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AQA A-Level Physics > Nuclear Physics > Flashcards

Nuclear Physics Flashcards

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1
Q

What did John Dalton think particles were like?

A

In 1804, John Dalton thought that atoms were tiny spheres that can’t be broken up.

He thought that everything was made up of indivisible particles.

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2
Q

What did J. J. Thompson think particles were like?

A

In 1897, J. J. Thompson thought that atoms were positive bodies of matter with negative electrons in them, named the plum pudding model.

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3
Q

What did the Rutherford Scattering (1909) experiment imply for the structure of atoms?

A

Rutherford Scattering suggested that atoms must have a small, positvely-charged nucleus at the centre.

  1. Most of the atom must be empty space because most of the alpha particles passed straight through the foil.
  2. The nucleus must have a large positive charge, as some positvely-charged alpha particles were repelled and deflected by a large angle.
  3. The nucleus must be small as very few alpha particles were deflected back.
  4. Most of the mass must be in the nucleus, since the fast alpha particles are deflected by the nucleus.
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4
Q

Implications of the Rutherford Scattering experiment:

  1. Most of the atom must be _____ _____ because most of the alpha particles passed _____ _____ the foil.
  2. The nucleus must have a _____ _____ charge, as some _____-charged alpha particles were repelled and deflected by a _____ angle.
  3. The nucleus must be _____ as very few alpha particles were _____ back.
  4. Most of the mass must be in the _____ , since the fast alpha particles are deflected by the _____.
A

Implications of the Rutherford Scattering experiment:

  1. Most of the atom must be (empty space) because most of the alpha particles passed (straight through) the foil.
  2. The nucleus must have a (large positive) charge, as some (positvely)-charged alpha particles were repelled and deflected by a (large) angle.
  3. The nucleus must be (small) as very few alpha particles were (deflected) back.
  4. Most of the mass must be in the (nucleus), since the fast alpha particles are deflected by the (nucleus).
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5
Q

Describe the plum pudding model.

A

The plum pudding model described an atom to be a ball of positive charge, with negatively charged electrons evenly distributed throughout it.

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6
Q

Describe the Rutherford Scattering experiment.

A
  1. In 1909, Rutherford tried firing a beam of alpha particles at thin gold foil.
  2. A circular detector screen surrounding the gold foil and the alpha source was used to detect alpha particles deflected.
  3. If the plum pudding model was true, alpha particles would be deflected by a very small amount.
  4. Instead, most alpha particles just went straight through the foil and a small number were deflected by a large angle.
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7
Q

How can you estimate the closest approach of a scattered particle?

A

An alpha particle that ‘bounces back’ and is deflected 180° will reverse direction when it’s close to the gold nucleus.

It does this at the point where electric potential energy equals its inital kinetic energy.

So, Ek = Coulomb’s Law/Eelec

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8
Q

Give the properties of alpha radiation.

A

Alpha Radiation:

  1. Strongly ionising
  2. Slow moving
  3. Stopped by a few centimeters of air or paper
  4. Positively charged (2p 2n)
  5. Deflected in a magnetic field
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9
Q

How do smoke detectors work?

A

Smoke detectors:

Alpha particles ionise the air particles between two metal plates, allowing a current to flow between them.

When smoke enters the detector, the alpha particles can’t ionise the air, causing a reduction in the current.

The reduction in the current triggers the alarm.

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10
Q

Give the properties of beta radiation.

A

Beta Radiation:

  1. Mildly ionising
  2. Fast moving
  3. Stopped by a few millimeters of aluminium
  4. Negatively charged
  5. Deflected in a magnetic field
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11
Q

How do thickness monitors work?

A

Thickness Monitors:

Beta particles are emitted on one side of the material and, detected on the other.

If the material is too thick, the number of beta particles detected will be too low, and will trigger the machine to reduce the material thickness.

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12
Q

Give the properties of gamma radiation.

A

Gamma Radiation:

  1. Weakly ionising
  2. Travels at the speed of light
  3. Stopped by several centimeters of lead or a few meters of concrete
  4. Chargeless
  5. Unaffected by magnetic and electric fields
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13
Q

What is gamma radiation used for?

A

Gamma radiation is used to sterilise medical equipment and kill cancerous cells, as well as being used as a medical tracer in diagnosis.

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14
Q

How can you be safe whilst dealing with radioactive sources?

A

Safe use of Radiation:

  1. Never directly handle the source
  2. Use long-armed tongs to increase your distance from the source
  3. Display signs warning others that radioactive sources are in use
  4. Keep the time that the sources are being used to a minimum
  5. Store in an appropriate lead box when not in use
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15
Q

If a beam of high-energy electrons is directed onto a thin film of material in front of a screen, a diffraction pattern is formed.

Give the equation for which the first minimum appears.

(This can be used to find the radius of a nucleus)

A

For high-energy electrons directed onto a thin film of material in front of a screen, first minimum:

sinθ = 0.61λ/R
not given in exam

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16
Q

When you plot a graph of radius of nucleus (R) against the cube root of of the nucleon number (A^1/3), what line do you get?

A

When you plot a graph of radius of nucleus (R) against the cube root of of the nucleon number (A^1/3), you get a straight line.

R ∝ A^1/3 not given in exam

Or

R = R0A^1/3 given in exam

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17
Q

Describe the inverse square law that gamma radiation obeys.

A

A gamma source will emit gamma radiation in all directions.

This radiation spreads out as you get further away from the source.

This means the amount of radiation per unit area (intensity) decreases the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it decreases by the square of the distance from the source.

Therefore:

I = k/x^2

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18
Q

Gamma Radiation Inverse Square Law:

A gamma source will emit gamma radiation in all _____.

This radiation _____ out as you get _____ _____ from the source.

This means the amount of radiation per unit area (intensity) _____ the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it _____ by the _____ of the distance from the source.

Therefore:

_____

A

Gamma Radiation Inverse Square Law

A gamma source will emit gamma radiation in all (directions).

This radiation (spreads) out as you get (further away) from the source.

This means the amount of radiation per unit area (intensity) (decreases) the further away you get from the source.

If you took a reading of intensity, I, at a distance, x, from the source you’d find that it (decreases) by the (square) of the distance from the source.

Therefore:

I = k/x^2

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19
Q

What does the gamma radiation inverse square law tell us?

A

The gamma radiation inverse square law of intensity with distance squared tells us that intensity is much higher with shorter distances to the source.

This means we should keep a long distance from the source to stay safe.

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20
Q

What is the rate of radioactive decay like?

A

Radioactive decay is completely random.

Every isotope decays at a different rate.

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21
Q

What is the decay constant?

A

The decay constant, λ, is the probability of a given nucleus decaying per second.

The bigger the value of λ, the faster the rate of decay.

Its units is s^-1

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22
Q

What is activity measured in?

A

Activity is measured in becquerels (Bq), where 1 Bq = 1 decay per second.

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23
Q

What is half life?

A

The half-life (T1/2) of an isotope is the average time it takes for the number of unstable nuclei to halve.

24
Q

What does a graph of number of unstable nuclei remaining, N, against time look like?

A

For a graph of number of stable nuclei remaining, N, against time, the line shows an exponential decay.

25
Q

How can you find the number of unstable nuclei remaining?

A

The number of unstable nuclei remaining, N, depends on the number originally present, N0.

N = N0e^λt given in exam

26
Q

How can you find activity for a point in decay?

A

As a sample decays, its activity goes down.

A = A0e^λt

Same as

N = N0e^λt

27
Q

How is gamma radiation used in medical diagnosis?

A

Radioactive tracers are used to help diagnose patients.

Radioactive sources that emit gamma radiation and have half-lives of around 6 hours (long enough for data to be recorded, short enough to limit radiation) are suitable.

28
Q

Why are long half-lives dangerous?

A

Isotopes used in nuclear power generation have very long half-lives, so we have to plan ahead about how the waste will be stored (in water tanks or sealed underground).

This is to prevent damage to the environment or people for the foreseeable future.

29
Q

What four things must be conserved in nuclear decays?

A

In every nuclear reaction energy, momentum, charge and nucleon number must be conserved.

30
Q

What is nuclear fission?

A

Nuclear fission occurs when large nuclei are unstable (usually with at least 83 protons) and can randomly split into two smaller nuclei.

31
Q

How is nuclear fission induced?

A

Nuclear fission can be induced by making a neutron enter a Uranium nucleus, causing it to become very unstable.

32
Q

Why is energy released during nuclear fission?

A

Energy is released during nuclear fission because the new, smaller nuclei have a higher binding energy per nucleon.

33
Q

What is spontaneous fission?

A

Fission is called spontaneous if it just happens by itself.

Large nuclei are more likely to spontaneously fission because the larger the nucleus, the more unstable it is.

34
Q

List the components of a nuclear reactor.

A

A nuclear reactor includes:

  1. Fuel rods (rods of uranium)
  2. Control rods
  3. Moderator (water)
  4. Pump
  5. Coolant
  6. Concrete case
35
Q

Nuclear reactors:

What is the critical mass?

A

The critical mass is the amount of fuel needed for the chain reaction to continue on its own at a steady rate.

Any less than the critical mass (sub-critical mass) means that the reaction will just die out.

36
Q

Nuclear reactors:

How does the moderator slow down the fission reactions?

A

The moderator, water, slows down neutrons so that they can be captured by the uranium nuclei and continue the chain reaction.

The slowed down neutrons are called thermal neutrons.

Collisions with particles of a similar mass to neutrons are more efficient at slowing them down - making water efficient as it contains hydrogen.

37
Q

Nuclear reactors:

What do control rods do?

A

Control rods control the chain reaction by limiting the number of neutrons in the reactor.

These absorb neutrons so that the rate of fission is controlled.

Control rods are made up of a material that absorbs neutrons, e.g. boron.

They can be inserted by varying amounts to control the reaction rate.

38
Q

Nuclear reactors:

What does a nuclear reactor do automatically in an emergency?

A

In an emergency, the reactor will be shut down automatically by releasing the control rods into the reactor.

This will stop the reaction as quickly as possible.

39
Q

Nuclear reactors:

What is the purpose of coolant?

A

Coolant is sent around the reactor to remove heat produced in the fission - often this is the same water that is being used as the moderator.

The heat from the reactor can then be used to make steam for powering electricity-generating turbines.

40
Q

Nuclear reactors:

What is the purpose of the concrete case?

A

Thick concrete casing acts as shielding - it prevents radiation escaping and reaching the workers.

41
Q

What are some advantages of using nuclear fission to produce electricity?

A
  1. Produces lots of energy

2. Creates less greenhouse gases than fossil fuels

42
Q

What is the biggest disadvantage to producing electricity with nuclear fission?

A

There are lots of dangerous waste products.

43
Q

What happens to the waste produced by nuclear fission reactors?

A

Some waste can be used for tracers in medical diagnosis.

Initally when material is removed from the reactor it is very hot, so is placed in cooling ponds until the temp falls to a safe level.

It’s then stored in sealed containers until its activity has fallen sufficiently. Areas are chosen where there is minimal impact. Residents are consulted.

44
Q

What is nuclear fusion?

A

Nuclear fusion is when two light nuclei are combined to create a larger nucleus.

45
Q

Why is so much energy needed for nuclear fusion?

A

All nuclei are positively charged, so there is a repulsive force between two nuclei.

Nuclei can only fuse once they have overcome this electrostatic force.

About 1MeV of kinetic energy is needed for nuclei to fuse together.

46
Q

What is the mass defect?

A

The mass of a nucleus is less than the mass of its constituent parts.

The difference is called the mass defect.

47
Q

What is binding energy?

A

Binding energy is the energy needed to separate all of the nucleons in a nucleus.

It is equivalent to the mass defect.

48
Q

What is the stability graph for radioactive nuclei?

A

You can get a stability graph by plotting Z (atomic number) against N (number of neutrons)
______________________________________x_________________________y

49
Q

Describe the line of stability on the stability graph for radioactive nuclei.

A

Straight line from the origin to (Z = 20, N = 20)

Curving upwards to Z = 80; N = 110 –130

50
Q

Where do you find alpha emitters on the stability graph?

A

Below the line of stability, N > 80 and Z > 60.

51
Q

Where do you find beta-minus emitters on the stability graph?

A

Any region above and close to the line of stability.

52
Q

Where do you find beta-plus emitters on the stability graph?

A

Any region below and close to the line of stability.

53
Q

What is the equation for electron capture?

A

Electron Capture

p + e- -> n + ν(e)

54
Q

What is the equation for beta-plus decay?

A

Beta-Plus Decay

p -> n + e+ + ν(e)

55
Q

What is the equation for beta-minus decay?

A

Beta-Minus Decay
——————_
n -> p + e- + ν(e)

AQA A-Level Physics (14 decks)

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