Physics Basics Flashcards
What type of radiation are x-rays?
electromagnetic
What are the properties of em radiation?
- No mass
- No charge
- Always travels at “speed of light”
- 3x108 ms-1 = 671 million mph
- Can travel in a vacuum
What are the 7 types of em radation?
- gamma ray
- x-ray
- ultraviolet
- visible
- infrared
- microwave
- radio
How is the wavelength calculated?
Divide distance over cycle
How is the frequency calculated?
Divided the cycles by the seconds
How is the frequency calculated?
how many times the wave’s shape repeats per unit time
measured in hertz Hz
What is the wavelength?
the distance over which the wave’s shape repeats
measured in metres, m
How is speed calculated and how does this affect other measurements?
Speed = Frequency x Wavelength
* BUT speed of all electromagnetic radiation is constant
* 3x108 ms-1
* Therefore if frequency increases then wavelength must decrease (& vice versa)
What are photons?
EM radiation involves the movement of energy as “packets of energy”
What is the energy usually measured in?
electron volts, eV
What is 1 ev?
energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
What are the values for x-ray photon energies?
range
~124eV – 124keV
What are the types of x rays?
- Hard X-rays (higher energies)
- Able to penetrate human tissues
- Soft X-rays (lower energies)
- Easily absorbed
What type of x-rays does medical imaging use?
hard >5kev
What are the properties of x rays?
- Form of electromagnetic radiation
- No mass, no charge, very fast, can travel in a vacuum, etc.
- Undetectable to human senses
- Man-made
- Note: gamma rays are identical except that they occur naturally (& generally have higher energies)
- Cause ionisation
- ie. displacement of electrons from atoms/molecules
How are X-rays produced?
electrons fired at atoms at very high speed
on collision the kinetic enegry of these electrons is converted to EM and heat
the x-ray photons are aimed at a subject
What is the structure of the atom (bohr)?
Atoms are the “building blocks” of matter
* Consist of:
* Central nucleus
* Protons (+ve charge)
* Neutrons (neutral)
* Orbiting “shells”
* Electrons (-ve charge)
What is the relative charge, mass and location of neutrons, protons, and electrons?
neutron = 0, 1, nucleus
proton = +1, 1, nucleus
electron = -1, 0, shells
What is the atomic number and what is the mass number?
- Atomic Number (Z) = number of protons
- Unique to each element
- Mass Number (A) = number of protons + neutrons
What does the number of electrons equal in a ground state atom?
number of electrons = number of protons
What is ionisation?
Ionisation = removing/adding electron(s) to an atom
* Atom - e- → positive ion
* Atom + e- → negative ion
What is the innermost shell called?
K
What are all the shells from innermost?
k, l, m, n, o
What is the max number each shell can hold?
- K=2, L=8, M=18, N=32
What force holds orbiting electrons?
electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus
What is the binding energy?
To remove an electron from its shell, a specific amount of energy is required to overcome this attraction
* Binding energy = additional energy required to exceed electrostatic force
What shells have the strongest binding energies?
The closer the electron is to the nucleus, the greater the electrostatic force (& therefore binding energy)
* K shell electrons have the highest binding energies
* Then L, then M,
What increases the electrostatic force?
the more positively-charged the nucleus, the greater the electrostatic force
What is the energy required to move an electron to a more outer shell equal to?
The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) equals the difference in the binding energies of the 2 shells
conversely if an electron drops to a more inner shell then this specific amount of energy is released
What are the components of an x-ray unit?
Tubehead
* Collimator
* Positioning arm
* Control panel
* Circuitry
What is current and what is it measured in?
- Flow of electric charge, usually by the movement of electrons
*SI unit: amp (or ampere), A - Measure of how much charge flows past a point per second
What are the two directions of current?
- Direct current (DC) = constant unidirectional flow
- e.g. batteries
- Alternating current (AC) = flow repeatedly reverses direction
What is AC?
How is the freqency found?
- Flow periodically reverses direction
- Number of complete cycles (reverse + reverse-back)
per unit time is the frequency - SI unit: hertz, Hz (cycles per second)
What current do x-rays require and what are they powered by?
X-ray production requires a unidirectional current
* But X-ray units are powered by mains electricity (AC)
How is the AC current in x-ray machines rectified?
X-ray units have generators which modify the AC so that it mimics a constant DC
* Process known as rectification
What is voltage and what is it related to?
Difference in electrical potential between 2 points in an electrical field
* Related to how forcefully a charge will be pushed through an electrical field
* SI unit: volt, V
What is the voltage and current of mains supply?
Alternating current (≤13 amps)
* 220-240 volts
What voltages does a dental x ray unit require?
One as high as 10s of thousands of volts
One as low as around 10 volts
What do transformers do?
alter the voltage (& current) from one circuit to another
What are the two seperate transformers required for x-ray unit?
x-ray tube (step up)
filament (step down)
What is the step up transformer?
↑ potential difference across X-ray tube
* Usually 60,000-70,000 volts (60-70 kV)
* Current reduced to milliamps (mA)
What is the step down transformer?
- Step-down transformer
- ↓ potential difference across filament
- ~10 volts
- ~10 amps
How do photons travel from the x-ray beam?
Photons effectively travel in straight lines but diverge from the X-ray source (i.e. do not travel in parallel)
What is the x-ray beam intensity?
Quantity of photon energy passing through a cross- sectional area of the beam per unit time
What is the x ray beam intensity proportional to?
current in filament (mA) & potential difference across X-ray tube (kV)
↑ number &/or energy of photons = ↑ intensity
What is the inverse square law?
Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source & the point of measurement
- Intensity ∝ 1/distance2
- Therefore, doubling the distance will quarter the dose
What are alpha, beta and gamma particles produced by and what are x-rays produced by?
All produced by radioactive decay of unstable atoms
- Unlike X-rays which are directly man-made
