Measuring radiation/radioactivity and LNT model Flashcards
Activity
= disintegrations per second.
Bq (Becquerel), Ci (Curie)
Bq
Becquerel (SI)
One disintegration per second
Used for measuring activity
Ci
Curie (non-SI)
3.7x10^10 Bq
Used for measuring activity
Exposure
= ionisation produced in the air by gamma.
Coulombs/kg (C/kg), Roentgen (R)
C/kg
Coulombs/kg (SI)
Number of gamma required to produce 1 C of electrical charge in 1 kg of air
Used for measuring exposure
R
Roentgen (non-SI)
2.58 x 10^-4 C/kg
Used for measuring exposure
Absorbed Dose
= energy deposition in matter
Gy (Gray), rad (Radiation Absorbed Dose)
Gy
Gray (SI)
Deposition of 1 J per kg of material
Used for measuring absorbed dose
rad
Radiation Absorbed Dose (non-SI)
0.01 Gy / 1.02 R in soft tissue (for gamma)
Used for measuring absorbed dose
Dose equivalent
= biological measure of damage
Sv (Sievert)
Sv
Sievert
1 Sv = QF x 1 Gy (QF = quality factor)
Used for measuring dose equivalent
1 Gy
Radiation sickness from intestinal damage
> 3 Gy
Blistering of the skin
> 8 Gy
Eventual death from indirect causes e.g. infection
> 500 Gy
Rapid death from CNS damage
Linear No Threshold Model
Used to calculate the probability of radiation-induced cancer
Assumes the long-term biological damage caused by ionising radiation is directly proportional to the dose - i.e. the sum of several small exposures is considered to have the same effect as one larger exposure (“response linearity”)
e.g. cancer risk from 0.001 Sv is 0.001 x the risk from 1 Sv
Controversy surrounding LNT model
Ramsar (Iran) has highest natural background radiation levels but local population do not seem to suffer ill effects
Low radiation doses may be beneficial (Hormesis model) - i.e. may activate repair mechanisms that protect against disease
2 main principles of radiation detection
- Electrical collection of ions from the ionisation of air
2. Scintillation (using a crystal to produce flashes of light)
Ionisation chamber
Detects/measures ionising radiation
Consists of a sealed chamber containing a gas and 2 electrodes between which a voltage is maintained by an external circuit
1. Ionising radiation enters the chamber through a foil-covered window
2. Ionises gas molecules
3. Ions are attracted to their oppositely-charged electrodes, leading to a momentary drop in voltage
4. This is recorded by an external circuit
5. The observed drop in voltage identifies the radiation, because the drop in voltage depends on the degree of ionisation, which depends on the charge/mass/speed of the photon
Advantages of ionisation chamber
Relatively inexpensive
Durable
Portable
Detects all radiation types
Disadvantages of ionisation chamber
Cannot differentiate types of radiation
Cannot determine exact energy of radiation
Low efficiency
Long dead time
Geiger counter
Measures the ionising radiation using the “ionisation effect” produced in a Geiger-Muller tube
Scintillation counters
- Charged particle strikes scintillator
- Atoms are excited and photons are emitted
- Emitted photons are directed at the photocathode
- Electrons are emitted by the photoelectric effect
- Electrons are electrostatically accelerated and focus by an electrical potential, striking the first dynode of the tube
- The impact of a single electron on the dynode release secondary electrons
- These electrons are accelerated to strike the second dynode
- Each subsequent dynode impact release further electrons at each dynode (“current-amplifying effect”)
- There is a measurable ‘pulse’ output at each anode for each photon originally detected at the photocathode - this pulse carries information about the energy of the incident radiation therefore the intensity and the energy of the radiation can be measured
Why does a scintillator need to be in complete darkness?
So visible light photons don’t swamp the individual photon events caused by the incident ionising radiation
Liquid Scintillation Counting
Active material mixed with liquid scintillator and the resulting photon emissions are counted
Generally used for alpha/beta detection
More efficient counting due to intimate contact of the radioactive material with the scintillator
Little background noise
Examples of scintillators
NaI (+small amount of Th) to detect gamma waves
CsI crystal to detect protons/alpha particles
Background radiation
55 % radon 18 % man-made 11 % internal 8 % cosmic 6 % terrestrial
Acute exposure to radiation
Large dose in a short time
Chronic exposure to radiation
Small doses over a long period of time
Examples of radioactive tracers
14C for photosynthesis research
3H for organic mechanisms
32P for uptake of fertilisers
