-1).
1 Bq = 1 disintegration per second
A Geiger-Müller tube measures the count rate C instead of the activity A. This is because the normal detectors do not usually surround the radioactive source but only captures a portion of the radioactive emissions from a sample of radioactive source. The count rate is a fraction of the activity, so C is directly proportional to A:
C ∝ A
Although radioactive decay is a random process, if the number of radioactive nuclei is large, the rate at which a particular decay process occurs in a sample is directly proportional[1] to the number of radioactive nuclei N present:
A ∝ N
The number of undecayed radioactive nuclei N in a sample decreases exponentially with time t. This is shown by the decay curve (graph of N against t) on the right.
As the activity is directly proportional to number of radioactive nuclei, the rate of decay also decreases exponentially with time.
Background Radiation
In the absence of a specific radioactive source, a radiation detector still picks up a non-zero count rate (typically 20 to 50 counts per minute) from the environment. This measurement is known as the background count.
Background count is due to ionising radiation that is constantly present in the environment. This radiation is known as background radiation and its sources can be natural or man-made.
Naturally occurring sources account for about 80% of our exposure, and is contributed by things such as rocks, radon (a colourless and odourless gas that is easily trapped in buildings) and cosmic radiation. Man-made sources include those used in nuclear medicine, nuclear power plants and in consumer products (such as tobacco, which contains radioactive Pb-210).
In experiments to study radioactivity, the background count is usually subtracted from the measurement, to eliminate the systematic error in the count rate.
Half-life t1/2
A parameter that is useful in characterising nuclear decay is the half-life t1/2. The half-life of a radioactive nuclide is the average time taken for half of the original number of nuclei in a sample of the radioactive nuclide to decay. It is also the average time taken for the activity of a sample of the radioactive nuclide to halve. The definitions assume a statistically large number of particles and the half-life represents an expectation over a long time.
Applications of radioactivity
Applications of radioactivity
The uses of radioactive sources include :
* Using the ability of radioactive substances to be detected
* Using the properties of the radiation that they emit
* Using the energy that they emit
The uses of radioactive sources include :
* Using the ability of radioactive substances to be detected
* Using the properties of the radiation that they emit
* Using the energy that they emit
Radioactive nuclides are used in some industries and in nuclear medicine for their chemical properties and as sources of radiation.
Radioactive nuclides are used in some industries and in nuclear medicine for their chemical properties and as sources of radiation.
Medicine
Medicine
- As a tracer. Any radioactive nuclide which is to be used as a tracer for diagnostic purposes must be incorporated into a pharmaceutical product which can be administered to the patient. The pharmaceutical product and the radioactive nuclide which it incorporates should have the following properties:
o They should concentrate in the organ or system which is to be imaged or assessed.
o They should concentrate in the organ or system which is to be imaged or assessed.
o They should not change the functioning of the organ (i.e., the pharmaceutical behaves in the same way as the substance it is replacing).
o They should not change the functioning of the organ (i.e., the pharmaceutical behaves in the same way as the substance it is replacing).
o The half-life of the radioactive nuclide should be as short as possible to reduce radiation damage, but sufficiently long to enable measurements to be made.
o The half-life of the radioactive nuclide should be as short as possible to reduce radiation damage, but sufficiently long to enable measurements to be made.
o For example, technetium-99 is used in the detection of tumours. When a small amount of the isotope is taken into the body, the gamma-ray emitted allow images of internal organs to be taken using a gamma camera. The isotope has a short half-life of 6 hours, so it does not remain in the body for too long.
o For example, technetium-99 is used in the detection of tumours. When a small amount of the isotope is taken into the body, the gamma-ray emitted allow images of internal organs to be taken using a gamma camera. The isotope has a short half-life of 6 hours, so it does not remain in the body for too long.
- Cancer treatment. Immature cells and cells that are growing or dividing most rapidly are most sensitive to radiation. This is used in radiation treatment of cancer (e.g., brain tumours). Often, cancer cells are growing rapidly. Hence, they are much more likely to be killed by a high dose of gamma radiation from a cobalt-60 source than normal cells that divide less frequently.
Industry
Industry
- As a tracer.
Tracers are used to detect leaks in underground pipes by doping the water in the pipes and using a detector to check where the water flows.
Tracers are used to detect leaks in underground pipes by doping the water in the pipes and using a detector to check where the water flows.
- To check for cracks and imperfections.
Gamma rays can be used in place of X-rays to photograph solid objects. Gamma rays are directed at the object and the image is captured on a photographic film placed at the other side of the object. The film can reveal any imperfections such as cracks. The technique has an advantage over X-rays as no high-voltage electrical sources are needed.
Gamma rays can be used in place of X-rays to photograph solid objects. Gamma rays are directed at the object and the image is captured on a photographic film placed at the other side of the object. The film can reveal any imperfections such as cracks. The technique has an advantage over X-rays as no high-voltage electrical sources are needed.
- Radiation can also be used to check and control the thickness of paper or plastic during the manufacturing process. The thickness of rolled sheets of plastic can be controlled by placing a beta source on top of the sheet and a detector beneath it. If the sheet is too thin, the count rate will rise. This rise in count rate can then be directly coupled to the rollers, so that their separation is increased. Note that α-particle is not suitable as they will be absorbed by the paper or plastic easily.
Agriculture
Agriculture
As a sterilisation tool.
As a sterilisation tool.
Radiation can also be used as a means of preserving food, because exposure to high levels of radiation can destroy or incapacitate bacteria and mould spores. Techniques include exposing foods to gamma rays, high-energy electron beams and X-rays. Food preserved this way can be placed in a sealed container (to keep out new spoiling agents) and stored for long periods of time.
Radiation can also be used as a means of preserving food, because exposure to high levels of radiation can destroy or incapacitate bacteria and mould spores. Techniques include exposing foods to gamma rays, high-energy electron beams and X-rays. Food preserved this way can be placed in a sealed container (to keep out new spoiling agents) and stored for long periods of time.
Archaeology
Archaeology
Carbon-14 dating is a way of determining the age of certain archaeological artefacts of biological origin, up to about 50,000 years old. Refer to the infographic on the right for more information.
Carbon-14 dating is a way of determining the age of certain archaeological artefacts of biological origin, up to about 50,000 years old. Refer to the infographic on the right for more information.
Using this technique, scientists have been able to identify samples of wood, charcoal, bone, and shell as having lived from 1000 to 25000 years ago. This knowledge has helped us reconstruct the history of living organisms, including humans, during this time span.
Using this technique, scientists have been able to identify samples of wood, charcoal, bone, and shell as having lived from 1000 to 25000 years ago. This knowledge has helped us reconstruct the history of living organisms, including humans, during this time span.
Nuclear power station
Nuclear power station
Nuclear power is the use of nuclear reactions to produce electricity. Nuclear reactors are machines that contain and control nuclear chain reactions while releasing heat at a controlled rate. A nuclear power plant uses the heat that a nuclear reactor produces to turn water into steam, which then drives turbine generators that generate electricity.
Harzards and precautions when using radioactivity
Harzards and precautions when using radioactivity
Radioactive emissions can cause damage to the cells in our bodies. Most of this damage is due to ionisation when the radiation passes. If levels of radiation are high enough, there can also be damage due to heating effects as the body absorbs the energy from the radiation.
Radioactive emissions can cause damage to the cells in our bodies. Most of this damage is due to ionisation when the radiation passes. If levels of radiation are high enough, there can also be damage due to heating effects as the body absorbs the energy from the radiation.
Ionising Radiation
Ionising Radiation
Ionising radiation encompasses energetic subatomic particles, ions and atoms moving at high speeds and electromagnetic waves at the high-energy end of the electromagnetic spectrum.
Ionising radiation encompasses energetic subatomic particles, ions and atoms moving at high speeds and electromagnetic waves at the high-energy end of the electromagnetic spectrum.
α-particle, β-particle and γ-ray are ionising radiation.
α-particle, β-particle and γ-ray are ionising radiation.
During an interaction with an atom, the ionising radiation may remove electrons from the atom, causing the atom to be electrically charged or ionised.
During an interaction with an atom, the ionising radiation may remove electrons from the atom, causing the atom to be electrically charged or ionised.
Damage to Cells
Damage to Cells
The nucleus of a biological cell has been shown to be the part of the cell most vulnerable to radiation damage. This is because the nucleus contains the DNA molecules which carry the organism’s genetic code.
The nucleus of a biological cell has been shown to be the part of the cell most vulnerable to radiation damage. This is because the nucleus contains the DNA molecules which carry the organism’s genetic code.
The damage to cells may be direct or indirect.
The damage to cells may be direct or indirect.
o Ionisation may cause direct damage to the DNA within the nucleus, when the whole DNA molecule may either be broken into fragments or have sections of it removed.
o Ionisation may cause direct damage to the DNA within the nucleus, when the whole DNA molecule may either be broken into fragments or have sections of it removed.
o Indirect damage occurs when the radiation interacts with other molecules in the cell, producing ions and radicals (e.g., H+, OH-), which can then “attack” other cell structures and DNA. The radicals can also combine to form toxic substances like H2O2, which could initiate harmful reactions within the cells.
o Indirect damage occurs when the radiation interacts with other molecules in the cell, producing ions and radicals (e.g., H+, OH-), which can then “attack” other cell structures and DNA. The radicals can also combine to form toxic substances like H2O2, which could initiate harmful reactions within the cells.
Effects of Damage to DNA
Effects of Damage to DNA
- Cell death:
If the damage within the cell is great, then cell death occurs immediately or within a very short time.
If the damage within the cell is great, then cell death occurs immediately or within a very short time.
2.Cell is unable to replicate:
2.Cell is unable to replicate:
The DNA in the cell nucleus may have been damaged so badly that the cell is unable to replicate itself by cell division. This may have serious long-term consequences for the organism.
The DNA in the cell nucleus may have been damaged so badly that the cell is unable to replicate itself by cell division. This may have serious long-term consequences for the organism.
3. Cancer, genetic mutation and sterility:
3. Cancer, genetic mutation and sterility:
At low doses, cellular reproduction may continue for many cell generations, giving rise to delayed effects such as enhanced risk of developing cancer. Rapidly multiplying cells are far more susceptible to damage, so special attention is paid in the use of ionising radiation on unborn children and young persons. The reproductive system may also be affected, giving rise to an increased risk of genetic mutations. If the damage is severe enough, it can cause sterility.
At low doses, cellular reproduction may continue for many cell generations, giving rise to delayed effects such as enhanced risk of developing cancer. Rapidly multiplying cells are far more susceptible to damage, so special attention is paid in the use of ionising radiation on unborn children and young persons. The reproductive system may also be affected, giving rise to an increased risk of genetic mutations. If the damage is severe enough, it can cause sterility.
Precautions
Precautions
We can protect ourselves from ionising radiation by
We can protect ourselves from ionising radiation by
o reducing exposure time.
o reducing exposure time.
o increasing our distance from radioactive source with long tongs or remote-controlled devices.
o increasing our distance from radioactive source with long tongs or remote-controlled devices.
o shielding with thick concrete walls and lead-lined doors for rooms in which ionising radiation is used.
o shielding with thick concrete walls and lead-lined doors for rooms in which ionising radiation is used.
o storing radioactive materials in sealed containers that will absorb the radiation from the source.
o storing radioactive materials in sealed containers that will absorb the radiation from the source.
o limiting contamination in the event of a nuclear accident (e.g., leave the area immediately, remove outer layer of clothing, wash all exposed parts of body with soap and water).
o limiting contamination in the event of a nuclear accident (e.g., leave the area immediately, remove outer layer of clothing, wash all exposed parts of body with soap and water).
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