Nuclear Physics Flashcards
Define Activity
the rate of decay of the radioactive nuclei in a given isotope. It is proportional to the total number of nuclei in the sample and is measured in becquerels.
Desceribe the Rutherford scattering experiment.
1) A beam of alpha particles was directed at a thin gold foil
2) occurs in a vacuum so that no collisions between air particles and alpha particles can occur.
3) The experiment w
What did the alpha particles do when fired at the gold foil.
Most just went straight through the foil
While some were deflected at an angle.
Some were deflected by more than 90 degrees - sending them back from where they came from.
What were conclusions of the Rutherford scattering experiment?
1) Most of the atom must be empty space as alpha particles passed straight through the foil
2) The nucleus must have a large positive charge, as some were repelled and deflected by a large angle.
3) The nucleus must be very small as very few alpha particles were deflected back
4) Most of the mass must be in the nucleus, since the fast alpha particles (high momentum) are deflected by the nucleus
What is the range of alpha, beta and gamma in air?
Alpha - 2-10cm
Beta - 1m
Gamma - infinite range:follows inverse square law
What are alpha, beta and gamma absorbed by?
Alpha - paper
Beta - Aluminium foil (3mm)
gamma - several metres of concrete or several inches of lead
Which types of radiation get deflected by magnetic fields?
Alpha and beta
What are some applications for alpha beta and gamma?
Alpha - Smoke alarms because they have a short range but highly ionising
Beta - controlling thickness of materials
Gamma - used in medicine
what is the inverse square law for gamma radiation?
I = k/x^2
Explain the experimental verification for the inverse square law.
As gamma radiation moves through the air it spreads out in all directions equally, as you move further away from the source. As you move further away the radiation per unit area will decrease.
The experiment: making measurements of intensity at different distance from the gamma source, using Geiger counter. If distance is doubled the intensity should fall to a quarter - verifying the inverse square law. It would form a straight line verifying the equation.
what are some origins of background radiation?
Also how do I get rid of it when I’m conducting an experiment?
The air - radon gas released from rocks.
The ground and buildings - all rocks contain radioactive isotopes
Cosmic radiation
Living things - carbon-14
Man-made radiation - medicines
Corrected count = total count rate - background count
What does it mean when radioactive decay has a random nature
Radioactive decay is random and you cannot predict when a nucleus will decay or which nucleus will decay next
Define the decay constant
the probability of a nucleus decaying per unit time.
How can decay constant be calculated?
Can be calculated by finding the change in number of nuclei (ΔN) of a sample over time (Δt), over the initial number of nuclei (N).
ΔΝ/Δt = -λN
N = N0e^-λt
Define half-life
The half-life (T1/2) of an isotope is then average time it takes for the number of unstable nuclei to halve.
Draw a graph of the number of unstable nuclei remaining against time.
Check notes
Draw the log graph of number of the activity against time.
Check notes
Give some applications of organic substances
Radioactive dating of objects - carob 14 used in radioactive dating. Living plants absorb carbon 14, which decays.
Medical diagnosis - radioactive tracers are used to help diagnose patients. Technetium-99m as a gamma source in medical diagnosis.
How can long half-lives be dangerous and how should they be stored?
Very long halve-lives mean they will be highly radioactive for a long time, so need to be sealed underground to prevent harm to the environment and people.
What does ‘Z’ stand for in element notation?
The proton number (atomic number)
What does ‘A’ stand for in element notation?
The nucleon number or mass number
How are nuclei held together?
Held together by the strong nuclear force, however protons feel a force of repulsion due to the electromagnetic force and so if the forces are out of balance, the nuclei will become unstable and will experience radioactive decay.
What’s are the 4 reasons why a nuclei will become unstable.
1) It has too many neutrons - decays via beta-minus emission
2) It has too many protons - decays via beta-plus emission or electron capture.
3) has too many nucleons - decays via alpha emission
4) too much energy - decays via gamma emission ( nucleus becomes excited and has excess energy).
