Nuclear Physics and Paper 2 Flashcards
(35 cards)
Where did Rutherford do his alpha scattering experiment?
In an evacuated chamber which was covered in a fluorescent coating.
What do we se to measure the count rate when the Source is not present (the background count rate)?
We use a Geiger-Muller (GM) tube to measure the background count rate.
Why does Gamma radiation follow an inverse-square law in air?
It spreads out in all directions equally and is not absorbed by air. Alpha and Beta radiation are absorbed by the air so do not follow an inverse-square law.
What are the uses of different types of radiation?
Alpha:
-Smoke detectors
-Thickness monitoring paper
-Kill cancerous tumours by being inserted into the tumour and killing all cells within.
Beta:
-Thickness monitoring of Aluminium sheets/foil.
Gamma:
-Thickness monitoring of Steel Sheets
-Detector/Radioactive tracer (Technetium-99m)
-Sterilise surgical equipment (as it is the most penetrating and kills all bacteria)
-Radiation therapy, can be used to kill cancerous cells if it is fired in a targeted region of the body such as a tumour however it will also kill any healthy cells in that region.
What safety measures do we take when handled radioactive sources?
-Use long-handled tongs
-Use a lead-lined container
-Keep sure as far away as possible from yourself and others
-Never point the source at anyone
Sources of background radiation?
-Radon Gas- released from rocks
-Cosmic Rays
-Nuclear Weapons testing and Nuclear Meltdowns
-Rocks- containing naturally occurring isotopes
What is lambda?
It is the decay constant and it means each nucleus has a constant probability of decay.
Decay Constant-Probability of a nucleus decaying over time.
Can be found by finding change in number of nuclei over initial number of nuclei.
Radioactive decay is a random and spontaneous process so we can never know which nuclei will decay next or when the next decay will be.
What is the activity of a sample?
Unit?
The number of nuclei which decay per second.
A= lambda x Number of nuclei
Unit: Bq Becquerel
As Activity is directly proportional to Number of Nuclei in the sample with the decay constant as the constant of proportionality it also follows a exponential decay and has the same half-life.
Usually activity is measured to find half-life as is much easier to measure.
Why can the decay constant only be used to model the decay when there are a large number of nuclei in the sample?
The decay constant models the probability of decay by statistical means so needs a large data set or large number o nuclei to be valid.
How does Carbon dating work?
Carbon-14 is radioactive and has a long half-life of 5730 years. It can be used to date organic objects because the percentage of carbon-14 initially in all organic objects is roughly the same. If we then measure the amount of carbon-14 in the organic object now, we can figure out how many half-lives it has been through and find out it’s age.
Why does electron diffraction give a far more accurate estimate of the nuclear radius than distance of closest approach?
Electrons are Leptons so will not interact with nucleons in the nucleus via the strong nuclear force as an alpha particle would.
Distance of closest approach method uses an alpha particle.
In order for electrons to undergo electron diffraction what must we do?
We must accelerate the electrons to very high speeds so that their De Broglie Wavelengths are around 10^-15. They are fired at a very thin film of the material with nuclei of that nucleon number which you are testing for. The nuclei of the atom sin the sample diffract the electrons.
Nuclear radius equation from electron diffraction?
sin theta=0.61 lambda/ R
What is roughly the radius of all nuclei?
1x10^-15 varies a bit with nucleon number but all around this order of magnitude.
Definition of binding energy?
The energy required to separate the nucleus into it’s constituent nucleons.
When is energy released in a reaction?
When the nuclei produced has a higher binding energy per nucleon. therefore energy is released because some mass is converted into energy therefore there is an “increase in energy”.
All reactions in nature will occur because etc reactants want to become more stable into a lower energy state. So nuclei will naturally want to form nuclei with higher binding energies per nucleon as these are more stable. This is why on the graph on the RHS the nuclei undergo fission because they want to become more stable e
by forming nuclei with higher binding energy so undergo fission. Also why on the LHS the nuclei undergo fusion because they also want to form more stable nuclei with a higher binding energy as this reaction releases energy so undergo fusion.
What is the critical mass ?
The minimum mass of fuel required to maintain a steady chain reaction. Using exactly this mass will mean a single fission reaction will follow with last while using less than the critical mass will lad to a reaction which will eventually stop.
What does the Moderator do?
The Moderator slows down the neutrons to thermal speeds through elastic collisions with the moderator atoms. the closer the moderator atoms are in size(mass) to the neutrons, the larger the portion of momentum which is transferred so the lower the number of collisions required to slow the neutrons down to thermal speeds (so they become thermal neutrons.)
Water I soften used as it contains hydrogen (which are close in size), is inexpensive and not very reactive. Graphite I also used sometimes.
Control Rods function?
Control the rate of fission in a reactor by being lowered. The height which they are lowered can be adjusted so that they absorb the right number of neutrons.
Absorb neutron in the reactor in order to control chain reactions. the height which the control rods are lowered to in the erector can be altered in order to control the rate at which fission occurs in the reactor core and therefore control the power output of the reactor of the amount of energy produced per second by the reactor.
Made of materials which do not become radioactive when they absorb a neutron such as Boron and Cadmium.
Purpose o the Coolant in a reactor?
Absorbs the heat released during fission reactions in the core of the reactor and transfers this out of the core so that it can be used to make steam and power a generator. Water can function as both a coolant and a moderator due to it’s high specific heat capacity which means it can transfer large amounts of thermal energy. However the primary function of the coolant is to absorb and transfer the thermal energy which is produced by the reactor out of the core in order to make steam and power a generator.
Molten salt or Helium gas can also be used.
What is the percentage of U-238 and U-235 in mined uranium and enriched uranium?
Mined URanium:
u-238 99%
U-235 1%
Enriched Uranium:
U-238 95%
U-235 5%
What is emergency shutdown?
When the control rods are dropped fully into the reactor core in order to stop the fission chain reaction occurring as soon as possible by absorbing all the free neutrons in the core.
What is the nuclear reactor surrounded by?
Thick concrete shielding in order to block radiation from escaping the core and affecting the workers ini the power station. This concrete may become radioactive after a while because the nuclei in the shielding will absorb neutrons from the reactor and become unstable so start decaying by beta-minus decay.
What is high-level waste?
Also how do we dispose o high level waste?
The spent fuel rods contain the daughter nuclei produced in the fission reactions of U-235 which are extremely unstable and radioactive and also have a very high activity. These daughter nuclei in the fuel rods will remain radioactive for thousands of years so have a very long-half life.
-The waste is removed and handled remotely
-The fuel rods are placed in cooling ponds because they are extremely hot due to the fission reactions occurring in the reactor. They can remain in these cooling ponds for up to a year. These Cooling ponds are usually found very close to the reactor so the fuel rods do not have to be transported through are distances which would increase risk of exposure.
-Any usable Plutonium or Uranium is extracted from the fuel rods to be recycled.
-The waste is then vitrified and stored in thick steel casks and stored in geologically stable locations deep underground so there is no chance of the radioactive high-level waste coming free of it’s casing. Locations are also chosen so they have minimal impact on the environment.
