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‫الرحيم‬
ّ ‫الرحمان‬
ّ ‫باسم اهلل‬
Course :
Health Safety & Radiation Protection (RAD453)
Chapter 5: External Radiation Protection
Omrane KADRI, Ph.D.
[email protected]
Office 2021
1
Outline
1. ICRP’s Three Types of Exposure
2. Basic Principals of Radiation Protection
3. Protecting against external radiation:
Methods of protection (ALARA principles)
Access Control
4. Personnel Monitoring
2
1. ICRP’s Three Types of Exposure
ICRP: International Commission on Radiological Protection
– 20 (1) mSv a year effective dose (averaged over 5 years, but <50mSv in a single
year)
– 150 (15) mSv a year to lens of eye
– 500 (50) mSv a year to 1 cm2 of skin, hands and feet
– 1 mSv to the embryo/fetus
•
Occupational (work related) and Public
•
Medical (diagnosis and treatment ): No dose limits apply  Consider dose
constraints
3
2. Basic Principals of Radiation Protection
• Justification
– Benefit > risk
• Optimisation
– Doses as low as reasonably achievable
• Limitation
– Absolute legal limits for staff and public
– Reference levels as guidance for patients.
Diagnostic objective
Medical
exposure
• “Any decision that alters
the radiation exposure
situation should do more
good than harm.”
• i.e. must be a net benefit.
4
The Optimisation of Protection
• “The likelihood of incurring exposures, the number of people exposed,
and the magnitude of their individual doses should be kept as low as
reasonably achievable, taking into account economic and societal
factors .”
ALARA
as low as reasonably achievable
.
5
Individual Dose and Risk Limits
• “The total dose to any individual from regulated sources in
planned exposure situations other than medical exposure of
patients should not exceed the appropriate limits
recommended by the Commission.”
• Prevent deterministic effects
• Limit risk of stochastic effects to acceptable level.
6
Example of Radiation Damage
700 CANCER CASES CAUSED BY X-RAYS
Erythema Of Right Hand 4th Day , 17th Day, 3 ½ Months , Five
Months , Seven Months After Accident
7
External Radiation Protection Consider This …
Exposure to a source of ionizing radiation is very similar to
the exposure from a light bulb (i.e. light and heat).
The closer you are
to the source, the
more intense the
light and heat are.
Likewise, if you
move away, the
intensity decreases.
If you put something
opaque between
you and the light
bulb, you effectively
eliminate the light.
The longer you are
close to the light
bulb, you begin to
feel the warming
effects of the light.
If however, you
move quickly to and
from the light, you’ll
not likely feel the
warming effect.
8
Exposure & Contamination
A difficult concept to understand is the difference between exposure
and contamination when we talk about radioactive materials.
To illustrate the difference, consider a burning candle.
• If you stand away from the candle, you
are being exposed to the candle’s light. If
you leave the room, your are no longer
exposed to the candle’s light.
• If you walk up to the candle, you are
being exposed to the candle’s light. If you
then reached out and grabbed the candle,
you would get hot wax on your hand. If
you left the room, you are no longer
exposed to the light, but the wax on your
hand (i.e. contamination) remains. If the
wax were radioactive, the “contamination”
would continue to expose your hand until
you washed it off.
Remember: Being
exposed by a
radioactive source
does not
contaminate you.
You must have
interacted with the
source to get some
of the source on
you. Once on you,
the contamination
will expose you
until it is removed.
9
General Safety Guides
• X-ray equipment should not be left unattended while in
operating mode.
• When in fixed radiographic rooms, operators shall remain
behind the protective barrier.
• If required to be in a room during a diagnostic x-ray
exposure (e.g. fluoroscopy), wear a lead coat or stand
behind a protective barrier.
• Wear your dosimeter, if applicable.
• Follow established procedures; when unsure, stop and
notify your supervisor.
• Keys MUST not be left in portable x-ray equipment.
10
Radiation Symbols
• Caution Radioactive Materials
• Caution Radiation Area
• Caution Radiation Area when
X-ray Energized
Cyclotron
“Hot” Lab
PET/CT Scanner11
3. Protecting against external radiation
Methods of Protection (ALARA)
•
Allow source to Decay
•
Decrease Time spent near source
•
Increase Distance between source & yourself
•
Absorb the radiation in Shielding
Access Control
• Access control measures are commensurate with the radiation hazard for
the area.
12
Decay
• Allow a cool down time between end of beam and entering
the radiation area
Ex: You have to work for half an hour near a beam line component which
is giving an equivalent dose rate of 3.2 mSv/h. The activation products
causing this field have an apparent half-life of 2 hours. How long should
you wait before doing the job so that the dose you receive will not be
greater than 0.1 mSv?
13
Decay
• Ex (cont’d):
•
Dose rate will decrease by a factor of 2 for every 2 hours.
• Initial dose for half hour = 3.2 mSv/h * 0.5 h
= 1.6 mSv
•
After 2 hours; = 0.8 mSv
•
After 4 hours; = 0.4 mSv
•
After 6 hours; = 0.2 mSv
•
After 8 hours; = 0.1 mSv
• Thus, if you wait for 8 hours after the beam is shut off you will only
receive 0.1 mSv if you work for half an hour.
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Time
•
•
Radiation exposure is controlled by limiting time spent in radiation area
Time can be limited by:
1. Planning job ahead of time
2. Task can be practiced on similar set-up or video used to
demonstrate task.
3. Tools required are prepared ahead
15
Distance
• Intensity of radiation from a point source obeys Inverse square
law.
• I1d12=I2d22
Source
d1
d2
d=50cm
150 mSv/h
0.06 mSv/h
16
Distance
• Extension tools can be used to distance oneself from hot spots
• Important to have survey map showing hot spots
• Localized hot spots may result in a high dose to part of the body which
the more distant dosimeter would not ‘see’.
• Beta sources with thin or no shielding can result in sizeable dose to
extremities. Use tongs when handling.
17
Shielding
18
Access Control
• Fields are posted to control access in these areas
1. Unmarked Areas
• Levels depend on occupancy factor
• Access via boot box
2. High Radiation Areas
• Work permit & survey map required
• Controlled by the Central Safety System
3. Exclusion Areas
- Primary beamline access requires 3 distinct elements
to be “off”
- Secondary channels require 2 distinct elements to be
“off”
- Residual monitors < safe level for entering
• Area Safety Unit
19
Access Control
A survey sign indicates a local
radiation field
1. Unmarked Areas
2. High Radiation Areas
A boot box marks a
high radiation area
3. Exclusion Areas
An Area Safety Unit
(ASU) ascertains
that all safety
precautions have
been followed for a
lockdown
20
4. Personnel Monitoring
Monitoring Required
Whole Body
Extremity
Internal Contamination
Monitoring Method
TLD or OSL Badge
Finger Ring TLD
Urinalysis or Bioassay
Ring Badge
Whole Body
Badge
Thyroid Bioassay
21
4. Personnel Monitoring
• Wear your own badge.
• Wear your whole body
(WB) badge whenever
working with radiation
sources
• Notify the concerned
person immediately when
a badge is lost.
• Wear ring badges under
gloves.
• Store badges in
designated areas at the
end of each day of work.
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4. Personnel Monitoring
• Dosimetry does not protect
you from radiation.
• Dosimetry is not a warning
device (i.e. it will not alarm,
beep or change color)
• Dosimetry documents the
radiation dose an individual
receives when working with
radiation sources.
• It is ILLEGAL to intentionally
expose an individual’s
dosimeter.
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4. Personnel Monitoring
• Each monitoring period dose report is reviewed by the Radiation
Safety Officer (RSO)
• The report is compared against the institution’s investigational levels:
>200 mrem/monitoring period to whole body
> 2000 mrem/monitoring period to extremities
> 800 mrem/monitoring period to the skin
Action Required: Written notification from RSO to worker and
investigation
24
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