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Physics year 2 sem 1 revision > Particles > Flashcards

Particles Flashcards

1
Q

For a reaction why is the mass of the products larger than the mass of the reactants

A

The K.E. of the reactants is converted into mass

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2
Q

What are Cosmic Rays

A

Charged particles which have the highest energies ever to be observed (10^8 times CERN)

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3
Q

What was the problem associated with the origin of cosmic rays

A

B-fields in extragalactic space, the milky way and surrounding the earth deflect and distort the parths of the charged particles

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4
Q

How was the origin of cosmic rays figured out

A

High-energy neutrinos produced by the same cosmic accelerators preserve their directional information. These were traced back to a Blazar.

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5
Q

What is a blazar and how does it produce cosmic rays

A

A feeding super massive black hole at the centre of galaxies which launch powerful jets which accelerate protons (ie produce cosmic rays). When it is directed in the direction of eath the feeding black hole is called a Blazar. The Collisions between accelerated protons produce pions which leave neutrinos and gamma rays as decays products which move in the direction of the jet.

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6
Q

What are the exchange particles of the strong force and when are the mediators used

A

Mesons - interact between nuclei

Gluons - interact between quarks

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7
Q

Why can bound n-p states exist but not p-p or n-n states not

A

Due to pauli’s exclusions principle for both identical fermions to exist in the same quantum state. Therefore the total spin will be equal to zero. S = 0 potential well, however, is not deep enough to form a bound state whereas an S = 1 state (like deuteron) is.

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8
Q

Definition: Binding energy

A

The energy required to split nucleus into its elementary constituents. (or energy released when free protons and neutrons form a nucleus)

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9
Q

What are the 3 types of beta decay

A

Beta minus
Beta plus
K capture

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10
Q 
In the Semi- empirical mass formula what do the following terms represent 
aA
-bA^2/3
-s(N-Z)^2 /A
-dZ^2 /A^1/3
Pairing term/A^1/2
A

aA - Bulk Term
a: Basic Binding energy per nucleon
analogy with bulk cohesive energy of liquid
-bA^2/3 - Surface term
Nucleons near the surface are less well bound (fewer
neighbours)
-ve as there is less binding energies for these nucleons
Contribution = proportional to SA = prop. to R^2 = prop
to A^2/3
-s(N-Z)^2 /A - Symmetry Term
Favours N = Z
Q-M explanation
-dZ^2 /A^1/3 coulomb term
Nucleus has a charge Ze in small vol. this is
energetically unfavourable. therefore this term is
proportional to the energy of the uniformly charged
sphere
Pairing Term
Empirical favours even-even nuclei over odd-odd and
odd-even

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11
Q

What does the liquid drop model not cover

A

Light Nuclei
Magic Numbers
Spin
Excited States

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12
Q

How is Nuclear spin-orbit coupling different to the one in atomic physics

A

i) not electromagnetic in origin
ii) Produces large splits which increase with l
iii) l +1/2 level is lower for nuclear coupling

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13
Q

What do magic numbers correspond to

A

Filling up to levels with a larger average gap to the next level there for it is more stable

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14
Q

Why does nuclear spin-orbit coupling explain the presence of magic numbers

A

S-O coupling splits the energy levels into discrete bands. Bands with a large gap in between are considered shells and therefore extra stable.

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15
Q

Rules when filling up shells of nuclear models

A

(a) Filled LEvels have a total angular momentum of 0
(b) Successive protons and neutrons pair off to give
angular momentum = 0
All even-even nuclei: J= 0
Even - odd: J=j of unpaired nucleon
odd - odd: no general rule

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16
Q

What are the different classification of particles

A 
fermions and bosons ( half and integer spin respectively) 
Bosons:
mesons and gauge bosons
Fermions:
Baryons, leptons,

Baryons and Mesons give hadrons

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17
Q

Explain why the cross section against energy spectrum (in neutron reactions) for light nuclei feature spikes at very large energies, compared to the heavy nuclei spectra which feature many peaks closely spaced but at much lower energies.

A

Heavy nuclei have more bound energy levels and so transitions between energy levels result in smaller changes in energies hence the spikes at smaller energies and more frequent spikes in the spectra.
Light nuclei feature less energy levels and so the jumps between give much higher energies.

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18
Q

How can the Omega - baryon (SSS ) exist? Why doesnt it violate paulis exclusion principle?

A

new quantum number ‘colour’ required to explain the existence. Each S quark in the omega - baryon has a different colour, so no violation of pauli as not all quantum numbers identical.

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19
Q

How do you work out baryon number and lepton number?

A

Baryon number= 1/3 ( no of quarks - no of anti quarks)
Electron Lepton number = Ne - Ne+ + N(electron neutrino) - N(electron anti neutrino).
etc

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20
Q

List the 4 fundamental forces and their associated properties

A

Gravity - mediator graviton - range inf
Couloumb force / electro magnetic force - mediator photons - range inf
Weak force - mediator w+ w- z0 gauge bosons - range 10^-18m
Strong force - mediators: gluons in quark and mesons in nuclei - range 10^-15m strong force additional property of asymptotic freedom (like a spring, increases in strength as separation increases).

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21
Q

Which of the fundamental forces can leptons feel

A

All except the strong force

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22
Q

What is the difference between a decay and a reaction

A

decay- going from 1 entity to multiple products

reaction - going from two entities to reaction products

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23
Q

List the conservation laws

A

Conservation of energy -angular momentum - quark flavour (except in weak interactions)- charge - baryon number - lepton number

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24
Q

Derive an expression for the lifetime of a virtual particle of mass m0, comment on the case of virtual photons.

A

HUP delta E delta T = h bar /2
delta E = m0c^2
solve for delta T

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25
Q

Describe the one quantum mechanical exception to the conservation of energy law

A

Can have extra energy for time delta T

if obeying the HUP

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26
Q

Explain why the product(s) can not have a greater mass energy for a decay than the reactant(s) but can for a reaction?

A

In a reaction the initial reactants can have some KE and so can compensate for the products greater total mass energy and so conservation of energy not violated.
In a decay, no

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27
Q

If a process does not conserve charge flavour, what does that imply?

A

It must be a weak force process

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28
Q

In neutron reactions, what is the difference for the peaks shown on a cross-section against kinetic energy plot

A 
Light Nuclei
  Widely spaced
  Broad
  Low ( few barns
Heavy Nuclei
  Closely spaced
  Narrow
  High (10^3 - 10^4 barns)
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29
Q

Why is there resonant peaks for a cross-section against energy graph for neutron reactions

A

There is a formation of a compound nucleus. The neutron is absorbed into the nucleus and shares the energy between the nucleons exciting the nucleus before the nucleons have enough energy to escape.

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30
Q

When does resonance occur

A

Initial KE ( in CM frame) matches the energy of the excited state of the compound nucleus.

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31
Q

What are the two cases for nuclear fission

A

Spontaneous Fission
Some heavy nuclei are unstable and decay
Induced Fission
Follows a nuclear reaction ( Usually neutron + heavy
nucleus) causing the nucleus to be more unstable

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32
Q

Considerations for a good moderator material

A
  • Inexpensive and easily obtained
  • Low radiative capture cross-section
  • Small A, Maximises energy transfer per scattering
  • High density
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33
Q

When does Fission become the dominant decay mode

A

A>= 260

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34
Q

What happens following fission

A
  • “Prompt” neutrons boil-off (fast ~ 10^-20 s)
  • gamma- decay to ground state (fast ~ 10^-16s)
  • Products = neutron-rich: beta decay to beta- stability
    line (slow mins/years)
  • ‘Delayed neutrons’ emitted sometimes after beta decay
    (important for control of nuclear power plants)
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35
Q

What controls the rate of the PP1 chain

A

Initial p + p interaction is controlled by the weak interaction which is slow and there for is the limiting step

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36
Q

What is the most suitable reaction for fusion reactors

A

Deuterium - Tritium (D-T) reaction

  • High Q (17.6MeV)
  • Lower Barrier
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37
Q

What are the requirements for a nuclear reactor

A

1) Plasma Temperature must be high (>10^8K)
2) Plasma density n (ions/m^3) must be high
3) Must hold plasma for sufficient time ( Confinement time)

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38
Q

Types of Confinement Methods for reactors

A 
Magnetic Confinement Fusion (MCF)
 Plasma confined in a magnetic field (in a tokomak)
  held in a torus
Inertial Confinement Fusion (ICF)
  DT mixture in a plastic sphere
  Vaporised by intense energy burst, applied 
  symmetrically
  Pellet implosion generates
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39
Q

Why is 238Uranium fissionable but not a fissile material whereas 235U is

A

U235 is a fissionable nuclide that can be induced to fission with low-energy thermal neutrons with a high probability (definition of fissile) whereas U238 can only be fissionable with high energy neutrons

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40
Q

How can neutrons interact with nuclei

A

Elastic Scattering
Overall KE conserved – Target recoils, causing neutron
to lose KE
Inelastic Scattering
Neutron loses KE ( daughter nucleus in excited state)
Radiative Capture
Incident neutron absorbed compound nucleus forms
and rapidly decays to ground state resulting in gamma
emission. Resonance occurs
Fission
Neutron leaves target at energy state above
spontaneous fission barrier. Short time scale

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41
Q

What is the role of the moderator

A

Slow down the fast neutrons to lower energies in the thermal regime where the cross-section for inducing nuclear reactions is higher

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42
Q

when is energy loss at a minimum/maximum for Elastic scatter between a neutron and a moderator

A 
Min. = grazing angle collisions
Max. = Head-on collisions
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43
Q

What is the average logarithmic decrement

A

Mean energy reduction per event

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44
Q

Considerations for a good moderator material

A
  • Inexpensive and easily obtained
  • Low radiative capture cross-section
  • Small A, Maximises energy transfer per scattering
  • High density
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45
Q

What does neutron density in a reactor depend on a balance of

A

-Production
i.e. the slowing down at the energies considered
-Absorption
radiative capture
-Leakage
Neutrons escaping, relates to the geometry (smaller
reactor core = more probable leakage) of the reactor
and to the properties of the material

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46
Q

How does neutron density vary with distance from the source

A

inversely

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47
Q

Explain Super-critical state and sub-critical state

A

-Super-crit
Not enough neutron leakage from the core. Neutron
level rises unless control measures are taken, i.e.
insertion of control rods
-Sub-crit
Leakage is too large, neutron population will decay
away, unless steps are taken to increase thermal
neutron production

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48
Q

What do baryons normally decay into, as compared to mesons?

A

Mesons decay to leptons and photons

baryons to protons or anti protons, leptons and photons

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49
Q

What is a cp transform?

A

flipping all the coordinates and charges

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50
Q

What does symmetry under CP transform mean?

A

A process and its CP transform equivalent is equally likely to occur, i.e its half life or cross section is equivalent.

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51
Q

How can you identify what reaction has occured?

A

Weak interaction - neutrino and anti neuntrinos involved and change of quark flavour

Em interaction - Real photon is emitted or absorbed

Quark flavour is conserved and no photon => strong force

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52
Q

Drawn a feynman diagram for a process involving weak force, em force and strong force

A

Here is the relevant bit in the lectures:
https://uniofbath.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=0b27e475-a1f2-4f2a-b471-aadb00a9deb3
@37:45

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53
Q

What process is not invariant under a CP transform

A

Weak decay

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54
Q

Define the decay constant lambda and its relation with the half life

A

Probability of a nuclei decaying per unit time. lambda = ln 2/ T halflife

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55
Q

What is inertial confinement fusion as compared to magnetic confinement fusion

A

Magnetic confinement fusion is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma.

Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium.

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56
Q

How are quarks held together as compared to nuclei?

A

Gluons hold quarks and mesons hold nuclei

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57
Q

Why can you not have a nucleus containing 2 protons only or 2 neutrons only?

A

Pauli exclusion principle, for a nuclei only containing N-N or P-P, the two nucleons would have to be off opposite spin, a system with S=0 is less bound than a system of S=1 and so systems of s=1 are more stable.

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58
Q

Roughly what energy would electrons that are to be used for determining the charge distribution of a nuclei be required to be at?E

A

E=p^2c^2+m^2c^4
lambda = h / p
lambda roughly equal to nuclear diameter 10^-15

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59
Q

what is the relationship between nuclear radius and atomic number?

A

R= 1.1A^1/3 fm

60
Q

Define BE

A

Energy required to split nucleus into constituent parts

61
Q

List the main decay modes of nuclei

A

Beta,alpha,fission and emission of photons

62
Q

What is K capture

A

proton in a nuclei turns into neutron as nuclei captures e

63
Q

What type of interaction mediates beta +- decay and K capture? Where are these decays typically observed?

A

Weak interaction, beta - observed in free space, all 3 observed in nuclei

64
Q

Describe the energy distribution of emitted electrons and positrons in beta -+ decay
Explain the curves and why all the electrons dont just end up with the energy released in the reaction.

A

beta minus decay produces a curve which starts with some positive y intercept and curves down to a max E, beta + has a bell shaped look with a defined maximum E.

No e+ with 0 E like the e- because of the coloumb force, upper E limit determined by Q (energy released in reaction). Distrubtion due to the ways in which the energy can be transferred to the 3 products.

65
Q

What were the early signs that neutrinos must exist?

A

Missing energy of electrons and positrons in beta decay.

66
Q

How much more massive must the parent atom be than the daughter atom to decay via K capture?

A

2m_e heavier

67
Q

What is the minimum Z value needed for alpha decay?

A

Z>=70, in practice Z>= 83:

Derived from considering the BE of a alpha decay and the constraint:
m_parent c^2 - m _ daughter c^2 - m alpha c^2 >0

68
Q

Describe the energy spectra of emitted alpha particles from alpha decay processes

A

Resonance peaks, peak at E max corresponds to alpha decay leaving with full energy released, other peaks correspond to daughter nuclei being left in excited states

69
Q

With what maximum energy can an alpha particle emitted from an alpha decay be observed with? (Relate Q energy released in reaction to E max, energy max of alpha)
What conservation law is responsible for this?

A

Emax=Q[ 1 - (m alpha/ m_daughter ) ]

Arises due to conservation of momentum

70
Q

What main process dictates alpha process decay rates?

A

Couloumb barrier, alpha particle needs to tunnel through coloumb barrier to decay.

71
Q

Describe what is meant by the valley of stability?

A

A characterization of the stability of nuclides to radioactivity based on their binding energy.
The shape of the valley refers to the profile of binding energy as a function of the numbers of neutrons and protons, with the lowest part of the valley corresponding to the region of most stable nuclei.[2] The line of stable nuclides down the center of the valley of stability is known as the line of beta stability.
The sides of the valley correspond to increasing instability to beta decay (β− or β+).

72
Q

Explain limitations of the liquid drop model

A

. light nuclei
• magic numbers
• spin
• excited states …

73
Q

What is the idea of self consistent fields?

A

Each neutron moves in the average potential well
caused by the other neutrons and protons.
Each proton moves in the average potential well of the
protons and neutrons (strong force) AND the average
Coulomb potential of the other protons

74
Q

What do the potential wells of neutrons and protons in nucel look like

A

Neutron potential well is square about x = 0, a proton well is similar except there are slight positive ‘hills’ due to coloumb interaction.

Check section 3 page 16

75
Q

What does apparent/absolute luminosity/magnitude mean

A

Apparent Luminosity- how bright the star appears to a detector here on Earth
Absolute Luminosity - the apparent magnitude an object would have if it were located at a distance of 10 parsecs

76
Q

What is the Hertzsprung- Russel Diagram a plot of?

A

a scatter plot of stars showing the relationship between the stars’ absolute magnitudes or luminosities versus their stellar classifications or effective temperatures.

77
Q

What is meant by a main sequence of a star

A

any star that is fusing hydrogen in its core and has a stable balance of outward pressure from core nuclear fusion and gravitational forces pushing inward.

78
Q

What is meant by Red giant

A

Large Starrs with high luminosity and low surface Temperature. With no hydrogen left at the core of a star to fuel the nuclear reaction that keeps it burning, the core begins to contract. The core’s contracting releases gravitational energy into the surrounding regions of the star, causing it to expand. Consequently, the outer layers cool down and the color of the star (which is a function of temperature) becomes red.

79
Q

What is meant by White Dwarf

A

Small Dense stars of low luminosity and high surface temp.

80
Q

What is meant by a Variable star

A

Star that varies its luminosity over a period. The period of variability is related to luminosity.

81
Q

Cosmic abundance of visible matter, how is cosmic abundance set

A

74% H
24% He
2% Metals
Set in the first few minutes after the big bang, if all the He was produced by nuclear fusion there would be less He and more metals

82
Q

Definition
Luminosity
Flux
Flux Density

A

1) Total power output (the rate at which energy is radiated)
2) Energy per unit time, crossing unit surface area
3) Flux per unit frequency interval

83
Q

What processes go on in the PPI chain

A

H + H –> 2^H + e^+ electron neutrino (1Mev + 0.4 MeV)
2^H + H –> 3^He + ɣ (5.5Mev)
3^He + 3^He –> 4^He + H + H + ɣ (12.9Mev)

84
Q

What is the net results of the PPI chain, what temperature is needed and what percentage of the sun does it occur in

A

core Temp = 10^7K

0.1R (Suns core)

85
Q

Why do we have resonant peaks in neutron reactions?

A

Formation of a compound nucleus. Resonance occurs when the absorbed neutron is able to exactly transfer enough energy to excite the compound nucleus. (Initial KE in CoM = energy of excited state of compound nucleus)

86
Q

What is the net results of the CNO Chain and how does it vary to PPI and where does the carbon come from

A

4 1^H +2e- –> 4^He +7ɣ + 2electron neutrinos +23.8Mev
CNO needs higher T then PPI ( Coulomb barrier is higher)
CNO Cycle is much more T sensitive
Carbon is used as a catalyst and comes from previous generations of stars

87
Q

Describe the cross section against energy diagram from charge selection.

A

Upwards slope with resonance peaks for KE» Coloumb barrier, same as neutron reactions.
Very poor cross section if KE < coloumb barrier as quantum tunelling required

88
Q

What is the pressure of a degenerate electron gas dependent on

A

Only dependent on the number density of electrons, an increase in p DOES NOT lead to an increase in T

89
Q

How does photodisintegration help produce larger elements in stars

A

Photodisintegration occurs when T~ 10^9 K. The high energy photon interacts with the nucleus causing it to break apart into two lighter nuclei. One of these light nuclei (ie neutron) can be captured to form heavier elements. Photodisintegration and capture reactions are competing processes and produce elements up to 56Ni

90
Q

What causes a Cepheid variable star

A

The sensitivity in T and p during He shell burning causes intense thermal pulsations where the star expands, cools, contracts, heats. The size (>10M.) of the star can maintain this process and hence becomes a Cepheid Variable. Whereas a lower mass star would eject the pulsating outer layers as a planetary nebula.

91
Q

What happens to a Massive star after the core has reached the Chandrasekhar limit

A

Photodisintegration of iron occurs which is an endothermic process (producing 13 4^He and 4n), this robs the core of energy causing a dramatic collapse (~1s)
electron capture of 4^He produces 2p and 2n. Electron capture turns protons into neutrons forming a degenerate neutron gas
Outlying material falls onto the neutron core
-energetic rebound causing a shockwave which is driven through the star causing heating.
-A huge onset of fusion reactions throughout the star occurs causing the star to blow itself apart (increases in luminosity by x10^8) \SUPERNOVAE

92
Q

What is the pressure of a degenerate electron gas dependent on

A

Only dependent on the number density of electrons, an increase in p DOES NOT lead to an increas in T

93
Q

Describe the features of a white dwarf

A

A sun-like star that has passed through H-burning and He burning to form a c-o core surrounded by He and H. The outer envelope forms a planetary nebula leaving the core as a W.D. Mass carys from 0.2M. to 1.4M. Core is supported by degenerate electron pressure. The more massive W.D has a smaller R. Pressure increases with density

94
Q

Under what conditions are species formed by either r-process or s-process

A

r-process
if they are immediately right of an unstable isotope
s-process
if shielded by a stable nucleus below and right of it

95
Q

What happens to a Massive star after the core has reached the Chandrasekhar limit

A

Photodisintegration of iron occurs which is an endothermic process (producing 13 4^He and 4n), this robs the core of energy causing a dramatic collapse (~1s)
electron capture of 4^He produces 2p and 2n. Electron capture turns protons into neutrons forming a degenerate neutron gas
Outlying material falls onto the neutron core
-energetic rebound causing a shockwave which is driven through the star causing heating.
-A huge onset of fusion reactions throughout the star occurs causing the star to blow itself apart

96
Q

How is most of the energy of the supernovae carried away

A

> 99% of the energy is carried away by electron neutrinos

97
Q

What does the word constant in hubbles constant mean

A

Hubbles constant changes with time as the scale factor does, instead it refers to it being constant across the universe

98
Q

Under what conditions are species formed by either r-process or s-process

A

r-process
if they are immediately right of an unstable isotope
s-process
if shielded by a stable nucleus below and right of it

99
Q

What were the two major 20th century discoveries for cosmology

A

-The universe is expanding
Uniform expansion
Universe had infinite density (14bn yrs ago)
-Whole of space is filled with thermal radiation at an equivalent blackbody T~ 2.73K
Very pure BB spectrum
Extremely Uniform across the whole sky (fluctuations 1
part in 10^5)

100
Q

Thermal History of the universe

A

-t<10^-10 T>10^15K
Unknown
-10^-10s light elements
10^11s

101
Q

What does the word constant in hubbles constant mean

A

Hubbles constant changes with time as the scale factor does, instead it refers to it being constant across the universe

102
Q

Describe difference between nuclear and atomic spin orbit effect

A

Not electromagnetic in origin

And as there are large splits which increase with L
and L+1/2 levels are lower in energy —-> Shells overlap

103
Q

What are magic numbers?

A

Magic numbers correspond to the filling up of levels with a larger than average gap to the next level.

104
Q

How are the distributions of the energy levels different for light nuclei are compared to heavy nuclei?

A

Light nuclei have few and very widely spaced energy levels as compared to the many dense spacing of energy levels in heavy nuclei.

105
Q

How does nuclear excitation to low levels compare to higher levels?

A

Excitations to low energy levels correspond to single nucleon excitations between energy levels.

High level excitations correspond to many nucleons being excited, not well described by the shell model.

106
Q

What are the values Q must take on for an endo thermic reaction? What are the values Q must take on for an exo thermic reaction?

A

Q for an endo thermic reaction < 0 , Q for an exo thermic reaction >0.

107
Q

What is the typical form of a reaction?

A

a + X -> Y + b

108
Q

How close must nuclei be to invoke consideration of the strong force?

A

1fm

109
Q

Explain the concept of cross section

A

Cross section represents the “effective” cross section of the nucleus. If large P of interaction larger cross section, i,e nucleus interacts with more particles so incoming flux of particles see a larger effective cross section of nucleus.

110
Q

Explain under what assumption is the probability of interaction equivalent to delta L / L _ 0

A

L_0»delta L

111
Q

Why is the KE in the CoM frame also equivalent to all the energy available to excite the target nucleus?

A

As all forces involved in the reaction are internal forces, the net change in momentum of the CoM is zero, This means that the CoM will have some KE that is not available to go to exciting the target nucleus as it must be conserved. The amount of KE that is available for excitation is found in the frame where the net momentum of the CoM = 0 .

112
Q

Why do we have resonant peaks in neutron reactions?

A

Formation of a compound nucleus. Resonance occurs when the absorbed neutron is able to exactly transfer enough energy to excite the compound nucleus. (Initial KE in CoM = energy of excited state of compound nucleus)

113
Q

What is vacuum energy density/ Dark matter

A

The energy density of empty pace caused due to zero-point energy which remains even when no particles are present.
Particle-antiparticle pairs are continually created/annihilated out of the vacuum giving the vacuum a non-zero potential energy

114
Q

What is the CMB

A

The leftover radiation following the big bang which decoupled from matter.

115
Q

Describe the 4 results of neutron reactions

A

Resonant elastic scattering
Inelastic excitation
Capture
Neutron capture

116
Q

Describe the cross section against energy diagram from charge selection.

A

Upwards slope with resonance peaks super imposed.

in the case E_k»E_coloumb

117
Q

Explain the difference between neutron reactions and charged reactions

A

Coloumb barrier provides opposition to reaction, more energy must be supplied.

118
Q

What is nuclear fission

A

Heavy nucleus into 2 light.

119
Q

Explain the potential energy against seperation distance for a fission reaction using the liquid drop model.

A

Nucleus seperates like a liquid drop, potential energy has some slight upwards bend before dropping. This barrier is due to the surface term increasing more than the coloumb term decreases in the semi emperical mass formula. So net BE increases when the fission starts to occur.

120
Q

Why is spontaneous fusion rare?

A

Quantum tunnelling through barrier ( barrier created by liquid drop like splitting of nucleus and the surface term increasing a lot compared to the coloumb term decrease).

121
Q

What happens after a fission reaction?

A

Prompt neutrons boil off, gamma decay to to ground states, neutron rich products decay via beta decay.

122
Q

What are the unanswered questions of the standard model

A

Why are there 3 generations of particles
Why do protons and electrons have equal and opposit charge
How does gravity fit in

123
Q

Why are the heaviest elements only created in super novae?

A

Coloumb barriers massive, need a lot of energy , supernovae

124
Q

Why is DT fusion more viable than P P fusion?

A

PP too slow, DT is releases more energy (large Q) has a lower coloumb barrier (more probable)

125
Q

What are the four fundamental problems of cosmology

A 
Problems the big bang couldn't explain
The Horizon Problem
The Flatness Problem
Matter-antimatter asymmetry
Formation of galaxies
126
Q

How does cosmic abundance support the big bang theory

A

The universe shows evidence for a large majority of H and He, with little to no trace of atoms with unstable mass numbers (eg 5 or 8). Therefore, the prominence of H and He could not undergo nucleosynthesis further with n and p. Giving evidence for an early universe consisting largely of subatomic matter seconds after the BB.

127
Q

What does Inflation help predict that the big bang couldn’t

A

-Horizon problem
Initially, the universe was causally connected
-Flatness Problem
The radius of curvature of geometry increased by an enormous factor
Matter-antimatter asymmetry
The initial bubble had slight imbalance of matter-antimatter

128
Q

How may the antimatter-matter asymmetry be explained?

A

the ratio of the number of photons to the number of nucleons ~ 10^9
if we can created (10^9 +1) protons for every 10^9 antiprotons it would solve it

129
Q

What is a mirror nuclei

A

The number of protons on one atom equals the number of neutrons in another atom

130
Q

Why do we believe the universe is flat?

A

Compare measurements of fluctuations in CBR with modelling of pressure waves in the early universe match results expected from a flat universe

131
Q

What are the unanswered questions of the standard model

A

Why are there 3 generations of particles
Why do protons and electrons have equal and opposit charge
How does gravity fit in

132
Q

What does the unification of forces mean

A

The coupling constants for the forces converge at high energies to the same value at a unification point making them indistinguishable from each other. This results in no distinction between hadrons and leptons, al masses are the same. (Symmetry is broken at low E)

133
Q

What is the Success of GUT

A

Unification of E/m, weak and strong interactions

Explains relationships between quarks, leptons and their charges

134
Q

What are the failures of GUT

A

Doesn’t explain number of generations
Predicts proton decay which hasn’t been seen
Predicts magnetic monopoles hasnt been seen

135
Q

What is Cosmic inflation

A

When the strong force separated, the material comprising the early universe underwent a phase transition(ie quarks condensing–>hadrons). This released a large amount of energy which drove an accelerated expansion of the universe. Phase transitions are controlled by scalar fields which can behave with -ve pressure(pressure goes up as density decreases) which can drive accelerated expansion

136
Q

What does Inflation help predict that the big bang couldnt

A

-Horizon problem
Initially, the universe was causally connected
-Flatness Problem
The radius of curvature of geometry increased by an enormous factor
Matter-antimatter asymmetry
The initial bubble had slight imbalance of matter-antimatter

137
Q

Evidence that Neutrino oscillations take place. (Neutrinos change from one flavour to another during flight).

A

Experimentally proven by using large detectors by studying:

Solar Neutrinos, less Ve observed than expected from sun.

Atmospheric Neutrinos, measuring changes in flux ratio over Ve and Vų over baseline radius of the earth.

Implication: Neutrinos have little mass.

138
Q

Wavelength for extremely relativistic effects. (KE»mc^2).

A

Wavelength = hc/KE.
From E = hc/wavelength
and assuming mc^2 is negligible to KE in the relativistic equation of energy, E=mc^2+KE.

139
Q

What is the mean lifetime of a source?

A

Mean lifetime = 1/decay constant

139
Q

What are the conditions for the strong force?

A

Quark flavour conserved
Lifetime < 10^-20 s
Acts on quarks and hadrons but not leptons
Mediator is a Gluon, which carries colour charge

140
Q

What semi-emperical relation does the nucleus follow and what can we deduce about the density of the nucleus?

A

R=RoA^1/3
Where A is the mass number, R the radius and Ro=1.33 fm.
The mass number A cancels out with density calculations, hence the density of the nucleus is constant = 1.6810^17 Kg/m^3

141
Q

Little riddle break:

What is greater than God,
more evil than the devil,
the poor have it,
the rich need it,
and if you eat, you'll die?
A

Nothing!

144
Q

Explain the neutron energy regimes in a fission reactor

A

Thermal neutrons
Epithermal and resonance neutrons
intermediate / slowing down neutrons
fast neutrons

145
Q

What 2 factors are to be maxamised when trying to pick a good fission reactor moderator material

A

High cross section sigma and large lograithmic decrement delta.

146
Q

what is critical,sub critical and super critical mass?

A

Number of neutrons produced in the i + 1 th stage = ith stage. > for sub critical and < for super critical

147
Q

In a nuclear reaction how is the energy realesed calculated and how can the nature of this reaction be determined?

A

Energy released, Q = Binding Energy of products - Binding Energy of reactants.
Endothermic if Q<0
Exothermic if Q>0

Physics year 2 sem 1 revision (3 decks)

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