5.2 Radioactivity (Cambridge (CIE) O Level Physics): Flashcards
Exam code: 5054
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Define count rate.
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Define count rate.
The count rate is the number of decays detected per second (or minute) by a detector and recorded by a counter. Not every decay is necessarily detected.
Which two detectors are used to detect alpha particles?
A cloud chamber or a spark counter.
How can an alpha particle's track be identified in a cloud chamber?
It produces a thick, short track that curls slowly in one direction.
Describe how a spark counter detects alpha radiation.
A high voltage is applied between a thin metal gauze and a nearby wire, reduced until no sparking occurs. When an alpha source is brought close, it ionises the surrounding air, creating sparks between the gauze and the wire.
Which detector is most commonly used to detect beta particles and gamma radiation?
The Geiger-Müller (GM) tube.
How does the count rate detected change as distance from a radioactive source increases?
The count rate decreases, because the radiation becomes more spread out further from the source.
A cloud chamber contains a thin film of ______ vapour, cooled using dry ice, which condenses to form visible tracks.
A cloud chamber contains a thin film of alcohol vapour, cooled using dry ice, which condenses to form visible tracks.
True or False?
A cloud chamber is set up with a magnetic field at right angles to the chamber.
True.
This is part of the standard cloud chamber set-up, alongside a cooled metal plate, alcohol vapour and the radioactive source.
Define background radiation.
Background radiation is the (ionising) radiation that is always present in the environment around us.
Name four natural sources of background radiation.
Radon gas, rocks and buildings, cosmic rays from space, carbon-14 in biological material, and radioactive material in food and drink.
Why is radon gas a significant source of background radiation?
It is produced by the natural decay of uranium in rocks and soil, and is particularly dangerous if inhaled into the lungs in large quantities.
Give two examples of man-made (artificial) sources of background radiation.
Medical sources (for example X-rays, CT scans, radiotherapy) and nuclear accidents (for example Chernobyl). Nuclear fallout and nuclear waste are also man-made sources.
Define corrected count rate.
The corrected count rate is the count rate due to a radioactive source alone. It is found by subtracting the background count rate (measured with no source present) from the count rate measured with the source present.
How can the background radiation count be found from a graph of count rate against distance from a source?
Identify the distance beyond which the count rate stops decreasing and levels off. At this point, the source's radiation has been fully absorbed by the air, so the remaining count rate is due to background radiation alone.
Cosmic rays entering the Earth's atmosphere collide with molecules in the air, producing ______ radiation.
Cosmic rays entering the Earth's atmosphere collide with molecules in the air, producing gamma radiation.
True or False?
Nuclear power stations are the largest contributor to background radiation.
False.
Radiation from nuclear power contributes less than 0.1% of the total; natural sources (radon, rocks and buildings, cosmic rays, food and drink) contribute the most.
Define radioactive decay.
Radioactive decay is the spontaneous and random process by which an unstable nucleus emits radiation (a high-energy particle or wave) to become more stable.
Define gamma radiation.
Gamma radiation (γ) is an electromagnetic wave with the highest energy of the different types of electromagnetic waves. It is emitted from unstable nuclei and has no charge.
What does it mean for radioactive decay to be a random process?
It is not possible to know exactly when, or in which direction, a particular nucleus will decay.
What does it mean for radioactive decay to be a spontaneous process?
It cannot be affected by environmental factors such as temperature, pressure, or chemical conditions.
State the nature and charge of an alpha particle.
An alpha particle (α) consists of two protons and two neutrons (the same as a helium nucleus) and has a charge of +2.
What is a beta particle, and how is it produced in the nucleus?
A beta particle (β⁻) is a fast-moving electron, produced when a neutron changes into a proton and an electron. It has a charge of -1.
Describe the trend in range, penetrating power and ionising power down the table from alpha to beta to gamma radiation.
Range increases, penetrating power increases, and ionising power decreases.
Alpha radiation is stopped by paper; beta radiation is stopped by a few mm of ______; gamma radiation is only partially stopped by a few mm of thick lead.
Alpha radiation is stopped by paper; beta radiation is stopped by a few mm of aluminium; gamma radiation is only partially stopped by a few mm of thick lead.
Why is gamma radiation not deflected by electric or magnetic fields, while alpha and beta particles are?
Gamma radiation has no charge, so it is unaffected. Alpha and beta particles are charged (+2 and -1 respectively), so they are deflected — in opposite directions, because their charges are opposite.
True or False?
Beta particles are deflected towards the negative plate in an electric field.
False.
Beta particles are negatively charged, so they are deflected towards the positive plate. It is alpha particles (positively charged) that are deflected towards the negative plate.
Define radioactive decay.
Radioactive decay is the process in which an unstable nucleus emits radiation (as alpha, beta or gamma) in order to become more stable.
Define random, in the context of radioactive decay.
It means the decay is spontaneous: it is impossible to know which particular nucleus will decay next or at what time it will decay, and the rate is unaffected by surrounding conditions.
Why is carbon-14 unstable while carbon-12 is stable?
Carbon-14 has two extra neutrons compared to carbon-12, which creates an imbalance in the forces within the nucleus.
How do the mass number and atomic number change during alpha decay?
The mass number decreases by 4 and the atomic number decreases by 2.
How do the mass number and atomic number change during beta decay?
The mass number stays the same; the atomic number increases by 1, because a neutron changes into a proton (which remains in the nucleus) and an electron (which is emitted).
How do the mass number and atomic number change during gamma decay?
Both stay the same — a gamma ray carries energy away but has no mass and no charge.
During alpha decay, the atomic number decreases by ______ and the mass number decreases by ______.
During alpha decay, the atomic number decreases by 2 and the mass number decreases by 4.
True or False?
The atomic (proton) number of a nucleus is determined only by the number of protons it contains.
True.
Only protons determine the atomic number; neutrons do not contribute to it, though they do contribute to the mass number.
Define nuclear fission.
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, releasing neutrons and energy.
Define nuclear fusion.
Nuclear fusion is when two light nuclei join to form a heavier nucleus, releasing energy.
Define a chain reaction.
A chain reaction is when the neutrons released by one fission reaction go on to cause further fission reactions, each releasing more neutrons.
Besides two smaller (daughter) nuclei, what else is produced when a large nucleus undergoes fission?
Two or three neutrons and gamma rays are also produced.
What is the purpose of control rods in a nuclear reactor?
To absorb neutrons (without becoming dangerously unstable themselves), controlling the rate of fission.
What is the purpose of the moderator in a nuclear reactor?
To slow down the fast-moving neutrons produced by fission, so they become thermal neutrons that react efficiently with the fuel.
In both fission and fusion, the mass of the products is slightly less than the mass of the starting nuclei because the missing mass has been converted into ______.
In both fission and fusion, the mass of the products is slightly less than the mass of the starting nuclei because the missing mass has been converted into energy.
True or False?
Nuclear fusion requires extremely high temperatures to occur.
True.
This is why fusion is very difficult to reproduce on Earth, despite it occurring naturally in stars.
Define half-life.
Half-life is the time taken for half the nuclei of an isotope in a sample to decay (equivalently, the time for the activity of a sample to fall to half its original level).
How is carbon-14 formed in the atmosphere?
A high-energy cosmic ray knocks a neutron out of a nucleus, and the neutron collides with a nitrogen-14 nucleus, forming carbon-14 (and a proton).
Why does the proportion of carbon-14 in a living organism stay constant?
Carbon-14 is constantly replaced while the organism is alive (through photosynthesis or eating plants); it only starts to fall after the organism dies and stops absorbing carbon.
What is the approximate half-life of carbon-14?
About 5730 years.
Over what approximate age range is carbon dating considered reliable?
From about 1000 years to 40 000 years old.
Why is carbon dating unreliable for samples older than 40 000 years?
So little carbon-14 remains that its activity is similar to background radiation, so very few decays occur — the carbon-14 to carbon-12 ratio is too small to determine an accurate age.
Each time one half-life passes, the activity of a radioactive sample falls to ______ of its previous value.
Each time one half-life passes, the activity of a radioactive sample falls to half of its previous value.
True or False?
The half-life of an isotope decreases as a sample gets older.
False.
Half-life is constant for a given isotope, regardless of the age of the sample or how much of it remains.
Why are alpha particles used in smoke detectors rather than beta or gamma?
Alpha ionises the air to create a small current, and is weakly penetrating so it is easily blocked by smoke, triggering the alarm. Beta or gamma would pass straight through the smoke without being stopped.
Why are beta particles (not alpha or gamma) used to monitor the thickness of a material?
Beta is partially absorbed by the material, so the amount detected changes with thickness. Alpha would be fully absorbed and gamma would almost all pass through, so neither would show a measurable change.
Why are gamma rays used in radiotherapy to treat tumours?
Gamma rays are able to penetrate the body, allowing them to reach a tumour inside it.
Define a tracer, in the context of medical uses of radiation.
A tracer is a radioactive isotope used to track the movement of a substance, such as blood, around the body — for example, detected using a PET scan.
Why is gamma radiation used to sterilise medical equipment?
Gamma is the most penetrating type of radiation, so it can irradiate all sides of the instruments and sterilise them without removing the packaging.
Give two uses of radiation besides smoke detectors, thickness gauging, cancer treatment and sterilising equipment.
Sterilising food (irradiation to kill microorganisms) and dating ancient artefacts (determining age).
Beta particles are used to measure material thickness because they are ______ absorbed by the material, unlike alpha (fully absorbed) or gamma (mostly passes through).
Beta particles are used to measure material thickness because they are partially absorbed by the material, unlike alpha (fully absorbed) or gamma (mostly passes through).
True or False?
During radiotherapy, the gamma beams are moved around the patient to minimise damage to healthy tissue.
True.
The beams are moved around while still being aimed at the tumour, to reduce harm to the surrounding healthy tissue.
List the four ways ionising radiation can damage human cells and tissue at high doses.
Cell death, tissue damage, mutations and cancer.
Why might a doctor recommend radiotherapy for a patient despite the risks of radiation?
Because the risk posed by the radiation is smaller than the risk of leaving a life-threatening tumour untreated.
How can ionising radiation lead to a mutation?
Radiation ionises the atoms in a DNA strand, damaging it. If the DNA is repaired incorrectly when it reforms, a mutation results — if the mutated cell replicates, a tumour can form.
What effect can acute radiation exposure have on the immune system?
It can reduce white blood cells, making the body more susceptible to infection.
How should a radioactive source be handled to minimise risk?
Handled with tongs at arm's length, for the minimum time possible, then returned immediately to its lead-lined box.
Why is radioactive waste with a long half-life buried deep underground?
It remains radioactive (and a contamination risk) for a very long time, so burial prevents it being released into the environment.
Radioactive sources should be stored in ______ boxes to shield people from radiation.
Radioactive sources should be stored in lead-lined boxes to shield people from radiation.
True or False?
Gloves and safety glasses are always required, in addition to tongs, when handling radioactive sources.
False.
Tongs held at arm's length are usually sufficient; gloves and specs are only needed if there is a risk of the material leaking onto other items.
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