Physics Flashcards
Terminology to characterise an atom?
A/Z(X) where A is number of nucleons, Z protons, X chemical symbol
The atomic number and mass numbers are denoted by which characters?
An element is defined by?
Atomic number = Z the number of protons
Mass number = A proton + neutrons
Element defined by Z, A defined the isotopes of X.
The nuclear binding energy means the mass of A is less than the sum of its parts
In the Bohr model each shell can hold how many electrons?
2(n^2) where n is the shell number
1st shell = k = 2
2nd = l = 8
Electron binding energy is the energy that be supplied to remove an electron from its shell.
Elevating an electron from K to l requires?
An amount of energy equal to the difference in binding energies
E>Ek
Define the electron volt
eV is the energy gained by an electron as it is accelerated by a potential difference of 1 volt
Characteristic radiation occurs when? Under what condition can this process be a cascade?
As a consequence is this emission continuous or discrete?
An outer shell electron falling to a more proximal shell - requires a vacancy.
High atomic number atoms have many shells and so the loss of a proximal election will trigger a cascade of electrons falling from higher shells. Each releasing a discrete amount of energy proportional to the binding energy of their shell.
Define radioactive decay and radioactivity?
Instability in ratio of neutrons to proton leads to a transition to a more stable configuration, this change emits particles and EMR with the energy corresponding to the increase in binding energy at final configuration. The change is referred to as decay, the emission as radioactivity.
In a sample of radionuclide the rate of decay is directly proportional to:
the number of atoms of the radionuclide present. A these change in the their progeny nuclide the rate decreases - is a decaying exponential function of initial number of atoms and a decay constant (ln2/T half)
Sometime used to determine dose delivery from permanent brachy implant Average life or mean life is defined as
1/the decay constant, where the decay constant is ln2/over half life = lamda). ie. 1/lamda = 1.44(T half)
Planck’s constant
h=6.62 x 10^-34 J.sec
Relate the energy of a photon to its wavelength
E=hv, where h is Planck’s and v frequency, where v=speed of light (c)/wavelength (i.e wavelength x v = c)
Production of gamma rays
By decay or capture. Decay can be either (or a mix of both - see decay schemes) betaminus (electron emitted by a neuron turning into a positron), or betaplus (when proton and electron combine to make a neutron).
In capture - inner orbital electron captured to make neutron from positron, also releases characteristic radiation (with possible cascade).
Principle elements of an Xray tube
1) Filament (cathode) - releases electron by thermionic emission
2) Target - Large atomic nuclei such as tungsten
3) Anode - At the site of the target,
4) High Voltage between cathode and anode accelerates electron onto focal spot (determined by size of filament) on target
In Kv imaging,what is the focal spot? How is it changed? What is the effect of shrinking the focal spot?
The focal spot is the site where accelerated electrons strike the target. The size of the filament/cathode determines the size of the focal spot (modern systems often have 2 or more filaments).
Smaller focal spots result in a smaller region from which Xrs are emitted (the apparent source) producing a more detailed picture. To produce sharp images, focal spots need to be small but able to withstand heat loading without melting the anode target. A small focal spot is used when spatial resolution is important, while a large focal spot is employed when a short exposure time is the priority.
The possible outcomes of XR/Gamma ray interaction w/matter?
Photon may be 1) Scattered 2) Absorbed 3) Transmitted without interaction Photons that traverse the medium uninterrupted are called primary radiation. Those that are scattered or absorbed are termed attenuated.
The amount by which a photon beam is attenuated is a characteristic of:
1) Attenuating material
2) Photon beam spectrum
What is HVL
Half value layer, in units of cm or mm is the thickness of the attenuating material that reduces the intensity of a radiation beam to half its original value. Sometimes measured in air kerma rate.
Under conditions of good geometry a monoenergetic beam will be attenuated ______ with increasing thickness of the absorber
Exponentially
A polyenergetic beam is not attenuated _____ as the absorber preferentially absorbs _______
Exponentially
Low energy photons
True or False: In the case of a polyenergetic beam the second HVL is equal to or less than the 1st HVL
False. to reduce from 1/2 to 1/4 A much larger HVL may be needed to attenuate high energy photons.
Define the linear attenuation coefficient u
u = 0.693/HVL
The transmitted beam intensity I for any thickness of absorbing material:
I transmitted = I0^-ux where I0 = initial beam intensity u = linear attenuation coeef 0.693/HVL x is thickness u is in units cm^-1
Define Coulomb
What is the charge of the following particles: Proton Neutron Election Positron
1 Columb = it is the charge carried by a current of 1ampres in 1 second. Proton = 1.602 x10^-19C neutron = 0 Electron = -1.602 x10^-19 C Positron = +1.602 x10^-19
Rest mass of an electron
9.109 x10^-31 hbpKg = 0.511 MeV