Radioactivity Flashcards
In Rutherford experiment what was the purpose of the lead container
The alpha particles were placed in a lead container. This is because alpha particles are emitted in all directions, and alpha particles are absorbed by lead so placing them in a lead container with a hole at the front allowed a beam of alpha particles parallel to each other.
Why was the gold foil in the experiment so thin?
A thicker gold foil would stop alpha particles completely so a really thin gold sheet had to be used.
Why was the experiment taken place in vacuum?
This is because alpha particles are highly ionising, so they travel short distances before colliding with air molecules. The vacuum ensure that alpha particles do not collide with any other air molecules on their way to the gold foil target.
What were the findings from the experiment?
Most of the alpha particles passed straight through, showing that an atom was mainly empty space. Some of the particles were deflected, and this showed that there was a positive nucleus at the centre, which repelled the positive alpha particles. The closer you are to the nucleus, the greater the amount of deflection, this is because the electrostatic force of repulsion between the positive nucleus and the postoperative alpha particles increases with a smaller distance. A small amount of alpha particles were reflected back, and this shows that the nucleus is small, and it is where most of the mass and charge is concentrated.
What is closest distance of approach?
This is the closest distance that an alpha particles can get to the nucleus before it is repelled by the electrostatic force of repulsion between the positively charged nucleus and the positively charged alpha particles.
The particle starts with max kinetic energy and when the alpha particle reaches the closers distance off approach, all this kinetic energy is converted into electrostatic potential energy.
An alpha particle with a greater kinetic energy will have a smaller least distance of approach before it is repelled, meaning it will come closer to the nucleus. This is because it has more energy to overcome the electrostatic force of repulsion.
Kinetic energy = Electrostatic potential energy (U)
1/2mv^2 = kQq/r
Describe the range and penetration of alpha, beta plus, beta minus, and gamma
Alpha particles have the shortest range, of around 5cm. They are the least penetrating and this is because they are quite large, and so travel a shorter distance before coming into contact with something. They can be stopped by paper. They are highly ionising (knock electrons out of atoms forming ions), as they have a charge of +2e.
A beta minus particle is a high energy electron, and a beta plus particle is a high energy positron. They are less ionising than alpha, but still ionising, due to they +1e charge. They are more penetrating than alpha radiation, and beta minus can travel around 1m before being stopped. They can be stopped by a few mm of aluminium foil.
Beta plus has a short range, because it will shortly meet its an electron and annihilation will occur.
Gamma radiation is high energy EM radiation emitted by a nuclei that needs to lose energy. They are the least ionising due to having no charge, but they are the most penetrating and can travel hundreds of metres/kilometres, and are stopped by many cm’s of lead.
How are alpha, beta and gamma radiated deflected by magnetic fields?
Alpha and beta radiation are both deflected by magnetic fields, and they are deflected in opposite directions. Beta radiation is more easily deflected by a magnetic field, and therefore it is deflected MORE. Gamma radiation is not deflected at all in a magnetic field.
What is binding energy?
This is the amount of energy required to separate the nucleus into its individual nucleons.
What is mass defect?
This arises from the fact that the total mass of the individual nucleons is greater than the mass of the overall nucleus. This difference is known as the mass defect.
This fall in mass, shows that energy is released, and as a result, the mass of the nucleus falls, according to the equation E=mc^2.
The fall in mass is because when nucleons join to form a nucleus, the strong nuclear force does work to pull the nucleons together and there is work done in overcoming the electrostatic force of repulsion between the positive protons. This energy released is equal to the binding energy.
What is binding energy per nucleon?
This is the work done PER NUCLEON, to remove all the nucleons from the nucleus.
A greater binding energy per nucleon shows a more stable nucleus, and is less likely to undergo fission or fusion. It shows more energy is required to split the nucleons from the nucleus.
What is nuclear fusion?
This is the process of fusing together two small nuclei to produce a larger nucleus. As a result, energy is released, and is equal to the increase in binding energy.
Why must the 2 nuclei have high kinetic energy to fuse?
For 2 nuclei to fuse, they must have high kinetic energy because for their strong nuclear force to kick in and hold them together, they must have a short range, so they must come close to each other but they repel each other due the electrostatic force of repulsion between the protons so it tastes work, so they must have high kinetic energy.
Fusion in the sun:
In the sun, temperatures are extremely high, and pressure is also very high and at such high temperatures, atoms are stripped of of their electrons, and plasma is created. The nuclei of plasma have enough kinetic energy to come close to each other and fuse to form helium, and during this process, a tremendous amount of energy, as solar energy, is released.
What is nuclear fission ?
This is the splitting of a large atomic nucleus into smaller nuclei. The free nuclei are called daughter nuclei. In this process, 2/3 free neutrons are also released.
What is induced fission?
This is when we bombard uranium 235 or plutonium 239 with neutrons. They then absorb these neutrons and become unstable. Now this unstable nucleus decays into 2 smaller, light daughter nuclei, releasing energy and 2/3 neutrons. The neutrons released can go on to be absorbed by another nucleus and this process is repeated.
Why is energy released during nuclear fission?
The nuclei absorbs the neutron, causing it to become unstable and now the strong nuclear force starts to lose its grip while the electrostatic force of repulsion becomes significant. The fission fragments overcome the strong nuclear force holding them together. When the nucleus splits, the fission fragments produced have a greater binding energy than the original nucleus and this difference in binding energy is the amount fo energy released in the fission process.
The energy is released as kinetic energy as the fragments move apart from each other, and also gamma radiation and neutrons are released which are apart of the energy released.
What is the moderator in a nuclear reactor ?
The moderator slows down neutrons. The moderator is made up of a material that are poor absorbers of neutrons. When neutrons collide with the material of the moderator, this causes the neutrons to lose some momentum, reducing their speed, creating thermal neutrons. The neutrons are slowed down, until they are in thermal equilibrium with their surroundings ( material of the moderator). Often, moderators are made out of water or graphite.
What are thermal neutrons?
These are neutrons that have a low kinetic energy, and are in thermal equilibrium with its surroundings. The neutron must be slow moving in a reactor core because if it had a great speed, this would mean that it would rebound when it hits the nucleus and not be absorbed to cause fission.
Thermal neutrons have kinetic energies association with 3/2kT.
What do control rods do in a nuclear reactor?
They are made out of a material that absorbs neutrons. The control rods are moveable and can be inserted or raised into the fuel assemblies. The control rods are typically made out of cadmium, or boron and these are materials that absorb neutrons. The control rods depth is varied to ensure that only 1 fission neutron goes on to cause 1 more fission event. This is a controlled reaction.
What is the role of the coolant in a nuclear recator?
This is the material that passes through the reactor core and absorbs the thermal energy released by fission. This heat is then used to produce steam, which drives a turbine, and generates energy.
The coolant must have good heat transfer characteristics such as a high specific heat capacity.
Another role of the coolant is to stop the reactor core from overheating.
The typical material used for a coolant is water, but co2 gas can also be used.
Why is enriched uranium used rather the natural uranium in a nuclear reactor, and what is the meaning of a fissile material?
Uranium - 235 is the fissile material, which means it easily undergoes fission when struck by a neutron. It is the material that causes fission in a nuclear reactor.
Enriched uranium is used rather than natural uranium in a nuclear reactor because enriched uranium contains a greater concentration of uranium-235 whereas natural uranium only has a small amount of uranium-235, therefore it is not enough to keep the reactor running.
Safety aspects of a nuclear reactor :
Radioactive substances have a long half life, meaning they emit harmful radiation for many years and therefore waste products must be stored safely i water tanks, or sealed underground.
The fuel rods are handled remotely by machines.
The nuclear reactors core is surrounded by thick lead or concrete to ensure no radiation escapes.
In emergency, control rods are automatically fully lowered to shut down the reaction.
Workers are protected by concrete shielding.
What are the adv and disadv of nuclear reactors
Uranium provides far more energy compared to fuel.
There are no polluting gases released.
It s highly reliable, unlike solar.
Only a small amount of uranium is necessary.
Dealing with radioactive substances is very dangerous and expensive.
A nuclear meltdown such as Chernobyl could happen.
What is the critical mass?
This is the minimum amount of fuel required to maintain a steady chain reaction.
Using the critical mass of fuel means a single fission event follows the other.
Using too much mass of fuel (supercritical mass) means too many fission events would follow the last leading to an uncontrolled reaction and an explosion.
Using too less fuel, means there is not enough to maintain a reaction.
