8: Nuclear Physics Flashcards
What was the Rutherford scattering experiment?
-A beam of alpha particles was fired at a piece of thin gold foil
-Occurred in a vacuum to avoid collisions with air particles
-Carried out to determine structure of nucleus
What was observed as a result of the Rutherford scattering experiment?
-Most particles went straight through
-However some particles were deflected or reflected back
What was inferred as a result of the Rutherford scattering experiment?
-Atom was mostly empty space
-Nucleus had a positive charge
What are the properties of alpha radiation?
-Strong ionisation
-Weak penetration
-Can be stopped by air or a thin sheet of paper
What are the properties of beta radiation?
-Medium ionisation
-Medium penetration
-Can be stopped by a few cms of aluminium foil
What are the properties of gamma radiation?
-Weak ionisation
-Strong penetration
-Can be stopped by lead/concrete
Why is ionising radiation dangerous?
-It can kill/mutate cells
-Leads to mutations and can cause cancer
Which type of radiation follows the inverse-square law?
-Gamma
What is the inverse square law?
-The intensity is inversely proportional to the square of the distance from the source
-I=k/r^2
What safety measures are involved when handling radiation?
-Store in lead-lined containers
-Handle with tongs or wear gloves
-Minimise the exposure time
What is an experiment that can be done to show that the inverse-square law applies to gamma radiation?
-Firstly measure background radiation without the gamma source in the room
-Then put the gamma source at a set distance from the GM tube and measure the count rate per minute
-Record 3 measurements for each distance and take an average.
-Do this for many distances going up in 10cm intervals –Take away the background radiation from each reading to find the corrected count rate
-Plot a graph of the count rate per minute against 1 over distance squared (1/d²)
-If it is a straight line through the origin then it confirms they are directly proportional
What is background radiation?
-Radiation that is constantly in the surrounding areas
What are some sources of background radiation?
-The air
-Medical procedures
-Buildings
-The sun
What is the decay constant (λ)?
-The probability of the nucleus decaying each second
What is the equation for random decay?
-Δ N / Δt = - λN
How is the number of atoms calculated for a radioactive sample?
-N=N0.e^(-λt)
How is the activity calculated?
-The decay constant multiplied by the number of atoms
-A=λN
-A=A0.e^(-λt)
What is the half-life of a substance?
-The time taken for the activity of a radioactive substance to half.
How is the half-life calculated?
T(1/2)=ln2/λ
Why is technetium used as a tracer?
-Releases gamma radiation
-Has a short half-life
-Easy to detect
How is the distance of closest approach calculated?
-All kinetic energy is transferred to potential energy
-r=(2kQq)/(mv^2)
How is electron diffraction used to determine the diameter of a radius?
-An electron beam is fired at a thin sheet of the atom
-A diffraction pattern is produced on a screen behind
-The angle to the minima can be used to find the diameter
How is nuclear radius proportional to the number of nucleons?
-The nuclear radius is proportional to the cube root of the number of nucleons
-R=r0.A^(1/3)
What is the typical value for the radius of one atom?
-r0=1.5fm
How can the relationship between the nuclear radius and the number of nucleons be shown?
-Measure the radius of the nucleus and record the number of nucleons for multiple elements
-Plot a graph of lnR on the y-axis against lnA on the x-axis
-The gradient should be a 1/3 with a y-intercept of r0
How is nuclear density calculated?
-Mass/volume
-(3A*u)/(4πr^3)
What equation converts mass to energy?
-E=mc^2
What is the mass defect?
-The difference between the total mass of the nucleons separately and the mass of those same nucleons when they are in the nucleus
What is the binding energy?
-The energy released when neutrons and protons are assembled into a nucleus
Why is there a mass defect?
-Energy is needed to bring the nucleons together
-As a result the mass equivalent of the energy is lost and the total mass decreases
What is nuclear fission?
-When an unstable nucleus splits into two smaller daughter nuclei
-The binding energy per nucleon increases as a result and so this causes a release of energy
What is nuclear fusion?
-Two smaller nuclei fuse together to create a single larger nuclei
-This larger nuclei has a greater binding energy per nucleon so energy is released
Why is it difficult to initiate fusion reactions on earth?
-There is a large repulsion due to the electrostatic force between the two nuclei
-Therefore a large amount of energy is required to fuse the two nuclei together
-A lot of heat is produced and it is hard and expensive to find a material that is suitable
What is induced fission?
-Thermal neutrons are fired at U-235
-This creates u-238 which is unstable
-The U-238 undergoes fission and splits into two daughter products releasing energy and three neutrons
-These neutrons then collide with more U-235 causing a chain reaction and more fission
What is the purpose of the moderator?
-To slow down the thermal neutrons
-The neutrons must be at a slow enough speed to be absorbed by the U-235
What type of collisions occur in a nuclear reactor?
-Elastic collisions
What is the purpose of the control rods?
-To stop the chain reactions
-They absorb neutrons to reduce the number of fission reactions
-Too many fission reactions would cause the reactor to overheat
What is a typical moderator used in reactors?
-Water
What is the purpose of the coolant?
-To reduce the temperature of the reactor so that it doesn’t overheat
What is the critical mass?
-The minimum mass that the U-235 must have to undergo fission
How is high-level waste disposed of?
-Stored in cooling ponds
-Then sealed in steel containers and placed deep underwater
How is low-level waste disposed of?
-Sealed in containers and placed underground
Information that can’t be put on flashcards
-The N-Z curve and what decay elements on the N-Z curve undergo
