13: Nuclear Physics Flashcards
What did John Dalton think about atoms?
Matter was made up of tiny spheres (atoms) that couldn’t be broken up
What was J.J Thompson’s suggestion about atomic structure?
Atoms were spheres of positive charge with tiny negative electrons stuck in them.
This is the Plum Pudding Model
What experiment did Ernest Rutherford do to prove the Plum Pudding model wrong?
Alpha scattering experiment
How did the alpha scattering experiment work?
- A stream of alpha particles from a radioactive source are fired at very thin gold foil
- when alpha particles strike a fluorescent screen a tiny visible flash of light is produced
- The fluorescent screen is circular and surrounds the experiment so that alpha particles can be detected from any angle
What conclusions were made from the alpha scattering experiment?
- The atom must be mostly empty space because most alpha particles pass straight through
- The nucleus must have a 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 an angle greater than 90 degrees
- Most of the mass must be in the nucleus, since the alpha particles (with high momentum) are deflected by the nucleus
What was the expected conclusion from the alpha scattering experiment?
If the Thomson model was right, all the flashes should have been seen within a small angle of the beam, because the positively charged alpha particles would be deflected by a small amount by the electrons
How did Rutherford and Kay discover the proton?
By firing high-energy alpha particles at different gases
How can you estimate the radius of an atomic nucleus
- Rutherford’s scattering experiment
- an alpha particle that ‘bounces back’ and is deflected through 180 degrees will have stopped a short distance from the nucleus
- this is at the point where the particles electric potential energy is equal to the initial kinetic energy
what is the equation to work out the radius of an atomic nucleus
Initial Ek = Eelec = Q(nucleus) x q (alpha)/ 4π x εo x r
where Q is the charge of the nucleus, q is the charge of the alpha particle, and r is the distance of closest approach
What is the distance of closest approach
an estimate of nuclear radius
what is a more accurate method of measuring the radius of a nucleus
electron diffraction
how can electron diffraction estimate the radius of a nucleus
- a beam of moving electrons has an associated de Broglie wavelength, which at high speeds is approximately = hc/E
- the wavelength must be tiny to investigate the nuclear radius, so electrons have a very high energy
- if 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
- the first minimum appears where sinθ = 1.22λ/2R
-using measurements from this diffraction pattern, you can rearrange the equation to find the radius of the nucleus
what is the radius of an atom
5 x10 ^-11
what is the radius of the smallest nucleus
1 fm
what does the graph of nuclear radius (R) against nucleon number look like (A)
lower case letter r
how do you get a straight line graph between nuclear radius (R) and nucleon number (A)?
Plot R against A^1/3
what is the equation between nuclear radius and nucleon number?
R = R0 x A^1/3
what does the equation for nuclear radius tell you about the nucleus
- the volume each nucleon takes up in the nucleus is about the same
- as the mass of each nucleon is about the same, the nuclear density of a nucleus is always constant, no matter how many nucleons make up the atom
what is the equation for nuclear density
3m(nucleon)/4πx(R0)^3
Nuclear density is significantly greater than atomic density. What does this suggest about the structure of the atom
- most of the 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 atomic nucleus releases energy/particles in order to become more stable
what is one atomic mass unit
1.661 x10^-27
what is the range of alpha radiation
absorbed by paper, skin or a few centimetres of air
what is the range of beta minus radiation
absorbed by about 3mm of aluminium
what is the range of gamma radiation
absorbed by many centimetres of lead, or several metres of concrete
what is the range of beta plus radiation
effectively zero, as the particle annihilate themselves with electrons almost immediately
how you identify the types of radiation emitted from a source
- record the background radiation count rate when there is no source present
- place an unknown source near to a Geiger-Muller tube and record the count rate
- place a sheet of paper inbetween the source and the Geiger-Muller tube. Record the count rate
- Replace the paper with a 3mm thick sheet of aluminium and record the count rate
- for each count rate that is recorded, take away the count rate of the background radiation to find the actual count rate
- depending on when the count rate significantly decreased, you can deduce what kind of radiation the source was emitting
how can you use magnetic fields to identify the type of radiation being emitted from a source
- charged particles moving perpendicular to a uniform magnetic field are deflected in a circular path
- the direction in which the particle curves depends on its charge
- radius of curvature can also tell you about charge and mass
uses for alpha radiation
smoke detectors
- they allow current to flow, but don’t travel very far
- when smoke is present, the alpha particles can’t reach the detector and this sets the alarm off
order of radiation by ionisation (h to l)
alpha
beta
gamma
order of radiation by penetration (h to l)
gamma
beta
alpha
when are alpha particles dangerous
- dangerous when ingested
- quickly ionise body tissue in a small area, causing lots of damage
what can beta radiation be used for
- can be used to control thickness of materials (paper, aluminium or steel)
- the material is flattened as it is fed through rollers
- a radioactive source is placed on one side of the material =, and a radioactive detector on the other
- the thicker the material, the more radiation is absorbs and prevents from reaching the detector
- if too much radiation is being absorbed, the roller move closer together to make the material thinner (or vice versa)
uses of gamma radiation
- used in cancer treatment
- a rotating beam of gamma rays is used to lessen the damage done to surrounding tissue, whilst giving a high dose of radiation to the tumour at the centre of the rotation
sources of background radiation
- air - radioactive radon gas is released from rocks, which emits alpha radiation
- ground and buildings - nearly all rock contains radioactive materials
- Cosmic radiation - cosmic rays are particles from space which produce radiation when they collide with particles in the upper atmosphere
- Living things - plants and animals contain carbon, and some of this will be carbon 14
- man-made radiation - radiation from medical or industrial sources
how do accurately get a measurement for background radiation
- take three readings of the count rate using a Geiger Muller counter without a radioactive source present
- average the readings and subtract that average from each measurement made of a radioactive source’s count rate
what is the intensity of radiation
amount of radiation per unit area
What is the equation for the inverse square law
I = k/x^2
- I is the intensity (Wm^-2)
- k is constant of proportionality, W
- x is the distance from source, m
how can you investigate the inverse square law
- set up a Geiger Muller tube with a metre ruler
- turn on the Geiger counter. Take three readings of the background radiation count rate and average them
- carefully place the radioactive source at a distance d from the tube
- record the count rate at that distance. Take three readings of the count rate at this distance
- move the source so the distance between it and the tube doubles and record the count rate
- repeat step 5 for distances 3d, 4d, etc
- put away the radioactive source once the experiment is finished
- average the count rates recorded for each distance and subtract the average background radiation count rate
- plot a graph of corrected count rate against distance
how to safely handle radioactive sources
- hold a source away from your body when transporting through the lab
- use long handling tongs to minimise the radiation absorbed by the body
- sources should always be stored in a lead box
what is meant by the activity
the number of nuclei that decay each second
what is the decay constant
λ
probability of a specific nucleus decaying per unit time, and is a measure of how quickly an isotope will decay
what is the equation linking activity to the number of unstable nuclei
A = λN
or
A = - ΔN/Δt
equation for the rate of change of the number of unstable nuclei
ΔN/Δt= -λN
what is the decay equation
N = N0 x e^-λt
what is the gradient of the graph of ln(N) against t
-λ
what is the y-intercept of the graph of ln(N) against t
ln(N0)
how to work out the number of atoms in a sample
N = n N(A)
where n is the number of moles
where N(A) is the Avogadro constant
what is the equation for activity in a source over time
A = A0 x e^-λt
A0 is the initial activity
what is the half-life of an isotope
the average time it takes for the number of unstable nuclei to halve
what is the equation for half life
T1/2 = ln(2)/λ
why can it be difficult to get a reliable age from radioactive dating
- man-made objects will only tell you the age of the material used, not the object itself
- the object may have been contaminated by other radioactive sources
- there may be a high background count that obscures the object’s count
- sample size may be small
when would a nucleus become unstable
- too many neutrons
- too few neutrons
- too many nucleons altogether (too heavy)
- too much energy
what kind of emission happens in heavy atoms
alpha
what kind of emission happens in neutron rich atoms
beta minus
what kind of emission happens in proton rich atoms
beta plus
how is gamma emission produced
- after alpha or beta decay, the nucleus often has excess energy, which is released as a gamma ray
- electron capture
what is meant by binding energy
the energy released when a nucleus forms, or the energy required to separate all the nucleons in that nucleus.
Equivalent to the mass defect of the nucleus
what is meant by the mass defect
the difference in mass between a nucleus and the individual nucleons
which has a smaller mass: nucleus or sum of mass of individual nucleons
nucleus
how do you work out the average binding energy per nucleon
binding energy (N)/ nucleon number (A)
what is nuclear fission
when large, unstable nuclei randomly split into two smaller nuclei
how do you know energy is released during nuclear fission
- the new smaller nuclei have a higher average binding energy per nucleon
how can you induce nuclear fission
by making a thermal neutron enter a 235U nucleus, causing it to become very unstable
what is nuclear fusion
when two light nuclei can combine to create a larger nucleus
only elements to the ….. of iron can release energy through nuclear fission
right
only elements to the ….. of iron can release energy through nuclear fusion
left
how are chain reactions used in nuclear reactors
- use rods of uranium rich in U235 as ‘fuel’ for fission reactions
- these reaction produce ore neutrons which then induce other nuclei to fission
what is the role of a moderator in a nuclear reactor
to slow down and/or absorb neutrons
- slows down neutrons through elastic collisions with the nuclei of the moderator material
example of a moderator
water
- contains hydrogen, which is a similar mass to a neutron, so neutrons are slowed down more efficiently
what is meant by the critical mass
the amount of ‘fuel’ you need for the chain reaction in a nuclear reactor to continue at a steady rate
what do control rods do
control the chain reaction by limiting the number of neutrons in the reactor
- absorb neutrons so the rate of fission is controlled
what does coolant do in a nuclear reactor
it is sent around the nuclear reactor to remove heat produced by fission
- the heat from the reactor can then be used to make steam for powering electricity-generating turbines
safety features of nuclear reactors
Reactor shielding: reactor is surrounded by a thick concrete case, to prevent radiation escaping
Emergency shut down: can be shut down automatically by releasing the control rods
Handling and storing fission waste products: unused rods only emit alpha so can be stored easily but used rods emit beta and gamma, which must be sealed in containers
pros of nuclear power
- doesn’t release greenhouse gases
- very efficient
- generates thousands times more energy per kg compared to fossil fuels
cons of nuclear power
- hard to deal with nuclear waste
- making sure it doesn’t harm people or the environment
