radioactivity unit 5 Flashcards
Why does rutherford scattering tell us that the nucleus is small and positive in charge?
Most of the alpha particles go straight through, so most miss.
Must be positive to repel the positive alpha particles.
How do you find the closest point an alpha particle can get to a nucleus?
It is the point where the electric potential of the particle is equal to the original KE.
This is the upper limit of the diameter of nucleus
Why is electron diffraction the most accurate way to find out about a crystals atomic structure?
They are leptons meaning that they don’t interact with the strong nuclear force that would interferer with them.
How can electron diffraction get used to examine a crystal.
An electron beam is shone through and this will diffract through the thin layer of crystal. This will produce a diffraction pattern which is like several rings inside each other.
How can the first minimum of a electron diffraction pattern get used to find the nuclear diameter?
The first minimum is the first and smallest ring. This will be produced by electrons that are diffracted by a certain angle from the line of no diffraction.
Use sinϴ=(1.22λ)/d and the equation for de broglie wavelength to find λ to then solve for d which is the nuclear diameter.
What is the typical diameter of the atom?
1 X 10^-10m
What is the typical diameter of the nucleus?
2 X 10^-15m
What relationship does the nuclear radius increase by?
By the cube root the the nucleon number
what is r0 in the equation for nuclear radius?
the value of r when A is 1. it is 1.4 fm. (1.4 X 10^-15m)
Find the radius of a nucleus with 16 nucleons
1.4 X 10^-15 X 16^(1/3)= 3.5 X 10^-15 m
Why is nuclear density much the same for nucleses?
Protons and neutrons have similar mass and volume.
What, approximately, is nuclear density?
1.46 X 10^17 Kg m^-3
Nuclear density is significantly larger than atomic density. What does this suggest?
Most of the mass of an atom is in the nucleus.
The nucleus is small compared to the atom.
An atom must contain a lot of empty space.
What can cause a nucleus to be unstable?
Having too many or too few neutrons and having too much energy.
What is a gamma particle?
Short wavelength, high frequency, electromagnetic wave.
What is a beta plus particle?
A positron.
Why is radiative emissions also known as ionising radiation?
A radioactive particle can hit an atom and remove an electron, creating an ion.
How fast is the alpha particle
slow
How fast is the beta particle
fast
what is the alpha particle stopped by?
a few cm of air or paper.
what is the beta particle stopped by?
5 mm of aluminium
What is the beta plus particle stopped by?
It annihilates with an electron so virtually zero range.
What is the gamma particle stopped by?
Many cm of lead or several m of concrete.
Why is the alpha particle highly ionising ?
They are strongly positive.
Why does the alpha particle have a small range?
They are highly ionising and each time they ionise something they give some energy to the ion. They quickly lose their energy and so stop.
Why are alpha particles used in smoke alarms?
They allow current to flow and have a small range.
Why is gamma radiation used in medicine?
It does the less damage to the body.
What is intensity?
radiation per unit area.
what is the unit for radioactivity?
the becquerel. One count per second.
what are the sources of background radiation?
The air. It contains radon gas that comes from rocks and emits alpha particles.
The ground.
Cosmic radiation. It collides with the atmosphere, making radiation.
Living things as they all contain carbon, meaning they all contain carbon 14.
Man made radiation from industry or hospitals.
what are the units of the decay constant?
s^-1
what is the decay constant a measure of?
The probability that an individual nucleus will decay in one second.
In the activity equation, what does the N mean?
The no. of atoms.
What is the half life?
The average time it takes for the number of undecayed atoms is halve.
How do you find the decay constant?
ln2 / half life.
How does carbon dating work?
Living things take in carbon meaning they take in carbon 14. When they die, the amount of carbon 14 starts to fall as it decays with a half life of 5730 years.
What are the qualities needed for a medical tracer?
Needs to have a half life long enough to detect it but short enough to limit the radiation exposer)
Emits gamma radiation.
Decays to a more stable isotope.
When does alpha emission happen?
In atoms that are too heavy (82 + protons)
When does beta emission happen?
Too many neutrons.
What happens in beta plus emission?
neutron becomes a proton, and electron and electron anti neutrino are emitted.
What is the symbol of the electron anti neutrino?
a curly v with a bar onto next to an e
What happens in beta plus emission?
A proton becomes a neutron, and and electron neutrino emitted.
How is gamma radiation produced?
An atom may have excess energy after beta or alpha decay which it will lose with gamma emission. The nucleus could also capture one of its own orbiting electrons. A proton and electron become a neutron and electron neutrino which will make it unstable and cause gamma emission.
In every nuclear reaction, what is conserved?
energy, momentum, charge, proton number and nucleon number.
What is mass defect?
The difference between the total mass of a particle and the sum of the masses of the constituent parts.
The mass gets lossed as energy emitted when they bond.
If a nucleus was pulled completely apart, what amount of energy would you need?
The same as the energy released when the particles bonded together.
What is the binding energy?
The energy needed to separate all of the nucleons in a nucleus.
How large is the Mev?
1.6 X 10^-13 joules.
How do you turn joules to Mev?
Divide by 1.6 X 10^-13
What does a high binding energy per nucleon mean?
More energy is needed to remove a nucleon.
Where is the peak for a graph of binding energy per nucleon against nucleon number?
Nucleon number of 56 and binding energy of 8.3 MeV.
Why does nuclear fusion release energy?
Combining small nuclei increases the binding energy per nucleon meaning energy is released.
In terms of binding energy per nucleon, how does nuclear fission give energy?
A large nucleus is split in two, meaning an increase in the binding energy per nucleon so energy is given out.
What is the least mass of a nucleus that can split apart?
- (uranium)
What is a thermal neutron?
A low energy neutron that can be captured by a nucleus causing it to split up. Only a slow moving one can be captured in this way.
How much energy is in one atomic mass?
931.3 MEv
What is critical mass?
The smallest amount of fissile material needed to sustain a nuclear reaction.
How is nuclear fission like a chain reaction?
Because when it happens, neutrons are released and these can take induce more reactions to happen.
What is the purpose of a moderator?
To slow down the neutrons so that they can be captured by other nucleolus.
Choosing a moderator that will absorb more as temperature increases will reduce the chance of meltdown.
What is a super critical mass?
One where each reaction will induce several more reactions. Super critical mass is used in nuclear reactors and control rods are used to control it.
What are control rods?
Rods that absorb neutrons (boron) and can be inserted by varying degrees to affect the number of reactions. In an emergency they are inserted all the way to stop the reaction.
What does the coolant in a nuclear reactor do?
It absorbs the heat from a nuclear reaction meaning it can be latter turned into steam to give energy.
Why is a nuclear reactor in a concrete case?
This acts as shielding and prevents radiation from escaping and killing the local populous.
Why are the waste products of a nuclear reactor unstable
They usually have a larger proportion of neutrons than nuclei of a similar atomic number, making them unstable.
what is one of the practical uses of radioactive waste?
Used as tracers in medical diagnosis.
What is done with radioactive waste?
They have to be handled with care(remote control, impact proof flasks). When removed from a reactor, they are originally very hot and they are put in cooling ponds until the temperture falls.(several months)
It is them stored underground in sealed containers until the activity has fallen.
uranium removed from active waste.
vitrification
Why is nuclear fusion hard to achieve?
All nuclei are positively charged meaning that the electrostatic force will repel them and they need to get close enough so that the strong nuclear force will pull them together. They need about 1 Mev of KE to achieve this which is a lot.
How do you investigate the inverse square law for gamma radiation.
Record the background radiation for a given time.
Place a sample directly in front of the GM tube and measure the count, taking away the background. Use a meter ruler to increase the distance and measure it.
Why in the Rutherford experiment, is the gold foil thin?
to prevent repeated scattering in the foil.
To let them get through
why is the Rutherford experiment done in a vacuum?
To prevent the particles colliding with and interacting with other particles.
What equation gives the relationship between intensity and distance and a different distance?
I₂/I₁ = r₁²/r₂²
Why in the Rutherford experiment, is the width of the beam of alpha particles thin?
So to define the precise location where scattering takes place.
What observation from the Rutherford experiment suggests that most of the mass of an atom is concentrated in one area?
Some particles are deflected by more than 90 degrees and it must have gone into something with a mass a lot bigger than its own.
What is activity equal to?
no. of counts / time
decay constant X no. of radioactive atoms.
what are the disadvantages of using alpha particles to find nuclear radi?
The strong nuclear force messes stuff up.
scattering caused by the distribution of protons, not the whole nucleon distribution.
alpha particles are massive and cause recoil which messes stuff up.
what is an equation that uses the number of half lives to find the amount of mass of a radioactive sample.
remaining mass=(0.5)^n*mₒ
mₒ was the starting mass.
n is the number of half lives. This equation also works for activity and concentration.
What is the atomic mass unit?
1/12 of the mass of a carbon 12 atom.
Where does the binding energy come from?
The strong nuclear force.
Why when measuring the activity of an element in a source might the actual activity of the source be greater?
The radioactive element might decay into other radioactive things.
The detector might not detect all the decays.
why do beta particles have a greater range than alpha particles?
They have no mass while the alpha has some. They will have fewer collisions and lose less energy.
If you had a sample of uranium, what would it contain?
235 and 238 isotopes of uranium. Only 235 takes part in fission
when a uranium nucleus absorbs an neutron, how many are later given out?
2 or 3
What does shielding around the core stop?
neutrons and gamma radiation.
how are artificial isotopes produced?
Nuclei are bombarded with neutrons and are absorbed.
What is induced fission?
A high energy neutron is fired into a nucleus, causing it it split into two different parts.
why does fission in uranium fuel rods cause them to get very hot?
fission fragments and neutrons collide with each other and other things and transfer ke. They vibrate more and this causes heat.
what is enriched uranium?
It has a higher proportion of 235 than natural uranium
Why is enriched uranium in nuclear reactors?
induced fission is more likely with 235 than 238
why is not all the uranium put in one fuel rod in a reactor?
Neutrons need to pass through a moderator so to react.
Neutrons will leave the fuel rod and not react.
Makes it harder to replace the fuel in stages.
why is it that after time, fuel rods become less efficient at making power?
The remaining fission fragments absorb neutrons and prevent more fission.
why is it that after time fuel rods become more dangerous?
They will contain unstable fragments that emit beta and gamma radiation. They have a short half life and will damage people
What is looked for in a material to act as shielding around a reactor?
Density, stand high temperatures, minimum fatigue problems after irigation
Why does reactor shielding become radioactive after time?
The nuclei will absorb neutrons and take part in beta and gamma decay.
what can a very high energy neutron do to the nucleus of a moderator?
It will excite the nucleus and cause it to emit gamma radiation.
It will de-excite in discrete energy levels to the ground state.
how many collisions does a neutron need to make with a moderator before it can be absorbed?
around 50
what can happen in a neutron in a reactor, other than get absorbed by 235?
Absorbed by 238, scattered by 235 or 238, could leave the reactor
Why do spent fuel rods present a greater radiation hazard than normal uranium?
Normal uranium is an alpha emitter which is easily shielded and got out of the range of.
It turns to fragments which have a short half life, give ionising beta and gamma radiation and can cause cancer
what is induced fission?
a split of a nucleus caused by neutron bombardment
how much energy does a thermal neutron have?
0.03 ev
what is a self sustaining chain reaction?
fission reactions give out neutrons
neutrons cause further fission
self sustaining when one fission leads to at least one further fission
what is involved in the process of moderation?
There is a large no. of collisions
there are elastic, ke is given to the moderator atoms
they don’t absorb neutrons
they should have a relatively low mass.
Why might carbon dating be inaccurate ?
The item may have not been made immediately after the wood was killed.
Can’t be sure of carbon 12 to 14 ratio many years ago.
Contamination
Small count rates are susceptible to statistical variation.
Why will carbon dating be unreliable for samples less than 200 years or older than 600000 years?
Not enough carbon 14 will have decayed to measure the difference
Most of the carbon 14 will have decayed and it would be too small to measure.
What can a chain reaction only happen when the uranium has a certain mass?
The number of escaping neutrons ∝ surface area
As mass increases surface/volume ratio falls
So less neutrons escape.
Fraction producing fission increases as mass increases
What is the typical range of an alpha and beta particle?
3cm-7cm
20cm-300cm
