P7 - Atoms and Radiation Flashcards
What does radioactive ‘radiation’ refer to? Where does it come from?
Radiation refers to the particles or waves emitted by radioactive substances.
Radiation is decay from the nucleus of a radioactive atom.
What does a radioactive atom contain?
It contains an unstable nucleus that becomes stable by emitting some kind of radiation.
What are the 3 types of radiation?
Alpha radiation, α
Beta radiation, β
Gamma radiation, γ
In terms of penetration, how do the 3 types of radiation compare?
Alpha is the least penetrating - stopped by paper.
Beta is the second most penetrating - goes through paper.
Gamma is the most penetrating - even more penetrating than beta.
What is meant by the term radioactive decay?
The process by which a radioactive atom’s unstable nucleus emits radiation.
Can the process of radioactive decay be predicted?
No.
It is a natural and spontaneous event - you can’t predict or influence when it will happen.
Who discovered that there is a positively charged nucleus at the centre of every atom?
Ernest Rutherford
What experiment did Rutherford use?
The Alpha, α particle scattering experiment.
He fired beams of positively charged alpha particles at some gold foil.
The experiment was done in an evacuated chamber (a vacuum) so no particles could interfere with the alpha particles.
What were Rutherford’s findings?
Most of the particles went straight through the gold foil - Evidence of empty space.
But a few were deflected and scattered in different directions - evidence of positive nucleus.
Even fewer deflected straight back - evidence that the mass and positive charge are concentrated in a tiny volume.
What model of the atom came before Rutherford and what did it show?
JJ Thompson’s Plum Pudding Model
Atom is a ball of positive charge with electrons embedded in them.
What model of the atom came after Rutherford’s model?
Niels Bohr’s model of the atom.
He put forward ideas about how the electrons orbit the nucleus at different distances with different energy levels.
Who discovered neutrons?
James Chadwick discovered neutrons after Bohr’s model.
What are the relative masses and charges of protons, electrons and neutrons?
Relative masses:
Protons - 1
Neutrons - 1
Electrons - 1/1836 (<1)
Charges:
Protons - +1
Neutrons - 0 (neutral)
Electrons - -1
What are Isotopes?
Isotopes are atoms of the same element with different numbers of neutrons.
E.g Normal Lithium = Li-7
Isotopes include Li-5, Li-6, Li-8, Li-9 etc…..
How do Isotopes compare to atoms of the same element?
The same chemical properties - electrons not changed.
Different physical properties - mass number different so physical properties like BP, MP, density are all different (density of nucleus varies - mass number changes).
What is a radioisotope?
An isotope which has an unstable nuclei. E.g Tritium (Isotope of Hydroyen, 2 neutrons - mass number of 3)
It will become more stable by emitting radiation.
Radiation is only emitted by radioisotopes.
What is the radius of an atom roughly?
1x10^-10 m (or 0.1nm)
What is the radius of a nucleus roughly?
1 x 10^-4m
What makes a nucleus unstable?
Either:
- Very big nucleus
- Too many neutrons.
- Too many protons
How does the nucleus of a radioactive isotope become stable?
By emitting either:
alpha radiation, α
Beta radiation, β
gamma radiation, γ
What is an alpha particle made up of?
α (4/2)
An alpha particle has 2 protons and 2 neutrons.
So:
- mass number = 4
- atomic number = 2
What atom’s nucleus is an alpha particle identicle to?
A helium atom’s nucleus.
In some nuclear equations, you might see it written as He (4/2) rather than α (4/2).
What happens to an unstable nucleus when it emits an alpha particle?
It loses 2 protons and 2 neutrons. Therefore:
- its mass number decreases by 4
- its proton number decreases by 2
Because the atomic number changes, so does the element. It shifts 2 places to the left on the periodic table.
What is the equation for alpha decay?
X(a/z) —> Y(a-4/z-2) + α(4/2)
What would a Thorium-228 isotope (228/90) form if it emitted an α-particle?
Th (228/90)
228-4 = 224 90-2 = 88
It forms the radioisotope Ra (224/88).
Summary:
Th (228/90) —> Ra (224/88) + α (4/2)
How do you check to see if you’re equation for alpha radiation is correct?
The numbers on the top should add to get the mass number of the isotope before the change.
The numbers on the bottom should also add to get the atomic number before the change.
What is a Beta, β particle made up of?
β (0/-1)
A Beta particle is essentially a high energy, fast moving electron emitted by a nucleus that has too many neutrons.
It effectively has a mass number of 0 as electrons have an insignificant mass. It has an atomic number of -1
How is a Beta particle emitted?
When a nucleus has too many neutrons, a neutron changes into a proton AND a Beta, β (electron). The electron is then instantly emitted by the nucleus as electrons cannot be in the nucleus.
What happens to an unstable nucleus when it emits a beta particle?
Overall, the nucleus, gains a proton as it emits the electron.
a - - 1 = a + 1 —> 2 negatives (–) make a positive.
Because the atomic number changes, the element changes. The mass number stays the same.
What is the equation for beta decay?
X (a/z) —> Y(a/z+1) + β (0/-1)
What would a Carbon-14 isotope (14/6) form if it emitted beta decay?
C (14/6)
14 = stays the same 6+1 = 7 protons
C (14/6) —> N(14/7) + β (0,-1)
What is gamma, γ decay?
γ (0,0)
Gamma decay is a high frequency electromagnetic wave. It is uncharged and has no mass.
IT DOES NOT CHANGE THE NUMBER OF PROTONS OR NEUTRONS IN A NUCLEUS.
How would a Thorium-234 isotope (234/90) decay if it emitted beta and gamma radiation?
Th (234/90) —> Pa (234/91) + β (0/-1) + γ (0/0)
The mass number stays the same. The nucleus gains an electron and forms Pa.
As you can see, the gama radiation has no input on the atom’s nucleus.
What is ionisation?
The process an atom is turned into an ion by a radioactive substance’s radiation, knocking electrons out of atoms.
What term is used to describe when an object is exposed to ionising radiation?
It is said to be Irradiated.
DOES NOT BECOME RADIOACTIVE.
What does Ionisation do to living cells?
It can damage or kill the cell.
How can you detect and measure the absorption and range of radiation (α, β, γ)?
Using a Geiger-Muller Counter and Tube.
How would you measure the absorbtion of different radiation (α, β, γ) by different materials?
Place each material between the tube and the Geiger counter. Measure and record the count rate.
Subtract the background count rate.
How would you measure the range of different radiation (α, β, γ)?
Move the Geiger tube away from the source until the count rate from the source is 0.
Which of the 3 radiations is the most penetrating (α, β, γ)?
1) Gamma - the most penetrating
2) Beta - medium
3) Alpha - least
Why is gamma radiation the most penetrating?
Gamma rays have a lower mass (nothing - 0) than beta particles (0.0005) and alpha particles (4).
What can alpha particles be stopped by?
Thin paper.
What can Beta particles be stopped by?
Aluminium
What can Gamma rays be stopped by?
Thick lead or very thick concrete.
What is the range of alpha radiation in air?
5cm of air.
What is the range of Beta radiation in air?
1m of air
Which of the 3 radiations (α, β, γ) is the most ionising?
1) Alpha - most ionising
2) Beta - medium
3) Gamma - least
Why is alpha radiation the most ionising?
Alpha radiation has the greatest charge (+2).
Beta radiation has a charge of (-1).
Gamma radiation has no charge (0) as it’s a wave not a particle.
Can Gamma radiation directly ionise atoms?
No.
It causes atoms to emit other particles which cause ionisation.
Why is alpha radiation more dangerous in the body?
It is the most ionising kind of radiation.
What is a common use of alpha radiation?
Smoke alarms.
How do smoke alarms work?
They contain a radioactive isotope which only emits alpha (α) particles into a gap in a circuit within the alarm.
The alpha particles ionise the air in the gap in the circuit, to enable a current across the gap.
In a fire, smoke absorbs the alpha particles preventing them from ionising the air and causing an electric current across the gap.
An alarm sounds.
How is Beta radiation used in the production of metal foils?
Beta radiation is used to detect the thickness of the foil. The detector measures the amount of Beta radiation passing through the foil.
If the foil is too thick, the detector reading drops and increases the pressure on the rollers.
α & γ can’t be used as α would be stopped and γ would go straight through.
What is the ‘activity’ of a radioactive source? What is its unit?
The number of unstable nuclei that decay every second.
Unit = Becquerels (Bq) = 1 decay per second
What is the ‘half-life’ of a radioactive source?
The time taken for the number of decays per second, to fall by half.
Why is a radioactive isotope’s half-life important?
It enables us to know when that radioactive source will stop emitting radiation.
E.g Uranium-235 has a half life of 700 million years. Meaning it will take several billion years for certain amounts of it to stop emitting radiation. This is why it has to be stored underground.
How can you calculate the half-life of radioactive isotopes? Observe the example below:
You can use a graph.
Measure the amount of time (width of the graph) for the radiation to half. It should be the same at every set interval.
Example (using mass - would be radiation):
Day 1 - 10g of I-131 has a half life of 8 days
Day 8 - 5g of I-131
Day 16 - 2.5g of I-131
Day 24 - 1.25g of I-131
You should notice that the graph gradually slows as the amount of decay get smaller at a more constant rate.
What is the count rate of a radioactive isotope?
The number of decays (counts) recorded per second by a Geiger Muller Tube.
What equation is used to calculate the count rate before and after decay?
Number of unstable nuclei after = number of unstable nuclei before / 2^n
n = half-life
Essentially, the decay (Bq) is 1/2^n of the original count rate.
E.g original decay = 8
3 half-lives = 1/2^3 x 8
A radioactive istotope has a half life of 6 hours. A sample of this isotope containing 60,000 radioactive nuclei is taken. Calculate the number of radioactive nuclei remaining after 24 hours.
24 hours = 4 half lives, so n = 4
= 60,000 / 2^4
= 60,000 / 16
= 3750 radioactive nuclei remaining.
Why is radioactivity useful in medicine?
Radioactive isotopes can be used for medical imaging, treatment of cancer, and as tracers to monitor organs.
What 2 factors must be taken into account when using radioactive sources in medicine?
The source’s half life and the type of radiation it gives out.
How do radioactive tracers work?
A patient will consume a tiny amount of a radioactive substance (usually with water). The substance will give out gamma rays (least ionising).
A detector is then used to see if the substance flows in and out of various organs (e.g kidney’s).
How does a gamma camera work?
Gamma cameras take images of internal organs.
Prior to the imaging, the patient is injected with a small amount of a radioactive solution. The organ absorbs the solution and a gamma camera can detect the rays emitted by the solution.
How is radiation used to kill cancer cells?
Gamma knives/beams and radioactive implants are both different ways of killing cancer cells.
Why is the half life of a radioactive source important in medicine?
If too long, it may do damage to health tissue.
It too short, it may not do its job.
What is Contamination?
Contamination is when a radioactive substance is inside the body.
What is Irradiation?
Irradiation is when you expose an object to beams of radiation.
How might somebody become contaminated with a radioactive source?
- Ingesting it (food or drink)
- Injection
How might somebody become irradiated with a radioactive source?
Being exposed to any kind of waves (x-rays), radon gas and cosmic rays.
What are the main sources of background radiation?
50% radon gas (from rocks) - radon emits alpha particles which can cause cancer. 11.5% food and drink 14% ground and buildings 10% cosmic rays 14% medical uses 0.2% nuclear weapons tests 0.1% nuclear power
What equation did Einstein use do express that a small amout of matter could release a large amount of energy?
E = mc^2
E-Energy
m-mass
c^2-speed of light^2
Where is the energy stored in atoms?
In the nuclei.
How can the energy trapped in the nuclei be released? 2 WAYS
1) Nuclear Fission
2) Nuclear Fusion
What is Nuclear Fission? (Simple Definition)
The splitting of nuclei.
What is Nuclear Fusion? (Simple Definition)
The joining of nuclei.
How does Nuclear Fission occur?
Nuclear Fission occurs when the nucleus of a stable atom is is struck by a neutron.
The nucleus of the atom absorbs the neutron, causing it to become an unstable isotope.
This unstable isotope then splits into 2 smaller fragments, releasing large amounts of energy.
How is the energy released from fission?
It is released in the form of gamma radiation, as well as the kinetic stored in the nuclei.
What else does a nucleus undergoing fission release?
2 or 3 neutrons at high speed.
What are the 2 or 3 neutrons released by a nucleus called?
Fission neutrons
What can the fission neutrons do after being released?
They often strike other nuclei, causing a ‘chain reaction’ of fission events.
What are 2 common fissionable isotopes used in nuclear reactors and weapons?
Uranium-235 and Plutonium-239
What is the purpose of a nuclear reactor?
To produce the energy needed to be used at a nuclear power station.
What is the core of a reactor?
The core is the main part of a nuclear reactor. It is made up of the moderator and contains fuel rods and control rods.
What are control rods?
Control rods are used to absorb some of the surplus neutrons released by fission. Only one neutron per fission is enabled to produce further fission.
What are the fuel rods?
The rods containing the nuclear fuel for the reactor.
What is a moderator in a nuclear reactor?
A substance which is used to slow down the fission neutrons so that they can be absorbed more easily.
The moderator is usually water or graphite.
Why is the reactor core located in a vessel made of steel and thick concrete?
Steel is used to maintain the high temperatures and pressures inside the core.
The concrete absorbs ionising radiation that escapes through the steel.
What are the advantages and disadvantages of nuclear reactors using fission?
Advantages:
- Uranium or Plutonium fuel is relatively cheap.
- Produces a large and steady amount of energy.
- No greenhouse gases.
Disadvantages:
- Power plants are expensive to build.
- Nuclear watse has to be stored.
- Nuclear Disasters can happen (e.g Chernobyl)
How does Nuclear Fusion occur? What must the mass of the nuclei be for fusion to occur?
When 2 small nuclei fuse together, they form a single larger nucleus and release energy.
Fusion can only happen if the relative mass of the nucleui is no more than about 55.
How can nuclear fusion be brought about?
Nuclear fusion can be brought about by making two light (mass less than 55) nuclei collide at very high speed.
How does nuclear fusion in the sun work to release energy (creating enormous heat)?
The sun is made up of hydrogen and helium. The core is made up of a plasma of bare nuclei (no electrons attached). These nuclei move about and fuse together when they collide. Energy is released when they fuse.
Why must the conditions of a fusion reactor be so hot?
The nuclei need to have enough energy so that they can overcome the force of repulsion due to the positive charge of the nuclei.
What is the fuel needed for nuclear fusion to occur?
-‘Heavy hydroyen’ (hydrogen with a mass number of 2) is readily available in seawater.
What is the product of nuclear fusion?
Helium gas - non-radioactive
Why could fusion reactors be better than the current fission reactors?
- No nuclear waste.
- Fuel is readily available.
What are 2 examples of nuclear disasters that show the dangers of nuclear power?
1) Chernobyl - reactor didn’t have a high-speed shut down system.
2) Fukushima - reactors cripked by an earthquake and a tsunami.
Why is nuclear waste an issue?
It has to be stored (expensive £) and sealed for many years. This damages plant and animal habitats.
Leaks can also occur.
Why is alpha radiation very dangerous if it is ingested (e.g eat a radioactive source)?
Alpha is the most ionising radiation and would affect all surrounding tissue.
Why is alpha radiation less dangerous if it is outside the body?
The skin can absorb it. We are exposed to background radiation.
How can exposure to radiation be limited?
- Lead/concrete barriers.
- Equipment that distances you from the source.
- Wear PPE.
- Limit exposure.
