Mr Palmer's AQA GCSE Physics Notes
Nuclear Radiation
Atomic Structure
Protons and neutrons make up the nucleus of an atom. Compared to the whole atom, the nucleus of the atom is a tiny, tiny fraction of the size.
The number of protons in an atom is called the atomic number. The total number of protons and neutrons is called the mass number.
In an atom:
number of protons + number of neutrons = mass number
number of protons = number of electrons
number of protons = atomic number
In an atom, the number of electrons and the number of protons is always equal. The overall charge of an atom is 0 (neutral). If an atom loses an electron or, more rarely, a proton, it becomes charged and is now called an ion.
All atoms in an element have the same number of protons. If the number of protons in an atom changes, that atom is now a new element.
Elements with different numbers of neutrons are called isotopes.
Alpha Radiation
In α-radiation, the nucleus of a large atom ejects a Helium nucleus. The atom, because it has lost two protons, becomes a different element.
An atom emitting an α particle.
Beta Radiation
In β-radiation, one of the neutrons transforms into an electron and a proton. The electron is ejected from the nucleus. The atom now becomes a different element because it has an additional proton.
An atom giving off a β particle and changing a neutron into a proton.
Gamma Radiation
In γ-radiation the nucleus gives off a short wavelength, high frequency burst of electromagnetic radiation. The atom does not become a new element as no protons have been gained or lost.
An atom emitting γ radiation.
Nuclear Radiation and Fields
Nuclear radiation in an electric field.
Because some forms of radiation have an electric charge, they are affected when they pass by other charged objects.
Gamma radiation does not have a charge. If it passes through an electric field, it is unaffected. Because beta radiation has a negative charge, when it passes through the field it is attracted to the positive plate and repelled from the negative plate. Because alpha radiation has a positive charge, it is attracted to the negative plate and repelled from the positive plate.
When radiation passes through a magnetic field the uncharged gamma is unaffected and travels in a straight path.
Nuclear radiation in a magnetic field.
However, the charged alpha and beta radiation are deflected from their paths in opposite directions.
In both electric and magnetic fields the path of the beta particles bends more than the alpha particles. This is because the beta particle are lighter and thus more easily moved by the field.
Half-Life
Half-life is the amount of time it takes half the radioactive atoms in a sample to decay. The following diagram starts with a sample of 16 undecayed radioactive atoms and shows how many are left after each half-life.
It is impractical to directly count the number of decayed and undecayed atoms in a sample, so we graph the radioactive counts from a Geiger counter instead. The data points do not fit the trend line exactly because radioactive decay is a random process.
Radioactive Dating
The older a sample of radioactive material is, the less radiation it emits. Scientists use this information along with the half-life of various atoms to determine the age of the objects they are found inside. Here is a common example:
Carbon-14 is a radioactive isotope of Carbon that living organisms take in while they are alive. Once they stop living, they no longer take in Carbon-14 from their environment.
Carbon-14 decays into Nitrogen and has a half-life of about 6,000 years.
A human skeleton is found on an archaeological dig. For every one atom of Carbon-14 it contains it also has three atoms of Nitrogen. How old is the skeleton?
First we must figure out how many half-lives have passed:
So, it takes two half-lives before we have one Carbon-14 atom for every three nitrogen atoms.
(2 half-lives)(6,000 years per half-life) = 12,000 years
Therefore the skeleton is about 12,000 years old.
Industrial Uses of Nuclear Radiation
Radiation is used to determine the thickness of materials with a high degree of accuracy with the following set up:
A ductile material, such a copper, needs to be pressed into a thin sheet. A thick sample of the material is drawn into the rollers from the left. A sample of radioactive material is placed above the thinner sheet, and a Geiger counter below. The thicker the sheet gets, the less radiation gets through. The thinner the sheet is, the more radiation gets through. The Geiger counter is then connected to a computer that automatically adjusts the pressure on the rollers to keep the sheet at a constant thickness.
If a material such as metal is being used, γ-rays must be used as they are the only one that can penetrate it.
If paper is being pressed into sheets, β-radiation is necessary because gamma would pass through the paper too easily.
α-radiation is useless in this task, as it is too easily absorbed.
Radiation is also used to detect cracks in underground pipes. Radioactive material is sent through the pipe and where the crack is leaking fluid, a Geiger counter above ground will measure a much higher count rate above the leak.
Medical Uses of Nuclear Radiation
Radioactive materials can be used as tracers in the body. A patient is injected with a radioactive material and doctors can create a three dimensional image of the inside of the body by measuring the amount of radiation given off.
A good radioactive tracer should emit γ-rays and should not emit α or β-particles.
γ-radiation is less likely to be absorbed by cells and γ-rays can penetrate through the body so they can be detected externally.
β and α-particles won't be detectable on the outside of the body because they will be absorbed by the cells, causing damage to them and increasing the chances of cancer.
The half-life needs to be long enough for the doctor to take a measurement (usually longer than a couple of hours) but not so long that it stays after the procedure. (A half-life of 100 days is much longer than the doctor needs)
γ-radiation is also used to sterilise medical equipment and treat cancerous tumours. (See electromagnetic spectrum notes for more detail)
Nuclear Radiation Safety
Radioactivity is all around us and comes from many sources. This constant level of exposure to radiation is called background radiation.
Sources of natural background radiation include: radon gas, granite and cosmic rays
Man-made sources of background radiation include x-rays and nuclear weapons testing.
Remember that mobile phones do NOT give off nuclear radiation.
Do not eat α emitters. Do not stand near β and γ emitters.
The effect that radiation has on living tissue depends on the kind of radiation and where it is located.
If the source of radiation is outside the body then β and γ are the most dangerous as they can penetrate the outer layers of skin and be absorbed by living cells deeper in the body. α radiation is the least dangerous outside the body as it will be absorbed by the air or dead skin cells.
If the source of radiation is inside the body, then α is the most dangerous as all of it will be absorbed by the living cells. β and γ are less dangerous (but still problematic) because they have a chance of passing through the body unabsorbed.
Radioactive badges use photographic film to detect radiation. When radiation strikes the film it turns darker. They are useful to prevent people from being exposed to too much radiation and to know how much they have already been exposed too.
Physics P1 Topics
Physics P2 Topics
- Motion
- Forces
- Energy
- Static Electricity
- Current Electricity
- Mains Electricity
- Momentum
- Nuclear Physics
Other