Topic 2: ionising radiation measurements + standards Flashcards
Who discovered X-rays?
- Wilhelm Conrad
- Father of radiography
- Mechanica engineer + physicist
- 1895
- By accident
Who discovered radioactivity?
- Henri Becquerel
- In 1896
- Accidentally detected radiation
- Within a year of X-ray discovery = used film + electroscope to detect ionising radiation
Define dosimeter
- Measures ionizing radiation
- Measuring assembly = electrometer + detector
- Monitoring equipment calibrated = f mrad/h or µGy/h
What are the 2 ways to measure the dose rate?
1) Geiger Muller tube = low level dose rate
2) Ionization chamber = more accurate + less effected by radiation = can measure high dose rate
What is the standard instrument for measuring dose rate?
- Ionization chamber system
- Solid state detector + scintillation counter not suitable for routine dose measurements
What is a TLD?
- Cheapest + most common personal monitor
- AKA thermoluminescent dosimetry
- Getting replaced by ionization chamber dosimeters
- Don’t offer protection/alert to exposure/not radioactive
Advantages of film dosimeters
- Film badge = very simple personal monitor = inexpensive
- Permanent record
- Very reliable
- Measure + record exposure = gamma + X-ray + beta particles
Disadvantages of film dosimeters
- Cannot be read on site = have to be developed
- 1 time use
- Exposure > 0.2 mSv gamma = not accurately measured
Advantages of TLDs
- Measure greater range of doses compared to film
- Doses easily obtained
- Read on site
- Reusable
Disadvantages of TLDs
- Each dose cannot be read more than once
- Readout = zeros TLD
Describe electronic personal dosimeters
- For occupational radiation workers in planned exposure situations = measure for regulatory compliance
- High range
- Alarming
- Displays dose + dose rate
- High level sensitivity
Explain lightning
- Electricity cannot flow in air = no free e- + ions to carry current
- If some atoms in air ionized = free e- produced = electric current can flow
- High potential gradient between cloud + ground
What is the use of ionization chambers?
- Measure ionizing radiation output of therapeutic + diagnostic radiation generators
- Used to make accurate measurements of patient radiation dose
Describe ionization chambers
- Ionizing radiation frees e- in air of chamber
- e- fill up chamber = allow current to flow
- Potential applied across metal plates
- Chamber = sealed/open
- When chamber exposed to radiation = +/- ions produced
- +ve attracted to -ve plate and vise versa
- This allows current to flow through chamber
- Current measured by sensitive ammeter because currents = 10^-9 A > 6x10^9 e- per sec
What is the use of GM counters?
- Detect + measure all types of radiation = alpha/beta/gamma
- Not for diagnostic radiology
Describe GM counters
- Very sensitive form of ionization chamber = can detect single ionizing particle entering tube
- Structure = central wire electrode inside hollow metal tube
- Difference from ionization chamber = filled with gas e.g. argon/neon instead of air
- Gas 1/5th atm
- Incident ionizing radiation = produce free e- in tube = attracted to central electrode = positive potential
- e- attracted towards central anode = accelerated by potential = gain energy to further ionize = chain reaction
- When all e- produced = hit central anode = photons emitted = cause more ionisation in chamber
How many e- can the original ionizing radiation produce in GM tube?
- 10^5
- Measured as a pulse current = 1µs
What is the disadvantage of the GM tube?
- Positive ions = travel slowly compared to e-
- Tube takes long to recover from recorded pulse
- Causes dead time = limits number of events that can be recorded each second
2 difficulties in diagnostic radiology with GM counter
1) Several-second response time
2) Strong energy dependence at low photon energy
