Nuclear Physics Flashcards
What did J.J. Thompson discover?
He discovered that electrons could be removed, so came up with the Plum Pudding model of the atom
What is the Plum Pudding model?
That atoms were spheres of positive charge with negative electrons stuck in them
What are alpha particles?
A Helium nucleus made up of 2 protons and 2 neutrons
Describe the Rutherford Alpha Particle Scattering experiment
A stream of alpha particles where fired from a radioactive source at a very thin gold foil
In the Rutherford Scattering experiment, how could they tell where the alpha particles were being projected?
They surrounded the experiment with a fluorescent screen, because when an alpha particle hits a fluorescent screen a tiny visible light flash is produced
If the Plum Pudding model had been corrected, what would Rutherford have found when he did the Alpha Particle Scattering Experiment?
All the alpha particles would’ve been within a small angle of the beam
How did the Alpha Particle Scattering Experiment show that the Plum Pudding model was incorrect?
Because most of the alpha particles passed straight through or were scattered at angles greater than 90°
What were 4 conclusions from the Rutherford Scattering experiment?
- The atom must be mostly empty space
- The nucleus must have a positive charge
- The nucleus must be tiny
- Most of the mass was concentrated at the centre
How did the Rutherford Scattering experiment show that the nucleus must be tiny?
Because very few alpha particles were deflected at an angle greater than 90°
How did the Rutherford Scattering experiment show that the atom must be mostly empty space?
Because most of the alpha particles just passed straight through
How did the Rutherford Scattering experiment show that the nucleus must have a positive charge?
As some of the positively-charged alpha particles are repelled and deflected at a large angle
How did the Rutherford Scattering experiment show that most of the mass was concentrated at the centre?
Because the fast alpha particles (with high momentum) are deflected by the nucleus
Who discovered the proton?
Rutherford and William Kay
Who discovered the neutron?
James Chadwick
How can you estimate the radius of an atomic nucleus using Rutherford’s Scattering Experiment?
An alpha particle which is deflected back through 180° will have stopped a very short distance away from the nucleus, giving you an estimate of the radius
At what point does an alpha particle stop when being deflected through 180° by the nucleus?
When its electric potential energy equals its initial kinetic energy
What is the equation for initial kinetic energy of an alpha particle in Rutherford’s Scattering Experiment?
Initial Ek = ((Qnucelus) x (q alpha)) / 4πε0r
Initial Ek: Initial kinetic energy Qnucelus: Charge on nucleus q alpha: Charge of alpha particle ε0: Permittivity of free space constant r: Distance of closest approach
What is the charge of an alpha particle?
+2, because it has 2 protons
How do you find the charge of a nucleus?
You need to find out the atoms proton number, then multiply this value by the magnitude of the charge of an electron (1.6x10^-19 C)
What is a better method of estimating the radius of a nucleus, other than using the distance of closest approach?
Electron diffraction
How do you convert between electronvolts (eV) and joules (J)?
Multiply by the charge of an electron
What type of particle are electrons?
Leptons
Why is electron diffraction a more accurate method of estimating the radius of a nucleus?
Because leptons (which an electron is a type of) don’t interact with the strong nuclear force (unlike neutrons and alpha particles)
What is the de Broglie wavelength?
The wavelength associated with a particle, as part of the theory of wave-particle duality
What is the equation for the de Broglie wavelength of an electron?
λ ≃ hc / E
λ: de Broglie wavelength
h: the Planck constant
c: Speed of light in a vacuum
E: Energy of the electron
What must the wavelength and energy of the electron be like to investigate the radius of a nucleus?
Must have a tiny wavelength and a very high energy
What happens when a beam of high-energy electrons is directed onto a thin film of material in front of a screen?
A diffraction pattern will be seen on the screen
When a beam of high-energy electrons is directed onto a thin film of material in front of a screen, why is the equation for a minimum rather than maximum (like a diffraction grating in Yr1)?
Because each high-energy electron is diffracted by an individual nucleus
What is the equation for the first minimum of electron diffraction?
sinθ ≃ 1.22λ / 2R
sinθ: Scattering angle for first minimum
λ: de Broglie wavelength of an electron
R: Radius of the nucleus the electrons have been scattered by
Describe the diffraction pattern of electron diffraction
A central bright maximum (containing most of the incident electrons) followed by other dimmer maxima
How is the intensity of the maxima affected by the angle of diffraction?
As the angle of diffraction increases, the intensity of the maxima decreases
What is the radius of an atom?
0.05 nm (5x10^-11 m)
What is the radius of the smallest nucleus?
1 fm (1x10^-15 m)
What does the size of a molecule equal?
The number of atoms in it multiplied by the size of one atom
What are nucleons?
Particles that make up the nucleus (protons and neutrons)
What is the mass number?
The number of nucleons that make up a nucleus
What is the relationship between nucleon number and radius of nucleus?
If you plot nuclear radius against the cube root of the nucleon number, the line of best fit is a straight line. So as the nucleon number increases, the radius of the nucleus increases proportionally to the cube root of the nucleon number
What is the equation for the radius of a nucleus?
R = R0 A^1/3
R: Radius of nucleus
R0: Constant = 1.4 fm
A: Nucleon number
What does the equation for the radius of a nucleus (R = R0 A^1/3) show us about nucleons?
They have roughly the same volume
What theory does the the equation for the radius of a nucleus (R = R0 A^1/3) provide evidence for?
That the density of matter is constant, regardless of how many nucleons make up the nucleus
What is the volume of a nucleon equal to (presuming that they’re spherical)?
4/3(π)(R^3)
What is the equation for the mass of a nucleus with a nucleon number A?
Mass = A x Mass of nucleon
How do you prove that two different nuclei with different nucleon numbers have a roughly equal density?
Volume of a nucleon = 4/3(π)(R^3)
Mass of nucleus = A x Mass of nucleon
Radius of nucleus = R0 A^1/3
Density = Mass / Volume
= (3 x Mass of nucleon) / 4π(R0)^3
=Constant
What assumption is made when calculating the volume of a nucleon?
That it’s spherical
Which is greater, atomic density or nuclear density?
Nuclear density
Due to nuclear density being much greater than atomic density, what 3 things does this tell us about the structure of an atom?
- Most of the atom’s mass is in the nucleus
- The nucleus is small compared to the atom
- An atom must contain a lot of empty space
What will happen to an atomic nucleus which is unstable?
It will decay to become more stable
How does an unstable nucleus ‘break down’ to become more stable, and what is this process called?
Radioactive decay
The nucleus decays by releasing energy and/or particles, until it reaches a stable form
What is an important characteristic of radioactive decay?
Each individual radioactive decay is random and can’t be predicted
What are the 4 types of nuclear radiation?
Alpha
Beta-minus
Beta-plus
Gamma
What are the constituents of alpha radiation?
A helium nucleus - 2 protons, 2 neutrons
What is the constituent of beta (beta-minus) radiation?
Electron
What is the constituent of beta-plus radiation?
Positron
What are the constituents of gamma?
Short-wavelength, high-frequency EM wave
Give 3 examples of things that absorb alpha radiation (linked to penetrating power)?
Paper
Skin
A few centimetres in air
Give an example of something that absorbs beta radiation (linked to penetrating power)?
Around 3mm of aluminium
Give 2 examples of things that absorb gamma radiation (linked to penetrating power)?
Many centimetres of lead
Several metres of concrete
How, and why, does the skin protect us from alpha radiation?
Alpha radiation is stopped by the layer of dead skin cells on a person’s body - the radiation is stopped before it has a chance to reach live cells which would be ionised
What 2 factors affect whether radiation can penetrate through it?
Thickness
Density
What is a Geiger–Müller tube?
A piece of equipment which produces a ‘count’ in the form of an electrical impulse each time radiation enters it
What is the count rate?
The number of counts per second from a Geiger–Müller tube
What are the 6 steps to test what kind of radiation is being emitted by a source?
- Record the background radiation count rate when no source is present
- Place an unknown source near a Geiger–Müller tube and record the count rate
- Place a sheet of paper between the source and Geiger–Müller tube and record the count rate
- Replace the piece of paper with 3mm of aluminium, record the count rate
- For each count rate, take away the measured count rate with no source present to find actual count rate
- If there is a big reduction in count rate, this will show you which types of radiation can penetrate through the paper or aluminium
Which type of radiation is no deflected by a magnetic field?
Gamma
Why is gamma radiation not deflected by a magnetic field?
It isn’t made up of charged particles
Describe the deflected path of charged particles when moving perpendicular to a uniform magnetic field
They’re deflected in a circular path
What determines the the direction of the deflected path of charged particles when moving perpendicular to a uniform magnetic field?
The charge
Describe a use of alpha radiation?
Smoke alarms
Why is alpha radiation used in smoke alarms?
The alpha particle ionises lots of atoms and then loses its energy quickly. This means that current can flow, but won’t travel very far
How does a smoke alarm work?
When smoke is present, the alpha particles won’t be able to get to the detector, setting off an alarm
Why are alpha particles very dangerous?
They can ionise body cells if ingested, causing lots of damage
Why can beta-minus particles still ionise electrons even though they’re not as ionising as an alpha particle?
They travel faster, even though they have a lower mass and charge
What are beta-minus particles used in?
The machinery used to control the thickness when making sheets of material (such as paper or aluminium foil)
How does the machinery used to control the thickness when making sheets of material work?
The material is flattened as it’s fed through 2 rollers. A radioactive source is placed on one side of the material, and a radioactive detector on the other side. The thicker the material, the more radiation it absorbs, so the less radiation that reaches the detector. If too much radiation is absorbed, the rollers will move closer to decrease the thickness of the material, and vice versa if too little radiation is absorbed
Describe a use of gamma radiation
In medicine, such as radioactive tracers and treating cancer
Why can gamma radiation be used in medicine, but beta and alpha can’t?
Gamma is very weakly ionising, so doesn’t do any damage to body cells
What are radioactive tracers used for?
To help diagnose patients without the need for surgery
How do radioactive tracers work?
A radioactive source with a short half-life is either eaten or injected into the patient. A detector (e.g. a PET scanner) is then used to detect the emitted gamma radiation.
How is gamma radiation used to treat cancer?
A rotating beam of radiation is targeted at the cancerous cells, which damages them. Gamma radiation is used because it doesn’t damage surrounding cells too badly
What is background radiation?
The weak level of radiation found everywhere
What do you need to do, when measuring the count rate of a radioactive source, to make sure you’ve measured the correct value?
Measure the amount of background radiation first before and then subtract this from the count rate of the radioactive source
What should you do to measure the amount of background radiation accurately?
Take 3 readings without a radioactive source present, then average these readings. You can then subtract this from the measurement of the radioactive source’s count rate
What are 5 sources of background radiation?
- The air
- The ground and buildings
- Cosmic radiation
- Living things
- Man-made radiation
Describe how the air is a source of background radiation?
Radioactive radon gas released from rocks emits alpha radiation, which is the main source of background radiation
Describe how the ground and buildings are a source of background radiation?
Nearly all rocks contain radioactive materials
Describe how cosmic radiation is a source of background radiation?
Cosmic rays (particles from space) collide with particles in the upper atmosphere, producing nuclear radiation
Describe how living things are a source of background radiation?
All plants and animals contain carbon, some of which is radioactive carbon-14
Describe how man-made radiation is a source of background radiation?
Radiation from medical or industrial sources make up a tiny amount of the background radiation
Describe the inverse square law for background radiation
A gamma source will emit radiation in all directions. The intensity of the radiation decreases by the square of the distance away
I ∝ 1/x^2
What is the intensity of radiation?
The amount of radiation per unit area
What is the equation linking intensity of radiation and distance from source?
I = k/x^2
I: Intensity
k: Constant of proportionality
x: Distance from source
What can be deduced from the inverse square law about the safety of handling radioactive sources?
The closer you get to the source, the more dangerous it becomes because the intensity of the radiation increases
What should sources of gamma radiation be transported in, and why?
A lead box because the radiation will be absorbed
Is radioactive decay constant or random?
Random
What is an isotope of an element?
An element with the same number of protons but a different number of neutrons
Even though radioactive decay is random, what can be said about the rate of decay of any sample of a particular isotope?
The rate of decay will be the same (the same proportion of atomic nuclei will decay in a given time)
What is the activity of a radioactive sample?
The number of nuclei that decays each second
What is the activity of a radioactive sample proportional to?
The number of unstable nuclei in the sample
What is the decay constant?
Is the probability of an unstable nuclei decaying per unit time
What does a bigger decay constant mean for an unstable nuclei?
The bigger the decay constant, the faster the rate of decay
What are the units of the decay constant?
s^-1
What is the equation for the activity of an unstable nuclei?
A = λN
A: Activity
λ: Decay constant
N: Number of unstable nuclei
What is the equation for the rate of change of number of unstable nuclei?
ΔN/Δt = -λN
ΔN/Δt: Rate of change of number of unstable nuclei
λ: Decay constant
N: Number of unstable nuclei in a sample
How do you derive the equation ΔN/Δt = -λN for the rate of change of number of unstable nuclei?
Because the activity is the number of nuclei that decay per second, you can write the equation for activity as the change in number of unstable nuclei during a given time
A = ΔN/Δt
Combine the equations, and add a minus sign because ΔN is decreasing, to derive the equation
Why is radioactive decay an iterative process?
The number of nuclei that decay in one time period controls the number that are available to decay in the next time period
What is the decay equation?
N = N0 e^(-λt)
N: The number of unstable nuclei remaining
N0: Original number of unstable nuclei
t: Time
λ: Decay constant
