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Physics > Nuclear Physics > Flashcards

Nuclear Physics Flashcards

1
Q

What did JJ Thomson propose?

A

Plum pudding model.

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2
Q

Describe properties of alpha particle:

A

Fast moving, positively charged.

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3
Q

What experiment did Rutherford do to expand on this model?

A

Alpha scattering experiment (fired alpha particles at metal foil all at same Ek and in an evacuated container. Ek must have long half-life to ensure consistency of readings)

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4
Q

What 2 things did this experiment show?

A

1) Mass concentrated in single point. 2) This point must be positively charged.

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5
Q

What else did he discover from using different metals and repeating it?

A

He discovered charge/size of the electron and charge/size of nucleus.

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6
Q

Who discovered the nature of radioactive materials?

A

Marie Curie and Henri Becquerel.

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7
Q

Most radioactive element?

A

Radium. Million times more radioactive than Uranium.

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8
Q

Why is alpha radiation the most ionising?

A

It has a charge of +2 and so reacts more strongly with other electric fields to knock out an electron despite having less energy than gamma rays.

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9
Q

Define count rate:

A

The number of counts in the time it was given in on a Geiger tube.

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10
Q

How can absorption be investigated?

A

Radioactive source pointed at a Geiger tube with a absorber between them. Vary thickness of absorber and plot against count rate. (Subtract background count rate from any count rate readings.)

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11
Q

How does a Geiger tube work?

A

Contains argon gas at low pressure with metal rod down the centre with a positive potential. When radiation enters, negative ions created are attracted to the wall and positive ions repelled to the wall. A pulse of charge is created from this and is then passed around the circuit, creating a voltage pulse across R which is counted as a single count rate.

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12
Q

Range of diff radiations:

A

alpha - 2-10 cm
beta - around 1m
gamma - infinite but follows inverse square law.

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13
Q

Deflection by magnetic fields:

A

alpha - yes
beta - yes
gamma - no (not charged)

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14
Q

application of diff. radiations:

A

can be used to measure thickness of paper, aluminium and concrete based on how much is absorbed by detector on other side of material.

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15
Q

Use of gamma radiation:

A

-As a detector: short half life injected into patient which emits radiation and be detected by cameras to help diagnose.
-Sterilise surgical equipment.
-Radiation therapy to kill cancer cells (also kills healthy cells)

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16
Q

Safety measures for radioactive exposure:

A

-Reduce exposure time.
-Shielding.
-Wash.
-Long tongs to move it.
-Store in lead container.

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17
Q

Direction of gamma radiation:

A

-spreads out in all directions.

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18
Q

Inverse square law:

A

I = k/x^2

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19
Q

Verifying ISL:

A

-measure count rate at various distances from GM tube.
-plot graph against 1/x^2 and it will be a straight line.

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20
Q

Corrected count rate:

A

Total count rate - background count rate

21
Q

Sources of background radiation:

A

Radon gas from rocks
artificial sources e.g. test sites or Chernobyl.
cosmic rays
rocks containing isotopes.

22
Q

Nature of decay and decay constant:

A

-random process.
-decay constant: probability of a nucleus decaying per unit time can be calculated by finding change in number of nuclei over time over the initial number of nuclei.

23
Q

exponential decay formula:

A

N = N0 e^-lt (l is decay constant)

24
Q

half life

A

time for number of nuclei to halve.

25
Q

most accurate way to measure half life:

A

-plot ln(N) against time where the gradient is negative decay constant and the y intercept is the N0.

26
Q

half life formula:

A

by substituting 0.5N0 for N in decay formula:

T1/2 = ln2/l

27
Q

activity definition and formula.

A

number of nuclei that decay per second.

A=lN as activity is proportional to number of nuclei left.

due to this ^

A=A0 e^-lt is also true. (time taken for activity to half is equal to the half life also which is much easier to measure)

28
Q

when is decay constant used:

A

only when there is a large number of nuclei in a sample as it is modelled by statistical means.

29
Q

uses of half life:

A

dating organic objects: comparing amount of carbon 14 in it with the original amount (roughly the same percentage in all living things)
medical diagnosis: short half lives are useful for diagnosis e.g. Techneutium-99m as it is a pure gamma emitter and has a half life of 6 hours.

30
Q

storing radioactive material:

A

-long half lives in steel caskets underground

31
Q

4 reasons a nucleus becomes unstable and the decays they go through:

A

-too many neutrons - beta minus emission.
-too many protons - beta plus emission .
-too many nucleons - alpha decay.
-too much energy - gamma emission (usually after another decay and the nucleus becomes too excited.)

32
Q

why is neutron and proton number not increase uniformly:

A

after around 20 of each the electromagnetic force of repulsion becomes larger so more neutrons are needed to increase distance between protons and electrons to decrease EM force.

33
Q

calculating distance of closest approach:

A

initial kinetic energy = electric potential energy when it stops.

therefore make the energies equal and rearrange for r (the distance from the centre of the nucleus)

this will always be an overestimate for the nuclear radius.

34
Q

nuclear radius and electron diffraction:

A

electrons accelerated until their wavelength is around 10^-15m.

intensity (of the rings)is plotted against diffraction angle

gives formula sin(theta) = 0.61l/R (where l is de Broglie wavelength)

35
Q

nuclear radius and nucleon number:

A

R = kA^n

so if you plot lnR against lnA you get a straight line from a point lnk on the y axis with a gradient = n =1/3

therefore R =R0 A^1/3

36
Q

nuclear density:

A

mass/volume - A *Mnucleon/ 4/3pi r^3

this is always a constant value for all nuclei (around 1.45 *10^-17)

this is much larger than density of an atom meaning an atom is mostly empty space.

37
Q

Binding energy:

A

energy required to separate nucleus into its constituents or energy released when it is formed from its constituents

38
Q

atomic mass units:

A

-1/12th of the mass of a carbon 12 atom
-convert from atomic mass units to kilogram by multiplying by 1.661 * 10^-27
-change of 1u is loss of 931.5 MeV

39
Q

nuclear fission:

A

splitting a large nucleus into 2 daughter nuclei.
-occurs in large unstable nuclei and is a random process but can also be induced.
-energy is released because the smaller nuclei have higher binding energy per nucleon.

40
Q

nuclear fusion:

A

two smaller nuclei form a larger nucleus.
-occurs in small nuclei
-energy released as the larger nucleus has a higher binding energy per nucleon.
-releases more energy than fission but only occurs at very high temperatures as lots of energy is needed to overcome electrostatic repulsion force between nuclei.

41
Q

binding energy per nucleon:

A

-binding energy of nucleus divided by number of nucleons.
-plotting binding energy per nucleon against nucleon number shows whether an atom undergoes fission or fusion and peaks at nucleon number of 56 (iron has the highest binding energy per nucleon - any larger undergoes fission, any smaller undergo fusion)

42
Q

uses of fission:

A

-create electricity without emission of greenhouse gases
-risks include that the daughter nuclei must be safely stored for thousands of years and risk of meltdowns.

43
Q

induced fission:

A

-fission can be induced by firing thermal neutron with low energy at a uranium nucleus (high energies would simply rebound)
-the fission causes 2 nuclei and at least 1 neutron, this causes more fission reactions by chain reaction of neutrons.

44
Q

critical mass:

A

minimum mass of fuel required to maintain steady chain reaction.
(using exactly this means 1 reaction follows the last whereas less than this would cause the reaction stop)

45
Q

features of a nuclear reactor:

A

-moderator: slows down neutrons released in fission to thermal speeds through elastic collisions between moderator atoms and neutrons. the closer the moderator atoms are in size to neutrons the greater the proportion transfer of momentum - water is often used as it is cheap, unreactive and contains lots of hydrogen. graphite is sometimes used.
-control rods: absorb neutrons to control chain reactions and control the amount of energy produced - made of materials that absorb neutrons without undergoing fission e.g. boron or cadmium.
-coolant: absorbs heat released in fission from the core which then creates steam to power turbines. sometimes water is both moderator and coolant as it has a high specific heat capacity meaning it transfers lots of thermal energy (sometimes molten salt or gas is also used)

46
Q

enriched uranium:

A

enrichment of mined uranium which consists of 99% U-238 which does not experience fission in order to increase percentage of U-235 to around 5%

-U-238 absorbs neutrons from fission and so controls rate of reaction. - fuel rods are inserted into reactor remotely to limit exposure.

47
Q

safety features:

A

-very thick concrete shielding around reactor to block radiation (the shielding may become radioactive over time and experience its own decay)
-in an emergency control rods are completely dropped in to stop the reaction (emergency shut down)

48
Q

waste removal:

A

-high level waste - very high activity e.g. spent fuel rods.
low level waste - short lived radioactivity e.g. gloves.

high level waste is 1) removed and handled remotely, 2) placed in cooling ponds for up to a year which are usually on site to prevent exposure by travel
3)any plutonium or uranium from fuel rods is removed to be recycled
4) waste is vitrified - encased in glass - and placed in thick steel casks and stored in deep caverns. the locations are chosen carefully so they have minimal environmental and human damage.

49
Q

positives and negatives of nuclear power:

A

-no pollution
-reliable for power
-need far less fuel

-produce radioactive waste
-nuclear meltdown

risks and benefits must be balanced to ensure it is safe and benefits outweigh the risks.

Physics (14 decks)

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