Nuclear Physics Flashcards
Describe Rutherford’s scattering experiment
- plum pudding model
- he fired alpha particles at a thin gold foil and most passed through with a only a few deflecting as they pass through the gold foil
- He observed :
*that the atom has mostly empty space - the nucleus must have a large positive charge
- the nucleus must be tiny
- most of the mass is in the nucleus
state the strength of ionisation and the range (penetration power), effect in magnetic and electric field for
a) alpha radiation
b) beta radiation
c) gamma radiation
How to detect radiation
- use a geiger counter
- As radiation enters the Geiger-Müller tube and passes through the inert gas it leaves behind a trail of ions. These ions create a charged path between the metal rod in the centre of the detector and the metal casing. This briefly allows current to flow and the circuit to be complete. Each time the circuit is completed the counter registers a count on the screen.
problems with geiger tube
cannot identify what the particles are
in areas of high radiation, cannot distinguish between successive pulses
How to measure count rate
measure background radiation and subtract it from its measurements
How do you stop alpha, beta and gamma particles
alpha : stopped by a few cm of air or a few mm or paper
beta : stopped by few mm of aluminium
gamma : never stoped completely, intensity can be reduced by a few cm of lead or couple of metres of concrete
How can we use geiger counter to determine the radiation
try placing paper between source and geiger counter. If the count does not change then try a few mm of aluminium. If count does not change you have gamma radiation.
Which out of alpha beta and gamma is the most dangerous
alpha most dangerous, gamma least dangerous
alpha difficult to get out of body unlike beta and gamma
Discuss how a beta source may be used to control the thickness of sheet metal or paper
count rate is directly proportional to thickness of paper
use count rate to work out thickness of paper
Discuss how smoke alarms may use an alpha source such as Americium 241 to detect smoke.
- alpha particles ionise the air particles
- positive particles from the air attracted to the negative plate, and negative particles to the positive plate
- a current flows between
- the smoke absorbs the alpha radiation and prevents the air being ionised
- so current is much smaller triggering an alarm
Define activity and count rate
activity : the actual number of nuclei in a source that decay per unit of time.
count rate : Count rate is the number of ionisation pulses recorded per unit time by a detector. This is usually a small fraction of the overall activity.
If you are given radius, how do you calculate count rate
like inverse-square law, count rate is inversely proportional to radius squared so count rate = constant / radius ^2
What is the effect on radiation on the human body
could cause cells to die or become defective where defective cells divide and produce more defective cells causing cancer. If large number of cells die, body may not be able to reproduce them quickly enough
What to do and not do when handling radioactive sources
What are medical tracers used for
a gamma emitter is first injected into the body
organs absorbs radionuclides and emit gamma radiation
gamma sources use das they freely pass out of the body
these sources are used because they have a short half life (exposure to patient decreases rapidly)
what is radiotherapy used for
- gamma radiation can kill cancer cells
- they’re contaminated so they can focus on specifically on the cancerous region
Describe why the line of stability trends the way it does
As the number of protons increases the number of neutrons needed to keep the nucleus stable also increases, but at a slightly greater rate. This is because the larger the number of protons in the atom the greater the electrostatic repulsion. Neutrons act to separate the protons and reduce the repulsion
Beta plus decay is one way to get a proton to turn into a neutron, state another
Electron capture
- a proton captures an electron from the electron cloud and turns it into a neutron
Prove that the density of a nucleus always remains constant
Describe Rutherfords alpha scattering experiment to find the diammeter of a nucleus
alpha particles are repelled by electrostatic repulsion as both the nucleus and alpha particle are both positively charged
force of replulsion found using equation F = Qq / 4Pi Eo (r^2)
use the fact that the distance of closest approach is when all kinetic energy of alpha particle is converted into potential energy
Describe electron diffraction experiment to find the diameter of the nucleus
Describe the accuracy of the alpha scattering to find the diammeter of a nucleus
- In the alpha scattering experiment the calculations only produce the distance of closest approach of the alpha’s not the diameter
- The experiment can’t detect alphas scattered by 180o
- The alpha’s have their own size which must be taken into account
Describe accuracy of electron diffraction experiment to find the diammeter of a nucleus
- Electrons need high speed because they need a wavelength similar to the diameter of the nucleus
- Both experiments need monoenergetic beams and need a thin sample of target material.
Advantages and disadvantages of alpha scattering and electron diffraction
Alpha scattering
* Results upset by nuclear recoil
* Results upset by the strong force as alphas contain hadrons
* The alphas are only affected by protons not neutrons
Electron diffraction
* Not affected by the strong force since they are leptons
* The 1st minimum in the scattered intensity can be difficult to detect
Why do isotopes in nuclear reactors need to have a long half-life
if half life was too short they would need to be replaced regularly slowing down productions and costing money to replace
Describe carbon dating
- used to determine the age of living materials such as plants, animals etc.
- All living things contain a proportion of carbon-14
- The activity of a living organism remains constant during their lifetime as the carbon-14 that decays is replaced.
- Once the organism dies the carbon-14 is no longer replaced. This means the activity of the sample begins to reduce
- By comparing the activity of a dead organism with that of a living organism you can deduce how long ago the material died.
Define binding energy (comes in two ways)
- energy released when nucleons come together to form the nucleus
- It is the energy needed to split an atom into its constituent parts
Define term atomic mass unit (U)
1 Atomic mass unit is defined as the mass of 1/12 of a carbon-12 atom.
How do you compare the stability of different nuclei
- compare the binding energy per nucleon
- the greater the binding energy per nucleon, the more stable it is
How does a nucleus become stable
Just like electrons, a nucleus has excited states which are unstable. By emitting one or more gamma photons a nucleus can lose energy and return to its ground state. This often occurs with a daughter nucleus after an alpha or beta decay
State use of technetium generator
- produces a source which emits gamma radiation.
- technetium used in medial diagnosis applications
- radioactive isotopes like technetium form in an excited state
- Nuclei of the technetium isotope has a half life of 6 hours and decays t the ground state by gamma emission
What is a metastable state, use technetium isotope as an example
Technetium isotope form in an excited state after an alpha or beta emission stay in the excited state long enough to be separated from the parent isotope (molybdenum isotope) which has a half life of 67 hours
What happens when technetium isotope is in the ground state
has very long half life and it forms a stable product. As molybdenum is not present, technetium only emits gamma photons.
What is nuclear fission
- when a heavy nucleus splits into two lighter and more stable nuclei with the release of energy and neutrons
What is induced fission
When an atom is bombarded by neutrons
e.g U-235 is bombarded by neutrons to make U-236
I-236 is unstable and wild undergo fission
Describe something that uses energy fission
Alpha particles have the highest ionising power per, beta and gamma have lower
Ionising power produced per mm is directly related to ionising power
Which has higher range between alpha radiation, beta radiation and gamma radiation and what steps them
Alpha lowest, beta, gamma has highest
Alpha : stopped by Paper or few cm of air
Beta : stopped by aluminum
Gamma : stopped by thick lead or concrete
Radiation is spread out evenly over large surface area
Inverse square law applies as count rate is proportional to 1/r^2
Dust May be breathed into body
Causing molecules to be made cancerous, damaged by ionization
- cosmic rays
- sun
- radon gas
- nuclear waste
Do part ii) using answer from part I)
Count rate when source was 0.18m away from source was 0.62 s^-1. Calculate count rate when detector is 0.10m away from the source
Why does gamma radiation obey an inverse square law but alpha and beta radiation cannot
Gamma radiation is not absorbed
Spreads uniformly from a point
Area over which it spreads is proportional to radius squared
Alpha and beta are absorbed as they spread out
A represents nucleon number in equation in formula sheet
Instead of naming materials just describe a random nucleus
A heavy nucleus undergoes nuclear fission as it splits into two smaller nuclei and a neutron. These two could continue by also splitting into two smaller nuclei and producing a neutron
Purpose of moderator in nuclear reactor
Moderator slows down neutrons and hence they have less kinetic energy
So the neutrons are then absorbed by a nucleus
Neutrons are absorbed and they collide with nuclei in the shielding
Converting the atoms of the shielding into unstable isotopes
What is meant by a thermal neutron
- slow moving neutrons / low kinetic energy neutrons
- are in thermal equilibrium with the moderator
Define binding energy of nucleus
Amount of energy required to seperate a nucleus into its seperate neutrons and protons
OR
Energy released on formation of nucleus from its seperate protons or neutrons
Discuss changes made inside a nuclear reactor to reduce it’s power output and explain the process involved
Insert control rods further into the nuclear reactor
Which will absorb more neutrons reducing further fission reactions
Main source of highly radioactive waste from a nuclear reactor
Fission fragments
I don’t know what happened here but potential energy radius is not squared
Suggest why method of carbon dating is likely to be unreliable if a sample is less than 200 years old
Difficult too measure accurately a small drop in count rate
The activity would be very small as there is little decay
What equation links power and activity
Power = energy (per decay) x activity
B is correct
C is not correct as there could be more neutrons
D not correct as neutrons (especially fast neutrons) could get lost
D
B is correct answer
A is true because control rods must reduce number of neutrons available for sustaining the chain reaction
C is true as this is critical for maintaining consistent neutron absorption
D is true as length is adjusted to control the rate of nuclear reaction
B is not true because moderators slow down neutrons, not control rods.
A
D - neutrons
B
Straight one or deflection of zero degrees
Most of the mass is concentrated in th nucleus
The nucleus contains a positive charge
I) electrostatic
ii) force should be going away from nucleus and be shortest distance
iii) new path should show less deflection
Why is it necessary to remove air from chamber in a Rutherford scattering experiment
So air molecules do not absorbs the alpha particles
Each decaying nucleus of Po is 11 Joules. State the form this energy initially appears
Kinetic energy
In E=mc^2 what does m represent
The DIFFERENCE in mass if theres like loads of nuclides involved. If it’s asking you for the energy is something is travelling in space then fair enough just use the mass
What is meant by random nature of radioactive decay
Which atom decays at which time is just chance
What is the use of the coolant
it transfers heat from fuel rods to the pipes containing water that passes into the reactor vessel.
It passes through heat exchangers to maximise the efficiency of heat transfer to the water in order to make sure it boils and is turned into steam before it reaches the turbines.
A suitable material for a coolant is carbon dioxide gas.
What is the use of control rods
On average 2.4 neutrons are released in each fission reaction. Therefore to maintain a steady rate of reaction 1.4 neutrons per fission need to be absorbed. The control rods are made of a material that absorbs neutrons.
- putting control rods further into reactor will decrease rate of reaction
- this is controlled by a computer to ensure that the rate remains at a constant level
- When one fission creates one further fission this is known as critical mass.
- In an emergency the control rods are inserted all the way in to stop the rate of reaction. A material suitable for control rods has to be capable of absorbing neutrons. Often Boron is used for this.
What are moderators used for
- Neutrons released from a fission reaction have large energy and travel too fast to be absorbed by another uranium nucleus.
Each fuel rod is inserted into a material that is reduces the energy of the neutron without stopping them - This is done by having a material where the neutrons can pass through it whilst undergoing collisions with the nuclei in the moderator
- each collision reduces energy of neutrons
What are thermal neutrons
Neutrons that have been slowed to a sufficient speeds to be absorbed by a uranium nucleus
How does a moderator excite and de-excite
- first few collisions between neutrons and the moderator transfer a large amount of energy to the nuclei in the moderator. -moderator nuclei excites into higher energy levels. This position is unstable so they fall back down again emitting photons of high energy electromagnetic radiation. This means the moderator can release gamma rays
What safety features does a nuclear reactor need to have
- The reactor core is a thick steel vessel designed to withstand the high pressure and temperature in the core. The thick steel vessel absorbs ß radiation and some of the γ radiation and neutrons from the core.
- The core is in a building with very thick concrete walls which absorb the neutrons and γ radiation that escape from the reactor vessel.
- Every reactor has an emergency shut-down system designed to insert the control rods fully into the core to stop fission completely.
- The sealed fuel rods are inserted and removed from the reactor by means of remote handling devices. The rods are much more radioactive after removal than before. This is because the fuel cans
a. before use contain U-235 and U -238 which emit only α radiation and this is absorbed by the fuel cans,
b. after use emit ß and γ radiation due to the many neutron-rich fission products that form.
Define nuclear fusion
two light nuclei combining to form one heavier nucleus with the release of energy.
How do you make nuclear fusion work when getting two nuclei to react together even though they are both positively charged
- we need nuclei close enough so the strong nuclear force overcomes the electrostatic repulsion
- we need to input some energy to get the nuclei close enough
How does fusion work practically
- two nuclei need to be in 1fm of range within each other so strong nuclear force holds nuclei together
- to get nuclei on their own, fuse needs to be ionised forming a plasma
- All nuclei therefore have a positive charge and will repel
- high kinetic energy needed to overcome this repulsion, so plasma needs high temperaure and high pressure
Why is nuclear fusion harder in manmade environment and where is it easy
- more energy is put into the system to achieve the high temperatures and pressures than is gained from fusion reactions
- much easier to achieve in centre of stars due to high temperataure and high pressure caused by large mass of star
What is the most stable element
Iron most stable
Why are high energy electrons used in determining nuclear size
The de Broglie wavelength of high energy electrons can be compared to radius of nucleus
What is meant by induced fission
A neutron strikes a nucleus
Nucleus splits into two fragments
Why method other than alpha particle scattering are used to determine nuclear radii
How can an atom release an X-ray photon when it undergoes electron capture
What type of radiation could be used to test for cracks in metal pipes
Gamma - they are highly penetrating and can pass through thick materials like metal
Alpha can’t as it is stopped by Paper or even a few cm of air
Beta can’t penetrate a few mm of metal, but not enough for thick or dense materials like industrial pipes
Briefly explain what is meant by nuclear fission and by nuclear fusion.
fission when nuclei splits into two nuclei
fusion when two nuclei join to form one nucleus
What is meant by induced fission
large nuclei splits into two nuclei brought about bombardment or collision
What makes one nucleus more stable than the other
Nucleus that is more stable has higher binding energy per nucleon
Explain what is involved in the process of moderation.
neutrons from fission are fast (high energy) neutrons
fission most favourable with low energy neutrons
moderation involves slowing down neutrons by collision with moderator atoms
large number of collisions required
collisions are elastic/k.e. transferred to atoms
suitable moderator material named e.g. graphite, water
moderator must not absorb neutrons
moderator atoms should have (relatively) low mass !
Describe how the rate of fission is controlled in a nuclear reactor
control involves limiting number of neutrons
excess neutrons absorbed by control rods
suitable control rod material named e.g. boron, cadmium
control rods inserted into reactor to slow reaction rate
What materials are commonly used for moderators, control rods and shielding
moderators : graphite
control rods : boron
shielding : concrete
Explain why nuclei in a star have to be at a high temperature for fusion to take place.
Nuclei need to be close together for fusion to take place
but electrostatic/ electromagnetic force is repulsive
If temperature is high then the nuclei have high kinetic energy/speed to overcome the repulsion
Describe the changes made inside a nuclear reactor to reduce its power output and
explain the process involved.
Insert control rods into reactor
Will absorb more neutrons, reducing further fission reactions
State the main source of the highly radioactive waste from a nuclear reactor
Fission fragments / daughter products
neutrons are released with high energies.
Describe and explain the nature of the radiation that may be emitted from an excited
nucleus of the moderator.
gamma radiation
As energy gaps are large in a nucleus
The subsequent collisions of a neutron with the moderator are elastic.
Describe what happens to the neutrons as a result of these subsequent collisions with the
moderator.
Kinetic energy is transferred to the moderator
A neutrons slows down / loses its kinetic energy
eventually reaching speeds associated with thermal random motion or reaches speed which can cause fission
A nuclear reactor core is contained in a steel vessel surrounded by concrete.
State and explain the purpose of the concrete other than its structural function.
It forms a shield to reduce the intensity of radiation
from/ for protection from
neutron (and gamma) radiation
