Lecture 14 Flashcards
Define non-ionising radiation
Any type of radiation less than the ionising potential (including visible light and UV light). It is less energetic than ionising radiation.
What is meta-stable state?
A particular excited state of an atom, nucleus, or other system that has a longer lifetime than ordinary excited states and (generally) a shorter lifetime than the ground state.
What is a population inversion?
A state of a system in which there are more members of the system in higher states than in the lower, unexcited states.
How does a LASER work?
A narrow beam of light is produced in which all photons have the same wavelength.
What is laser surgery and how does it work?
High-power, very focused beams of infrared light are directed at a target in the body using a carbon dioxide laser. These beams vaporise the tissue (tissue ablation).
Give an example of when lasers are used in healthcare.
UV lasers are used for corrective eye surgery by emitting very powerful but short pulses of light.
What is a negative side effect of laser surgery?
The high power of the laser can cause unwanted damage to the skin, corneas, lens or retina. The greater the light intensity, the more photochemistry occurs and the more damage happens as a result.
What is the maximum permissible exposure (MPE) for ocular exposure to visible light?
For a 400 - 700 nm wavelength, the continuous (1hr) limit is 1 mW/cm².
What is the maximum permissible exposure (MPE) for ocular and skin exposure to HeNe (632 nm)?
- Occular: 2.5 mW/cm² for a 0.25s blink reflex
- Skin: 200 mW/cm² for 1hr
What are the class categories for lasers?
Class 1
Class 1C
Class 1M
Class 2
Class 2M
Class 3R
Class 3B
Class 4
What are the allowed powers for the 4 main laser categories?
Class 2: 0-1 mW
Class 3R: 1-5 mW
Class 3B: 5-500 mW
Class 4: 500+ mW
What is the wavelength of UVA?
320 - 400 nm
What is the wavelength of UVB?
280 - 320 nm
what is the wavelength of UVC?
100 - 280 nm
What is the energy of UVA at 400nm (in eV)?
3.1 eV
What is the energy of UVB at 320 nm (in eV)?
3.9 eV
What is the energy of UVC at 280 nm (in eV)?
4.4 eV
Rank UV radiation from least to most ionising
UVA
UVB
UVC
What are sources of UV radiation?
- Mercury lamps
- Black light lamps
- Laser
- LED
- Arc lamps and welding
A fluorescent tube is a __ _______ source.
UV radiation
How do fluorescent tubes work?
They rely on the inelastic scattering of electrons. The electrons collide with atoms in the gas which causes a temporary jump of one of the gas electrons to a higher energy level and light is emitted when they return to their original state.
What are the wavelengths of fluorescent light?
253.7 nm to 185 nm
Give two examples of UV detectors
- Semiconductor devices (cheap and sensitive)
- Thermopile (true radiometric measurements)
How do semiconductor diode devices detect UV?
1) The n-type semiconductor acts as a detector of photons.
2) Incoming photons are prevented from recombining with the material by an imposed electric field, generating electron-holes pairs.
3) A current flows proportionally to the incident photon flux, showing the location of the beam.
4) The p-type semiconductor acts as a voltage multiplier so that the location of each photon is clearer.
