Dose monitors, guidelines and shielding

Question 1

Name monitoring situations

Answer 1

• general external gamma-ray exposure
• extremity exposures
• contamination monitoring
• leakage from irradiating apparatus
• neutron monitoring

Question 2

What is the primary radiation hazard in many cases?

Answer 2

External gamma ray exposure

Question 3

Name 4 typical personal dosimeters

Answer 3

1. film badges
2. thermoluminescent dosimeters
3. optically stimulated devices
4. pocket electronic dosimeters.

Question 4

Where are monitors worn?

Answer 4

At chest or waist level

Question 5

Key characteristics of personnel monitors

Answer 5

small and light
robust
reusable
cheap

Question 6

What is the most common dosimeter

Answer 6

Film badges

Question 7

Features and process of film badges

Answer 7

Film dosimeters incorporate photographic emulsion of (typically) silver halide crystals and gelatin, which is coated onto a transparent polyester plate.

Incident radiation causes the release of electrons (remember photoelectric effect, Compton scatter), which are then trapped. Try to rejoin an atom and gets trapped in an imperfection.

At the locations where the electrons are trapped, electrostatic charges attracts silver atoms (in the silver halide).

As long as the film isn’t overexposed or underexposed, the number of electrons should be proportional to the radiation dose.

Question 8

TLD process

Answer 8

Exposure of some crystals to IR results in excitation of electrons (or holes) from their normal (stable) position in the crystal lattice.

The excited electrons may be captured at lattice imperfections or impurity atom sites in the crystal.

We can later heat the TLD to release the electron from the trap back to the conduction band.

The electron returns to the valence band and gives off light in the process, which we measure (it’s too hot to be trapped)

Question 9

Glow curve

Answer 9

A measurement made of the light output versus temperature

Glow curves show several peaks corresponding to the depths of the various traps

Deeper traps require more energy to empty them and are seen at higher temperatures.

The peaks seen at lower temperatures, being more shallow traps, may ‘fade’ if the TLD is left for a time between exposure and reading.

Question 10

3 ways to control exposure to radiation in a workplace

Answer 10

Administrative Controls

Engineering Controls

Quality Assurance

Question 11

Which exposures can be minimised by administrative controls

Answer 11

1. Medical exposure - minimise probability of accidents. Follow ALARA
2. Public exposure - constrained optimised approach
3. Occupational exposure - PPE (last line of defence), reduction in time of exposure

Question 12

What areas are facilities divided into?

Answer 12

1. Public
2. Supervised
3. Controlled

Question 13

Controlled Area

Answer 13

Areas of high dose rates or potential exposure is high

Designated by signs

Well established work procedures
must be in place

Only radiation
workers are authorised to enter

Annual dose limit typically 5 mSv (but varies with jurisdiction)

Question 14

Supervised area

Answer 14

Working conditions kept under review but special procedures not required
Designated by signs
Workers designated as such and monitored

Question 15

What is shielding

Answer 15

Shielding a radiation source means putting a barrier of some type between the source and the area you want to shield

Question 16

3 considerations needed for protection from external radiation

Answer 16

1) Time: limit the exposure time to the minimum
2) Distance: use inverse square law (radiation dose decreases proportionally to the square of the distance) – get some distance between you and the source
3) Shielding: attenuate the beam by putting some sort of absorber between it and you (like a lead shield)