Measuring radiation/radioactivity and LNT model

Question 1

Activity

Answer 1

= disintegrations per second.

Bq (Becquerel), Ci (Curie)

Question 2

Bq

Answer 2

Becquerel (SI)
One disintegration per second
Used for measuring activity

Question 3

Ci

Answer 3

Curie (non-SI)
3.7x10^10 Bq
Used for measuring activity

Question 4

Exposure

Answer 4

= ionisation produced in the air by gamma.

Coulombs/kg (C/kg), Roentgen (R)

Question 5

C/kg

Answer 5

Coulombs/kg (SI)
Number of gamma required to produce 1 C of electrical charge in 1 kg of air
Used for measuring exposure

Question 6

R

Answer 6

Roentgen (non-SI)
2.58 x 10^-4 C/kg
Used for measuring exposure

Question 7

Absorbed Dose

Answer 7

= energy deposition in matter

Gy (Gray), rad (Radiation Absorbed Dose)

Question 8

Gy

Answer 8

Gray (SI)
Deposition of 1 J per kg of material
Used for measuring absorbed dose

Question 9

rad

Answer 9

Radiation Absorbed Dose (non-SI)
0.01 Gy / 1.02 R in soft tissue (for gamma)
Used for measuring absorbed dose

Question 10

Dose equivalent

Answer 10

= biological measure of damage

Sv (Sievert)

Question 11

Sv

Answer 11

Sievert
1 Sv = QF x 1 Gy (QF = quality factor)
Used for measuring dose equivalent

Question 12

1 Gy

Answer 12

Radiation sickness from intestinal damage

Question 13

> 3 Gy

Answer 13

Blistering of the skin

Question 14

> 8 Gy

Answer 14

Eventual death from indirect causes e.g. infection

Question 15

> 500 Gy

Answer 15

Rapid death from CNS damage

Question 16

Linear No Threshold Model

Answer 16

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

Question 17

Controversy surrounding LNT model

Answer 17

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

Question 18

2 main principles of radiation detection

Answer 18

1. Electrical collection of ions from the ionisation of air

2. Scintillation (using a crystal to produce flashes of light)

Question 19

Ionisation chamber

Answer 19

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

Question 20

Advantages of ionisation chamber

Answer 20

Relatively inexpensive
Durable
Portable
Detects all radiation types

Question 21

Disadvantages of ionisation chamber

Answer 21

Cannot differentiate types of radiation
Cannot determine exact energy of radiation
Low efficiency
Long dead time

Question 22

Geiger counter

Answer 22

Measures the ionising radiation using the “ionisation effect” produced in a Geiger-Muller tube

Question 23

Scintillation counters

Answer 23

1. Charged particle strikes scintillator
2. Atoms are excited and photons are emitted
3. Emitted photons are directed at the photocathode
4. Electrons are emitted by the photoelectric effect
5. Electrons are electrostatically accelerated and focus by an electrical potential, striking the first dynode of the tube
6. The impact of a single electron on the dynode release secondary electrons
7. These electrons are accelerated to strike the second dynode
8. Each subsequent dynode impact release further electrons at each dynode (“current-amplifying effect”)
9. 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

Question 24

Why does a scintillator need to be in complete darkness?

Answer 24

So visible light photons don’t swamp the individual photon events caused by the incident ionising radiation

Question 25

Liquid Scintillation Counting

Answer 25

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

Question 26

Examples of scintillators

Answer 26

NaI (+small amount of Th) to detect gamma waves

CsI crystal to detect protons/alpha particles

Question 27

Background radiation

Answer 27

55 % radon
18 % man-made
11 % internal
8 % cosmic
6 % terrestrial

Question 28

Acute exposure to radiation

Answer 28

Large dose in a short time

Question 29

Chronic exposure to radiation

Answer 29

Small doses over a long period of time

Question 30

Examples of radioactive tracers

Answer 30

14C for photosynthesis research
3H for organic mechanisms
32P for uptake of fertilisers