19: Using the Atom Flashcards
Describe the number of protons and neutrons in isotopes of the same element
Same number of protons, different number of neutrons
Same Z, different A
What is the Z number in standard notation?
The proton number or atomic number
What is A in standard notation?
The nucleon number or mass number
What forces are balanced inside a stable nucleus?
Strong nuclear force holding it together
And the electrostatic force pushing the protons apart
What happens if the nucleus is unstable, how does it transform?
It transforms into a more stable isotope by emitting radiation
What can cause the nucleus to be unstable?
Too many neutrons
Too few neutrons
Too many nucleons in total, it’s too massive
Too much energy in the nucleus
What are the four types of nuclear radiation with symbol?
consitiuents
charge
mass
Alpha, beta–minus (beta), beta–plus, gamma
What is an atomic mass unit, u?
Roughly the mass of a proton or neutron
What is alpha radiation made of?
Two protons and two neutrons
Helium nucleus
What is the relative charge of each type of nuclear radiation?
Alpha: +2
Beta–minus: -1
Beta–plus: +1
Gamma: 0
What is the mass, in atomic mass units, or each type of nuclear radiation?
Alpha: 4
Beta–minus: negligible
Beta–plus: negligible
Gamma: 0
What is beta–minus radiation made from?
An electron
What is beta-plus radiation made from?
A positron
What is gamma radiation made from?
Short–wavelength, high–frequency EM wave
Describe the ionising power, speed, and penetrating power of alpha radiation
when does alpha radiation occur
Strong ionising power
Slow speed
Absorbed by paper or a few cm of air
occurs when nuclei have too much mass
Describe the ionising power, speed, and penetrating power of beta-minus radiation
when does it occur
Weak ionising power
Fast speed
Absorbed by ~3 mm of aluminium
occurs when nuclei have too many neutrons
Describe the ionising power, speed, and penetration power of beta-plus radiation
when does this occur
Annihilated by electron – so virtually zero range
occurs when nuclei have too many protons or too few neutrons
Describe the ionising power, speed, and penetrating power of gamma radiation
when does this occur
Very weak ionising power
Speed of light
Absorbed by many cm of lead, or several m of concrete
when nuclei has too much energy
how does intensity of gamma radiation decrease with distance and so what law does it follow
intensity decreases with the square of the distance
according to the inverse square law
Which types of nuclear radiation are affected by magnetic fields?
Alpha and beta because they carry a charge
Gamma radiation isn’t affected by the magnetic field
What is the relationship between penetrating power and ionising power, of radiation? Why?
The penetrating power of radiation decreases with increasing ionising power.
This is because radiation loses energy as it ionises atoms. This means that the higher the ionising power of the type of radiation, the more energy it loses in a given distance, so the shorter its range
What safety procedures should be put in place when doing experiments with a radioactive sources?
Radioactive sources should be kept on a lead lined box when they’re not being used
They should only be picked up using long handled tongs or forceps
Take care not to point them at anyone
Describe the investigation for the penetration of different kinds of radiation in the lab
Measure the background count.
Calculate a count rate.
Subtract this from all your results.
Set up the equipment.
Insert different materials between the source and tube, and record the count rate over a sensible time interval.
How can you get an accurate reading for the background count rate of radiation when investigating kinds of penetration?
Radioactive decay is random, so to get an accurate reading the count needs to be measured over a long enough time interval
How do you calculate an accurate count rate, given the background count of radiation?
you have the background count rate when no source is present,
calculate the count rate when the source is present.
subtract background count rate from measured count rate to find the count rate from the source (the corrected count rate)
Describe the setup for the investigation of the penetration of different kinds of radiation
how to setup with alpha particles testing
Radioactive source positioned a distance away from an absorber which is a distance away from a Geiger-Müller tube which is connected to a Geiger counter
the geiger tube needs to be closer to the source for alpha radiation as alpha radiation is readily absorbed by the air
Describe the possible outcomes of the investigation of penetration of different kinds of radiation
If the count rate remains about the same, then the radiation can penetrate the material.
If the count rate drops by a large amount, then the radiation is being absorbed and blocked by the material.
If the count rate drops to 0, the radiation is being completely absorbed
How can you repeat this experiment with different sources?
Investigation of the penetration of different kinds of radiation
You’ll need to change the distance between the source and the Geiger-Müller tube for each source, as different kinds of radiation have different penetrating power in air
how does the thickness of a material reduce the intensity of gamma rays getting though the material
the intensity of the radiation of gamma ray photons getting through decreases exponentially with thickness
what is half-thickness, and what is it a result of
half thickness is the thickness of the absorbing material required to halve the number of gamma ray photons that get through the material
this arises due to the exponential decrease in gamma ray intensity with increasing thickness
intensity of gamma ray vs thickness equation
I = I0e-ux
x = thickness
I = intensity with material of thickness x present
I0 = intensity with no material of thickness x present
u = absorption coefficient (m-1)
thickness x equation
thickness x = ln2 / u
u = absorption coefficient
what does a cloud chamber do
a cloud chamber reveals the path of particles that pass through it, can be used to obcerve pair production and annihilation
what does a cloud chamber contain and what does it do
a cloudchamber contains supersaturated vapour
when a particle of ionising radiation passes through the vapour it produces ions
these ions act as centres on which the vapour condenses to form the vapour trail
Nuclear decay equations:
What is the parent nucleus?
The nucleus you start off with
Nuclear decay equations:
What is the daughter nucleus?
The nucleus it decays to
What must be conserved in the decay equations?
Charge, nucleon number and lepton number
Energy and momentum as well
How are beta-minus particles written in the decay equations?
They have a negative charge. They are written with a negative proton number
0-1B
What variable does not have to be conserved nuclear equations?
Mass
Is an alpha particle heavier or lighter than its constituents added together?
The mass of an alpha particle is less than the individual masses of two protons and two neutrons
What is the mass defect, in nuclear equations?
The difference in mass on the different sides of the equation
What accounts for the missing mass in nuclear equations?
The energy released when the nucleons bond together to form the alpha particle
When does alpha emission happen?
Only happens in very heavy atoms, like uranium and radium
The nuclei of these atoms are too massive to be stable
What happens to the proton number and the nucleon number when an alpha particle is emitted from a heavy atom?
The proton number decreases by two, and the nucleon number decreases by four
What is beta-minus decay? When does it occur?
The emission of an electron (lepton) from the nucleus along with an antineutrino(antilepton).
It happens in isotopes that are ‘neutron rich’,
One of the neutrons in the nucleus decays into a proton and ejects a beta particle and an antineutrino
What happens to the proton number and nucleon number when a beta-minus particle is emitted?
Proton number increases by one
Nucleon number stays the same
When does beta-plus emission occur?
In proton rich nuclei
A proton gets changed into a neutron, releasing a positron (antilepton) and a neutrino (lepton)
What happens to the proton number and nucleon number when a beta-plus particle is emitted?
Proton number decreases by one, and the nucleon number stays the same
Describe gamma radiation, when it occurs, what happens
happens in nuclei with too much energy.
This energy is lost by emitting a gamma ray.
This often happens after an alpha or beta decay has occurred.
What happens to the proton number and nucleon number when a gamma ray is emitted?
There is no change to the nuclear constituents – the nucleus just loses excess energy
What are some useful things, that radiation can be used for?
Generate power,
in medicine for diagnosis and treatment,
and to kill harmful microorganisms that might contaminate our food
What can harmful radioactive materials cause?
Cancerous tumours, skin burns, sterility, radiation sickness, hair loss and even death
The result of the advantages and disadvantages of radiation, is that it is only used when?
When the benefits outweigh the risks
What two parts are there to the risks of radiation?
How likely it is that the radiation will cause a problem, and how bad the problem would be if it happened
probability * consequence
Describe the risk for a nuclear reactor
A nuclear reactor melting down would be catastrophic but it’s also very unlikely, so the risk might be acceptable
Describe the risk of ionising radiation in medicine
Ionising radiation can cause cancer, but it can also be used in cancer treatments to destroy tumours.
The risk of serious damage caused by the treatment is considered acceptable if the treatment is likely to prolong or improve a patient’s life
What does ionising radiation do? Briefly
Knock electrons of atoms, creating ions
What happens to the cells in your body, when affected by ionising radiation?
how does changing energy of ionising radiation affect damage to your cells
It can damage cells, and cause them to mutate or even kill them
the more energy transferred from radiation to your cells, the more damage
What is the absorbed dose?Units?
assumption made
The amount of energy absorbrd per kilogram of tissue (J Kg-1)
absorbed dose = energy absorbed / mass of tissue
Measured in grays (Gy)
all energy of particles transferred to thyroid
how to find energy absorbed
number of particles * energy per particle
activity * time * energy per particle (* number if finding energy absorbed from multiple things eaten)
Why can radiation treat cancer? What needs to be minimised or maximised?
The ability radiation to damage or kill cells
Minimise the dose absorbed by healthy tissue at the same time as maximising the dose absorbed by the cancer cells
Describe how radiation can be used to treat cancer?
Place radioactive sources inside the patient, in or next to the tumour
Sources can either be inserted next to the tumour for a short period of time, or left within the patient permanently
Describe the dosage and half life of sources left in the body to treat cancer
Sources left within a patient permanently are chosen to deliver a low-dose per hour than sources inserted for a shorter duration, to minimise damage to healthy tissue.
They will also have a very short half life. This means they remain active for long enough for the cancer cells to be killed, typically a few months, without causing unnecessary damage to surrounding tissue
What does the amount of tissue damage caused by exposure to radiation depend on?
The amount of energy absorbed, the type of ionising radiation, and the type of body tissue
What does the effective dose measure allow you to do?
another phrase for it
Lets you compare the amount of damage to body tissues that have been exposed to different types of radiation
dose equivalent
Effective dose equation and units
effective dose = Absorbed dose * quality factor
effective dose = Sv sievert
equation for risk for a population of 100000 people
risk = effective dose (Sv) * incidence per sievert
multiply this % by number of people to see how many get it
Describe the relationship between a radiation’s quality factor and its ionisation
The higher the radiation’s quality factor, the more ionising it is
When beta or alpha particles ionise an atom, they transfer…
They transfer some of their energy to the atom being ionised
Why can alpha particles easily pull electrons off atoms? What does this mean for ionisation and damage to the body?
They are strongly positive
An alpha particle quickly ionises many atoms (~10,000) and loses all its energy – that’s why it causes so much damage to body tissue
Describe beta-minus ionisation and therefore the damage on the body compared to alpha particles
Has lower mass and charge than the alpha particle, but a higher speed.
This means it can still knock electrons off atoms. Each beta particle ionise about hundred atoms, losing energy each interaction.
This lower number of interactions means that beta radiation causes much less damage to body tissue then alpha radiation
Describe the intensity and ionisation of gamma radiation
Most weakly ionising form of nuclear radiation, but also the most penetrating.
Intensity of gamma radiation decreases with distance.
Although its quality factor is generally low, it is more difficult to shield yourself from.
What is the relationship between intensity and how far gamma radiation travels through an absorbing material?
When it travels through and absorbing material, its intensity, the amount of radiation per unit area, decreases exponentially
Why does the intensity of gamma radiation decrease as you get further away?
A gamma source will also emit gamma radiation in all directions.
However this radiation spreads out as it gets further away from the source, so the intensity will decrease.
The radiation spreads out over a larger area.
What is the half life of a source?
The time it takes for the number of radioactive nuclei in the source to halve. Or the amount of time it takes for the activity of the source to halve
What needs to be considered when deciding whether or not to use nuclear power, and deciding where nuclear waste should be stored?
Radioactive waste often has a very long half life
What are radioactive tracers?
Radioactive sources that are ingested or injected into patients. The radiation release as they move through the body can then be used to generate images that are useful for medical diagnostics
Describe the half life of a radioactive isotopes used as medical tracers
They need to have a short half life to minimise exposure of patients to radiation, and the chance of environmental contamination when the body excretes them
What is the mass defect?
The mass of the nucleus is less than the mass of its constituent parts – the difference is called the mass defect
What does Einstein’s equation mean?
Mass and energy are equivalent
What happens to mass and energy when nucleons join together?
The total mass decreases – this ‘lost’ mass is converted into energy and released
explain What is the binding energy?
If you pull the nucleus completely apart into it separate nucleons, the energy you’d have to use to do it would be the same as the energy released when the nucleus formed. This is called the binding energy. The binding energy is equivalent to the mass defect.
Are binding energy and mass negative or positive. Why?
They are both negative quantities. That’s because they correspond to mass, and thus energy, lost from the nucleus
What value is used to compare the binding energies of different nuclei?
Look at the binding energy per nucleon
binding energy per nucleon equation (in MeV)
binding energy per nucleon = binding energy (B) / number of nucleons (A)
Describes a graph of binding energy per nucleon against nucleon number
The curve is called a nuclear valley
The graph curve steeply down from the origin to a minimum at N = 50 then the curve slowly rises again.
how does energy required to remove a nucleon change as the binding energy per nucleon number gets more negative
the more negative the binding energy per nucleon, the more energy is needed to remove a nucleon
so its also more stable (the nucleus)
Where do you find the most stable nuclei occur on the binding energy per nucleon against nucleon number graph?
Around the minimum point (so the maximum binding energy per nucleon), which is it nucleon number 56 – iron.
Nuclei with the nucleon number close to 56 are bound most strongly
Nuclei undergoing nuclear reactions will tend to move where on the graph of binding energy per nucleon against nucleon number? Why?
the nuclear reactions that tend to happen are those that make nuclei more stable, so nuclei undergoing nuclear reactions tend to move towards this valley of stability. (the nuclear valley)
What is nuclear fusion, what happens and why to the binding energy per nucleon and energy released?
nuclear fusion is Combining small nuclei to create a larger nucleus
This makes the binding energy per nucleon much more negative, this is because of mass defect, the nucleus will have less mass than the individual nucleons, this lost mass is converted to energy and released.
this means a lot of energy is released during nuclear fusion
What is fission? What happens to the binding energy per nucleon, and energy.
Large nuclei are split into two
The nucleon numbers of the two new nuclei are smaller than the original nucleus. so lost mass is converted to energy and released. This makes the binding energy per nucleon more negative.
So, energy is also released during nuclear fission (but not as much energy per nucleon is a nuclear fusion).
2 ways to calculate the energy released in a nuclear fission or nuclear fusion reaction?
The energy released is equal to the difference between the binding energy of the parent nucleus and the daughter nuclei (parent BE - daughter BE, can be positive)
Or, you can calculate the difference in mass between the parent nucleus and the daughter nucleus, then using E=mc2 to find the energy that this change in mass is equivalent to.
how to use graph to calculate energy released in reaction such as nuclear fusion
work out total binding energy of each thing involved using the binding energy per nucleon (y-axis) * nucleon number (rearranged equation for BEPN)
-BE1 + -BE2 -> -BE3 + energy
find be1 and be2 total and add be3 to get energy released
how to work out mass of nucleus if given mass of atom
you need the mass of nucleus in BE calculations
so if given mass of atom, subtract total mass of electrons
Describe nuclear fission in more detail. What is the difference between spontaneous and induced (how does this happen) nuclear fission?
Heavy nuclei are unstable and some can randomly split into two smaller nuclei, and sometimes several neutrons.
This process is called spontaneous if it just happens by itself, or induced if we encourage it to happen. in induced, we make a neutron enter a uranium nucleus causing it to be unstable
what type of neutronis needed to induce nuclear fission
a low energy neutron called a thermal neutron
Explain why energy is released during nuclear fission
Because the new, smaller nuclei have a higher binding energy per nucleon and a lower total mass
Are larger or smaller nuclei more likely to spontaneously fission?
What does this mean for nuclear fusion in all elements?
Larger, because they are more unstable
Spontaneous fission limits the number of nucleons that a nucleus can contain without being unstable so it limits the number of possible elements
what are nuclear reactors used for?
What do nuclear reactors use as fuel?
controlled nuclear reactors allow us to harness the energy released in nuclear fission for power
Fuel rods of uranium that are rich in uranium 235
Describe the nuclear reactions that happen inside a uranium nuclear reactor?
nuclear fission of uranium produces more neutrons which then induce other nuclei to fission – this is called a chain reaction
The neutrons will only cause a chain reaction if they are slowed down to thermal neutron energy levels, so they can be captured by the uranium nuclei.
To do this the fuel rods need to be placed in a moderator, water,. You need to choose a moderator that will slow down some neutrons enough so they can cause further fission, keeping your reaction going at a steady rate
how is energy produced in fission used to generate electricity
energy is transferred to pressurised water in cooling circuit, heating the water to produce steam driving the generator, producing electricity
how much uranium235 fuel does a reactor use in a year for a required power output is 800MW and the efficiency is 33%
each fission provides 3.2 * 10-11J
mass of uranium 235 is 4*10-25kg
3.2 * 107 seconds in a year
3% of uranium fuel is uranium235 so how much enriched uranium fuel is used in a year
What is the critical mass?
The amount of fuel you need to keep the chain reaction going on its own at a steady rate, where one fission follows another.
This means that for each nuclei that fissions, exactly one of the neutrons released triggers another fission.
What happens if you have any less than the critical mass (sub-critical mass)?
sub critical mass is when you have less fuel than the critical mass, this means too few neutrons will be captured by other nuclei and the reaction will just slow down to nothing
what is a supercritical mass of fuel
a supercritical mass of fuel is one where the amount of fuel you use that means several new fission follow each fission
Describe the sub/super/critical mass used in nuclear reactors
Nuclear reactors use a super critical mass of fuel, where several new fissions normally follow each fission, and control the rate of fission using control rods
How do control rods control the chain reaction, in a nuclear reactor?
how can they be used in an emergency
They limit the number of neutrons in the reactor. They absorb neutrons so that the rate of fission is controlled.
Control rods are made up of material that absorbs neutrons (boron), and they can be inserted by varying amounts to control the reaction rate.
in an emergency, reactor is shut down by the release of control rods into the nuclear reactor stopping the reaction ASAP
What is coolant used for, in a nuclear reactor?
It is sent around the reactor to remove heat produced in the fission
often the coolant is the same water that is being used in the reactor as a moderator (the substance that slows down neutrons to thermal energy levels so they can cause nuclear fission).
Briefly, how is heat used from the nuclear reactor?
The heat produced from the reactor due to nuclear fission can be used to make steam for powering electricity-generating turbines
Explain what could happen if the chain reaction in a nuclear reactor is not effectively managed
ineffective management could be control rods not absorbing enough neutrons
Large amounts of energy are released in a very short time.
Many new fissions will follow each fission, causing a runaway reaction which could lead to an explosion.
This is what happens in a fission (atomic) bomb
why do you need a high temp in nuclear fusion
Need a high temperature so strong nuclear force can grab protons to fuse together working against electrostatic forces of repulsion
what are the equations for the fusion equations that happen in a star
1: 11H + 11H -> 21H + e+ + v ( 1 proton decayed into a neutron giving positron and neutrino)
2: 21H + 11H -> 32He + gamma ray photon
these equations happen twice so that you have 2 32He
32He + 32He -> 42H + 11H + 11H
OVERALL: 4 * 11H -> 42He + (2e+ + 2gamma + 2v(antineutrino))
equations in fusion reactor
you pump in 21H and 31H -> 42He + 01n
it was 52He but then neutron ejected
What is the advantages of using nuclear fission over fossil fuels or renewable sources?
nuclear fission doesn’t produce carbon dioxide, unlike burning fossil fuels, so it doesn’t contribute to global warming.
It also provides continuous energy supply, unlike many renewable sources like wind and solar
Describe the disadvantages of nuclear reactors
Some of the waste products are highly radioactive and difficult to handle and store.
When waste material is removed from the reactor, it is initially very hot, so it is placed in cooling pond until the temperature falls to a safe level, its then placed in sealed containers until its activity has fallen sufficiently. but This can take many years, so there is a risk that material could escape from these containers.
What could happen if radioactive material escapes the container where it is stored?
A leak of radioactive material could be harmful to the environment and local human populations both now and in the future, particularly if the material were to contaminate water supplies
What events pose a risk to nuclear reactors?
Accidents or natural disasters
When can nuclei fuse?
They can only fuse if they have enough energy to overcome the electrostatic repulsion between them and get close enough to the strong interaction to bind them
Describe the difference in conditions in order for nuclear fission and fusion to take place
Fusion reactions require much higher temperatures than fission, as well as higher pressures or densities
Describe the energy released in nuclear fusion
A lot of energy is released because the new, heavier nucleus has a much more negative binding energy per nucleon and, and so a lower total mass (nucleus weighs less than constituents so a lot of energy given out, so higher binding energy so equation to give BEPN is much higher now).
The energy released helps to maintain the high temperatures needed for further fusion reactions
Why does fusion release more energy than fission, considering the energy released per reaction is generally lower in nuclear fusion than fission?
The nuclei used in fusion have a lower mass, so a mole of the reactants in the fusion reaction weighs less than a mole of the reactants in the fission reaction, so gram for gram a fusion reaction releases more energy than a fission reaction
What would be the advantage of a nuclear fusion reactor over a fission reactor?
We could generate nuclear electricity without the waste you get from fission reactors
how to calculate energy released in a fission reaction
energy released = binding energy before fission - binding energy after fission
mass of helium = 4
mass of neutron = 1
why does neutron carry away 4/5 of this energy using momentum
helium and nucleus go off in opposite directions so momentum is equal and opposite
He is 4 times heavier
so 4mV = m4V
KE:
1/2m(4v)² = 1/2 * 4mv²
8 : 2
4 : 1
so neutron has 4/5 of energy
what happens if body bombarded with neutrons
can damage you by joining nuclei and causing them to be radioactive
problems and solutions with a fusion reactor
problem: for a nuclear reactor to be viable, the energy produced must be more than the energy needed, but that energy is huge
problem: need strong magnetic fields to keep particles in centre of reactor spiralling around but its hard to keep them in the centre because they often drift to the side and lose energy
problem with solution: 21H, it can be extracted from water for the reactor but 31H is very rare, solution is a blanket of lithium aroudn the reactor, when neutron hits lithium it creates 31H (10n + 63Li -> 42He + 31H)
