Nuclear physics Flashcards
Describe the history of the atom from 500 BC to 1984
- Democritus proposed that matter was made up of clumps called atomos in 500 BC
- Dalton believed that matter was made up of atoms can couldn’t be broken down and each element was made up of a different type in 1984
Describe the recent history of the atom
- JJ Thompson discovered electrons so Dalton was wrong as atoms could be broken up. He proposed a positively charged sphere with electrons scattered throughout
- Rutherford discovered the nucleus so discovered that the charge and density were not distributed equally
Describe the method of Rutherford’s experiment
- Fire beam of alpha particles at a thin gold sheet from an alpha source
- Put a circular detector behind to detect the angle they are detected
What were Rutherford’s predictions for his experiment
All alpha particles would be deflected by a very small amount due to positively charged sphere proposed by JJ Thompson (small amount because charge is very spread out)
What actually happened during Rutherford’s experiment
- Most of the alpha particles were not deflected at all
- However, some were deflected by large amounts with a few even by more than 90 degrees i.e. sent back where they came
What were the conclusions drawn from the results of Rutherford’s experiment
- Most of the atom is empty space as most weren’t deflected
- The nucleus must be very small as only a few alpha particles we deflected
- The nucleus must have a very large positive charge as some alpha particles were deflected by more than 90 degrees
What is the closest approach in Rutherford’s experiment
- Closest approach is the closest distance that the alpha particle gets to the gold nucleus before being deflected back
- This distance is an estimate for the nuclear radius
- At this point the alpha particle is stationary as it needs to stop for a split second before turning back (where the distance would start to increase again)
How can you calculate closest approach
It has stopped because the KE of the alpha particle is now = to the Electrical energy between the alpha particle and the nucleus
So put KE = EE and solve for r
How can you find the nuclear radius more accurately using electron diffraction
- Find the de broglie wavelgnth of the elctrons using λ = hc / E where E is the energy of electrons
- Fire high E electrons (so low λ) at a thin film and electrons will diffract through gaps between nuclei
- First minimum appears where sinθ = 1.22λ / 2R where R is the nuclear radius
What does an electron diffraction interference pattern look like
Equally spaced maxima of decreasing intensity
Why does radioactive decay occur
Nuclei become unstable due to too many protons, neutrons or just energy in general so release energy (alpha beta or gamma) until they become stable
Describe the differences in ionising power of alpha, beta and gamma
Alpha is strongly ionising
Beta is weakly ionising
Gamma is very weakly ionising
Describe the differences in speed of alpha, beta and gamma particles
Alpha is slow
Beta is fast
Gamma moves at speed of light
Which of the 3 types of radiation is not affected by a magnetic field
Gamma
What are the relative charges and masses of each type of radiation
- Alpha has a charge of +2 and mass of 4 (2ps 2ns)
- Beta (-) has charge of -1 and mass of 0 (1 e)
- Gamma has charge 0 and mass 0 (EM wave)
What are the penetrating powers of each type of radiation
Alpha can travel 2 - 10 cm in air and absorbed by paper
Beta can travel 1 m in air and absorbed by 3mm of aluminium foil
Gamma has infinite range in air and absorbed by several metres of concrete or several inches of lead
Describe a use of alpha particles
Used in smoke alarms as they are able to ionise air molecules causing current to flow but when smoke slows down ions, current stops and alarm sounds
Describe a use of beta particles
Control thickness of aluminium material in manufacturing process by flattening if too little beta is being detected on the other side and pulling apart if too much
Describe 2 uses of gamma radiation
PET scanners where a patient consumes a radioactive substance with a short half life and the scanner detects the gamma rays emitted to produce a detailed image
Killing cancerous cells (but could have side affects by damaging healthy cells)
What is background radiation
A Geiger counter would detect some radiation even if there is no source present known as background radiation
What are the sources of background radiation
Air, rocks (e.g. in ground and buildings), cosmic rays, all living things
What is the inverse square law during the emission of gamma radiation
Intensity is inversely proportional to square of distance from source
so I = k / x^2
This is why we should stay far away from source for safety
What is the relationship between nuclear radius and nucleon number
R = R0 A^1/3 where R is nuclear radius, A is nucleon number and R0 is a constant
Describe a graph of R against A and R against A^1/3
R against A is curved with decreasing gradient as R ∝ A^1/3 so as A increases R doesn’t increase by as much
R against A^1/3 is a straight line passing through the origin
