Personal Dosimetry

Question 1

What is Absorbed dose?

Answer 1

The energy absorbed by a medium per unit mass (measured in J/Kg).

Question 2

What unit is Absorbed dose measured in?

Answer 2

GRAY

Question 3

What is Equivalent Dose?

Answer 3

The dose obtained by multiplying the absorbed dose (Gy) by a radiation weighting factor (WR)

Question 4

What unit is Equivalent Dose measured in?

Answer 4

Sievert (Sv)

Question 5

What unit is Effective Dose measured in?

Answer 5

Sievert (Sv)

Question 6

What is Tissue weighting factor (WT)?

Answer 6

A relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue

Question 7

Which organs/tissues has a Tissue Weighting Factor (WT) of 0.20?

Answer 7

Gonad

Question 8

Which organs/tissues has a Tissue Weighting Factor (WT) of 0.12?

Answer 8

Red Bone Marrow
Colon
Lung
Stomach

Question 9

Which organs/tissues has a Tissue Weighting Factor (WT) of 0.05?

Answer 9

Bladder
Breast
Liver
Oesophagus
Thyroid

Question 10

Which organs/tissues has a Tissue Weighting Factor (WT) of 0.01?

Answer 10

Skin

Question 11

What is the Linear No-Threshold (LNT) Dose-effect Model

Answer 11

If all radiation dose is minimised then individuals are unlikely to be exposed to any unnecessary risk

Question 12

What’s the drawbacks of the LNT Model

Answer 12

It does not consider human defense mechanisms

Question 13

What is a classified person?

Answer 13

Employees who are likely to receive:

An effective dose greater than 6 mSv per year or

An equivalent dose greater than 15mSv per year for the lens of the eye or

A dose greater than 150mSv per year for the skin or the extremities (hands, forearms, feet or ankles

Question 14

What is Effective Dose?

Answer 14

The dose given to the whole body that would produce the same biological effect as the equivalent dose(s) received by each organ or tissue

Question 15

Women who are pregnant or breastfeeding should not recieve an equivalent dose to the fetus at above what dose?

Answer 15

1 mSv

Question 16

Name the 5 Characteristics of Dosimeters

Answer 16

Able to measure doses over the whole range of radiation types & energies encountered by the wearer

Show an appropriate response over that range

Provide sufficient information to enable the radiation type & energy to be identified and suitable correction factors applied

Capable of measuring doses in the range of fractions of milli Sieverts to tens of Sieverts.

Be issued for a specific period

Question 17

What does TLD stand for?

Answer 17

Thermoluminescent Dosimeter Badges

Question 18

How does a Thermoluminescent Dosimeter Badge (TLD) work?

Answer 18

When the radiation falls on TLD, the electrons are excited and store energy.

The TLD reader consists of a heater. On getting heated, the excited electrons again come back to the ground state and emit light which is then read by a photomultiplier.

The light output is proportional to the radiation exposure.

Question 19

What dose range can TLDs measure in Sieverts and Gray?

Answer 19

0. 01 mGy to 10 Gy^3

0. 05 mSv to 10 Sv

Question 20

Where are Extremity TLD Dosimeters worn?

Answer 20

Worn on fingers or wrists

Question 21

What dose range can Extremity TLD measure in Sieverts?

Answer 21

0.2 mSv to 10 Sv

Question 22

What radiation type does TLD’s not read?

Answer 22

Alpha radiation

Question 23

What dose range can Photographic Film Dosimeters measure in Sieverts for beta and gamma compared to x-rays?

Answer 23

0. 1 mSv to 10 Sv for beta & gamma radiation

0. 1 mSv to 400 mSv X-ray radiation

Question 24

How does a Photographic Film Dosimeter work?

Answer 24

The badge consists of two parts: Photographic film and the film holder.

The silver film emulsion is sensitive to radiation and forms the latent image in the film.

Exposed areas increase in optical density (film blackening) in response to incident radiation.

The photo densitometry analysis is used to compare the worker’s exposed film with the standard films exposed to calibrated radiation doses

Question 25

How does an Optically Stimulated Luminescence (OSL) Badge Dosimeter work?

Answer 25

Uses a lase to stimulate an aluminumoxide material. A tiny crystal traps and stores energy from exposure to ionizing radiation Amount of exposure can be determined by shining a green light on the crystal and measuring the intensity of the blue light emitted

Question 26

What dosimeter badges can be re-used and which cannot be?

Answer 26

Thermoluminescent Dosimeter (TLD) badge - Cannot be re-used Optically Stimulated Luminescence (OSL) badge dosimeters - Can be re-used

Question 27

What dose range can Direct Ion Storage (DIS) Dosimeter measure in Sieverts?

Answer 27

0.01 mSv to 5 Sv

Question 28

Give 8 characteristics of A Geiger counter (Geiger-Muller tube)

Answer 28

- GM inexpensive, since amplification in detector means simple amplification post detector - GM tubes are more robust than ionisation chambers - GM tubes have reasonable response time - End mica window is fragile - Relationship between counts per second and dose rate is energy dependent - Cannot differentiate different radiations being detected - Cannot be used to determine exact energy of the detected radiation - Have a smaller size and very low efficiency.

Question 29

Describe how a Geiger-Muller Tube (Counter) works

Answer 29

Ionising radiation particles enter the tube and ionise some of the gas molecules in the tube. The tube consists of a pair of electrodes surrounded by a gas, with a high voltage across the electrodes. Filled with Helium or Argon An electron is knocked out of the ionised atom, and the remaining atom is positively charged. The high voltage in the tube produces an electric field inside the tube. The electrons that were knocked out of the atom are attracted to the positive electrode. Positively charged ions are attracted to the negative electrode. This produces a pulse of current in the wires connecting the electrodes, and this pulse is counted

Question 30

What 3 advantages does an Ionisation Chamber have over a GM tube?

Answer 30

Have linear response Have a dose rate scale Usually more robust than end-window GM tubes

Question 31

List 4 characteristics of Ionisation Chambers

Answer 31

- Sensitive to humidity, temperature & shock - Expensive - Response time slow - For given detector size, less sensitive than other types

Question 32

Describe how a Ionisation Chamber works

Answer 32

Ionising radiation passing through the gas produces free electrons and positive ions. These are attracted towards electrodes and generate current and measures the direct dose rate

Question 33

Describe how a Scintillation Detector works

Answer 33

When an ionising particle passes into the scintillator material, atoms are ionised along a track The photon from the scintillation strikes a photocathode and emits an electron which accelerated by a pulse and produces a voltage across the external resistance This voltage is then amplified and recorded by the photomultiplier tube (Electronic counter).

Question 34

Give 3 advantages of Scintillation Detectors

Answer 34

- Very sensitive for given detector volume - Alpha and Beta particles can be detected - Fast response time

Question 35

Give 3 disadvantages of Scintillation Detectors

Answer 35

- No dose scale because signal varies with radiation energy - Window vulnerable to damage - Expensive