Exposure Limits and Shielding

What are the 3 Cardinal Principles of ALARA

1. Time

2. Distance

3. Shielding

What is time affected by

• Exposure rate

• time around radiation

Occupational Annual (in Rem)

5

Occupational Cumulative (in Rem)

1 Rem x Age

Occupational Lens (in Rem)

15

Occupational: Skin, Hands, Feet (in Rem)

50

Public: Frequent Annual (in Rem)

0.1

Public: Infrequent Annual (in Rem)

0.5

Public: Lens (in Rem)

1.5

Public: Skin, Hands, Feet (In Rem)

5

Embryo/Fetus: Total (in Rem)

0.5

Embryo/Fetus: Monthly (in Rem)

0.05

When do HCP get dosimeters

When expected to get >10% of the effective dose

What is the most common personnel Monitoring device used

OSL

Area limits are measured suing what unit

mSv

An individual in an area should not receive more then the ________ ________ _________

Maximum permissible dose

MPD Controlled Area

• 0.1 mSv/week

• 5 mSv/year

MPD Uncontrolled Area

• 0.02 mSv/week

• 1 mSv/year

Unctrilled areas have less MPD because…

They're not expected to get any exposure

Radiation area signs placed when expecting >_____ mSv in ____ hour at ____ cm from the source

• 0.05 mSv

• 1 hr

• 30 cm

High Radiation area signs placed when expecting >_____ mSv in ____ hour at ____ cm from the source

• 1 mSv

• 1 hour

• 30 cm

Very High Radiation area signs placed when expecting >_____ Gy in ____ hour at ____ meter from the source

• 5 Gy

• 1 hour

• 1 meter

5 Gy = ____ rads

500

Are barriers the same for each radiation type?

No

Barrier needed for alpha particles

Paper

Barrier needed for Beta particles

• Aluminum

• Plastic

Barrier needed for x-rays and Gamma rays

Lead

Barrier needed for Neutrons

Concrete

Barriers must protect against what 2 things

1. Primary radiation

2. Scatter

What 5 factors go into barrier thickness?

1. P (allowed dose equivalent)

2. W (Workload)

3. U (Use Factor)

4. T (Occupancy Factor)

5. d (Distance)

P represents what

Allowed dose equivalent for those who are in controlled and uncontrolled areas

W represents what

• Dose delivered per week at 1 meter

• (# of patients treated per week) x (Dose per patient at 1 meter)

U represents what

How long the beam is aimed at a barrier

Use Factor for 0° gantry

31%

Use Factor for 90°/270° gantry

21.3%

Use Factor for 180° gantry

26.3%

T represents what

How long adjacent rooms are occupied

Full-time occupancy:

T = ___

1

Adjacent room occupancy:

T = ___

1/2

Corridor occupancy:

T = ___

1/5

Tx vault doors occupancy

T = ___

1/8

Public Bathroom/Storage

T = ____

1/20

Outdoor area

T= ___

1/40

d represents what

Distance from source to the area shielded in meters

What do you take into account for distance?

Inverse square law

What is the most common shielding method?

Concrete

What are the 2 main barrier types

1. Primary

2. Secondary

What is a primary barrier

The barrier the useful beam is aimed at and hits

Examples of primary barriers

• Side walls

• Ceilings

• Floor

Examples of secondary barriers

• Back wall

• Front wall

What hits the secondary barrier

• Scatter

• Leakage

The maximum photon Energy that scatters 90° is _____ KeV

500

• 1MeV / 2 = 500 KeV

If photons scatter <90°, it has _______ (lower/higher) energy and ______ (less/more) penetrating power

Higher; more

If photons scatter >90°, it has _______ (lower/higher) energy and ______ (less/more) penetrating power

Lower; Less

What is the average concrete primary barrier thickness? What energy?

2 meters

• 15-18 MV

What is the average concrete secondary barrier thickness? What energy?

1 meter

• 18 MV

Why are some mazes longer?

Neutron scatter

If there is no maze, what are the doors covered with

Polyethylene