8. Nuclear Physics Flashcards
What did the alpha particle scattering experiment enable?
The calculation of the size of the nucleus
What was the set-up for the alpha scattering experiment?
- monoenergetic alpha particles were fired at a thin gold foil
- zinc sulphide screen flashed when alpha particles hit it
- vacuum
What was the screen in the scattering experiment made out of?
Zinc sulphide
What were the paths of the particles in the scattering experiment?
- most passed straight through
- some displayed a small deflection
- 1 in 10000 were deflected by angles > 90°
What did the results from the alpha scattering experiment show?
The atom must contain a small concentrated positive charge with mass
What charge do alpha particles in the scattering experiment have?
Positive
In nuclear physics, what can Coulomb’s law be used to calculate?
The distance between two particles when they have an electrostatic force
In the scattering experiment, at what point will an alpha particle scatter back?
When its kinetic energy equals its electric potential energy
What law can be used to find the distance between two charged particles?
Coulomb’s
What does 1u mean?
One atomic mass unit
Does the strong force only affect adjacent nucleons?
Yes
Approximately, how many times bigger is the diameter of a uranium atom than its nucleus?
23,000 x
Approximately, how many times bigger is the diameter of a hydrogen atom than its nucleus?
145,000 x
What does it mean, that radioactive decay is spontaneous?
The rate cannot be changed by heating/cooling, dissolving in acid etc.
What will NOT change the rate of radioactive decay?
- heating/cooling
- dissolving in acid
- applying pressure
- applying a magnetic or electric field
Is radioactive decay continuous?
No
What happens in alpha decay?
A nuclei decays into a new nuclei and emits an alpha particle
What happens in beta decay?
A nuclei decays into a new nuclei by changing a neutron into a proton and electron
What happens in gamma decay?
After alpha or beta decay, surplus energy is sometimes emitted
Is the atom changed when it emits gamma?
No
What are the properties of gamma radiation?
High frequency, short wavelength
What is the most ionising type of radiation?
Alpha
Why can alpha only travel a few cm in air?
It is highly ionising
Why do alpha particles from the same source all travel the same distance in air?
They have the same energy if they are from the source, so they travel the same distance before they have lost all their energy
Why do alpha particles ionise air?
To gain the electrons they need to become a helium atom
What can alpha radiation be blocked by?
A sheet of paper
What can beta radiation be blocked by?
A few mm of aluminium
What can gamma radiation be blocked by?
A few cm of lead
Why does each beta particle travel a different distance?
It has a range of energies
Why can gamma rays travel large distances?
They barely interact with air molecules
Why does gamma radiation intensity decrease?
They spread out
intensity ↓ as beam area ↑
What equation shows how the intensity of gamma rays varies with distance?
I = k / x2
Brief outline of an experiment to verify the 3 types of radioactive emission?
- measure activity of background radiation
- place geiger count within 2cm of source then measure count rate again
- deduct backgound count - does reading change when tube is moved to distance of 10cm?
- leave tube at this distance and place aluminium instead - count rate ↓ then beta
- repeat with lead sheet - count rate should drop to background count
What are some sources of background radiation?
- radon gas from ground
- human body and food
- rocks
- cosmic rays
- artificial sources (e.g. medical, nuclear power and weapons)
How should sources of radiation be stored?
In a lead box
What are some steps for safe handling of radioactive sources?
- use handling tool e.g. tongs
- use lowest activity source possible
- keep 2m away from others
What are alpha particles used in?
Smoke alarms
Why are alpha particles used in smoke alarms?
Allow current in air to flow, but don’t travel very far
How do smoke alarms work?
- alpha particles ionise many atoms and lose energy quickly
- allow current to flow
- when smoke present, alpha particles can’t reach detector and this sets alarm off
What is beta radiation used in?
Control thickness of sheets of material e.g. paper, Al foil or steel
What is gamma radiation used in?
- radioactive tracers - help diagnose patients without need for surgery
- treatment of cancerous tumours
What law does gamma follow?
Inverse square
What is the activity of a source?
The average number of undecayed nuclei which decay per second
If a source has one nucleus decay per second, what is its activity?
1 Bq
What is the unit for activity?
Bq = Becquerels
What is the symbol for activity?
A
What is the decay constant?
The probability of a given nucleus decaying in the next second
What is the symbol for the decay constant?
λ
What is the equation for activity?
A=λN
What does A stand for in A=λN?
Activity (Bq)
What does λ stand for in A=λN?
Decay constant (s-1)
What does N stand for in A=λN?
Number of undecayed nuclei
What is half life?
The time taken for half of the radioactive nuclei to decay into other nuclei
Graphically, what does radioactive decay look like?
An exponential curve
What are the radioactive decay equations?
N = N₀e^-λt
A = A₀e^-λt
What does N₀ mean in N = N₀e^-λt?
Initial number of radioactive nuclei present
What does N mean in N = N₀e^-λt?
Number of radioactive nuclei remaining at time t
What does A₀ mean in A = A₀e^-λt?
The initial activity of the sample
What does A mean in A = A₀e^-λt?
The activity at time t
What is Avogadro’s constant used for?
To calculate the number of atoms/nuclei that are present in a known mass of an element
What is the equation using Avogadro’s constant?
N = mNₐ / M
How is the equation for half life (T₁/₂ = ln2/λ)?
A = A₀e^-λt
at half life, A = A₀/2
A₀/2 = A₀e^-λt₁/₂
1/2 = e^-λt₁/₂
take natural logs: ln2 = λt₁/₂
How does carbon dating work?
- whilst living, plants take in CO2
- small fraction of carbon atoms is radioactive C-14
- ratio of C-14 to C-12 increases with time
- enables age of plant to be calculated
What does C-14 decay to in B- decay?
N-14, electron and an anti-neutrino
What does C-14 decay to in B+ decay?
N-16, positron and a neutrino
Equation for electron capture of C-14?
C-14 + e- → B-14 + neutrino
What counts as ‘light’ isotopes?
With proton number from 0-20
For light isotopes on the N-Z (neutron-proton) graph, what pattern do they follow?
Follow the straight line N=Z
What happens to stable nuclei as Z number increases beyond about 20?
They have more neutrons than protons
Why do larger nuclei have more neutrons than protons?
Extra neutrons help to bind nucleons together without introducing the repulsive electrostatic forces than protons would
What type of nuclei are often alpha emitters?
With proton number beyond about 60 (but most with > 80p and 120n)
Why are very large nuclei, with more neutrons than protons, often unstable?
Strong nuclear force between nucleons is unable to overcome the electrostatic force of repulsion between the protons
In a N-Z graph, where do B- emitters lie?
To the left of the stability belt
Why, on an N-Z graph, do B- emitters lie to the left of the stability belt?
These isotopes are neutron rich
How do the nuclei to the left of the stability belt on an N-Z graph become more stable?
Neutron rich, so they convert a neutron to a proton and electron
In a N-Z graph, where do B+ emitters lie?
To the right of the stability belt
Why, on an N-Z graph, do B+ emitters lie to the left of the stability belt?
These isotopes are proton rich
How do the nuclei to the right of the stability belt on an N-Z graph become more stable?
Proton rich, so they convert a proton to a neutron and positron
On an N-Z graph, which region does electron capture take place in?
To the right of the stability belt
On an N-Z graph, what does electron capture lie in the same region as?
B+ emission
On an N-Z graph, where does a nucleus that emits an alpha particle move to?
Moves diagonally downwards to the left
On an N-Z graph, where does a nucleus that emits a B+ particle move to?
Moves diagonally upwards, left
On an N-Z graph, where does a nucleus that captures an electron move to?
Moves diagonally upwards
On an N-Z graph, where does a nucleus that emits a B- particle move to?
Moves diagonally downwards, right
What is the technetium generator used for?
In hospitals, to produce a source which emits gamma radiation only
What is a metastable state?
A long-lived excited state in radioactive nuclei
In the technetium generator, how is Tc-99 formed?
- Tc-99 forms in an excited state after alpha/beta emission
- it stays in the excited state long enough to be separated from its parent isotope
- decays to ground state by gamma emission
What is the half life of Technetium-99?
6h
Is the Tc-99 used in technetium generators in the ground state?
No
What state is the Tc-99 used in technetium generators?
Metastable
What are the uses of Tc-99 in the metastable state?
Diagnosis;
- monitoring blood flow
- gamma camera - image internal organs and bones
When might an unstable nucleus emit gamma radiation?
When the ‘daughter’ nuclei is formed in an excited state after it emits an alpha or beta particle or undergoes electron capture
What is binding energy?
The energy required to separate an atom into its constituent parts
How can it be shown that Ca has binding energy?
- 20p, 20e, 20n
- total mass of protons, neutrons and electrons is 40.34u
- however Ca has a mass of 39.96u
- use E=mc² to calculate binding energy
How would you calculate the binding energy of an atom?
- add up masses of constituent parts
- take away mass on periodic table
- multiply mass difference by unified mass constant
- E=mc² in J
- change to MeV
What does a graph of binding energy per nucleon against nucleon number reveal?
The stability of the elements
On a graph of binding energy per nucleon against nucleon number, which are the most stable elements?
Those with a nucleon number around 56
Which type of elements release energy from fusion versus fission?
- smaller nucleon number - fusion
* high nucleon number - fission
Why do we know energy is released in fusion?
Binding energy per nucleon increases. Mass defect is greater. Energy has been released
Why do we know energy is released in fission?
As a heavy nucleus split binding energy of each fragment is greater. Mass defect is greater therefore energy has been released
What is fusion?
The process by which light nuclei join together forming heavier nuclei
Where does fusion happen?
In stars
What temperatures are required for fusion?
Above 8 million K
At 8 million K for fusion, how will positive nuclei be?
In a plasma, moving at very high speeds
When, in fusion, will nuclei fuse?
When they overcome the electrostatic repulsion
What happens, in fusion, after nuclei overcome the electrostatic repulsion?
The strong nuclear force holds them together
What is induced nuclear fission?
The process by which energy is released when a radioactive isotope is forced to split
What is used in induced nuclear fission and why?
Uranium 235 - long half life and abundance mean it is found in large quantities
How is nuclear fission undergone?
The radioactive nucleus absorbs a slow neutron, causing it to become unstable and split
Why is energy released in induced nuclear fission?
Due to change in mass
What does the chain reaction that is nuclear fission consist of?
- when a nucleus is split, more neutrons are released
- these can then split other uranium nuclei
- the process keeps going
In induced nuclear fission, why do neutrons need to be slowed down?
Or they will bounce off the (uranium) nuclei
In induced nuclear fission, what are neutrons slowed down using?
A moderator
In induced nuclear fission, why do extra neutrons need to be absorbed?
So the reaction stays at a constant rate
In induced nuclear fission, how are extra neutrons absorbed?
Using control rods
What is the critical mass of a fuel?
The minimum mass required to establish a self-sustaining chain reaction
What does the reactor core contain?
- fuel rods
- control rods
- coolant
What is the coolant in a nuclear reactor?
Water at high pressure
What is the reactor core connected to in a nuclear reactor?
A heat exchanger, via steel pipes
What is function of the control rods?
To absorb neutrons
What does the depth of the control rods control?
The number of neutrons in the core
What happens if the control rods are pushed in further?
They absorb more neutrons so that the number of fission events per second is reduced
What condition must be true, in a nuclear reactor, for a chain reaction to occur?
The mass of the fissile material (e.g. U-235) must be greater than a minimum mass (the critical mass)
Why does the mass of the fissile material need to be greater than the critical mass for a chain reaction to occur?
- some fission neutrons escape form the fissile material without causing fission
- if mass of fissile material < critical mass, too many fission neutrons escape as SA to mass ratio is too high
What are the safety features of nuclear reactors?
- reactor core is a thin steel vessel
- core is in a building with thick concrete walls
- every reactor has an emergency shut down system
- the sealed fuel rods are inserted and removed from the reactor by remote handling devices
How is the reactor core being a thick steel vessel a safety feature?
- to withstand high pressure and temperatures in the core
* absorbs beta emission and some gamma radiation and neutrons from the core
How is the reactor core being in a building with thick concrete walls a safety feature?
Absorb neutrons and gamma radiation that escape from the reactor vessel
How is every reactor having an emergency shut-down system a safety feature?
Control rods are inserted completely into the core to stop fission when needs be
How is radioactive waste categorised?
High, intermediate or low level, depending on its activity
Example of high level radioactive waste?
Spent fuel rods
How are spend fuel rods stored (high level waste)?
- stored underwater in cooling ponds for a year as they continue to release heat
- then stored in sealed containers in deep trenches in Sellafield
How is intermediate level radioactive waste stored?
Sealed in drums that are encased in concrete then stored in special buildings with walls of reinforced concrete
How is low level radioactive waste stored?
Sealed in metal drums and buried in large trenches
Examples of low level radioactive waste?
Lab equipment and protective clothing
What does 1 u equal in MeV?
931.5 MeV
