3.8 Nuclear Physics Flashcards
What is meant by activity?
- The rate of decay of the radioactive nuclei in a given isotope
- It is proportional to the total number of undecayed nuclei in the sample
What is activity measured in?
Becquerels
Alpha decay:
What is it?
How ionising?
Stopped by?
- The emission of an alpha particle (2 protons and 2 neutrons) from an unstable nucleus (usually one with too many nucleons) to make it more stable.
- Alpha radiation is strongly ionising and is stopped by a few centimetres of air or a sheet of paper.
Beta decay:
What is it?
How ionising?
Stopped by?
- The emission of a beta particle when a proton turns into a neutron (or vice versa) in an unstable nucleus.
- Beta minus radiation is weakly ionising.
- stopped by 3mm of aluminium foil but beta plus radiation is immediately annihilated by an electron
What is binding energy?
The amount of energy required to split a nucleus into all its separate constituent nucleons. It is equivalent to the mass defect.
What is a chain reaction?
The process by which neutrons released by a fission reaction induce further fissile nuclei to undergo fission.
What can closest approach be used for?
Estimating a nuclear radius (by firing a alpha particle at it)
What is contamination?
The introduction of radioactive material to another object. The object is consequently radioactive.
What is meant by the critical mass?
The minimum mass of fissile material required in a fission reactor for a chain reaction to be sustained.
What is electron capture?
A process that occurs in proton-heavy nuclei, in which an electron is drawn into the nucleus, causing a proton to transition into a neutron. An electron neutrino is also produced.
Describe the process of fission
- An unstable nucleus absorbs a thermal neutron
- The nucleus of fuel splits into two smaller daughter nuclei.
Releasing two or three fast moving neutrons, and a lot of energy.
What is meant by fusion?
The joining of two smaller nuclei to form a larger nucleus and to release energy.
Gamma radiation:
What is it?
How ionising?
Stopped by?
- The emission of Gamma rays from an unstable nucleus that has too much energy.
- Gamma radiation is very weakly ionising but requires several centimetres of lead to be stopped.
Define half-life
The average time it takes for the number of radioactive nuclei in a sample to halve
What is irradiation?
The exposure of an object to radiation. The exposed object does not become radioactive.
What is the mass defect?
The difference in mass between a nucleus and the sum of the masses of its constituent nucleons.
What is radioactive dating? What isotope is commonly used?
- The use of radioactive isotopes with known half-lives to date objects
- The isotope that is usually used is Carbon-14
What does Rutherford scattering show?
- The existence and nature of the nucleus.
- That the nucleus is small, dense and positively charged
3 observations and conclusions from Rutherford scattering experiment
Most alpha particles passed straight through the foil with no deflection
- atom is mostly empty space (and not a uniform density)
A small amount of particles were deflected by a large angle
- centre of the atom is positively charged
Very few particles were deflected back by more than 90 degrees
- centre of the atom was very dense - charge is concentrated in centre
Applications of beta radiation
Thickness measurements of paper and aluminium foil
Which radiation follows inverse square law?
Gamma radiation
How to verify inverse square law of gamma radiation?
- Measure the count rate of a gamma source at different distances from the GM tube (making sure to adjust for the background radiation)
- then plot a graph of corrected count against 1/x^2 , which will form a straight line verifying the above equation.
Danger of radiation if gets in body
Can cause mutations and damage to cells (which can lead to cancer etc.)
3 sources of background radiation
● Radon gas - which is released from rocks
● Cosmic rays - which enter the Earth’s atmosphere from space
● Artificial sources - caused by nuclear weapons testing and nuclear meltdowns
How can waste with a long half-life be stored?
Underground in steel casks (to prevent damage to environment and people in future)
Decay that happens if nucleus has too many neutrons
Beta minus decay (so that a neutron changes into a proton)
Decay that happens if nucleus has too many protons
Beta-plus emission or electron capture
Decay that happens if nucleus has too many nucleons
Alpha emission
What happens if nucleus has too much energy?
Gamma emission (usually occurs after a different type of decay, as nucleus becomes excited)
4 reasons technetium-99m is useful in medical diagnosis
- It only emits gamma rays (which are weakly ionising)
- Half-life (6hrs) short enough to not remain in body for too long after the medical examination
- Half-life (6hrs) long enough to complete the diagnosis
- Can be made near to the hospital
How to estimate nuclear radius using closest approach
- All kinetic energy is converted into electric potential energy
- (gives an overestimate - doesn’t include distance between alpha particle and nucleus, which won’t touch)
2 ways to calculate nuclear radius
- Distance of closest approach
- Electron diffraction
Why is energy released during nuclear fission?
The smaller daughter nuclei have a higher binding energy per nucleon than the original unstable nucleus
Why is energy released during nuclear fusion?
The larger nucleus has a much higher binding energy per nucleon
After what element do elements begin to undergo fission as opposed to fusion? Why?
Iron - it has the highest binding energy per nucleon
How to calculate energy released during fission?
Mass defect = Mass before - Mass after
Mass defect * 931.5MeV (or use E = mc^2)