A LevelPhysicsAQAFlashcardsNuclear PhysicsAlpha, Beta & Gamma Radiation

Alpha, Beta & Gamma Radiation (AQA A Level Physics): Flashcards

Exam code: 7408

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  • FrontRutherford Scattering

    Define collimated beam.

Cards in this collection (57)

  • Define collimated beam.

    A collimated beam is a narrow, concentrated beam of particles all travelling in the same direction, produced in Rutherford's experiment by passing alpha particles through a narrow hole in a lead container.

  • What four components made up the apparatus in Rutherford's alpha-scattering experiment?

    • A source of alpha particles in a lead container

    • A thin sheet of gold foil

    • A movable detector

    • An evacuated chamber

  • Why did the gold foil have to be extremely thin (about 10-6 m)?

    A thicker foil would have completely stopped the alpha particles. Gold was also chosen because it is malleable and easy to hammer into thin sheets.

  • Why was Rutherford's apparatus placed inside an evacuated chamber?

    Alpha particles are highly ionising and only travel about 5 cm in air before interacting with air molecules, so a vacuum ensured they reached the foil without colliding with air particles first.

  • Most alpha particles passed straight through the gold foil undeflected, which suggested that the atom is mostly ...........

    Most alpha particles passed straight through the gold foil undeflected, which suggested that the atom is mostly empty space.

  • What did the small number of alpha particles deflected through angles greater than 90° suggest about the nucleus?

    The nucleus is extremely small and contains most of the atom's mass and positive charge concentrated in one place.

  • True or False?

    In Rutherford's scattering experiment, alpha particles were deflected through small angles because the nucleus is negatively charged.

    False.

    The nucleus is positively charged. Alpha particles, also positively charged, were deflected through small angles because two positive charges repel each other.

  • Roughly how many times larger is an atom than its nucleus?

    An atom is around 100,000 times larger than its nucleus.

  • Define plum pudding model.

    J.J. Thomson's model of the atom, proposing a sphere of positive charge with uniformly distributed density, with negatively charged electrons stuck in it like currants in a pudding.

  • What did Dalton's atomic model (1803) propose?

    • Atoms are the smallest constituents of matter and cannot be broken down further

    • Atoms of a given element are identical; atoms of different elements are different

    • In chemical reactions, atoms rearrange to form different substances

  • What result from Rutherford's gold foil experiment disproved the plum pudding model?

    According to the plum pudding model, alpha particles should have passed straight through the foil, but many were deflected, or even backscattered, showing the model was wrong.

  • What did Rutherford's model (1909-1911) propose about the structure of the atom?

    • Atoms are mostly empty space

    • Atoms have a central, positively charged nucleus containing most of the mass

  • Define planetary model (Bohr, 1913).

    Bohr's model in which electrons orbit the nucleus, like planets around a star, occupying well-defined shells or energy levels.

  • What did Schrödinger's quantum mechanical model (1926) propose about electron position?

    Electrons form a cloud around the nucleus; where the cloud is most dense, the probability of finding an electron is greatest.

  • James Chadwick discovered the .......... in 1932, completing the atomic model used today.

    James Chadwick discovered the neutron in 1932, completing the atomic model used today.

  • True or False?

    J.J. Thomson's plum pudding model proposed that the atom has a small, dense, positively charged nucleus.

    False.

    That description is Rutherford's model. Thomson's plum pudding model proposed the positive charge was spread uniformly throughout the atom, not concentrated in a small nucleus.

  • Define alpha particle.

    A high-energy helium nucleus, containing two protons and two neutrons, with a mass of 4u and a charge of +2e.

  • What is emitted by a nucleus with too many neutrons, and what is emitted by a nucleus with too many protons?

    • Too many neutrons: a beta-minus particle (a high-energy electron)

    • Too many protons: a beta-plus particle (a high-energy positron)

  • Rank alpha, beta and gamma radiation from most to least ionising.

    • Alpha (most ionising, around 10,000 ion pairs per cm)

    • Beta (moderate, around 100 ion pairs per cm)

    • Gamma (least ionising, around 1 ion pair per cm)

  • What thickness of material is needed to absorb each type of radiation?

    • Alpha: a single sheet of paper

    • Beta: a few millimetres of aluminium foil

    • Gamma: several metres of concrete or several centimetres of lead

  • How are alpha and beta particles deflected in an electric field, and what happens to gamma radiation?

    Alpha particles deflect towards the negative plate, beta particles deflect towards the positive plate, and gamma radiation is uncharged so it is not deflected.

  • Although alpha particles have a higher charge than beta particles, they are deflected .......... than beta particles in a given field because of their much greater mass.

    Although alpha particles have a higher charge than beta particles, they are deflected less than beta particles in a given field because of their much greater mass.

  • True or False?

    Gamma radiation is the most penetrating type of radiation because it has the greatest mass.

    False.

    Gamma radiation is the most penetrating because it is uncharged electromagnetic radiation, so it rarely ionises or interacts with matter, not because of any mass property.

  • Why is beta radiation, rather than alpha or gamma, used to control the thickness of materials such as foil?

    Alpha would be absorbed completely by the material, and gamma would pass straight through undetected, whereas beta is only partially absorbed, so changes in thickness can be detected.

  • Why is Americium-241 suitable for use in a smoke detector?

    It is an alpha emitter, whose ionising particles are absorbed by smoke to reduce the current and sound the alarm; its short range keeps it safe within the device, and its long half-life of 460 years means it rarely needs replacing.

  • True or False?

    Gamma radiation would be a suitable choice for monitoring the thickness of thin aluminium foil.

    False.

    Gamma radiation is too penetrating and would pass straight through the foil undetected, regardless of its thickness, so beta radiation is used instead.

  • Define the inverse-square law of gamma radiation.

    The intensity, I, of gamma radiation is inversely proportional to the square of the distance, x, from the source: I = \frac{k}{x^{2}} where k is a constant of proportionality.

  • Write the equation relating gamma intensity at two different distances from a source.

    \frac{I_{1}}{I_{2}} = \left(\frac{x_{2}}{x_{1}}\right)^{2}

  • Why does gamma radiation obey an inverse-square law, but alpha and beta radiation do not?

    Gamma radiation is not easily absorbed by matter and so can spread out uniformly as a sphere, whereas alpha and beta radiation are absorbed quickly before they can spread out.

  • What are the units of intensity, I, in the inverse-square law equation?

    Watts per square metre, \text{W m}^{-2}

  • The inverse-square law applies to gamma radiation but .......... to alpha or beta radiation.

    The inverse-square law applies to gamma radiation but not to alpha or beta radiation.

  • True or False?

    If the distance from a gamma source is doubled, the intensity is halved.

    False.

    Because intensity follows an inverse-square law, doubling the distance reduces the intensity to one quarter of its original value, not one half.

  • Define background radiation.

    The ionising radiation present in the environment.

  • Name four natural sources of background radiation.

    • Radon gas from rocks and buildings

    • Cosmic rays from space

    • Carbon-14 in biological material

    • Radioactive material in food and drink

  • Name three artificial (man-made) sources of background radiation.

    • Nuclear medicine (for example X-rays, tracers, radiotherapy)

    • Nuclear waste

    • Nuclear fallout from weapons, or nuclear accidents

  • In the UK, which single source contributes the largest proportion of background radiation?

    Radon gas.

  • Define corrected count rate.

    The count rate of a source with the background count rate subtracted, found by taking readings with no source present and subtracting this from readings taken with the source present.

  • A Geiger counter records 285 counts in one minute with a source present, and 24 counts per minute with no source present. What is the corrected count rate, in counts per second?

    (285 - 24) \div 60 = 4.35 \text{ counts per second}

  • Radon gas is produced when .......... in rocks and soil decays, and it is an alpha emitter.

    Radon gas is produced when uranium in rocks and soil decays, and it is an alpha emitter.

  • True or False?

    Radon gas can easily be detected by its smell.

    False.

    Radon gas is tasteless, colourless and odourless; it can only be detected using a Geiger counter.

  • Define dose equivalent.

    A measure, in Sieverts (Sv), of the biological damage caused by radiation exposure. It depends on the energy absorbed per kilogram of the body and the type of radiation absorbed.

  • Give some precautions used to reduce risk when handling radioactive sources.

    • Keeping sources shielded when not in use, for example in a lead-lined box

    • Wearing protective clothing

    • Keeping personal items outside the room

    • Limiting exposure time

    • Handling sources with long tongs

    • Monitoring exposure with detector badges

  • Why are short-lived isotopes and small sample sizes preferred to minimise received radiation dose?

    They reduce the total amount and duration of ionising radiation the body is exposed to.

  • What precautions protect the radiographer during radiotherapy?

    • Handling the source remotely with tongs or a machine

    • Being protected by a screen

    • Standing a long way from the source

    • Storing the source in its lead case immediately after use

  • Why are radioisotopes with a short half-life preferred as medical tracers?

    Their high initial activity means only a small sample is needed, and they decay quickly, posing a lower risk to the patient.

  • Why is Technetium-99m widely used as a medical tracer?

    It is a gamma emitter, so it is highly penetrating and can be detected outside the body, and it is the weakest ioniser. It has a short half-life of six hours and can be prepared easily at the hospital.

  • Why doesn't gamma-sterilised medical equipment itself become radioactive?

    Becoming radioactive requires the nucleus to be affected, but ionising radiation only affects the outer electrons, not the nucleus.

  • Iodine-131 emits .......... particles, meaning it stays concentrated on the thyroid rather than spreading through the body.

    Iodine-131 emits beta particles, meaning it stays concentrated on the thyroid rather than spreading through the body.

  • True or False?

    Sterilising medical equipment with gamma radiation makes the equipment itself radioactive.

    False.

    Ionising radiation only affects outer electrons, not the nucleus, so the equipment does not become radioactive. The radioactive source is also kept sealed away from the equipment.

  • Define the inverse square law for gamma radiation.

    The intensity, I, of gamma radiation from a point source is inversely proportional to the square of the distance, x, from the source:

    I \propto \frac{1}{x^{2}}

  • What are the independent and dependent variables in the inverse square law experiment for gamma radiation?

    • Independent variable: distance between source and detector, x

    • Dependent variable: count rate (activity) of the source, C

  • To obtain the corrected count rate, the .......... must be subtracted from each raw count rate reading.

    To obtain the corrected count rate, the background radiation must be subtracted from each raw count rate reading.

  • Why is a graph of \frac{1}{\sqrt{C}} against x plotted, rather than C against x?

    Since C \propto \frac{1}{x^{2}}, plotting \frac{1}{\sqrt{C}} against x gives a straight line through the origin, which confirms the inverse square relationship.

  • What does it mean if the straight-line graph of \frac{1}{\sqrt{C}} against x does not pass through the origin?

    It indicates a systematic error in the measurement of distance, x, because the exact positions of the source and detector inside their sealed tubes are not known.

  • True or False?

    The Geiger-Muller tube should be shielded with aluminium to stop gamma radiation reaching the detector.

    False.

    The aluminium shielding stops alpha and beta radiation from reaching the detector, not gamma radiation, so that only gamma radiation from the source is measured.

  • Why should the count be measured over as long a time span as possible?

    A longer count time gives a larger total count, which reduces the percentage uncertainty, since percentage error is proportional to the inverse square root of the count.

  • Define dead time.

    Dead time is the short period (around 100 μs) during which multiple counts occurring simultaneously are registered by the Geiger-Muller tube as only one count.

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