Radioactivity Flashcards
Explain what is meant by background radiation?
Background radiation is radiation produced by radioactive material in the Earth and in the Earth’s atmosphere. It should be measured and taken into account when measuring the activity of radiation from a particular source.
Explain the difference between natural background radiation and artificial radiation?
Natural background radiation is due to the decay of naturally occurring radioactive isotopes in the Earth that were formed when the Solar System was created. Natural radiation also results from high-energy particles that bombard the Earth. Artificial background radiation comes from
man-made sources, rather than from the rocks that make up the Earth.
Give three examples of sources of BR?
Uranium in rocks in the Earth’s crust (natural). Radioactive materials that have escaped into the environment from nuclear power stations (artificial). Fallout from atmospheric nuclear weapon
testing in the 1950s and 1960s (artificial).
Explain simply, the principle of the GM tube?
Ionising radiation causes ionisation of a low-pressure gas inside the tube. The ionised gas allows a current to flow between two electrodes, and the current is detected by an electronic circuit.
What does a rate meter measure in a GM tube?
A rate meter gives an indication of the number of decays occurring per second.
Rate meter is calibrated at kBq. How is this unit defined?
A bequerel is a rate of decay of one disintegration per second. 1 kBq is 1000 disintegrations per
second.
Define what is meant by the half life of a radioactive material?
The half-life of a radioactive sample is the average time taken for half the unstable atoms in the sample to undergo radioactive decay.
Radioactive decay is a random process. Explain this?
Random means that the decay of an individual atom is unpredictable; we cannot say when any particular atom will undergo decay.
A student wants to measure the half life of a radioactive isotope. He is told that is has a half life of between 10 and 20 minutes.
What measurements should he take?
How should he use the measurements to find the half life?
Students should measure the background radiation. They should then measure the radioactivity of the sample using a GM tube and rate meter at regular intervals of, say, 5 minutes for a period
of 30–40 minutes.
The readings of the activity of the sample should be corrected by subtracting the average background radiation count. The corrected readings should then be plotted on a graph of count rate against time. The time taken for the
initial activity to fall to half can then be measured from the graph.
Explain why a particles half life of 6 hours and decay by emission of low energy gamma and beta particles is good in medicine?
It has a short half-life, so its activity drops to a negligible level in a day or two. Beta particles and low energy gamma rays penetrate soft tissue easily, so the progress of the isotope through the body can be monitored easily. The emitted radiation is not strongly ionising, so the risk of
tissue damage is acceptably small. (It is also relatively easy to produce.)
How can an isotopes’ progress in the body be monitored?
By using a detector such as a GM tube.
What can cause an isotope to decay to another isotope of a different element with the same mass no?
Beta decay. The process involves a change in element but with a negligible change in mass.
How can iodine isotope be used to identify an over-active thyroid gland?
Iodine-131 is taken up by the thyroid gland in the same way as ordinary iodine. An overactive thyroid concentrates more iodine – if the concentration of I-131 is greater than normal, this can be detected by measuring the activity and comparing it with the expected take-up from a normal thyroid gland.
How can iodine isotope help to treat an over-active thyroid?
I-31 is a high-energy beta-emitter. The radiation is sufficiently ionising to destroy cells in the thyroid, reducing its activity.
Explain the advantages of ionising radiation for sterilizing surgical equipment
Surgical instruments may be sealed into wrappers and then sterilised using radiation. The radiation passes through the wrapper, destroying any organisms on the instrument, which then remains sterile within its wrapper.
How can you use ionizing radiation to check the thickness of paper during production?
type of radiation
how it will be used to measure paper thickness
what checks should be made to make sure measurements are accurate
safety procedures
Beta radiation is used. It can pass through paper (unlike alpha particles) but the thicker the paper, the greater the amount of beta radiation absorbed.
A beta-emitting source is placed above the paper as it emerges from the rollers used to press it to
the required thickness and a detector is placed beneath the paper in line with the source. The count rate will decrease as the thickness of the paper passing between the emitter and the
detector increases.
To ensure accuracy, the background radiation count should be taken regularly so that the reading from the detector can be corrected. The half-life of the beta-emitting source needs to be quite long (so that the count rate does not fall significantly over short intervals), but the apparatus will need to be recalibrated regularly using paper of known thickness.
Care should be taken to ensure workers cannot come within range of the radiation from the source. The source must be stored in a secure container that is lined with lead to ensure no ionising radiation can escape. The storage area and the part of the paper-making works in which
the radiation source is being used should be clearly identified with standard signs. The source must be selected and screened to ensure that it emits only beta radiation.
How is C-14 made?
Cosmic radiation causes C-14 to form from nitrogen in the atmosphere.
Why does all living matter contain C-14?
Carbon-14 has the same chemical behaviour as the abundant stable isotope C-12. Carbon reacts
with oxygen to form carbon dioxide. Through photosynthesis, carbon-14 enters the food chain
and therefore all living material.
What happens to the proportion of C-14 in an organism when it dies?
Once an organism is dead, no new C-14 is taken in via photosynthesis (plants) or food
(animals). The proportion of C-14 in organic material decreases as the C-14 decays.
What assumptions are made in radiocarbon dating?
The principal assumption is that the rate of production of C-14 through cosmic rays has remained constant over time. In practice this is not so, but it is possible to make adjustments for the variation in C-14 by taking samples from objects of known age.
Why is carbon dating not suitable when measuring material older than 50,000 years?
The half-life of C-14 is roughly 5600 years. After 10 half-lives have elapsed, the activity remaining C-14 is too small for accurate measurement.
An isotope that decays by alpha emission is relatively safe when outside the body by dangerous if absorbed.
Why?
Alpha radiation has a very limited range. After alpha particles have travelled only a few centimetres in air, they have lost most of their kinetic energy and thus lose their ionising power. Alpha particles are stopped completely by quite thin layers of material that are not very dense.
Thus alpha radiation is not particularly dangerous to living cells, as in the human body, unless the source is very close to living tissue. If a source is very close to the skin it may, if the exposure is prolonged, cause burns and other tissue damage.
The greatest danger is when alpha emitting material is absorbed into the body. Inside the body, cells do not even have the protection of a layer of skin and fat, so are readily affected by the highly ionising alpha particles. This will result in cell destruction or mutation.
Why is radon-220 a v.dangerous isotope?
Radon-220 is an alpha-emitter. It is a dense gas and therefore accumulates in the lower parts of buildings, etc. As it is a gas, it is readily inhaled and thus comes into close contact with internal cells.
As described above, this is the most hazardous condition for alpha sources.
Give one difference between Thomson’s and R’s model?
In Thomson’s model, the mass of the atom is uniformly distributed through the whole of the atom.The atom was thought to be a positively charged sphere with negative charges distributed evenly throughout the sphere.
In Rutherford’s model, nearly all the mass of the atom and all its positive charge is concentrated in the small central core or nucleus.
What did the results of G and M’s experiment reveal about the atom?
That most of the volume of a gold atom is empty space, and most of the mass and positive charge is concentrated in a very small volume at the centre of the atom.
How would deflection change if particles had only half the electric charge?
The particles would have travelled closer to the nucleus before suffering a rebound and be deflected through smaller angles at any specific distance from the nucleus than the more highly charged particles.
What is a fissile material?
The nuclei of its atoms can be split apart by a nucleus.
What is a chain reaction?
A reaction that produces further reactions – in this case, neutrons produced when a uranium nucleus splits can cause further nuclei to split.
Why does a chain reaction depend on the amount of fissile material in one piece?
If there is only a small amount, neutrons can escape the material without hitting any more nuclei.
Advantages and Disadvantages of nuclear fission as an energy resource?
Advantages:
virtually inexhaustible supply of energy
does not produce ‘greenhouse’ gases.
Disadvantages:
produces waste that remains extremely dangerous for thousands of years
very highset-up and decommissioning costs.
What are alpha particles?
Alpha particles are helium nuclei (1) ejected at high speed from the decaying nucleus (1).
Describe the results of G and M’s expand what R drew from the results?
Most alpha particles passed straight through, undeflected (1). Some were deflected (1) and a
very small proportion actually rebounded (1).
Rutherford deduced that the mass and charge of
an atom was concentrated in a very small central core or nucleus (1)
Describe the plum model?
In the plum pudding model, the mass is uniformly distributed throughout the volume of the atom
(1). The atom was considered to be a positively charged sphere with electrons studded in it like
plums in a pudding or raisins in a cake (1).
Will an experiment with more or less particle representers be better?
The 200-dice experiment is more likely to produce the best model (1). In radioactive decay, millions of atoms are involved. The larger the number of dice, the more statistically valid the
experiment will be (1).
