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a-level physics > 13: Nuclear Physics > Flashcards

13: Nuclear Physics Flashcards

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1
Q

What did John Dalton think about atoms?

A

Matter was made up of tiny spheres (atoms) that couldn’t be broken up

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2
Q

What was J.J Thompson’s suggestion about atomic structure?

A

Atoms were spheres of positive charge with tiny negative electrons stuck in them.
This is the Plum Pudding Model

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3
Q

What experiment did Ernest Rutherford do to prove the Plum Pudding model wrong?

A

Alpha scattering experiment

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4
Q

How did the alpha scattering experiment work?

A
  • A stream of alpha particles from a radioactive source are fired at very thin gold foil
  • when alpha particles strike a fluorescent screen a tiny visible flash of light is produced
  • The fluorescent screen is circular and surrounds the experiment so that alpha particles can be detected from any angle
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5
Q

What conclusions were made from the alpha scattering experiment?

A
  • The atom must be mostly empty space because most alpha particles pass straight through
  • The nucleus must have a large positive charge, as some of the positively charged alpha particles are repelled and deflected by a large angle
  • The nucleus must be tiny as very few alpha particles are deflected by an angle greater than 90 degrees
  • Most of the mass must be in the nucleus, since the alpha particles (with high momentum) are deflected by the nucleus
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6
Q

What was the expected conclusion from the alpha scattering experiment?

A

If the Thomson model was right, all the flashes should have been seen within a small angle of the beam, because the positively charged alpha particles would be deflected by a small amount by the electrons

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7
Q

How did Rutherford and Kay discover the proton?

A

By firing high-energy alpha particles at different gases

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8
Q

How can you estimate the radius of an atomic nucleus

A
  • Rutherford’s scattering experiment
  • an alpha particle that ‘bounces back’ and is deflected through 180 degrees will have stopped a short distance from the nucleus
  • this is at the point where the particles electric potential energy is equal to the initial kinetic energy
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9
Q

what is the equation to work out the radius of an atomic nucleus

A

Initial Ek = Eelec = Q(nucleus) x q (alpha)/ 4π x εo x r

where Q is the charge of the nucleus, q is the charge of the alpha particle, and r is the distance of closest approach

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10
Q

What is the distance of closest approach

A

an estimate of nuclear radius

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11
Q

what is a more accurate method of measuring the radius of a nucleus

A

electron diffraction

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12
Q

how can electron diffraction estimate the radius of a nucleus

A
  • a beam of moving electrons has an associated de Broglie wavelength, which at high speeds is approximately = hc/E
  • the wavelength must be tiny to investigate the nuclear radius, so electrons have a very high energy
  • if a beam of high energy electrons is directed onto a thin film of material in front of a screen, a diffraction pattern will be seen on the screen
  • the first minimum appears where sinθ = 1.22λ/2R
    -using measurements from this diffraction pattern, you can rearrange the equation to find the radius of the nucleus
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13
Q

what is the radius of an atom

A

5 x10 ^-11

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14
Q

what is the radius of the smallest nucleus

A

1 fm

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15
Q

what does the graph of nuclear radius (R) against nucleon number look like (A)

A

lower case letter r

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16
Q

how do you get a straight line graph between nuclear radius (R) and nucleon number (A)?

A

Plot R against A^1/3

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17
Q

what is the equation between nuclear radius and nucleon number?

A

R = R0 x A^1/3

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18
Q

what does the equation for nuclear radius tell you about the nucleus

A
  • the volume each nucleon takes up in the nucleus is about the same
  • as the mass of each nucleon is about the same, the nuclear density of a nucleus is always constant, no matter how many nucleons make up the atom
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19
Q

what is the equation for nuclear density

A

3m(nucleon)/4πx(R0)^3

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20
Q

Nuclear density is significantly greater than atomic density. What does this suggest about the structure of the atom

A
  • most of the atom’s mass is in its nucleus
  • the nucleus is small compared to the atom
  • an atom must contain a lot of empty space
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21
Q

what is radioactive decay

A

when an unstable atomic nucleus releases energy/particles in order to become more stable

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22
Q

what is one atomic mass unit

A

1.661 x10^-27

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23
Q

what is the range of alpha radiation

A

absorbed by paper, skin or a few centimetres of air

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24
Q

what is the range of beta minus radiation

A

absorbed by about 3mm of aluminium

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25
Q

what is the range of gamma radiation

A

absorbed by many centimetres of lead, or several metres of concrete

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26
Q

what is the range of beta plus radiation

A

effectively zero, as the particle annihilate themselves with electrons almost immediately

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27
Q

how you identify the types of radiation emitted from a source

A
  • record the background radiation count rate when there is no source present
  • place an unknown source near to a Geiger-Muller tube and record the count rate
  • place a sheet of paper inbetween the source and the Geiger-Muller tube. Record the count rate
  • Replace the paper with a 3mm thick sheet of aluminium and record the count rate
  • for each count rate that is recorded, take away the count rate of the background radiation to find the actual count rate
  • depending on when the count rate significantly decreased, you can deduce what kind of radiation the source was emitting
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28
Q

how can you use magnetic fields to identify the type of radiation being emitted from a source

A
  • charged particles moving perpendicular to a uniform magnetic field are deflected in a circular path
  • the direction in which the particle curves depends on its charge
  • radius of curvature can also tell you about charge and mass
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29
Q

uses for alpha radiation

A

smoke detectors
- they allow current to flow, but don’t travel very far
- when smoke is present, the alpha particles can’t reach the detector and this sets the alarm off

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30
Q

order of radiation by ionisation (h to l)

A

alpha
beta
gamma

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31
Q

order of radiation by penetration (h to l)

A

gamma
beta
alpha

32
Q

when are alpha particles dangerous

A
  • dangerous when ingested
  • quickly ionise body tissue in a small area, causing lots of damage
33
Q

what can beta radiation be used for

A
  • can be used to control thickness of materials (paper, aluminium or steel)
  • the material is flattened as it is fed through rollers
  • a radioactive source is placed on one side of the material =, and a radioactive detector on the other
  • the thicker the material, the more radiation is absorbs and prevents from reaching the detector
  • if too much radiation is being absorbed, the roller move closer together to make the material thinner (or vice versa)
34
Q

uses of gamma radiation

A
  • used in cancer treatment
  • a rotating beam of gamma rays is used to lessen the damage done to surrounding tissue, whilst giving a high dose of radiation to the tumour at the centre of the rotation
35
Q

sources of background radiation

A
  1. air - radioactive radon gas is released from rocks, which emits alpha radiation
  2. ground and buildings - nearly all rock contains radioactive materials
  3. Cosmic radiation - cosmic rays are particles from space which produce radiation when they collide with particles in the upper atmosphere
  4. Living things - plants and animals contain carbon, and some of this will be carbon 14
  5. man-made radiation - radiation from medical or industrial sources
35
Q

how do accurately get a measurement for background radiation

A
  • take three readings of the count rate using a Geiger Muller counter without a radioactive source present
  • average the readings and subtract that average from each measurement made of a radioactive source’s count rate
36
Q

what is the intensity of radiation

A

amount of radiation per unit area

37
Q

What is the equation for the inverse square law

A

I = k/x^2

  • I is the intensity (Wm^-2)
  • k is constant of proportionality, W
  • x is the distance from source, m
38
Q

how can you investigate the inverse square law

A
  • set up a Geiger Muller tube with a metre ruler
  • turn on the Geiger counter. Take three readings of the background radiation count rate and average them
  • carefully place the radioactive source at a distance d from the tube
  • record the count rate at that distance. Take three readings of the count rate at this distance
  • move the source so the distance between it and the tube doubles and record the count rate
  • repeat step 5 for distances 3d, 4d, etc
  • put away the radioactive source once the experiment is finished
  • average the count rates recorded for each distance and subtract the average background radiation count rate
  • plot a graph of corrected count rate against distance
39
Q

how to safely handle radioactive sources

A
  • hold a source away from your body when transporting through the lab
  • use long handling tongs to minimise the radiation absorbed by the body
  • sources should always be stored in a lead box
40
Q

what is meant by the activity

A

the number of nuclei that decay each second

41
Q

what is the decay constant

A

λ
probability of a specific nucleus decaying per unit time, and is a measure of how quickly an isotope will decay

42
Q

what is the equation linking activity to the number of unstable nuclei

A

A = λN

or

A = - ΔN/Δt

43
Q

equation for the rate of change of the number of unstable nuclei

A

ΔN/Δt= -λN

44
Q

what is the decay equation

A

N = N0 x e^-λt

45
Q

what is the gradient of the graph of ln(N) against t

A

-λ

46
Q

what is the y-intercept of the graph of ln(N) against t

A

ln(N0)

47
Q

how to work out the number of atoms in a sample

A

N = n N(A)

where n is the number of moles
where N(A) is the Avogadro constant

48
Q

what is the equation for activity in a source over time

A

A = A0 x e^-λt

A0 is the initial activity

49
Q

what is the half-life of an isotope

A

the average time it takes for the number of unstable nuclei to halve

50
Q

what is the equation for half life

A

T1/2 = ln(2)/λ

51
Q

why can it be difficult to get a reliable age from radioactive dating

A
  • man-made objects will only tell you the age of the material used, not the object itself
  • the object may have been contaminated by other radioactive sources
  • there may be a high background count that obscures the object’s count
  • sample size may be small
52
Q

when would a nucleus become unstable

A
  • too many neutrons
  • too few neutrons
  • too many nucleons altogether (too heavy)
  • too much energy
53
Q

what kind of emission happens in heavy atoms

A

alpha

54
Q

what kind of emission happens in neutron rich atoms

A

beta minus

55
Q

what kind of emission happens in proton rich atoms

A

beta plus

56
Q

how is gamma emission produced

A
  • after alpha or beta decay, the nucleus often has excess energy, which is released as a gamma ray
  • electron capture
57
Q

what is meant by binding energy

A

the energy released when a nucleus forms, or the energy required to separate all the nucleons in that nucleus.
Equivalent to the mass defect of the nucleus

58
Q

what is meant by the mass defect

A

the difference in mass between a nucleus and the individual nucleons

59
Q

which has a smaller mass: nucleus or sum of mass of individual nucleons

A

nucleus

60
Q

how do you work out the average binding energy per nucleon

A

binding energy (N)/ nucleon number (A)

61
Q

what is nuclear fission

A

when large, unstable nuclei randomly split into two smaller nuclei

62
Q

how do you know energy is released during nuclear fission

A
  • the new smaller nuclei have a higher average binding energy per nucleon
63
Q

how can you induce nuclear fission

A

by making a thermal neutron enter a 235U nucleus, causing it to become very unstable

64
Q

what is nuclear fusion

A

when two light nuclei can combine to create a larger nucleus

65
Q

only elements to the ….. of iron can release energy through nuclear fission

A

right

65
Q

only elements to the ….. of iron can release energy through nuclear fusion

A

left

66
Q

how are chain reactions used in nuclear reactors

A
  • use rods of uranium rich in U235 as ‘fuel’ for fission reactions
  • these reaction produce ore neutrons which then induce other nuclei to fission
67
Q

what is the role of a moderator in a nuclear reactor

A

to slow down and/or absorb neutrons
- slows down neutrons through elastic collisions with the nuclei of the moderator material

68
Q

example of a moderator

A

water
- contains hydrogen, which is a similar mass to a neutron, so neutrons are slowed down more efficiently

69
Q

what is meant by the critical mass

A

the amount of ‘fuel’ you need for the chain reaction in a nuclear reactor to continue at a steady rate

70
Q

what do control rods do

A

control the chain reaction by limiting the number of neutrons in the reactor
- absorb neutrons so the rate of fission is controlled

71
Q

what does coolant do in a nuclear reactor

A

it is sent around the nuclear reactor to remove heat produced by fission
- the heat from the reactor can then be used to make steam for powering electricity-generating turbines

72
Q

safety features of nuclear reactors

A

Reactor shielding: reactor is surrounded by a thick concrete case, to prevent radiation escaping

Emergency shut down: can be shut down automatically by releasing the control rods

Handling and storing fission waste products: unused rods only emit alpha so can be stored easily but used rods emit beta and gamma, which must be sealed in containers

73
Q

pros of nuclear power

A
  • doesn’t release greenhouse gases
  • very efficient
  • generates thousands times more energy per kg compared to fossil fuels
74
Q

cons of nuclear power

A
  • hard to deal with nuclear waste
  • making sure it doesn’t harm people or the environment

a-level physics (17 decks)

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