Nuclear Physics Flashcards
What is the Rutherford Scattering Experiment ?
It provides evidence for an atomic structure
- Scattering of alpha particles by thin metal foils
- alpha particles from a radioactive source fired at very thin gold foil
- strikes a fluorescent screen (circular) a tiny visible light if produced
What are the conclusions made from the Rutherford Scattering Experiment ?
1) ATOMS MUST BE MOSTLY EMPTY SPACE
- most of the alpha particles just pass straight through
2) NUCLEUS MUST HAVE A LARGE POSITIVE CHARGE
- as some alpha particles are repelled + deflected by large angle
3) NUCLEUS MUST BE TINY
- as very few particles are deflected by angle < 90
4) MOST MASS MUST BE IN THE NUCLEUS
- since fast alpha particles are deflected by the nucleus
How the structure of the nucleus has changed over time ?
- Matter was made of little lumps ‘atomos’
- Tiny spheres that couldn’t be broken down
- Plum pudding
- Rutherford scattering - protons and electrons
- Evidence for the neutron
What experiments can you measure the Nucleus radius ?
1) Close approach of a scattered particle
2) Electron diffraction
How to estimate the Radius of the nucleus using the Close approach of a scattered particle method ?
Initial Ek = E(electric potential energy)
E = Qq/4πEor
How to estimate the Radius of the nucleus using the electron diffraction method ?
- more accurate that ‘close approach’
- can be diffracted due to wave-article duality
- λ = hc/E
- FIRST MINIMUM (from diffraction pattern)
SinѲ = 1.22λ/2R
How does Intensity vary with diffraction angle ?
- central bright maximum containing the majority of the incident electrons
- intensity decrease as the angle of diffraction increases
- NEED TO KNOW GPAPH
How to calculate the Nuclear Density ?
R = RoA^1/3 -> R^3∝A -> V∝A
- p = m/v - assume nucleus is spherical
- p = A x M / 4/3πR^3
- p = A x M / 4/3π(RoA^1/3)^3
- p = 3M/4πRo^3 = constant
How to show the nuclear radius (R) is proportional to the cube root of the nucleon number (A)
As the nucleon (atomic mass) increase the nuclear radius increases - R/A graph
- linear relation between R and A^1/3
R = RoA^1/3
(Ro = 1.4x10^-15)
- show evidence that nuclear material have a CONSTANT DENSITY
What are the four types of Nuclear Radiation ?
1) Alpha
2) Beta-Minus
3) Beta-Plus
4) Gamma
What are the properties of Alpha Radiation (α) ?
CONSTITUENT - 2 Protons + 2 Neutrons (He) IONISING POWER - Strong SPEED - Slow PENETRATING POWER - Absorbed by paper/skin or few cm of air AFFECTED BY MAGNETIC FIELD - Yes
What are the properties of Beta-Minus Radiation (β-) ?
CONSTITUENT - Electron IONISING POWER - Weak SPEED - Fast PENETRATING POWER - Absorbed by 3mm of aluminium AFFECTED BY MAGNETIC FIELD - Yes
What are the properties of Gamma Radiation (γ) ?
CONSTITUENT IONISING POWER SPEED PENETRATING POWER AFFECTED BY MAGNETIC FIELD
What are the properties of Gamma Radiation (γ) ?
CONSTITUENT - Electromagnetic Wave IONISING POWER - Very Weak SPEED - Speed of Light PENETRATING POWER - Absorbed by thick lead or concrete AFFECTED BY MAGNETIC FIELD - No
What is Alpha Radiation used for ?
Is used for Smoke alarms because they allow current to flow but won’ travel far
- only need to amount of source
How to identify Radiation ?
Use a GM-tube ( GEIGER-MULLER )
- record count-rate/ mins^-1
- use different materials to receive count-rate
- repeat 3 times and average
- Corrected count-rate = CR - BCR
What is Gamma Radiation used for ?
- Radioactive tracers - help diagnose patients without the need of surgery
- radioactive source injected into body with short half-life to prevent prolonged radiation exposure
- PET scanner used to detect emitted gamma rays
- treat cancer by damaging cells (but all cells)
What is Beta Radiation used for ?
Is used for measuring the thickness of a material
- too little radiation being absorbed -> rollers move part so make material thicker
- too much radiation being absorbed -> rollers move closer so make material thinner
How to safely handle radioactive sources ?
- exposure time is minimum
- shielding during treatment
- inside lead box
- handle with long handling tongs
What is Background Radiation (BR) and type of sources of it ?
Is the low level of radiation always present
SOURCES
- Air (radon gas released from rocks)
- Ground and Buildings
- Cosmic radiation
- Living things e.g. plants/animals
- Man-made radiation e.g medical/nuclear power
What is the INVERSE SQUARE LAW ?
States the intensity of gamma radiation decreases with distance from a source
ɪ = K/x^2
- Intensity of radiation is the amount of radiation per unit area (Wm^-2)
- K - constant of proportionality (W)
- Graph of CCR/distance - ∝1/d^2
What is RADIOACTIVE DECAY ?
Is an unstable nucleus that breakdowns by releasing energy and/or particles to become more stable
- is random and can’t be predicted
What has the same rate of radioactive decay ?
ISOTOPES have the same rate of radioactive decay - constant decay probability
What is the ACTIVITY (A) of a sample ?
Is the number of nuclei that decay each seconds
- Measured in Bq
A = λN
- λ = decay contsant (s^-1)
- N = number of unstable nuclei in sample
What is the Decay constant (λ) ?
Is the probability of a specific nucleus decaying per unit time
- measure of how quickly an isotope will decay
- bigger λ - faster the rate of decay
How else can Activity be written as ?
A = -ΔN/Δt - Is the number of nuclei that decay each seconds - minus sign as always decreasing ΔN/Δt = -λN - model by using spreadsheet modelling
]What is the Decay Equation ?
N = Noe^-λt
A = Aoe^-λt
- Both N and A for a radioactive source both decrease exponentially
- can get Linear graph In(N) = -λt + In(No)
What is the DICE SIMULATION of radioactive decay ?
- Radioactive decay is a random process
- there is a constant probability that an unstable nucleus will decay
- constant probability of dice is 1/6
- 100 sample - number of 6’s represent the nuclei that have decayed
- repeat process but remove the decayed dice
What are the calculations involving molar mass and Avogadro constant ?
N = nNa ( Number of atoms = Number of moles x Constant)
- Molar mass of a substance is the mass that 1 mole of the substance would have (gmol^-1) and is equal to reative atomic mass
What is HALF-LIFE of a radioactive isotope ?
is the time taken for number of unstable nuclei in a sample to halve (T1/2)
How to calculate half-life from decay curves ?
- read off the value of unstable nuclei when t=0
- go to half original number on the y-axis
- draw a horizontal line to curve the vertical line down and read off half-life
- repeat for quarter of original then divid time by two to get half-life
How to derive Half-life equation ?
- When t = T1/2 so 1/2No = Noe^-λT1/2
- 1/2 = e^-λT1/2
- In(1/2) = -λT1/2
- In(1/2) = -In(2)
T1/2 = In(2)/λ
What are the types of Applications of radioactive Isotopes ?
1) Radioactive dating
2) Medical diagnosis
3) Storage of radioactive waste
What is Radioactive dating ?
- Uses Carbon-14
- Living plants take in CO2 from the atmosphere as part of photosynthesis
- When plants die, Activity of C-14 in the plant starts to fall, with half-life of 5730 years
- find made from once-living material can be tested to find current amount of C-14 in them, and date them
Why is it difficult to get a reliable age from radioactive dating ?
- Man-made objects crafted from natural materials e.g. wood, you can only find age of material not object
- object way be contaminated by other radioactive sources
- may be high background count
- sample/count rate is small so might be unreliable
What is Medical Diagnosis ?
- Technetium-99m used in medical tracers - show tissue or organ function
- tracer injected into body and move around to point of interest
- radiation emitted is recorded and image is produced
- Technetium-99m is suitable because has a half-life of 6 hours so can record data but safe
Why and how do you need to store radioactive waste ?
- nuclear fission reactors use Uranium-235 to generate electricity which decays into several different radioactive isotopes - emit radiation (alpha to gamma)
- has very high half-life so radioactive for long time so stored for years unit activity falls to safe level
What makes a Nucleus Unstable ?
- too many neutrons
- too few neutrons
- too many nucleons altogether e.g. too heavy
- too much energy
What does the Nuclear Stability Graph look like ?
- N against Z ( Neutrons against Proton number)
- Line of stability curves off from N=Z
- above line of stability - too many neutrons so undergo β- decay
- below line of stability - too may protons so undergo β+ decay
- right of line of stability - too my nucleons so undergo α decay
How do Protons and Nucleons number change during α, β-, β+ and electron capture (γ) ?
APLHA - very heavy atoms - N -> N-4 + α - Z -> Z-2 + α BETA MINUS - neutron rich isotopes - N -> N + β- - Z -> Z+1 + β- BETA PLUS -proton rick isotopes - N -> N + β+ - Z -> Z-1 + β+ GAMMA - excess energy from previous decay - no change in nuclear constituents - nucleus just loses excess energy - radiation produced via ELECTRON CAPTURE
What is the conservation rule in nuclear reaction ?
Is Momentum, charge, nucleon number, and lepton number must be conserved
How to interpret energy level diagrams showing the radioactive decay of a nucleus ?
- energy level as horizontal lines
- energy released from vertical lines
What equation applies too all energy changes ?
E = mc^2
What is the atomic mass unit (1u) ?
1/12th mass of Carbon-12 nucleus
What is 1u equivalent to ?
Is equivalent to a binding energy of 931.5 Mev
What is Mass Defect (Δm) ?
is the Mass of the Constituent nucleons - Mass of the nucleus
- Measured in u
Why is the Mass of the Constituent nucleons > Mass of the nucleus ?
When the nucleons joint together, the total mass decreases - lost mass in converted into energy and released
What is the Binding energy (BE) of a nucleus ?
is the amount of energy need to separate all the nucleons in a nucleus
- Measured in Mev
- equal to the mass defect
How to calculate the average binding energy per nucleon ?
= BE/A
binding energy/Nucleon number (A)
What does the Average binding energy per nucleon against nucleon number (A) Graph look like ?
- Curve increases till Fe then gradually decreases
- Y-axis up to 10 and X-axis to 250
- Fe is turning point as most stable nucleus - highest BE/A
What part of the graph represents Nuclear Fission ?
- Right side towards Fe
- Average binding energy per nucleon decreases gently for heavier nuclei
What part of the graph represents Nuclear Fusion ?
- Left side towards Fe
- Average binding energy per nucleon increases rapidly for heavier nuclei
What is Nuclear Fusion ?
Is combining of two smaller nuclei into one large nucleus
- a-lot of energy released due to new nucleus having higher BE/A
- energy released as kinetic energy of particles
What is Nuclear Fission ?
is Splitting up of large nuclei into two smaller nuclei’s
- a-lot of energy released due to new smaller nuclei’s having higher BE/A
- energy released as kinetic energy of particles
- can be SPONTANEOUS or INDUCED
What must nuclei overcome for Fusion to occur ?
- all nuclei are positively charged - will be electrostatic force of repulsion between them
- only fuse if they overcome this electrostatic force and get close enough for the attractive force of the strong interaction to hold them together (1Mev of Ek)
- Low-energy nuclei are deflected by electrostatic repulsion
What occurs in Induced fission - chain reaction ?
- Neutron fired at Uranium which creates daughter Products and 2 neutrons which are slowed down by Moderator to form thermal neutrons for chain reaction t occur
How do Nuclear Fission Reactors consist of ?
- Concrete case
- Fuel rods
- Control rods
- Moderator
- Coolant which turn into Steam (turn turbines)
What is chain reaction ?
at least one fission neutron subsequently goes onto produce another fission event
- induce other nuclei to fission
What is the Purpose of the Moderator in Reactor ?
- Fuel rods placed in Moderator e.g Water/Graphite
- Slow down fission neutrons to become thermal neutrons (chain reaction)
- Slow down by ELASTIC COLLISIONS
How is Moderation made from Elastic collisions ?
- Momentum and kinetic energy conserved
1) MnVn = MnV’n+ MmV’m (Momentum)
2) 1/2MnVn^2 = 1/2MnV’n^2+ 1/2MmV’m^2 (Kinetic Energy) - Mn = Mm = M -> V’m = Vn
- Similar masses cause all energy transferred to moderator particles
What is Critical Mass ?
the amount of fuel needed for chain reactions to continue on its own at a steady rate
- < Critical mass - reaction will just petal out
What is the Purpose of the Coolant in Reactor ?
- help remove heat produced by fission
- material which is efficient at transferring heat - make steam for turbines
What are the Safety aspects of the Nuclear reactor ?
1) Reactor shielding
2) Emergency Shut-down
3) Handling + Store Fission waste products
What are the Safety aspects of the Nuclear reactor ?
1) Reactor shielding
2) Emergency Shut-down
3) Handling + Store Fission waste products
What is the Emergency Shut-down of a Reactor ?
- shut down automatically by the release of control rods into the reactor
- slow down the reaction as quickly as possible
- prevent melt-down
How to Handle + Store Fission waste products ?
HANDLING
- placing in cooling ponds as very hot (serval months)
- reprocessing
- stored in sealed containers until activity has fallen sufficiently
What is Society View on Nuclear Power ?
PRO’S
- long lasting than fossil fuels
- no greenhouses gases released
- very efficient (generate more electrical energy per kg)
CON’S
- built and designed extremely carefully to minimise the danger of natural disaster
What is Society View on Nuclear Power ?
PRO’S
- long lasting than fossil fuels
- no greenhouses gases released
- very efficient (generate more electrical energy per kg)
CON’S
- built and designed extremely carefully to minimise the danger of natural disaster
- radioactive waste products - storage
