- Protons have spin or they behave like tiny magnets
- Protons precess about the magnetic field produced by a stong electromagnetic at a larmor frequency
- Transmitting coil produces a radio frequency equal to larmor frequency at the larmor frequency
- protons resonate and flip to a higher energy state
- when they relax to a lower energy state they emit radio waves
- relaxtion time depends on surrounding tissue this is the average time taken for the protons to return back to their normal energy state
- radio waves picked up by receiving colis
- gradient coils alter magnetic flux density through the body
- larmor frequency and radio waves given of during relaxtion times varies through the body so the position of tissues can be identified
- computer processes the signals and generates an image
Disadvantage Patient with metallic objects cannot be scanned: Patient has to remain still:confined space. Advantage: non-ionising: non invasive: better contrast between soft tissues.
PET (Positron Emission Tomography)
- A positron emitting source is used
- The positron annihilates with an electron in the patient
- This produces two gamma photons
- The gamma photons travel in opposite directions
- The patient is surrounded by a ring of gamma detectors
- The arrival times of the photons / delay time indicates the location of where they were emitted from inside the body
- A 3-D image is created by a computer
- An EMF pulse is put across a piezoelectric crystal which turns the pulse into ultrasound
- Ultrasound waves are reflected at boundaries
- Intensity of reflected signal depends on acoustic impedance of the materials at the boundary
- time from pulse to detection is used to determine depth of boundary in the body
- A-scan is one dimensional
- B-scan involves different directions and angles producing a 2-D or 3-D scan
- B-scan consists of many A-scans
Speed of blood
- Ultrasound is refelected by moving blood cells
- the frequency / wavelength of ultrasound is changed
- the change of frequency is related to speed of blood / change of wavelength is related to the speed of blood
Computerised Axial Tomography (CAT)
- Electrons are accelerated through a high voltage and the decelerated rapidly some of the kinetic energy is transferred to X-rays
- A thin fan shaped beam of X-rays is rotated around the patient and the X-rays detected by a ring of detectors
- An image of a cross sectional 'slice' of the body is made from the detected X-rays by a computer these slices can be put togeter to form a 3D image with better tissue contrast than normal X-rays.
- Advantage 3D image; better contrast between different soft tissues.
- Vertical tubes in lead that allow only parallel gamma photos to travel up them along their axis, gamma photos not emitted at the desired point are absorbed by the lead this makes the image sharper
Scintillator / Sodium Iodide Crystal
- contains fluorescent material that emits a light photon when struck by a gamma phton
- gives out an electrial pulse for every flash of light from the scintillator
- Generated an image from the electrical pulses
Quality of image improved by narrower thinner longer collimators or longer scanning time.
- Gas is drawn together by gravitational forces
- The loss in gravitational potential energy increases the kinetic energy increasing the temperature
- Fusion of protons hydrogen nuclei produces helium nuclei and energy
- A stable star is formed when radiation pressure is equal to gravitational pressure
- When hydrogen runs out the outer layers of the star expands the core shrinks
- A red giant is formed
- For a smaller star this then becomes a white dwarf
- For a larger star the super red giant explods in a supernova and the core becomes a neutron star or a balck hole
Evidence of hot big bang
- Galaxies are moving apart meaning the universe is expanding
- If galaxies have always been moving apart then at some stage they must have been closer together or started from a point
- Evidence in red shift either optical/ microwave
- Further away the galaxy the faster the speed of recession
- The existance of a (2.7 K) microwave background radiation
- There is more heliu in the universe than expected
- Intensity of the microwaves is the same in all directions
- These microwaves correspond to a temperature of 2.7 K or the temperature of the universe is 2.7 K
- The expansion of the universe following the big bang led to cooling and hence we observe microwaves rather than short wavelength
Open, Closed, Flat
- Open: Density of universe is smaller that the critical density the universe will expand forever.
- Closed: Density of universe is larger than the critical density hence the universe will eventually stop expanding and then contract/ big crunch
- Flat: Density of university is equal to the critical density hence the universe will expand towards a finite limit as the rae of expansion tens to zero
- Unpredictible: Existance of dark matter, Black holes, Neutrinos, Dark energy, hubbles constant is not known accurately
Olbers' Paradox and Hubbles Law
- The night sky should be bright/ have uniform brightness but it is not
- The line of sight ends on the surface of a star or number of stars is directly proportional to r squared and intensity 1/r squared
- infinite universe/ uniformly distrbuted matter or stars throughout / static / infinite age
- Hubbles Law recessional speed of galaxy is directly proportional to its distance from the Earth
- the universe is finite it is expanding it has a beginning visible light is red-shifted because of expansion of space
The Big Bang
- Big bang: creation of the universe from which space/time evolved
- At the start the universe was very hot and extermely dense / singularity
- All forces were unified
- Epansion led to cooling
- Quark / leptons soup plus photons
- more matter than antimatter
- quarks combine to form hadrons
- imbalance of neutrons and protons / primordial helium produced
- atoms formed
- idea of gravitional force responsible for formation of star/ galaxies
- temperture becomes 2.7 K / 3 K or the universe is saturated with cosmic microwave background radiation
- Evidence: specra from galaxies show shft to longer wavelengths suggesting galaxies are moving away from earth / The more distant galaxies are moving faster / Existance of microwave background radiation / the temperture of universe being 3 K / existance of primordial helium / temperture fluctuations (predicted and observed)
Nuclear Fusion in the core of the sun
- Protons / hydrogen nuclei to produce He nuclei ( prostirons and neutrinos)
- There is electrostatic repulsion between the prodonts / the protons repel each other becuase of their positive charge
- High tempertures / 10^7 K needed
- At high tempertures some of the fast moving protons come close enough to each other for the strong nuclear force to overcome the electrostatic repulsion
- High density / pressure in the core of the sun
- There is a decrease in mass, hence energy is released / products have greater binding energy.
- Production: Electrons are accelerated through high voltage
- high speed electrons hit metal
- kinetic energy of electrons produced X-Ray
- X-rays are produced by large deceleration of electrons
- Interation with matter: Compton / scattering: incoming photon collides with an electron, the electron is ejected and the photon is scattered / has lower energy. / Pair Production: incoming x-ray photon (disappears) interacts with nucleus and produces electron-positron pair. /Photoelectric effect: an orbital electron is ejected from atom / atom is ionised
- Intensifiers: Absorption of X-rays by silver halide molecules by a photographic film. /uses of flurescent / scintillator / phosphor. /Photon releases electron that is accelerated onto a fluorescent screen. /number of electrons increased /multipled. /Phosphor / intensifier / it converts X-ray photon into increased number of 'visible' photons
- Contrast Media: Different soft body tissue produce little difference in contrast / Attenuation. /contrast media with high atomic number / Z used / iodine or barium used to give greater contrast. / Liquids injected or swallowed ino soft tissue areas.
- Non-invasive means less chance of infecion
- Diagnose the condition of organs: tracer is injected into the body / placed inside the body
- Tracer is absorbed by organ / shows blockage
- Beta detector / gamma camera is used to detect radiation from the body.
Fuel rods, Control rods and Moderator
- Contain the uranium nuclei / fissile material
- Absorb some of the neutrons.
- Controlled chain reaction: The control rods are inserted into the reactor so as to allow on average one neutron from previous reactions to cause subsequent fission.
- Slows down the fast-moving neutrons / lowers the KE of fast moving neutrons / makes the fast moving neutron into thermal neutons.
- Slow moving neutrons have a greater chance of causing fission / of being absorbed by U-235 / sustaining chain reaction.
- Plants / living things take in cardon dioxide and stop taking in carbon dioxide after death.
- The ratio of carbon-14 to carbon-12 nuclei for the relic sample is determined.
- The current ratio of carbon-14 to carbon-12 is determined.
- The age of the relic is found using the decay constant formula
Limitation: The ratio of carbon-14 to carbon-12 is assumed to be constant / count-rate from relic may be comparable to background count-rate
Alpha-Particle Scattering Experiment
- Most of the alpha particles went straight through and some deviated through small angles.
- Hence most of the atom is empty space.
- Some, a very small number, of alpha particles were scattered / repelled through large angles / angles more that 90 degrees.
- This showed the existance of a tiny positive nucleus.
- The size of the nucleus is about 10^-14 m.
The Forces on Protons and Neutrons.
- This force is attractive and is long-ranged / obeys '1/r^2 relationship'.
Strong nuclear force / interaction
- This force is attrative at larger distances or repulsive at short distances and is short-ranged ~ 10^-14 m.
Electrostatic / electrical force / coulomb force
- This force is repulsive between protons / zero between neutrons / zero between protons and neutrons and is long-ranged / obeys the '1/r^2 relationship'.