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Nuclear and Particle Physics > Week 5-9 > Flashcards

Week 5-9 Flashcards

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1
Q

What is radioactivity?

A

The spontaneous change of nuclei to more stable state

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2
Q

Where do stable nuclides lie?

A

They lie on the valley of stability

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3
Q

When is decay possible?

A

When a nuclei is energetically favourable

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4
Q

For stability what should the masses of the parent nuclei and the masses in the final state be?

A

Mass of the parent nuclei needs to be GREATER than the sum of masses in final state

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5
Q

How can beta decay be characterised?

A

It doesn’t change the mass number of nucleon number for nuclides

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6
Q

Can nuclides decay to isobars?

A

Yes if favoured

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7
Q

Where do the stablest nuclides lie?

A

They are the closest integer to the minimum

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8
Q

What happens if A is odd?

A

The most stable nuclei is the value closest to the stable nuclei found using the SEMF

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9
Q

What happens if A is even?

A

We either have:

Odd number of protons and odd number of neutrons

Even number of protons and even number of neutrons

This changes the value of the pairing term so not everything is on the one parabola

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10
Q

Where is the stable nuclei when A is even?

A

It should lie on the lower parabola (even number) as if it is on the upper parabola it always has somewhere to decay to

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11
Q

When does the asymmetry term dominate the pairing term?

A

When A is small

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12
Q

When does the pairing term dominate asymmetry term?

A

When A is large

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13
Q

How many stable odd-odd nuclides are there?

A

4 and they are only stable because they are small (asymmetry dominates)

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14
Q

What happens in electron capture?

A

Nucleus captures electron from inner shell

proton + electron -> neutron +neutrino

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15
Q

How can electron capture be observed?

A

The electron captured leaves the atom in excited state and the outer shell electron will drop into the gap left by the captured electron
De-excitation energy is given off as an x-ray

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16
Q

What adds a sharp peak to the beta spectrum?

A

From the auger effect: if the inner electron which has dropped down gets absorbed by nucleus and leaves a gap for high energy electrons to dip down to the lower state and give off energy in the process. Any other electrons can be knocked out from nucleus entirely to liberate this extra energy

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17
Q

Which form of decay is most likely?

A

beta plus decay as it doesn’t require any input from the electron initially, just the proton decaying (final product will be ion with negatively charge)

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18
Q

What does electron capture rely on?

A

An electron being in the right place at the right time. It is dominant if the difference between the parent and the daughter is sufficiently small that they are missing the extra me + me

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19
Q

When is electron capture dominant?

A

If there is insufficient energy for beta plus decay

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20
Q

What do photon decays involve?

A

Excited states of nucleons

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21
Q

What happens during alpha decay?

A

Emission of alpha particle (very stable particle)

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22
Q

What does a large N value mean?

A

Neutrons are in high energy states, loosely bound and can “drip off” (neutron drip)

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23
Q

What does a large Z value mean?

A

Proton drip

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24
Q

What does a large A value mean?

A

Alpha drip (as it it energetically favourable for some protons and some neutrons to fall off at the same time and they fall off as an alpha particle)

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25
Q

When is a process energetically favourable for alpha decay?

A

When Q value is a positive number

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26
Q

When at a reasonable distance from the nucleus, what does the alpha particle feel?

A

Coulomb repulsion because of the charge on the alpha particle and the charge on the nucleus (1/r)

27
Q

What does an alpha particle need to do to be removed from the nucleus?

A

It has to overcome the potential barrier and so alpha decay can only occur through quantum tunnelling

28
Q

What happens during fission?

A

Roughly equal split of nucleus into fragments (usually 2)

29
Q

What can be deduced from the SEMF about attraction?

A

The more surface area there is the less attraction there is between the nucleons

30
Q

When does the mass of the system increase?

A

When there is a deformation so the binding energy increases

31
Q

What happens during deformation in fission?

A

The distance between the positive charges at the far ends increases and less repulsion occurs leading to a lower mass and increasing the binding energy

32
Q

If the change in energy of the surface energy and coulomb energy is positive, what does this mean (in fission)?

A

Energy needs to be put in to change system from spherical to ellipsoidal

33
Q

If the change in energy of the surface energy and coulomb energy is negative, what does this mean?

A

System is in lower energy state and the natural shape of it is slightly ellipsoidal so it is more stable

34
Q

When does instant fission occur?

A

Very large nuclei (Z^2/A > 47)

35
Q

How many nuclides do fissile materials have?

A

An odd number of nuclides because of the pairing term (mass of the system will be lower that it would be otherwise)

They have an even number of protons (and therefore an odd number of neutrons)

36
Q

How can an odd-odd state be made?

A

If there is an odd number of protons and even number of neutrons and a neutron is added, it goes to an odd-odd state so the mass is higher compared to an even-even state

37
Q

What does gamma decay allow?

A

To probe for actual energy levels

38
Q

In the shell model what are atomic states labelled by?

A

Principle quantum number: n (distance to nucleus)
Azimuthal quantum number : l (measure of orbital angular momentum of particular electron)
Magnetic quantum number: m (orientation of electron’s orbital angular momentum) in terms of l
Spin quantum number: ms

39
Q

How many electrons are in s state?

A

2 electrons

40
Q

How many electrons are in p state?

A

6 electrons (m can be 1,0,-1)

41
Q

How many electrons are in d state?

A

10 electrons (m can be 2,1,0,-1,-2)

42
Q

What changes the energy of an electron?

A

If the spin changes due to a magnetic field so it is either parallel to magnetic field or anti parallel

43
Q

What contributes to the overall spin of a nucleus?

A

Combined spin and orbital angular momentum of nucleons give overall angular momentum (spin)

44
Q

What is the nuclear shell model successful at explaining?

A

Predicting nuclear spin (except odd-odd nuclei)
Energy levels fill from bottom up
Independent sets of energy levels for protons and neutrons
Even number of nucleons in a level cancel out each other’s

45
Q

When is the nuclear model not successful?

A

When 2 energy levels are close where one has a higher angular momentum and the other has a low angular momentum
when there is an odd-odd nuclei

46
Q

What is an isomer?

A

When other decays occur, nucleus is left in excited state (isomer)

47
Q

What is an isomeric transition?

A

Nucleons de-excite to ground state and energy is given off as gamma radiation and electron from inner orbital (x-ray emission and Auger electrons)

48
Q

What do decay schemes show?

A

Shows information about decay model (parent nucleus -> end product)

If proton number is reduced, a nucleus would move to the left of diagram

49
Q

What is a decay chain?

A

Nuclides often decay to other unstable nuclides

But no decay lies along the valley of stability so there must be decay chain

50
Q

Which interaction dominates at high energy?

A

Pair production (photon creates electron and positron)

51
Q

Why is a second body needed in pair production?

A

The photon is massless and the incoming energy and momentum won’t balance with the outgoing energy and momentum (electron and positron)

52
Q

What is the photoelectric effect?

A

Photon gets absorbed by an electron in an atom and the electron is removed from the atom

53
Q

What happens if the photon doesn’t have enough energy to remove electron but it has the exact energy of a discrete energy level?

A

Electron will move to higher energy level

In conduction band, free electrons can absorb some energy from photon and move the electron in the band by changing energy of it which could lead to a photon being emitted from the electron with different energy

54
Q

Which interaction dominates at low energy?

A

Compton scattering: photon gets deflected by angle after process leading to trajectory change

55
Q

What is Bremsstrahlung radiation?

A

Braking radiation, emitted by high energy charged particles

Carries energy away and slows particle

56
Q

What is the characteristic “radiation length”?

A

The mean free path because once one particle emits a photon it will emit another photon

57
Q

What causes an EM shower?

A

Bremsstrahlung and pair production

58
Q

What is Cherenkov radiation?

A

“Photonic boom” which is due to charged particle travelling faster than (local) speed of light which causes ripples

Emitted at specific angle (useful for detection)

59
Q

When do ionisation losses increases?

A

At lower energy when stopping power increases as more time is spent near the atoms so there is higher chance to strip the electrons off

60
Q

In relativistic effect when do ionisation losses increase?

A

The higher the energy the faster the particle, the more concentrated it’s electric field becomes so higher the ability the particle has to strip off the electrons

61
Q

What happens above the critical energy?

A

Radiation losses dominate

62
Q

What happens below the critical energy?

A

Ionisation losses dominate

63
Q

What spin does an even-odd nucleus have?

A

It leads to half integer spin

64
Q

What spin does an odd-odd nucleus have?

A

It leads to integer spin

Nuclear and Particle Physics (2 decks)

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