8: Nuclear Physics Flashcards by harry Chapman

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

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What was observed as a result of the Rutherford scattering experiment?

-Most particles went straight through
-However some particles were deflected or reflected back

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What was inferred as a result of the Rutherford scattering experiment?

-Atom was mostly empty space
-Nucleus had a positive charge

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What are the properties of alpha radiation?

-Strong ionisation
-Weak penetration
-Can be stopped by air or a thin sheet of paper

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What are the properties of beta radiation?

-Medium ionisation
-Medium penetration
-Can be stopped by a few cms of aluminium foil

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What are the properties of gamma radiation?

-Weak ionisation
-Strong penetration
-Can be stopped by lead/concrete

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Why is ionising radiation dangerous?

-It can kill/mutate cells
-Leads to mutations and can cause cancer

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Which type of radiation follows the inverse-square law?

-Gamma

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What is the inverse square law?

-The intensity is inversely proportional to the square of the distance from the source
-I=k/r^2

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What safety measures are involved when handling radiation?

-Store in lead-lined containers
-Handle with tongs or wear gloves
-Minimise the exposure time

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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

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What is background radiation?

-Radiation that is constantly in the surrounding areas

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What are some sources of background radiation?

-The air
-Medical procedures
-Buildings
-The sun

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What is the decay constant (λ)?

-The probability of the nucleus decaying each second

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What is the equation for random decay?

-Δ N / Δt = - λN

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How is the number of atoms calculated for a radioactive sample?

-N=N0.e^(-λt)

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How is the activity calculated?

-The decay constant multiplied by the number of atoms
-A=λN
-A=A0.e^(-λt)

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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