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A-Level Physics > 12. Nuclear Physics > Flashcards

12. Nuclear Physics Flashcards

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1
Q

What happened in Rutherford’s scattering experiment?

A
  • Rutherford fired alpha particles from a radioactive source at a thin piece of gold foil
  • this was surrounded by a circular fluorescent screen
  • the screen would glow green if it was struck by any particles
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2
Q

Why was Rutherford’s scattering experiment surrounded by a circular fluorescent screen?

A

so that alpha particles scattered by any angle could be detected

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3
Q

Why was Rutherford’s scattering experiment done in a vacuum?

A

to prevent alpha particles being stopped by air molecules

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4
Q

State the three conclusions that can be made from where the particles ended up in Rutherford’s scattering experiment?

A
  1. most of the atom must be empty space as most particles pass straight through
  2. most of the mass and charge of the atom must be contained within a very small ‘nucleus’
  3. these ‘nuclei’ must be positively charged to repel positive alpha particles (some particles came straight back)
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5
Q

How can Rutherford’s scattering experiment be used to determine the radius of a nucleus? What is this method called?

A

CLOSEST APPROACH METHOD:
1. fire an alpha particle straight at a nucleus
2. the particle will be repelled
3. kinetic energy turns into electrical potential energy
4. where speed = 0 gives an overestimate of the radius (r)

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6
Q

What is electrical potential measured in?

A

Joules per Coulomb/Volts

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7
Q

How can you calculate electrical potential energy?

A

Electrical potential (Ep) = Electrical Potential (V) x Charge (q)

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8
Q

How can you calculate electrical potential?

A

1 Q
——— x —
(4πε0) r

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9
Q

What is the closest approach method?

A

Initial KE = Electrical Potential Energy

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10
Q

What are the disadvantages of the closest approach method?

A
  • measures closest approach so overestimate
  • alpha particles have a finite size that must be taken into account
  • need for very thin samples
  • need to have monoenergetic beams (all have same kinetic energy)
  • cannot detect particles with 100% scattering
  • measurements are disturbed by the nucleus recoiling
  • affected by SNF at very close ranges
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11
Q

What are the two methods that can be used to calculate the radius of a nucleus?

A
  • closest approach method
  • electron diffraction method
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12
Q

How can the electron diffraction method be used to find the radius of a nucleus?

A
  • fire electrons at the metal foil
  • as electrons can show wave like properties, they will diffract through gaps
  • this produced an interference pattern which can be measured
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13
Q

What is the equation that can be used to find the diameter of a nucleus from electron diffraction?

A

1.22 λ
sinθ = ———
d

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14
Q

How do you calculate the De Broglie wavelength of an electron?

A

h
λ = ——
mv

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15
Q

When can the De Broglie wavelength be used?

A

for low speed (low kinetic energy) electrons

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16
Q

What equation must be used if the electrons are going very quickly (typically with KE’s above 250 keV)?

A

hc
λ = ——
KE

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17
Q

What are the advantages of the electron diffraction method over the closest approach method?

A
  • electrons are leptons and not affected by SNF
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18
Q

What are the disadvantages of the electron diffraction method?

A
  • need to be very high speed to have a wavelength the same order of magnitude as the diameter of the nucleus
  • importance of monoenergetic beams
  • first minimum difficult to detect
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19
Q

How does intensity vary with diffraction angle?

A
  • central bright maximum (circle) containing the majority of the incident electrons, surrounded by other dimmer rings (maxima)
  • the intensity of the maxima decreases as the angle of diffraction increases
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20
Q

How does radius vary with nucleon number?

A

as more nucleons are added to a nucleus, its radius gets larger

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21
Q

What is radius directly proportional to?

A

cube root of A (nucleon number)

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22
Q

Why is the radius directly proportional to the cube root of A?

A
  • the volume of a nucleus will be directly proportional to the number of nucleons
  • volume of a nucleus is 4/3πr^3
  • therefore radius cubed is directly proportional to the number of nucleons
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23
Q

What equation represents the graph of radius plotted against the cube root of radius?

A

r = r0A^1/3

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24
Q

Why does the nucleus of any element have the same density?

A
  1. if you make two balls it of the same material it doesn’t matter how big the balls are, they will have the same density as they are made of the same material
  2. the number of nucleons in the equation for the density of a nucleus cancel out proving that density is independent of nucleon number
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25
Q

Nuclear density is significantly higher than atomic density. What three conclusions can be made from this?

A
  1. most of an atom’s mass is in its nucleus
  2. an atom must contain lots of empty space
  3. the nucleus is small compared to the whole atom
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26
Q

State the symbol and constituents of an alpha particle

A
  • α
  • a helium nucleus (two protons, two neutrons)
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27
Q

State the relative charge and relative mass of an alpha particle

A

relative charge: +2
relative mass: 4

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28
Q

State the range of an alpha particle in air

A

5cm

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29
Q

What is the penetrating power of an alpha particle?

A

blocked by paper/skin

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30
Q

State the symbol and constituents of a beta-minus particle

A
  • β-
  • electron
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31
Q

State the relative charge and relative mass of a beta-minus particle

A

relative charge: -1
relative mass: roughly 1/2000

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32
Q

State the range of a beta-minus particle in air

A

1-2m

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33
Q

What is the penetrating power of a beta-minus particle?

A

blocked by a few mm’s aluminium

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34
Q

State the symbol and constituents of a beta-plus particle

A
  • β+
  • positron
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35
Q

State the relative charge and relative mass of a beta-plus particle

A

relative charge: +1
relative mass: roughly 1/2000

36
Q

State the range of a beta-plus particle in air

A

0

37
Q

What is the penetrating power of a beta-plus particle?

A

annihilates with an electron almost instantly

38
Q

State the symbol and constituents of gamma radiation

A
  • γ
  • high frequency EM wave
39
Q

State the relative charge and relative mass of gamma radiation in air

A

relative charge: 0
relative mass: 0

40
Q

State the range of gamma radiation in air

A

infinite in air

41
Q

What is the penetrating power of gamma radiation?

A
  • 10s of cms of lead
  • metres of concrete
42
Q

What makes an atom radioactive?

A

something is radioactive if it is unstable because:
- it has too much mass
- an imbalance of protons and neutrons
- too much energy

43
Q

How can a radioactive atom become more stable?

A

by emitting particles or energy

44
Q

Why do we say radiation is ionising?

A

when a radioactive particle hits an atom, it can remove electrons (making ions)

45
Q

What are the ionising properties of alpha particles?

A
  • alpha particles are strongly positive and can rip electrons off atoms
  • ionising an atom transfers some energy from the alpha particle to the ion
  • alpha particles ionise lots of atoms (10000 ionisations per alpha particle) very quickly before they run out of energy
  • move relatively slowly
46
Q

What are the ionising properties of beta-minus particles?

A
  • have lower mass and charge than alpha, but travel faster
  • they can still knock electrons off atoms
  • each beta particle will ionise about 100 atoms before losing all its energy, making them less dangerous
47
Q

What are the ionising properties of gamma radiation?

A
  • gamma rays are an EM wave
  • even less ionising that beta
  • gamma rays excite electrons causing them to leave the atom
48
Q

What are the three types of experiments that allow you to identify the type of radiation?

A
  • penetrating power
  • range in air
  • deflection in a magnetic field
49
Q

What equipment do all three radiation experiments use?

A
  • Geiger Muller (GM) tube
  • counter
50
Q

How can you find the type of radiation of an unknown source from penetrating power?

A
  • involves placing various materials between GM tube and the unknown source
  • e.g. if you placed paper between the source and the GM tube and the count rate stayed the same, you can say alpha is not present, but if it did drop, you would know alpha is present
51
Q

How can you find the type of radiation of an unknown source from range in air?

A
  • involves changing the distance between the GM tube and the source
  • e.g. if you moved the source and the GM tube further than 5cm apart and the count rate stayed the same then you can say alpha is not present, but if it did drop, you would know alpha is present
52
Q

How can you find the type of radiation of an unknown source from deflection in a magnetic field?

A
  • involves placing a magnetic field between the source and the GM tube
  • using Fleming’s left hand rule you can work out which way the charges will be deflected
  • you then move the GM tube to different positions to see whether you detect any radiation there
53
Q

State four positive uses of nuclear radiation

A
  1. smoke alarm
  2. pipe leak detector
  3. medical tracers
  4. monitoring material thickness
54
Q

How do smoke alarms utilise nuclear radiation?

A
  • smoke alarms contain an alpha source (typically Americium) and a detector
  • the detector expects to detect a certain amount of alpha particles every second
  • the large particles in smoke (and steam) block the alpha radiation, causing a drop in the number of particles detected which sets off the alarm
55
Q

How long is the half life of the radioactive source in smoke alarms? Why?

A
  • roughly 10 years
  • so you don’t have to replace your alarm regularly
56
Q

How do pipe leak detectors utilise nuclear radiation?

A
  • pipes that carry water under roads and pavements often leak
  • engineers will inject a radioactive liquid into the water
  • the radioactive particles will accumulate where the leak is
  • they will dig at the place where they pick up the biggest reading
57
Q

What radioactive source do pipe leak detectors use? Why?

A
  • gamma source
  • to penetrate the ground
58
Q

How long is the half life of the radioactive source in pipe leak detectors?

A

a least a day

59
Q

How do medical tracers utilise nuclear radiation?

A
  • a radioactive liquid is injected into the patient
  • this then accumulates in areas of high blood use (e.g. organs and tumours)
  • a commonly used isotope is Technetium-99m which is a gamma emitter
  • this allows the radiation to penetrate the skin
60
Q

What time scale should the half-life be for the radioactive source in medical tracers?

A

short half-life

61
Q

How can nuclear radiation be used to monitor material thickness?

A
  • the material is passed through a set of adjustable rollers which flatten it to a desired thickness
  • a beta source is placed under the material with a counter above it
  • if the material gets too thick then the counter will pick up less beta particles
  • a signal would be sent to move the rollers closer together
62
Q

How is damage to healthy cells reduced when using gamma radiation in medicine?

A

sometimes a rotating beams of gamma rays is used to lessen the damage done to surrounding tissues whilst giving a high does of radiation to the tumour at the centre of rotation

63
Q

What are the risks of using gamma radiation in medicine?

A
  • damage to other, healthy cells is not completely prevented
  • treatment can cause patients to suffer side effects, such as tiredness and reddening or soreness of the skin
64
Q

Why won’t gamma radiation be deflected in a magnetic field?

A

gamma radiation isn’t made up of particles

65
Q

How can risks to medical staff giving treatments with radioactive tracers be kept as low as possible?

A
  • exposure time to radioactive sources is kept to a minimum
  • generally staff leave the room (which is itself shielded) during treatment
66
Q

How can you measure background radiation?

A
  • take three readings of the count rate using a Geiger counter without a radioactive source present
  • average these three readings and subtract this average from each measurement you take of a radioactive source’s count rate
67
Q

Name five sources of background radiation

A
  • radon gas released from the ground/rocks
  • buildings and the ground
  • cosmic rays
  • living things
  • artificial sources/man-made radiation
68
Q

What is the largest contributor to background radiation?

A

radon gas

69
Q

What type of radiation does radioactive radon gas emit?

A

alpha radiation

70
Q

How do cosmic rays produce radiation?

A
  • cosmic rays are particles (mostly high-energy protons) from space
  • when they collide with particles in the upper atmosphere, they produce nuclear radiation
71
Q

What does the inverse square law tell us about how the intensity of gamma radiation changes with distance?

A

the intensity of the radiation is inversely proportional to the square of the distance from the centre of mass

72
Q

What is the intensity of radiation?

A

the amount of radiation per unit area

73
Q

Which direction does a radioactive source emit radiation?

A
  • a source emits radiation in all directions, not just one
  • the inner sphere will have a higher intensity per metre squared than the outer sphere
74
Q

What implications does the inverse square law have for safety when handling radioactive sources?

A
  1. hold source away from body when transporting
  2. never point the source at a person as radioactive sources are designed to focus most of the radiation in one direction
  3. use long tongs to hold the source when moving it to increase the distance between your body and the source
  4. for those not involved in the experiment keep well away
  5. only keep the source in the room for as long as you need it to minimise exposure time
75
Q

What is meant by the random nature of decay?

A
  • you can’t predict which nuclei will decay next
  • but the same proportion of nuclei will decay per second
76
Q

What is the decay constant (λ)?

A

the probability of decay

77
Q

What is the activity (A) of a sample?

A

the number of nuclei that decay each second

78
Q

How do you calculate the activity of a sample?

A

A = λN

79
Q

What does a bigger decay constant (λ) mean?

A

a faster rate of decay

80
Q

State another equation for calculating activity

A

ΔN
A = - ——
Δt

81
Q

Why does the equation for rate of decay have a negative sign?

A

ΔN is always decreasing

82
Q

What type of graph is produced when number of unstable nuclei is plotted against time?

A

exponential decay

83
Q

How can the number of unstable nuclei after t seconds be calculated?

A

N = N0 e^(-λt)

84
Q

What is molar mass?

A

mass of 1 mole of that substance

85
Q

What is the link between molar mass and mass number?

A

molar mass is equal to mass number, but measured in grams

86
Q

How do you calculate number of moles?

A

N
n = ——
NA

87
Q

How do you calculate the half-life of a radioactive isotope?

A

ln2
T1/2 = ——
λ

A-Level Physics (15 decks)

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