Nuclear Physics Flashcards
Describe the ‘plum pudding’ model of the atom
Sphere of positive charge with tiny negatively-charged electrons stuck in it
Describe the setup of Rutherford scattering experiment
- A stream of alpha particles from a radioactive source are fired at a very thin gold foil
- Alpha particles from a radioactive source strike a fluorescent screen, producing a tiny visible flash of light
- The screen is circular, surrounding the experiment to detect alpha particles scattered at any angle
What was the predicted result of the Rutherford scattering experiment?
All the flashes would have been seen within a small angle of the beam
What was the true result of the Rutherford scattering experiment?
Most particles passed through the gold foil, some scattered at angles greater that 90 degrees, and few reflected backward
What were the conclusions of the Rutherford scattering experiment?
- The atom must be mostly empty as most of the particles passed through the foil
- The nucleus must have a very large positive charge as some of the positively charged alpha particles are repelled and deflected by a large angle
- The nucleus must be tiny as very few alpha particles are deflected by angles greater than 90 degrees
- Most of the mass must be in the nucleus, since fast moving alpha particles are deflected by the nucleus
What letter is used for the atomic number of an atom?
A
What letter is used for the proton number of an atom?
Z
What are the 2 methods of estimating values for nuclear radii?
- By using Rutherford’s scattering experiment to calculate the distance to closest approach of a scattered alpha particle
- Electron diffraction
State the conversion for eV to J
1eV = 1.6x10^-19 J
State the closest approach equation
Initial KE = Electric Potential = Qq/4πε0r , where r is the distance of closest approach
Why do the electrons from the electron beam have a very high energy in electron diffraction?
As the wavelength must be very tiny, 10^-15, to investigate the nuclear radius. λ = hc/E
What is the equation for the first minimum in electron diffraction?
sinθ = 1.22λ/2R, where R = the radius of the nucleus the electrons have been scattered by
Describe the graph showing the variation of intensity in an electron diffraction pattern
- A central bright maximum containing the majority of the incident electrons
- Maximum surrounded by other dimmer maxima
- The intensity of the maxima decreases as the angle of diffraction increases
State the approximate radius of an atom
0.05nm = 5 x 10^-11m
State the approximate radius of an atomic nucleus
1fm = 1 x 10^-15m
As the nucleon number increases, what happens to the radius of the nucleus
Radius of nucleus increases
What is the relationship between nuclear radius and the cube root of the nucleon number?
Directly proportional
What is the value of the constant Ro?
1.4fm = 1.4x10^-15m
What is the relationship between nuclear volume and nucleon number, giving the reason?
- Nuclear volume is directly proportional to the nucleon number
- If R ∝ 3√A, then R³ ∝ A = V ∝ A
Derive the proof that density of nuclear mass is constant
- p = m/V
- p = A x nucleon mass/4/3πr³
- p = A x nucleon mass/4/3π(Ro3√A)³
- p = A x nucleon mass/4/3πRo³A
- p = 3 x nucleon mass/4πRo³ = constant
Which is greater, nuclear density or atomic density?
Nuclear density
State the 3 conclusions about nuclear density being greater than atomic density
- Most of an atom’s mass is in its nucleus
- The nucleus is small compared to the atom
- An atom must contain a lot of empty space
What is radioactive decay?
When an unstable atom breaks down to become more stable, by releasing energy and/or particles
What is the constituent of alpha radiation?
A helium nucleus. 2 neutrons and 2 protons
What is the relative charge of alpha radiation?
+2
What is the approximate mass of alpha radiation, in u?
4u
What is the constituent of beta-minus(beta) radiation?
An electron
What is the relative charge of beta-minus radiation?
-1
What is the mass of beta-minus radiation, in u?
Negligible
What is the constituent of beta-plus radiation?
A positron
What is the relative charge of beta-plus radiation?
+1
What is the mass of beta-plus radiation, in u?
Negligible
What is the constituent of gamma radiation?
A short wavelength, high frequency EM wave
What is the relative charge of gamma radiation?
0
What is the mass of gamma radiation, in u?
0
What is the penetrating power of alpha radiation?
Absorbed by paper, skin or a few centimetres of air
What is the penetrating power of beta-minus radiation?
Absorbed by about 3 millimetres of aluminium
What is the penetrating power of gamma radiation?
Absorbed by many centimetres of lead, or several centimetres of concrete
Which radiation can pass through thin mica?
All of the radiatons
Why does beta-plus radiation have no penetrating power?
As beta-plus particles almost immediately annihilate with electrons, so they effectively have zero range
Describe how to identify the nuclear radiation of an unknown nuclear source
- Record the background count rate with no source present
- Place the unknown source near a Geiger-Muller tube and record the count rate
- Place a sheet of paper between the source and tube and record the count rate
- Replace the paper with a 3mm thick sheet of aluminium and record the count rate
- For each count rate recorded, subtract the count rate of background radiation
What is the ionisation rate of alpha radiation?
10,000 ionisations per mm of air
State an application of alpha radiation, explaining how it works
Smoke alarms: Alpha sources allow current to flow as they can quickly ionise many atoms, but they won’t travel far. When smoke is present, alpha particles can’t reach detector which sets off alarm
What is the ionisation rate of beta-minus radiation?
100 ionisations per mm of air
State an application of beta-minus radiation, explaining how it works
Creating sheets of material: A beta source and beta detector are placed either side of a material. The material is passed through adjustable rollers and if too little beta particles are detected, the rollers move closer together and vice versa
What is the ionisation rate of gamma radiation?
<100 ionisations per mm of air
State an application of gamma radiation, explaining how it works
PET scan: A radioactive tracer is injected into the bloodstream. Positrons from the tracer annihilate with electrons from the body, producing gamma radiation that can be detected by the scanner
Which radiation type has strong ionising power?
Alpha radiation
Which radiation type has moderately weak ionising power?
Beta-minus radiation
Which radiation type has very weak ionising power?
Gamma radiation
Which radiation type has no ionising power?
Beta-plus radiation
Which radiation type travels at a fast speed?
Beta-minus radiation
Which radiation type doesn’t have a speed?
Beta-plus radiation
Which radiation type travels at the speed of light?
Gamma radiation
Which radiation type travels at a slow speed?
Alpha radiation
Which radiation types are affected by magnetic fields?
- Alpha radiation
- Beta-minus radiation
Which radiation type are not affected by magnetic fields?
- Beta-plus radiation
- Gamma radiation
Define background radiation
The weak level of nuclear radiation found everywhere
State 5 types of sources background radiation and what caused them
- The air: Radioactive radon gas is released from rocks, emitting alpha radiation
- The ground and buildings: Nearly all rock contain radioactive materials
- Cosmic radiation: Cosmic rays are particles from space which collide with particles in the atmosphere to produce nuclear radiation
- Living things: All plants and animals contain carbon, some being radioactive carbon-14
- Man made radiation: Radiation from medical or industrial sources
What is the largest contributor to background radiation?
The air: Radioactive radon gas released from rocks, emitting alpha radiation
Define intensity of radiation
The amount of radiation per unit area
State the units for intensity
W/m²
What is the relationship between intensity and distance from the source
- The intensity is inversely proportional to the square of the distance from the source
- i = k/x² , where k =constant of proportionality
Describe the graph of how intensity varies with distance from the source
- As distance from the source increases, the intensity decreases by the square of the distance moved
- L shaped
- intensity never touches zero
State 4 ways of handling radioactive sources safely
- Keep the source as far away as possible when transporting it
- Use long handling tongs to minimise radiation absorbed
- Store the source in a lead box
- Remove the source from the lead box for the shortest time possible, only when needed
What are isotopes?
Elements with the same number of protons but a different number of neutrons
Define activity
The number of unstable nuclei in a radioactive sample decaying per second
State the units for activity and what it equates to
- Bq
- 1Bq = 1 decay per second
What is the decay constant?
The probability of an unstable nuclei decaying per unit time
What does “N” stand for in the equation for activity, A = λN?
Number of unstable nuclei in sample
What are the units of the decay constant?
1/s
State the decay equation
N = Noe^-λt
What is the relationship between the number of unstable nuclei and time?
As time increases, the number of unstable nuclei exponentially decreases
What is binding energy?
Amount of energy needed to divide a nucleus into its constituents
Describe the graph of the natural log of the number of unstable nuclei against time
- y intercept is the natural log of the original number of unstable nuclei
- Constant negative gradient
- Gradient = -λ
What is corrected count rate?
The count rate of a radioactive source one the value background count rate is removed
What is the decay constant?
The probability of a decay happening in a second
What is the role of a moderator in a nuclear reactor?
To slow down the neutrons to the equivalent of thermal neutrons, to allow fission to occur. Usually water.
What is the role of control rods in a nuclear reactor?
To absorb excess neutrons to prevent too many fission reactions happening. Usually made of boron.
Define half life
The average time taken for the number of unstable nuclei in an isotope to half
What is a radioisotope?
A radioactive isotope
State 3 applications of radioactive isotopes
- Radioactive dating
- Medical diagnosis
- Storage of radioactive waste
What radioisotope is used in radioactive dating?
Carbon-14
Describe how carbon-14 is used in radioactive dating
- Living plants take in carbon dioxide and carbon-14 from the atmosphere as a part of photosynthesis
- When they die, the activity of carbon-14 starts to fall
- The current amount of carbon-14 in the plant is found and this is used to date them
State 3 problems of radioactive dating
- The object may have been contaminated by other radioactive sources
- There may be a high background count that obscures the object’s count
- There may be uncertainty in the amount of carbon-14 that existed thousands of years ago
- The sample size or count rate may be too small, and so might be statistically unreliable
What radioisotope is mostly used in medical diagnosis?
Technetium-99m
Describe how medical diagnosis using radioisotopes works
A tracer containing technetium-99m is ingested by the patient
- The radiation emitted as it passes through the patient’s body to the region of interest is recorded and an image of inside the patient is produced
What is the half-life of technetium-99m?
6 hours
What is the half-life of carbon-14?
Around 5730 years
Describe how storage of radioactive waste with radioisotopes works
- Uranium-235 is used in nuclear fission reactors and the radioisotopes it decays into have different half lives
- These isotopes emit alpha, beta and gamma radiation and must be stored carefully for hundreds of years until their activity falls to safe levels
- Their long half lives make them dangerous as they make them highly radioactive for a long time
State 4 reasons why a nucleus will be unstable
- Too many neutrons
- Too few neutrons
- Too much energy
- Too heavy due to too many nucleons
What does alpha emission occur for?
Very heavy atoms
What does beta-minus emission occur for?
Neutron rich isotopes
What does beta-plus emission occur for?
Proton rich isotopes
What does gamma emission occur for?
- Atoms with too much energy
- There is no change to the nuclear constituents
