Nuclear Flashcards
Who posited the plum pudding model?
J J Thomson.
Describe the plum pudding model.
A cloud of positive charge with negative electrons inside of it.
Who disproved Thomson’s plum pudding model?
Rutherford.
How did Rutherford disprove Thomson?
The gold-foil experiment.
How does the gold foil experiment work?
Get a thin sheet of gold
Fire + charged alpha particles at it
What is expected to happen in the gold foil experiment according to Thomson?
The alpha particles should go straight through.
What actually happened in the gold foil experiment, why?
Some alphas bounce back, hit the positive nucleus.
How do we know the nucleus is small and dense?
Because most alphas pass straight through the nucleus.
Who discovered neutrons?
Chadwick.
What is r?
Shortest distance between the nucleus and alpha particles.
When alpha is scattered through 180 degrees what = KE?
EPE, particle loses KE and gains EPE.
What speed is the alpha particle at just before it bounces back and changes direction?
0.
Ek=Ep= ?. Define all terms.
Qn(qa) / 4π(ε0)r
Qn= charge of nucleus qa = charge of alpha ε0 = permittivity of free space
What is permittivity of free space?
A constant, the capacity of an electric field to permeate a vacuum.
What is the charge of an alpha?
2 x 1.6 x10^-19
What is electron diffraction used to measure?
The size of the nucleus.
How do we do electron diffraction in this context? What is observed?
Shoot an electron beam through thin metal foil at a screen. Minima and maxima are observed like light diffraction experiments.
What is the equation for the first minimum in electron diffraction? Define all terms.
Sinθ = 1.22λ / 2R
λ = wavelength (duh) R = radius of the nucleus of the atoms that scatter the e-s
What is the equation for the de Broglie wavelength of electrons?
λ = hc / E
What does the graph of relative intensity against angle of diffraction look like?
Quadrants 1 and 2. Big peak in the centre becomes tiny peak becomes even tinier. Symmetrical.
What is the size of the radius of a typical atom?
5 x10^-11m
Why is the nucleus so small?
Because SNF only works at very short distances (<3fm).
Describe the graph of nuclear radius against mass number A.
Steep increase then plateaus.
What is the equation for nuclear radius? Define all terms. What does this show?
R = R0 A^1/3
R = nuclear radius R0 = 1.4fm A = number of nucleons/mass number
R is proportional to the cube root of A.
What is the equation for nuclear density? Define all terms.
ρ = 3m / 4π(R0)^3
ρ = density m = mass R0 = 1.4fm
Why does radiation happen, what does the atom actually do?
When the nucleus of an atom is unstable, atom chucks something out of the nucleus.
How do we speed radioactive decay up?
We can’t.
On an A-Z graph what nuclear decay happens above the line of stability?
Beta minus, nuclei have too many neutrons.
On an A-Z graph what nuclear decay happens below the line of stability?
Beta plus, nuclei have too many protons.
What happens to very heavy nuclei?
Alpha decay, nuclei have too many nucleons.
What does every decay conserve?
Energy, charge, momentum, baryon number, lepton number.
How do you find unknowns from energy level diagrams?
Find equations for delta E, list all values given, rearrange equations and solve for unknown.
What does more ionising mean, how does this affect range and penetration depth?
Reacts with more matter, shorter range and penetration depth.
Why is radiation being in cells bad?
Once in cells can damage DNA.
What does high radiation do to cells?
Kills them completely.
What does low radiation do to cells?
Damages DNA, causes mutations and can lead to cancer.
What medical applications do we use radiation for?
See how fluids move around the body, radiotherapy.
How do we use radiation to see how fluids move around the body?
Give patient a small amount of radiation. detect the radiation as it’s being pushed around the body.
How do we use radiotherapy to treat cancer patients? Is this easy?
Using radiation to kill cancer cells, not easy, can’t kill too many normal cells as well.
What non medical uses are there for radiation (not power either)?
Detecting leaks in underground pipes, smoke alarms.
Define irridation.
Objects near a radioactive source get exposed to radiation (they do not become radioactive).
Define contamination.
When you get a radioactive atom on you (bad), atoms decay and cause you harm.
How can we reduce exposure to radiation?
Using lead shielding, remote controlled robot arms etc.
Define radiation dose, what is this measured in?
Amount of radiation you’ve been exposed to, sieverts Sv.
Where does background radiation come from?
Radioactive rocks in the earth
Space, radiation from sun/ cosmic rays
Man made, nuclear power stations, waste, accidents
What does background radiation depend on?
Location and occupation.
How does location effect background radiation?
Granite rocks, give off radioactive radon gas.
High altitude, less protection from cosmic rays by the atmosphere.
How does occupation effect background radiation?
Nuclear power plant workers
Radiographers
What is the power output of a blackbody?
Amount of energy it emits per second, P = E/t.
I = ? using terms. Define all terms.
P/A.
I = Intensity, W/m^2 P = power, W t = time, s
I = ? using a constant. Define all terms.
I = k/r^2
I = Intensity, W/m^2 k = constant r = distance from source, m
What is the law connecting intensity and distance called?
Inverse square law.
Is intensity and distance an inverse square law in actuality? Why or why not?
Not really, only gamma looks like this. Alpha and beta are absorbed easily so radiation drops off more quickly.
Define activity, give the units.
Number of decays per second. Becquerels, Bq or counts per minute, cpm.
How do we measure decay?
In half life.
Define half life.
Time it takes for amount of radioactive stuff to go down by half.
Time for activity to go down by half.
N(t) = ? Define all terms and give all units.
N(t) = No e^-λt
N(t) = mass after time, kg No = starting mass, kg λ = decay constant, s^-1 t = time, s
N = ? using moles. Define all terms and give units.
N = n NA
N = number of atoms n= number of moles NA = Avagadro's constant
Give activity = ? as rate of change.
- rate of change of unstable nuclei
- A = -dN/dt
A = ? using N. Define all terms and give units.
A = λN
A = activity, Bq λ = decay constant, s^-1 N = number of atoms
A = ? using Ao. Define all terms and give units.
A = Ao e^-λt
A = activity, Bq Ao = activity we started with, Bq λ = decay constant, s^-1 t = time, s
T1/2 = ?. Define all terms and give units.
ln(2)/λ = T1/2
T1/2 = half life, s λ = decay constant, s^-1
When plotting ln(N) over t what is the gradient?
λ
How does carbon dating work?
Living things add C from food
Some is C-14 (radioactive, therefore decays)
Thing dies (aah) and stops adding C
Amount of C goes down by 1/2 every 1/2 life (5700 years)
Look at how much C-14 is left (by percentage)
Work backwards to find age
What is the limit to carbon dating, why?
Around 50,000 years, not enough C-14 left to get an accurate age.
What does E=mc^2 show about the relationship between mass and energy?
They are interchangeable.
When protons and neutrons are together in a nucleus is their combined mass slightly more, less or the same as the sum of their masses? What is this called?
Slightly less, mass defect.
How can we explain mass defect?
Lost mass released as energy when nucleons bind together.
What do we measure mass defect in terms of?
u.
Define binding energy.
The energy needed to separate all the protons and neutrons.
b = ? = ?. Define all terms and give units.
b = B/A = Δmc^2/A
b = average binding energy per nucleon, J B = binding energy, J A = nucleon number Δm = mass defect, kg c = speed of light, ms^-1
What element is at the peak of the binding energy per nucleon curve? What does this mean it is?
Iron, most stable element, most binding energy per nucleon.
Below iron on the B per nucleon curve what do we have to do to get energy? What is this called?
Adding extra nucleons. fusion.
Above iron on the B per nucleon curve what do we have to do to get energy? What is this called?
Taking away nucleons/splitting apart nuclei, fission.
Define fission.
An atom splits into two to become more stable.
Define fusion.
Two atoms join together to become more stable.
What does a heavy unstable nucleus split into?
Two light, stable nuclei, energy and fast moving neutrons.
Because fission releases neutrons we can get what? Why?
A chain reaction. Neutron hits another nuclei (eg. Uranium) and becomes a heavy unstable nuclei, decays.
What is conserved in fission?
Mass number, atomic number.
How do we use controlled and uncontrolled fission chain reactions?
Controlled = nuclear power station Uncontrolled = atomic bomb
How do nuclear power stations control chain reactions?
By slowing it down, by absorbing neutrons.
What is the function of a control rod? What are they made of?
Made of neutron poisons, they absorb neutrons.
Do control rods slow neutrons down?
No, average KE stays the same.
What slows down a chain reaction in a thermal neutron reactor? How?
The moderator, slows down fast neutrons and make fission more likely to happen. Slows down by neutrons losing KE in elastic collisions with the molecules in the moderator.
What speed are neutrons in a thermal reactor slowed down to, what do we call these?
Until they are in thermal equilibrium with the moderator. have the same average KE as surrounding particles. Thermal neutrons.
How does the mass of the moderator affect the slowing of the neutrons?
Closer in mass to neutrons more they are slowed. Want moderator to be almost the mass of a neutron.
Why do fission reactors need shielding? What do we use as shielding?
To protect from alpha, beta, gamma and neutron radiation. Concrete or water.
What happens to the shielding after time? Why?
Becomes radioactive itself, it absorbs neutrons and becomes unstable and decays.
What are the two types of radioactive waste?
Front end and back end waste.
Where does front end waste come from, what does it emit?
From the fuel, unused uranium. Emits alpha radiation.
Where does back end waste come from, what does it emit?
From spent fuel rods. Emits beta and gamma radiation.
What type of radioactive waste is more dangerous?
Back end waste.
What do we do when waste is removed?
Place in water to cool (1 year in a ‘spent fuel pool’), then stored in sealed containers (‘dry cask’), then put in steel cylinders surrounded by inert gas, each cylinder surrounded by concrete for radiation shielding.
Where does fusion happen?
Stars and thermonuclear bombs.
What is conserved in fusion?
Atomic and neutron numbers.
Is the physical mass of a heavy nucleus the same as the sum of the physical masses of the lighter nuclei? Why?
No, it’s slightly less. Lost mass goes into energy released.
What conditions do we need for fusion?
Very high temperatures.
What are the benefits of fusion?
Almost 0 radioactive waste.
