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aqa a level physics > nuclear physics > Flashcards

nuclear physics Flashcards

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1
Q

© What was J.J Thomson’s model of the atom?

A

‘Plum Pudding Model’

spheres of positive charge, with negative electrons embedded, overall uniformly distributed positive charge

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2
Q

© Describe the Rutherford scattering experiment setup

A

● A beam of alpha particles was directed at a thin gold foil.

● Occurs in a vacuum so that no collisions between air particles and alpha particles can occur.

● fluorescent screen

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3
Q

© what was the expected result of the alpha scattering experiment?

A

all flashes on the fluorescent screen should’ve been seen within a small scattering angle.

all the positively charged alpha particles would be deflected by a small amount

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4
Q

© what were they recording in the alpha scattering experiment

A

the number of alpha particles scattered at different angles

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5
Q

★ In experiments carried out to determine the nature of atoms, alpha particles were fired at thin metal foils. Describe how the alpha scattering experiments provide evidence for the existence, charge and size of the nucleus [5]

A
  • Most of the alpha particles went straight through
  • Hence most of the atom is empty space
  • A small proportion of the particles were scattered through large angles
  • This showed the existence of a tiny positive nucleus
  • approx. 10^-15m
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6
Q

★ Describe the nature and range of the three forces acting on the protons and neutrons in the nucleus [5]

A
  • The Electromagnetic force acts between the similarly charged protons causing them to repel one another
  • This has an infinite range
  • The gravitational force is attractive (acts between the nucleons due to their mass)
  • This has a long range
  • The strong nuclear force is attractive (acts between nucleons to overcome the repulsive and force)
  • short-ranged (Repulsive <0.5fm and attractive up to 3fm, negligible after)
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7
Q

★ Explain the term binding energy of a nucleus [2]

A

The minimum energy required to separate nucleons from one another in a nucleus

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8
Q

© How can you estimate the radius of an atomic nucleus?

A

Rutherford’s scattering experiment

When the particle is deflected by 180

Reaches closest to nucleus when Initial Kinetic energy = Electric potential

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9
Q

✪✪ Electric potential energy

A

= (Qnucleus*Qalpha)/4πε0r

ε0 - permitivity of free space - 8.85*10^-12Fm^-1

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10
Q

© what is the more accurate method for measuring nuclear radius and why?

A

using electron diffraction

electrons are leptons and so don’t interact with strong force

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11
Q

✪ De Broglie wavelength

A

λ = h/mv

or

( E = hc/λ )

λ = hc/E

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12
Q

★ An alpha particle moves directly towards a nucleus of Aluminium-27.

The alpha particle comes to rest instantaneously a short distance away from the aluminium nucleus. It then reverses its direction of travel.

Describe and explain the motion of the aluminium nucleus at the instant the alpha particle is at rest. [2]

A
  • The aluminium nucleus moves to the right,
  • There is a repulsive force on the nucleus ( conservation of momentum )
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13
Q

© How small must the de broglie wavelength of the electrons be while investigating nuclear radius?

How is this achieved?

A

similar order of magnitude to the nucleus

10^-15m

The electrons must have have a very high energy (high velocities)

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14
Q

© Graph of intensity against angle for electron diffraction by a nucleus

A

intensity of maxima decreases as angle of diffraction increases

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15
Q

✪✪ First minimum for electron diffraction by a nucleus

A

sinθ ≈ 1.22λ / 2R

R -radius of the nucleus the electrons have been scattered by

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16
Q

☢ approx. size of an atom

A

radius of 10^-15m

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17
Q

© What is nucleon number known as?

A

A

number of neutrons and protons in a nucleus

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18
Q

© How does nuclear radius increase with increasing nucleon number? (graph)

A

as number of nucleons increase, radius increases
__ _______
/
|

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19
Q

© How can the relationship between A and and R be made linear?

A

Plotting R against A^1/3

Shows a linear relationship between R and A^1/3

R ∝ A^1/3

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20
Q

✪ nuclear radius

A

𝑅 = 𝑅0 * 𝐴1/3

R0 - constant - approx 1.4fm

A - nucleon number

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21
Q

© What does the equation for nuclear radius suggest?

A

𝑅 = 𝑅0 * 𝐴1/3

R ∝ A^1/3

R^3 ∝ A

V ∝ A

V = 4/3(πr^3)

m = A * m.nucleon

ρ = m/v = constant

provides evidence that the density of nuclear matter is constant, regardless of A (nucleon number)

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22
Q

© What does nuclear density suggest?

A
  • Nuclear density&raquo_space;> atomic density
  • This suggest most of an atom’s mass is in its nucleus
  • Nucleus is small compared to the atom
  • Atom must be mostly empty space
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23
Q

© the four types of nuclear radiation

A

beta minus

beta plus

alpha

gamma

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24
Q

© What is radioactive decay?

A

The emission of particles from an unstable nuclei to become more stable

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25
Q

© penetration power of the different types of radiation, range in air

A

alpha - weakly penetrating, stopped by a piece of paper
a few cm range in air

beta minus- moderately penetrating
stopped by few millimetres of aluminium
a metre in air

beta plus - immediately annihilate with electrons so effectively zero range

gamma - strongly penetrating
several cm of lead stops gamma
long range

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26
Q

© Describe how you would identify nuclear radiation

A

record background count rate when no source is present

place unknown source near a geiger muller tube and record the count rate

place various materials and thicknesses between the tube and the source

record count rate

subtract background radiation

amount by which count rate decreases determines which kind of radiation the source was emitting

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27
Q

© what are two applications of gamma radiation in medicine

A

causes least damage to body tissue as it is weakly ionising and highly penetrating so passes straight through body causing minimal damage

radioactive tracers

treating cancer

rotating beams of gamma rays,

radiation damages all cells - both cancer and non-cancerous

damage to healthy cells - side effects - tiredness etc.

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28
Q

How do radioactive tracers work

A

The patient is injected with a radioactive isotope with a short half-life.

A detector is used to detect the emitted gamma rays

29
Q

© describe an application of beta radiation

A

controlling the thickness of material production

source and detector placed opposite one another with material in between

the thicker the material, the more radiation it absorbs, so less is detected. Adjustable rollers change position accordingly

30
Q

© Why is alpha radiation especially dangerous in the body?

A

Cause localised damage

ionise body tissue in a small area causing a great amount of damage

weakly penetrating meaning it can’t escape the body easily so remains

31
Q

© describe an application of alpha radiation

A

smoke alarms

alpha particles are strongly positive, so can easily pull electrons off atoms (ionise)

strongly ionising - loses its energy quickly

allows current to flow and won’t travel far out of detector

when smoke is present the alpha particles can’t reach the detector and this breaks the flow of current and sets the alarm off

32
Q

© What happens when you place a radioactive source in magnetic fields

A

charged particles emitted by different radiations travel through magnetic fields differently

charged particles moving perpendicular to a uniform magnetic field are deflected in a circular path, direction depends on charge

33
Q

© What is background radiation and how is it dealt with?

A

the radiation that is always present from sources such as rocks, cosmic radiation, radon gas in the air, living things (c-14)

take 3 readings of count rate with no radioactive source present

Take average and subtract from each measurement taken with source present

34
Q

© What is the intensity of radiation

A

amount of radiation per unit area

35
Q

© What does the inverse square law state

A

a gamma source will emit gamma radiation in all directions

As distance from the source increases,

the intensity of radiation decreases proportional to the square of the distance

36
Q

✪ inverse square law formula

A

I = k/x^2

intensity (Wm^-2) = K(constant of proportionality (W))/x^2 (distance^2)

37
Q

© graph of number of remaining unstable nuclei against time

A

decreases exponentially

\

  \

        \

            \_\_\_

                   -\_\_\_\_\_\_\_\_\_\_\_\_\_

plot ln(N) against time

y - intercept = ln(N0)

gradient = -λ

38
Q

✪ the decay equation

number of unstable nuclei remaining

A

N = N0e^-λt

number of unstable nuclei remaining = Original number of unstable nuclei * e^-decayconstant(s^-1)*time(s)

39
Q

© Decay constant

A

constant of proportionality

probability of a specific nucleus decaying per unit time

any sample of a particular isotope has the same rate of decay - each nucleus will have a constant decay probability

40
Q

© Activity of a radioactive source

A

The number of nuclei that decay per second

 ΔN
  • _____ΔT

( negative since number of nuclei is decreasing)

41
Q

✪ Activity formula

A

A = λN

activity(Bq) = decay constant (s^-1) * number of unstable nuclei in a sample

42
Q

© Number of atoms in a sample

A

N = n*Na

number of atoms in a sample = moles*avogadro constant

43
Q

✪✪ Activity and the decay equation formula

A

A = A0 * e^-λt

activity(Bq) = activity (at time = 0)* e^-decayconstant*time

44
Q

✪ Energy

A

E = mc^2

Energy(j) = mass(kg) * c(ms^-1)^2

45
Q

© What is mass defect

A

the mass of a nucleus is less than the combined mass of its constituent nucleons

the difference between these two values = mass defect

46
Q

© Binding energy per unit mass defect

A

1u ≈ 931.5Mev

47
Q

© How do you calculate average binding energy per nucleon?

A

Binding energy / Nucleon number

48
Q

© Describe the graph of average binding energy per nucleon against Nucleon number

A

_Fe(56) —-_______
/ —–U(238)-
/
|
|
H(2)

average binding energy per nucleon increase rapidly for light nuclei

Iron is the most stable nucleus

Average binding energy per nucleon decreases gently for heavier nuclei

49
Q

©Nuclear fission

A

Large nuclei are unstable and sometimes randomly split into two smaller, less heavy nuclei

spontaneous process

releases energy since new, smaller nuclei have a higher average binding energy per nucleon (more energy

50
Q

© What is a condition for nuclear fusion to occur

A

Need to get close enough for attractive strong force (0.5fm - 3fm)

all nuclei are positively charged, resulting in electrostatic force of repulsion

Nuclei can only fuse if they can overcome this electrostatic force. Requires an initial energy input

51
Q

© Nuclear fusion

A

When two light nuclei combine to create a larger nucleus

Releases a lot of energy because new, heavier nuclei have a higher average binding energy per nucleon.

52
Q

© in fission and fusion, when is energy released

A

when the average binding energy per nucleon increases

53
Q

© For which elements are nuclear fission and fusion energetically favourable by looking at graph

A

elements to the right of Fe(56), fission releases energy

to the left - fusion release energy

54
Q

© in fission and fusion, when is energy released

A

when the average binding energy per nucleon increases

55
Q

© Which process do thermal nuclear reactors take advantage of

A

nuclear fission

56
Q

© What type of reaction occurs in nuclear reactors

A

chain reaction

fission reactions produce more neutrons which then induces other nuclei to fission

neutrons will only cause a chain reaction if they are slowed down so they can be ‘caught’ by uranium nuclei

57
Q

© what are the slowed down neutrons in nuclear reactors called

A

thermal neutrons

58
Q

© Fuel rods - moderator

A

rods of uranium that are rich in U-235 (or plutonium rods rich in Pu-239) as ‘fuel’ for fission reactions

fuel rods are places in a moderator e.g water
this slows down/ absorbs neutrons
needs to slow the neutrons down by right amount to keep chain reaction going at a steady rate

59
Q

© How does a moderator slow down neutrons?

A

Through elastic collisions with nuclei of the moderator material.

When neutrons collide with particles of similar mass, they are slowed down most efficiently, Water works because of H-1

60
Q

© control rods - what do they do and how?

A

ensuring chain reaction continues on its own at a steady rate
limits number of neutrons in the reactor
they absorb neutrons so rate of fission is controlled
made of materials that absorbs neutrons (e.g boron)

61
Q

© what is critical mass

A

amount of ‘fuel’ needed to ensure chain reaction continues on its own at a steady rate
where one fission follows another
any less than the critical mass and the reaction dies out

supercritical mass - where several new fissions normally follow each fission

62
Q

© What is half-life?

A

The half life of an isotope is the average time it takes for the number of unstable nuclei in the sample to halve

63
Q

© Describe how you’d obtain a half-life value from a graph

A

graph of number of unstable nuclei remaining, (N) against time(t)

  • read off N0, - N at time t = 0
  • Divide this by two and go to this value on the y-axis
  • draw a line across to the curve and draw a vertical line down where it meets the curve, read off x-value, half-life
  • repeat at least thrice and calculate an average half-life
64
Q

✪ Half-life formula

A

𝑇½ = ln2 / λ

half-life = ln2 / decay constant

65
Q

© How is radioactive waste stored, and what is an example of when this is implemented

A

radioactive waste has to be stored carefully for hundreds of years, only then would their activity have fallen to safe levels

radioactive waste has a very long-half life, stays highly radioactive for a long time

U-235 used in fission reactors

Decays into several different radioactive isotopes with different half-lives

66
Q

© How are radioactive isotopes used in medical diagnosis

A

Technetium - 99m

medical tracers

they have a short half-life, therefore suitable.

Also decays to a much more stable isotope

radioactive substance injected/ingested by a patient. Radiation emitted is then recorded and image produced

67
Q

© What are some possible problems with radioactive dating

A

For man-made objects, only gives the age of the material not the object itself

may have been contaminated by other radioactive sources

A high background count rate may skew readings

Uncertainty in amount of carbon-14 that existed thousands of years ago

68
Q

© How are radioactive isotopes used in radioactive dating

A

All living things contain a small percentage of Carbon-14

When they die the activity of the carbon-14 begins to fall

69
Q
A

aqa a level physics (10 decks)

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