3.8 - Nuclear Physics Flashcards
Who discovered radioactivity?
Marie Curie and Henri Becquerel
What are some ways you can safely handle radioactive sources
use long handled tongs to move it around
keep it as far away as possible
don’t point the source towards people
store in a lead-lined container when not using
How do you figure out the distance of closest approach for a particle?
The electrical potential when the particle is at its closest approach is equal to the initial kinetic energy so equate these two [PE= (Q1Q2)/4(pi)r] and rearrange to make r (distance) the subject
Why is the closest approach not the most accurate calculation of nuclear radius?
it is the distance between the atoms’ centres, so will always be an overestimate
If a gold nucleus was changed for an atom with fewer protons, what would change about the alpha particles distance of closest approach?
it would travel closer to the atom as there is less of an electrostatic repulsive force so needs to travel further to lose the same initial KE
what are the a, b, g radiation ranges in air
alpha: a few cm (2-10)
beta: 1m
gamma: infinite
How ionising are a, b, g radiation and how dangerous does this make them to the human body
alpha: highly/ very dangerous if ingested or inhaled
beta: weakly/ damage more spread out than alpha but more penetrating
gamma: very weakly/ not dangerous, small chance of interaction with body cells
which radiation are/ not deflected in an electric field
alpha: yes
beta: yes
gamma: no
What are a, b, g radiation absorbed by?
alpha: paper
beta: 3mm aluminium foil
gamma: several m of concrete OR several inches of lead
How does gamma radiation spread out?
as a sphere, intensity follows inverse square law
What are some natural sources of background radiation?
radon gas from rocks, cosmic rays
What are some man-made sources of background radiation?
medical x-rays, fallout from nuclear weapons testing
What is a better estimate of nuclear radius? How does it work?
High-energy electron diffraction.
A beam of high-energy electrons are accelerated to make their De Broglie wavelength in order of 10^-15, around same as diameter of nucleus. They’re fired at thin film of a material in front of a screen where they produce a diffraction pattern of concentric circles; bright centre and getting dimmer as you move away.
What are the benefits of using electrons instead of alpha particles in e- diffraction?
no electrostatic repulsion with e- so it can get closer
alpha particles give an overestimate (closest approach)
e- are easier to produce
What should a tracer isotope have?
half life stable enough for necessary measurements to be made but also short enough to decay quickly after use
What factors should you consider when using radioactive isotopes?
the half-life and type of radiation needed
toxicity
whether or not a stable product is needed
What are uses of radioactive isotopes?
carbon dating
alpha particles for smoke detectors
What is an isotopes activity proportional to?
its mass
Why could it be difficult to obtain a reliable age of an ancient boat by comparing the wood with living wood
There is uncertainty in the ratio of carbon-14 in the atmosphere thousands of years ago
The boat may have been made with the wood some time after the tree was cut down
When can it be unreliable to use carbon dating? Why?
When a sample is less than 500 years old since the change in activity is too small to be accurately measured.
When a sample is more than 50,000 years old since the activity of remaining C-14 isotopes would be too small compared to background radiation to be accurately measured.
What are the equations relating intensity and distance
I=k/r^2 where k=nhf/4pi
I1/I2=(r1/r2)^2
When does gamma radiation happen?
following alpha or beta decay
What is activity dependent on?
The number of atoms of the isotope present in the sample (N)
The probability that an atom will decay in the next second
What does the decay constant always need to be between?
0 and 1(its a probability)
What is the nature of radioactive decay?
random
What is a nuclear radius dependant on?
the nucleon number
What did the Greeks believe everything was made of?
earth, air, fire, water
Who first suggested things were made of particles?
Democritus, 460 BC
Who first realised there were many elements?
John Dalton, 1800s
Who produced the plum pudding model?
J J Thomson, early 1900s
What did Rutherford do and what did it explain
fired positive alpha particles at thin gold foil.
found that most went straight through: atom is mostly empty space.
some were deflected: the nucleus is positively charged
very few bounced back completely: the nucleus is extremely small compared to the size of the atom
What is the approx. size of the atom
10^-10m
What is the approx. size of a nucleus
10^-15m
What are some medical applications of a, b, g radiation?
Tracers: inject patient with gamma, this is detected outside, can locate certain things e.g. a tumour
Radiotherapy: attacking tumours with radiation from outside at many angles
Brachytherapy: alpha source injected into tumour, damage to tumour from inside but little damage to surrounding healthy tissue
What are some industrial applications of a, b, g radiation?
Thickness measurement: the amount of beta radiation going through paper/ aluminium depends on its thickness
Smoke alarms: alpha source, could be blocked by smoke particles
Tracer techniques: detecting leaks, pollution monitoring etc.
What is -λN also equal to?
dN/dt (the rate of change of N)
What does a graph of N against time look like?
exponential decay
How does carbon dating work?
Living things contain the same proportion of the unstable isotope carbon-14. When a living thing dies, it stops taking in atmospheric carbon, and the proportion of C-14 falls exponentially.
What is a commonly used isotope for medical imaging? Why? What is the half-life?
Technetium-99
It has a toxicity that can be tolerated by the body
6 hours
How can you work out the half-life from a graph?
plot ln(count rate) against time to get a line of best fit with equation ln(N)=-λt+ln(N0). The gradient=-λ so use this to find the half life.
What type of nuclei are above the stability line on an N-Z graph? Why?
Beta minus emitters.
Neutron heavy so turns some into protons.
What type of nuclei are below the stability line on an N-Z graph? Why?
Beta plus (positron) emitters.
Neutron deficient so turns some protons into neutrons.
Up to which values of N and Z does the stability line have a gradient of 1?
N=20 and Z=20
Where are alpha emitters on N-Z graph? Why?
at the top; heavy nuclei so decreases mass to become more stable
Where are electron capture nuclei on an N-Z graph? Why?
Below the stability line.
Neutron heavy
Why does the neutron to proton proportion increase as the nuclei gets bigger?
They hold the nucleus together - against repelling electrostatic forces of protons.
The act as spacers between the protons.
How do nuclei fall to lower energy levels?
by emitting gamma photons
What are the energy transfers in estimating the nuclear radius?
The alpha particle’s initial kinetic energy = potential energy at distance of closest approach
What is the volume of a nucleus proportional to? What does this tell us?
The number of particles in it; nuclear material has constant density
What is the approximate density of nuclear material?
2.5x10^17 kgm^-3
What is 1u in MeV?
931.5 MeV
Why is the total mass of constituent sub-atomic particles more than the mass of the whole atom?
Pulling an atom apart requires work against both the strong force and to pull away the electrons. The system of separate particles has more energy than the system of the entire atom still together. This difference in energy = a difference in mass.
Binding energy is always…
negative
What is a big binding energy indicative of?
Greater stability
What is the most stable nucleus? Why?
Iron-56; it has the greatest binding energy per nucleon
What happens when a very heavy nucleus spits into two smaller nuclei (fission)?
The binding energy per nucleon of the fragments is bigger (look at graph). The total binding energy is greater, so energy must be released.
How do you work out the energy released in a reaction?
Calculate the mass before and after the reaction (in atomic mass units). Find the difference between these. Convert from atomic mass units to MeV (x931.5)
What happens when two smaller nuclei come together (fusion)?
The binding energy per nucleon of fused nucleus is bigger (look at graph), so the total binding energy is greater. This means energy would be released
What is true for all nuclear decays?
There is a move towards stability
What can be used as a coolant?
pressurised water
carbon dioxide
(large specific heat capacity)
What material is used for fuel rods?
Uranium-235
What material is used for moderators?
Graphite (preferred: cheap and light)
Heavy water (very good but expensive)
One which doesn’t absorb neutrons; one with a nucleus about the same size as a neutron
How do moderators work?
the fissile neutrons collide with the nuclei of moderator atoms and lose kinetic energy without being absorbed. It increases the chance of them interacting with another U-235 nuclei
How do you move control rods to affect the rate of reaction?
Put them further in to decrease reaction.
Take them further out to speed reaction up.
Put them in all the way to stop reaction.
What material is used for control rods?
Boron (better for cost and availability)
Hafnium
Wy does the shielding around a nucelar reactor core become radioactive?
Neutrons collide with the nuclei in the shielding. This converts the nuclei into unstable isotops.
Why are fission fragments unstable? What radiation are they likely to initially emit?
They are more likely to be above the stability line, meaning they are neutron-rich. They are most likely beta-minus emitters
How does electron capture work?
An inner-shell electron captured by a proton in the nucleus, releasing a neutron and an electron neutrino.
What is the relationship between half-life and energy
inverse
What are two nuclei liekly to be used for the plasma of a fusion reactor?
Deuterium
Tritium
How do you raise the temperature of a nuclear reactor plasma?
inducing a current
