Radiation for OCR A-level Physics
This page covers the following topics:
1. Types of radiation
2. Experimental identification
3. Background radiation
4. Inverse square law
There are three types of radiation: alpha, beta and gamma. An alpha particle is a helium nucleus and is made up of two protons, two neutrons and no electrons. A beta particle is an electron. Gamma radiation is a high energy electromagnetic wave with really short wavelength and high frequency. Gamma rays are released when an atom is in a high energy state after emitting an alpha or beta particle to return to a stable state.
Ionising power is the ability of a particle to turn an uncharged atom into a charged one by removing or adding electrons. An alpha particle has the highest ionising power of the three types of radiation due to having the largest charge, however it has the smallest range in air of less than 5 cm and the lowest penetration power, which ranges only up to going through skin or paper. Gamma radiation has the largest penetrating power of the three, as it can penetrate through up to lead or concrete and the largest range in air, as it can travel through distances greater than 1 km. It however has a very low ionising power since it doesn't have a charge. Beta radiation has a greater penetrating power than alpha radiation but less than gamma radiation, as it can penetrate through up to 3 mm aluminium foil. It has a higher ionising power than gamma radiation, but lower than alpha radiation, since it has a greater charge than a gamma ray but a smaller one than an alpha particle. Its range is approximately a metre in air. Knowing this information allows us to check experimentally what type of radiation a radioactive sample is emitting.
Background radiation is radiation that exists around us at all times. This can be from both naturally occuring or artificial sources. Some natural sources of background radiation are radon gas from radioactive emitting rocks and soil in the ground, cosmic rays that reach the Earth from space and living things, as plants absorb radioactive materials from the soil they grow in and pass them up through the food chain. Some artificial sources of background radiation are medical procedures and nuclear power and weapons. When conducting experiments involving radiation, the background radiation must first be calculated and deducted from all results so as not to interfere with them.
A source of gamma radiation emits gamma rays equally in all directions. This means that for a distance r from the source, the gamma rays are spread over the surface of a sphere with radius r and surface area 4πr². The intensity of the source is defined as the energy per unit time per unit area and can be calculated using I = E/4πr², where E = energy of the gamma rays and r = radius of sphere. As the distance from the source increases, the intensity of the gamma rays decreases, since they will be spread over a greater area. According to the formula for intensity, gamma radiation follows an inverse square law, meaning that the intensity of the source is directly proportional to the inverse of the square of the distance from the source.
What are the three types of radiation?
Radiation can be emitted as an alpha particle, a beta particle or a gamma particle.
The range of the radiation that is being emitted by a radioactive sample is being measured. The radiation is detected up to 50 m away from the sample. Suggest what type of radiation is being emitted.
Alpha radiation has a smaller range in air than beta radiation. Beta radiation can travel through air for up to a metre, therefore since 50 m exceeds this, the radiation is not alpa or beta. Therefore, the sample is emitted gamma radiation.
Fill in the given table for the intensity of a gamma radiation source for the given distances from it.
By the inverse square law, A: I/4, B: 4r, C: 1/25.
State what type of radiation the given diagrams represent.
Diagram A shows a wave, therefore it represents gamma radiation. Diagram B shows two protons and two neutrons, therefore it represents alpha radiation. Diagram C shows an electron, therefore it represents beta radiation.
What is a beta particle?
A beta particle is an electron.
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