Nuclear Radiation Flashcards
What happens to nuclei that are unstable
They break down to become more stable, making them radioactive.
What could instability within a nucleus be caused by
too many neutrons
too few neutrons
too many nucleons (nucleus is too heavy)
too much energy in the nucleus
What is radioactive/nuclear decay
The nucleus decays by releasing energy and/or particles, until it reaches a stable form
What is the nature of radioactive decay
It is random and spontaneous, it can’t be predicted
what is an alpha particle (α) made of
A helium nucleus, 2 protons, 2 neutrons & 2 electrons
What is the relative charge on an alpha particle
+2
What is the mass of an alpha particle
4 u (atomic units)
What is Beta-minus (β−), which is the normal Beta particle, and it’s properties
An electron, -1 charge, negligible mass
What is Beta-plus (β+) particle and it’s properties
A positron, +1 charge, negligible mass
What is gamma particle (γ) and it’s properties
Short-wavelength, high-frequency EM wave (gamma wave). Charge = 0, mass = 0
What can all waves, like gamma, act as
As a particle. You can have gamma photons
What are radioactive emissions known as and why
Ionising radiation because when radiation hits an atom, it can knock off electrons, creating an ion.
How are alpha, beta and gamma radiation tested to see if they penetrate
They are fired at a variety of objects with detectors placed on the other side. If they are detected, that means they have penetrated that object
What is the ionising ability of alpha, beta-minus and gamma radiation
alpha - strong
beta-minus - weak
gamma - very weak
What is the speed of alpha, beta-minus and gamma radiation
alpha - slow
beta-minus - fast
gamma - speed of light
What is the penetration/range of alpha, beta-minus and gamma radiation
alpha - very small, absorbed by paper or a few cm of air
beta-minus - small, absorbed by ~3 mm of aluminium
gamma - very large, absorbed by many cm of lead, or several meters of concrete
Is alpha, beta-minus and gamma radiation affected by magnetic field
Alpha and Beta-minus have a charge, so they are affected by a magnetic field. Gamma is not affected as it has a charge of 0.
Why does beta-plus radiation not have the properties of the others
It is annihilated by an electron, so it has virtually zero range.
How are ionising ability, charge and penetration range related
The more charge the radiation has, the more ionising ability it has. A particle with high ionising ability will have a low penetration range.
How are ionising ability, charge and penetration range related in alpha radiation
Alpha particles are strongly positive, +2 charge, so they can easily pull electrons off atoms, ionising them. Ionizing an atom transfers some of the energy from the alpha particle to the atom (you are pulling off an electron which takes energy). The alpha particle quickly ionises many atoms (about 10,000 ionizations per alpha particle) and loses its energy, meaning alpha particles don’t travel very far, they have low penetration.
How are ionising ability, charge and penetration range related in beta-minus radiation
Beta-minus particles have lower mass and charge than alpha particles, but a higher speed, so they also have a significant amount of energy. This means they can still knock off electrons from atoms. Each beta-particle will ionise about 100 atoms, losing energy at each ionisation. Since beta-minus has lower number of ionisations than alpha radiation , it travels further than alpha radiation, meaning it has higher penetration
How are ionising ability, charge and penetration range related in gamma radiation
Gamma radiation is even more weakly ionising than beta-minus as it has 0 charge and 0 mass. This means it has very high penetration, as it has a lot of energy for traveling/penetrating since it isn’t losing it by ionising a bunch of atoms
What must you do when you take a reading from a radioactive source (CORE PRAC ADVICE)
You need to measure the background radiation separately and subtract it from your measurement
What are the sources of background radiation
The air: Radioactive radon gas released from rocks. It emits alpha radiation. The concentration of this gas in the atmosphere varies a lot from place to place, but it’s usually the largest contributor to background radiation.
The ground and buildings: all rock contains radioactive isotopes
Cosmic radiation: Cosmic rays are particles (mostly high-energy protons) from space. When they collide with particles in the upper atmosphere, they produce nuclear radiation
Living things: All plants and animals contain carbon, and some of this will be the radioactive isotope carbon-14
Man-made radiation: In most areas, radiation from medical or industrial sources make up a tiny, tiny fraction of the background radiation.
How do you write nuclear decay equations
In standard chemistry style notation. You write the proton number on the bottom and the nucleon number on top
What must be conserved in decay equations
Charge and Nucleon number. Decay equations need to be balanced, in every nuclear reaction, including fission and fusion, charge, and nucleon number must be conserved.
How do you check that charge is conserved in a nuclear decay equation
The total proton number before and after an interaction must be the same. This will tell you whether charge is conserved
How are beta-minus particles written in nuclear decay equations
0
β
-1
What else, other than nucleon number and charge, must be conserved in all nuclear reactions
Energy and momentum
Why does mass not have to be conserved in nuclear reactions
The mass of an alpha particle is less than the individual masses of 2 protons and 2 neutrons due to the mass deficit. The energy is released when nucleons bind together to form alpha particles, and this accounts for the missing mass
What is the concept of mass deficit in nuclear physics
Making bonds is exothermic, so it releases energy. This means that particles together in a nucleus have less energy, and therefore less mass than the same particles separated. 2 protons and 2 neutrons weigh 100g, but a nucleus with 2 protons and 2 neutrons weighs 70g. Because when the bonds are formed, energy is released in the form of mass, which is what the difference in mass is. This is the mass deficit. When you want to split the nucleons apart, you have to put energy in.
Where does alpha emission only happen and why
Alpha emission only happens in the nuclei of very heavy atoms like uranium and radium. This is because the nuclei of these atoms are too massive to be stable.
Uranium Atomic mass: 238
Radium Atomic mass: 226
How do proton number and nucleon number of the atom change when an alpha particle is emitted
Proton number decreases by 2, nucleon number decreases by 4
What does beta-minus decay involve
The emission of an electron from the nucleus along with an antineutrino
Where does beta-minus decay happen
In isotopes that are neutron rich, meaning they have many more neutrons than protons in their nucleus
What happens in beta-minus decay
One of the neutrons in the nucleus decays into a proton and ejects a beta-minus particle (an electron) and an antineutrino
How do proton number and nucleon number of the atom change when a beta-minus particle and an antineutrino is emitted
Proton number increases by 1. Nucleon number stays the same.
What happens in beta-plus emission
A proton gets changed into a neutron, releasing a positron and a neutrino.
How do proton number and nucleon number of the atom change in beta-plus emission
Proton number decreases by 1. Nucleon number stays the same
When does gamma radiation occur
This often happens after an alpha or beta decay has occurred.
How do proton number and nucleon number of the atom change in gamma emission and why
During gamma emission, there is no change to nuclear constituents. The nucleus just loses excess energy
What happens in gamma radiation
An excited nucleus, nucleus with excess energy, loses this energy by emitting a gamma ray. Gamma rays can be emitted from a nucleus with excess, too much energy.
What is the general overview of the absorption of gamma radiation by lead experiment
- Choose radioactive source, use micrometer to verify all lead sheets are same thickness, check background radiation with Geiger counter, place radioactive source 15cm from Geiger counter.
- Place lead into clamp perpendicularly so it completely blocks off straight line between source and counter. Record count rate, then add another lead sheet to clamp to increase combined thickness of lead sheets. Continue the experiment for up to 10 lead sheets.
- Subtract background count rate from results and plot the graph of corrected count rate (y-axis) vs thickness of lead (x-axis). Reduction in count rate means increase in absorption.
In the absorption of gamma radiation by lead experiment, what should you check the lead sheets for and how
Your lead sheets should all be the same thickness. Use a micrometer to measure their thickness and verify this.
In the absorption of gamma radiation by lead experiment, how do you check for background radiation, and why do you need to do this
Turn on the Geiger counter and take a reading of the background count rate (in counts per sec). Do this 3 times and take an average. You’ll need to subtract this from your count rate measurements to get the corrected count rate.
In the absorption of gamma radiation by lead experiment, how should you position your lead sheets in relation to the radiation source
When placing the piece of lead in the clamp, make sure it is perpendicular to the Geiger counter source so it completely blocks the straight line between the source and the counter.
In the absorption of gamma radiation by lead experiment, when measuring the count rate for each thickness of lead used, what should you do
Measure the count rate 3 times then take an average for each thickness
In the absorption of gamma radiation by lead experiment, how many lead sheets should you test up to
10 lead sheets is a good amount
In the absorption of gamma radiation by lead experiment, what must you do once the experiment is finished for safety reasons and why
Once the experiment is finished, put away the gamma source immediately. You don’t want to be exposed to more radiation than you need to be.
In the absorption of gamma radiation by lead experiment, what must you do to your recorded values before plotting them on a graph
Correct your data by subtracting the background radiation from your results.
In the absorption of gamma radiation by lead experiment, once you initially position the radioactive source, what must you not do and why
Once you’ve placed the radioactive source at about a 15cm distance from the tube, don’t move it again or it could impact the count rate on the Geiger counter.
In the absorption of gamma radiation by lead experiment, what is an example of a radioactive source you can use
Cobalt-60
In the absorption of gamma radiation by lead experiment, how should you set up the apparatus
In the absorption of gamma radiation by lead experiment, what graph should you plot, and what results do you expect to see
Then plot a graph of corrected count rate against thickness of lead. We should see that the count rate reduces with each sheet of lead. This means the absorption increases.
In the absorption of gamma radiation by lead experiment, what is the count rate
The count rate is the number of counts detected per second
Define Mass Defect
The difference in mass between a nucleus and it’s constituent parts
Define binding energy
Energy equivalent to the mass deficit/difference when nucleons bind together to form an atomic nucleus
What is rate of radioactive decay measured by
The decay constant
How can you predict how many nuclei will decay in a given amount of time
Using the equation for nuclear activity
What is activity (A) measured in
Activity is measured in becquerels (Bq).
What is activity (A) in nuclear radiation
The number of nuclei that decay each second
What is activity in nuclear radiation proportional to
Activity is proportional to the size of the sample
What does a value of 1 Bq mean
1 Bq means that 1 nucleus decays per second (s^-1).
What is the decay constant (λ)
The probability that a given nucleus will decay each second
What does a larger value of decay constant (λ) mean
The bigger the value of λ, the faster the rate of decay. Its unit is s^-1.
What are the units for decay constant (λ)
Its unit is s^-1.
What is the equation linking nuclear activity and decay constant
A = λN
activity = decay constant x number of undecayed nuclei
How else can you write represent activity and why can you do it this way
Since it is the rate of nuclei decay, it can be represented as a derivative. It will be a negative derivative as the number of decayed nuclei will be decreasing, so we write it as.
Because activity, A, is number of nuclei that decay each second, it is equal to minus the rate of change in the number of undecayed nuclei, which is dN/dt
-dN/dt
A = -dN/dt = λN
Why is there a negative sign in front of dN/dt
The number of undecayed nuclei (N) is decreasing, so dN/dt is negative. A negative sign must be put in front of it to make sure A is positive.
How can you model radioactive decay using a spreadsheet
- Set up spreadsheet columns for t, ΔN, N, Δt and λ.
- Decide on Δt interval to use depending on λ.
- Enter formulas into spreadsheet to calculate N in sample after each interval. Use ΔN = -λ xNx t
- Plot graph of N over t, changing Δt slightly to get a nice exponential shape if needed.
TABLE AND GRAPH FROM PG 152
What is Δt in the experiment with setting up a spreadsheet to model -ΔN/Δt = λN
This is the time interval between the values of N that the spreadsheet will calculate. The most sensible time interval will depends on your decay constant.
When setting up a spreadsheet to model -ΔN/Δt = λN, how should your columns be set up
Set up a spreadsheet with column headings for total time (t), change in number of number of undecayed nuclei (ΔN), and number of undecayed nuclei (N), and data input cells for Δt and λ
What do you need to model how an isotope sample will decay on a spreadsheet
ΔN, Δt λ, N_0.
You need these values to model -ΔN/Δt = λN.
How can you model the rate of radioactive decay and why
The rate of change of N, dN/dt, is close to the change in N, ΔN, divided by the change in t, Δt, provided that Δt is small. So you can model radioactive decay as: -ΔN/Δt = λN.
What kind of process is radioactive decay
Radioactive decay is an iterative process (the number of nuclei that decay in one time period controls the number that are available to decay in the next)
What is the equation to work out the number of undecayed nuclei remaining, N
N = N_0 e^-(λt)
N_0, initial number of undecayed nuclei
t, time, is measured in seconds
This is the equation of the exponential graph generated by the spreadsheet.
What does the value of N depend on and why
N, number of undecayed nuclei, depends on N_0, number of undecayed nuclei originally present, because radioactive decay is an iterative process.
What is the equation for how a radioactive sample’s activity goes down as it decays
A = A_0 e^(-λt)
A: Activity
A_0: Initial activity at t=0
Both A and A_0 are measured in Bq.
What is the half-life of an isotope, and which isotopes have it
The half life, t_(1/2), is the average time it takes for the number of undecayed nuclei to halve. All radioactive isotopes have a half-life
How is half-life measured
By measuring the time it takes the activity or count rate to half. We do it this way because measuring the number of undecayed nuclei is difficult.
What does a longer half-life mean
The longer the half-life of an isotope, the longer it stays radioactive.
What is the count rate
The number of decays detected per second (this is lower than activity)
How do you use the graph of N against t, number of undecayed nuclei against time, to find the half-life
Read off the value of N when t=0 (this is N_0). Go to half the original value of N (half of N_0). Draw a horizontal line to the curve, then a vertical line to the x-axis. Half-life is where this line meets the x-axis. Check your answer by checking the value for 1/4 of N_0 and 3/4 of N_0
Does half life change as the number of undecayed nuclei decreases
No, half life stays the same. It takes the same amount of time for half the nuclei to decay regardless of the number of nuclei you start with.
How to measure half-life of a radioactive material
Using a proctactinium generator, a bottle containing a uranium salt, the decay products of uranium (including protactinium-234) and two solvents, which separate out the two layers.
In a nutshell, how to do protactinium generator experiment
- Shake bottle to activate generator
- Wait for layer separation, p-234 in top layer. Uranium salt in bottom. Point geiger-muller tube at top layer to measure activity of p-234
- When layers separate, measure count rate asap. How many counts you get in 10 seconds of measuring, do this every 30 seconds
- Record all data, leave bottle for 10 mins, then take count rate again. This gives you background radiation.
- Subtract background from count rate, then plot graph of count rate against time. Should be negative exponential.
How do you start activate the protactinium generator so you can measure the count rate
Shake the bottle to mix the solvents, then add it to the equipment shown
Wait for the liquids to separate. Protactinium-234 will be in top layer of solution, the uranium salt in the bottom layer. Point the Geiger-Muller tube at the top layer to measure activity of the protactinium-234.
How does the graph of count rate against time from a protactinium generator look like, and what does it help you find
Negative exponential. You can use this graph to find the half-life. In this case the half-life is the time taken for the count rate to halve.
In the protactinium generator, what how do you account for background radiation
First measure count rate of generator and count rate of background. Subtract value of background from your measured count rates, then plot a graph of count rate against time. It should look like the graph on the right. You can use this graph to find the half-life in exactly the same way as above. In this case the half-life is the time taken for the count rate to halve.
In the protactinium generator, what should you do once you have recorded the count rate
Once you’ve collected your data, leave the bottle to stand for at least 10 minutes, then take the count rate again. This is the background count rate corresponding to the background radiation (You could also do this at the beginning of the experiment before shaking the bottle).
What does the protactinium generator look like
Bottle contains uranium salt, decay products of uranium, including protactinium-234, and 2 solvents that separate into layers
In the protactinium generator, when should you record the count rate
As soon as the liquids separate, record the count rate (e.g how many counts you get in 10 seconds). Re-measure the count rate at sensible intervals (e.g every 30 seconds)
What is the equation to calculate decay constant (λ), from half-life (t_1/2)
λ = ln2 / t_1/2
λ units are s^-1
t_1/2 units are seconds
You can rearrange this to find half life from decay constant as well
How to take logs of exponential decay equations
REMEMBER to subtract the background activity from any measurement first.
The gradient of the line is -λ (decay constant). From this you can calculate the half-life, t_1/2, of the sample. This works for graphs of activity against time too.
A = A_0 e^-λt —> ln (A) = -λt + ln(A_0).
Define binding energy
The energy needed to separate all of the nucleons in a nucleus. This is equivalent to the mass deficit.
What is binding energy measured in
MeV
How do you convert from atomic mass, u, to kg
Multiply the u value by 1.66 x 10^-27. To go the other way, divide the mass in kg by 1.66 x 10^-27
How do you convert from Joules to MeV
1MeV = 1.60 x 10^-13 J
Divide the value in joules by 1.60 x 10^-13. If you want to go from MeV to Joules, multiply the value in joules by 1.60 x 10^-13.
What is the principle of mass deficit
The mass of a nucleus is less than the mass of its constituent parts. The difference in mass between the nucleus and the nucleons separated is called the mass deficit.
What happens when nucleons join together
Total mass decreases, this lost mass is converted into energy and released. The amount of energy released is equivalent to the mass deficit.
How do you calculate the energy released when nucleons join together to make a nucleus
Use E=mc² and take the value for m as the mass deficit.
What are the units for E=mc²
m is in kg
E is in joules
c is in ms^-1
How much energy would you have to use to split the nucleons in a nucleus completely apart
If you pulled the nucleus completely apart, the energy you’d have to use to do it would be the same as the energy released when the nucleus is formed.
How do you caluclate the binding energy per unit of mass deficit, and what are the units
binding energy / mass deficit
Units, MeV u^-1
What does binding energy per unit of mass deficit of 930 MeV u^-1 mean
This means that a mass deficit of 1u is equivalent to about 930 MeV of binding energy. You can use this approximation to check your answer.
What is the equation and units for binding energy per nucleon
binding energy (B) / Nucleon Number (A)
Measured in MeV
What does a high binding energy per nucleon mean
More energy is needed to remove nucleons from the nucleus
Where do the most stable nuclei occur
Around the maximum point on the graph of binding energy per nucleon against nucleon number, which is at nucleon number 56, Fe - Iron.
How does nuclear fusion affect binding energy per nucleon
Nuclear fusion is when you combine nuclei. This increases the binding energy per nucleon dramatically, which means a lot of energy is released during nuclear fusion.
How does nuclear fission affect binding energy per nucleon
Fission is when nuclei are split in two. The nucleon numbers of the two new nuclei are smaller than the original nucleus, which means there is an increase in the binding energy per nucleon. So energy is also released during nuclear fission, but not as much energy as fusion.
Why are elements with the highest binding energy per nucleon the most stable, like Fe
Highest binding energy per nucleon means it takes that the bond between nucleons has the most energy out of any element. To split up the nucleons and separate the bonds, a lot of energy is needed. Since it is difficult to do, it is the most stable element, as not many things can cause the nucleons to split up.
What does the graph of binding energy per nucleon, in MeV, against Nucleon Number, no units, look like
How do you use the binding energy per nucleon graph to estimate the energy released from nuclear fusion
How do you use the binding energy per nucleon graph to estimate the energy released from nuclear fission
Why does fission happen
Because heavy nuclei are unstable. This means some can randomly split into 2 smaller nuclei (and sometimes several neutrons)
Which gives you more energy generally, fusion or fission
Fusion gives you more energy per nucleon, but fission generally gives you more energy per reaction.
What is the difference between spontaneous and induced fission
Spontaneous is when it happens by itself, induced is if we encourage it to happen
Why is energy released during nuclear fission
Because the new, smaller nuclei have a higher binding energy per nucleon and a lower total mass, { so some energy is released due to conservation of mass energy } - idk if this part is right though, ask chris or paulio
What relationship does the size of the nucleus have to fission
The larger the nucleus, the more unstable it will be, so the large nuclei are more likely to spontaneously fission.
What does spontaneous fission limit
Limits the number of nucleons that a nucleus can contain, in other words, it limits the number of possible elements, because if we try to get an even larger nucleus than we have already discovered, the element is very likely to have spontaneous fission occur.
What is an example of fission
Fission can be induced by making a neutron enter a U-235 nucleus, causing it to become very unstable.
Only low energy neutrons can be captured this way. A low energy neutron is called a thermal neutron.
Which nuclear process do nuclear power stations use
Nuclear power stations generate electricity from nuclear fission reactions
What is nuclear fusion
Nuclei can fuse together, increasing the binding energy per nucleon and releasing a lot of energy.
What force exists between nuclei that we must overcome to do fusion
All nuclei are positively charged, so there will be an electrostatic (otherwise known as Coulomb) force of repulsion between them
How can fusion happen if there is a repulsive force between nuclei
Nuclei can only fuse if they overcome this electrostatic force and get close enough for an attractive force called the strong interaction force to hold them together.
What is the force which holds nucleons together
Strong interaction force
Which conditions does fusion require for it to happen
Very high temperatures, and a high density of matter.
Why does fusion require very high temperatures to occur
Typically fusion reactions need about 1 MeV of kinetic energy. That is a very large amount of energy relatively. So very high temperatures are required
Why does fusion require a high density of matter to occur
Higher densities increases the likelihood of collisions between nuclei.
Where does fusion occur
In the core of stars
Where does the energy emitted from stars come from
Nuclear fusion reactions. This includes the energy from the Sun
Why is fusion able to happen in stars
Because the temperature in the core of stars is so high, the core of the sun is about 10^7 Kelvin.
What happens with the atoms in the core of a star
Because of the extremely high temperatures, atoms don’t exist. The negatively charged electrons are stripped away, leaving positively charged nuclei and free electrons. The resulting mixture is called plasma
How does is nuclear fusion maintained in stars
A lot of energy is released during nuclear fusion because the new, heavier nuclei have much higher binding energy per nucleon. This helps to maintain the temperatures for further fusion reactions to happen.
